Digitized by the Internet Archive in 2015 https://archive.org/details/painterscoloursoOOhurs_0 PAINTERS' COLOURS, OILS, AND VARNISHES: A PRACTICAL MANUAL. GEOEGE H. HURST, F.C.S., \LMBER ''F THE SOCjEVY OF CtEJKCL IN DUbTR L '. LECTURER ON THE TECHNOLOGY OF PAINTERS' COLOURS, OILS, AND VARNISHES, AT THE MUNICIPAL TECHNICAL SCHOOL, MANCHESTER. TOitb IRumerous illustrations* TP 1692. LONDON: CHARLES GRIFFIN & COMPANY, LIMITED. PHILADELPHIA : J. B. LIPPINCOTT COMPANY. < i « < THE GETTY CENTER LIBRARY PEE F AC E. In offering the following pages to Practical Workers and others interested in the wide subject of "Painters' Colours, Oils, and Varnishes," my aim throughout has been to combine theory and practice, and to show the scientific principles that underlie the methods in constant use. Naturally — and one may say unavoidably — there has grown up in the course of years, in connection with Colour-making, as with every other industry, a good deal of what is known as " Pule of thumb " procedure. The amount of this that prevails, however, has been greatly overrated, and we are not far distant from the day when " Rule of thumb " will be generally supplemented among us by an intelligent appreciation of the scientific principles involved. To give the rationale of every technical process is, nevertheless, by no means an easy task, and all that I can hope to have effected is the placing before the reader such a description of the various processes and their underlying principles, as shall be really helpful in practical work. The information given as to the properties and preparation of Pigments, is either based on my own experience, or drawn from the most trustworthy sources. For a revision of the chapter on Varnishes, and many excellent suggestions, I am indebted to a personal friend, practically engaged in their 3 6 2, / vi PREFACE. manufacture. My best thanks are due to him, and also to Messrs. Brinjes & Goodwin, Follows & Bate, Ritchie & Co., Rose, Downs & Thompson, and Rushton, Irving & Co., who have kindly furnished for the work illustrations of the newest types of Paint and Oil Machinery. GEORGE H. HURST. Chemical Laboratory, 22 Blackfriars Street, Manchester, October, 1892. GENERAL CONTENTS. CHAPTER L INTRODUCTORY. Colour, the Spectrum Colours, White from Coloured Light, Light from Coloured Bodies, Cause of Colour in Coloured Bodies, Colour Theories, Pigments, Paint, Varnishes, CHAPTER II. WHITE PIGMENTS. White Lead; Manufacture of White Lead; Dutch Method of White- Lead making; Chemistry of White-Lead making; Chamber Methods of White-Lead making ; Creed or German Process, Hatfield Process, Thompson's Process, Gardner's Electric Process ; Precipitation Processes of White-Lead making; the Kremnitz, Thenard, Cory, Milner, Martin, Fourmentin, Spence, Maclvor, Dundonald, Pattinson, Dale & Milner, Watt & Tebbutt, Delafield, Rowan, Lowe, and Condy Pro- cesses ; Miscellaneous Processes of Making White Lead ; the Torassa, Mullins, Martin, Lewis, Button & Dyar, Brown & Young, Maxwell Lyte, Woolrich, Ozouf, and Cookson Processes; Composition and Properties of White Lead; Analyses of White Leads; Assay and Analysis of White Lead; Analysis of Dry White Lead ; Analysis of Paste White Lead. Sulphate of Lead Pigments ; Sublimed White Lead, Freeman's Non-poisonous White Lead, Hannay's Caledonian White Lead, Maxwell Lyte White Lead. Sulphite of Lead White ; Zinc Whites, Zinc White ; Sulphide of Zinc Whites, PAGES 1-8 viii CONTENTS. Orr's White, Charlton White, Griffith's Zinc White, Knight's Zinc White, Lithophone; Barytes, Blanc Fixe, Gypsum, Strontian White; Whiting; Magnesite; China Clay; Manufacture of China Clay, Composition and Pro- perties of China Clay, Assay and Analysis of China Clay; Wilkinson's White Lead ; Pattinson's White Lead, . 9-87 CHAPTER III. RED PIGMENTS. Vermilion; Manufacture of Vermilion, Properties of Vermilion; Red Lead, Manufacture of Red Lead, Properties and Com- position of Red Lead, Assay and Analysis of Red Lead; Orange Lead ; Red Oxides, Oxides of Iron, Manufacture of Oxide Reds, Properties of Red Oxides, Analysis and Assay of Red Oxides, Analyses of Red Oxides ; Antimony Vermilion, Properties of Antimony Vermilion, Composition of Anti- mony Vermilion, Analysis and Assay of Antimony Vermilion; Brilliant Scarlet; Chromate of Mercury; Chromate of Silver; Chromate of Copper ; Magnesia Pink, . . . 88-114 CHAPTER IV. YELLOW AND ORANGE PIGMENTS. The Chromes, Manufacture of Lead Chromes, Preparation of Chrome Yellows, Preparation of Chrome Oranges and Scarlets, Preparation of Chrome Red, Properties of the Lead Chromes, Assay and Analysis of the Lead Chromes ; Zinc Chrome, Preparation of Zinc Chrome, Properties of Zinc Chrome, Assay and Analysis of Zinc Chrome, Lemon Chrome; Barium Chrome; Ochres and Siennas, Properties of Ochres and Siennas, Assay and Analysis of Ochres and Siennas, Composition of Ochres and Siennas, Burnt Sienna ; Mars Colours, Turner's Yellow, Naples Yellow, King's Yellow, Realgar, Indian Yellow, Cadmium Yellow, Aure- olin, ........ 115-150 CONTENTS. ix CHAPTER V. GREEN PIGMENTS. PAGES Brunswick Green, Chrome Green, Copper Greens, Verdigris, Scheele's Green, Emerald Green, Mineral Green, Green Verditer, Bremen Green, Terre Verte, Cobalt Green, Brighton Green, Douglas Green, Chinese Green, Sap Green, Manganese Green, Titanium Green, Zinc Green, . . 151-179 CHAPTER VI. BLUE PIGMENTS. Ultramarine, Natural Ultramarine, Artificial Ultramarine, Manu- facture of Ultramarine, Properties of Ultramarine, Com- position of Ultramarine, Constitution of Ultramarine ; Assay and Analysis; Ultramarine Derivatives, Violet Ultramarine, Red Ultramarine; Prussian Blues, Chinese Blue, Soluble Blue, Antwerp Blue, Brunswick Blue, Properties of Prussian Blues, Assay and Analysis of Prussian Blues; Cobalt Blues; Smalts, Manufacture of Smalts, Composition and Properties of Smalts, Assay and Analysis of Smalts; Cobalt Blue, Properties and Composition of Cobalt Blue, Assay and Analysis of Cobalt Blue; Copper Blues, Mountain Blue, Bremen Blue, Blue Verditer, Lime Blue, Properties of Copper Blues; Coeruleum, Manganese Blue, . . 180-223 CHAPTER VII. BROWN PIGMENTS. Umber, Composition and Properties of Umbers, Assay and Analysis of Umbers, Vandyke Brown, Sepia, Cappagh Brown, Man- ganese Brown, Bistre, ..... 224-231 CHAPTER VIII. BLACK PIGMENTS. Lamp and Vegetable Blacks, Properties and Composition of Lamp- Blacks, Assay and Analysis of Lamp-Blacks; Bone-Black, X CONTENTS. PAGES Properties and Composition of Pone -Black, Assay and Analysis of Bone-Black, Ivory-Black ; Animal Black, Frank- fort Black, Miscellaneous Blacks, .... 232-248 CHAPTER IX. LAKES. Red Lakes, Carmine, Carmine Lake, Florentine Lake, Brazil- Wood Lakes ; Rose Pink, Red Lake, Yellow Lakes, Orange Lake, Madder Lakes, Madder Red Lakes, Green Lakes, Violet Lake ; Analysis of Lake Colours, Reactions of Natural Dye- stuffs, Aniline Lakes, Coal- Tar Colouring Matters, Precipi- tating Agents for Aniline Lakes, Manufacture of Aniline Lakes, Vermilionettes, and Royal Reds, Scarlet Lakes, Orange Lakes, Yellow Lakes, Blue Lakes, Brown Lakes, Violet Lakes, Black Lake, Green Lake, Alizarin Lakes, . 249-281 CHAPTER X. ASSAY AND ANALYSIS OF PIGMENTS. Colour and Hue, Brilliancy or Luminosity, Colouring Power, Cover- Power or Body, Durability or Permanence, Mixability, Fineness, ....... 282-293 V CHAPTER XI. COLOUR AND PAINT MACHINERY. Levigation, Drying of Pigments, Preparing Pigments or Colours by Precipitation, Filtering, Grinding, Mixing, . . 294-330 CHAPTER XII. PAINT VEHICLES. Paint Oils, Drying Oils, Linseed Oil, Boiled Oil, Rosin Oil, Turpen- tine, Kosin Spirit, Shale Spirit, Benzoline, Coal-Tar Naphtha, Methylated Spirit, ...... 331-383 CONTENTS. CHAPTER XIII. DRIERS. PAGES Red Lead, Litharge, Manganese, Mixed Driers, . . . 384-388 CHAPTER XIV. VARNISHES. Varnish Materials, DryingOils, Resins, Oil-varnish Resins, Ethereal - varnish Resins, Spirit-varnish Resins; Gums; Colouring Matters; Artificial Colouring Matters, Coal-tar Colours; Varnish Making; Natural Varnishes, Oil Varnishes, Spirit Varnishes, Water Varnishes, .... 389-450 Tndex, 451 PAINTEBS' COLOURS, OILS, AND VARNISHES. CHAPTER I. INTRODUCTORY. COLOUR, COLOURS, PAINTS, AND VARNISHES. Colour is a term used by persons in several senses; hence confusion sometimes arises, although, as a rule, the context leaves no doubt as to the particular sense intended. When a beam of white light is made to pass through the angle of a tri- angular prism in a certain manner, and the light which has passed through is received upon a screen, we find that it has under- gone a wonderful change ; instead of being one uniform colour, as it was originally, it is spread out into a band of many colours, of which seven can readily be distinguished — viz., red, orange, yellow, green, blue, indigo, and violet. We see these colours by the effect or sensation produced by their action on the retina of the eye ; in a sense, therefore, these colours have an abstract existence only, we can see them by the eye, but we cannot handle them as we can a piece of cotton. When we speak of a red colour or a green colour, we use the term " colour " in an ab- stract sense to indicate the sensation which these colours create in our eyes. On the other hand, we often speak of coloured bodies (that is bodies which give the sensation of being coloured when we look at them) as " colours," especially when (as with vermilion, chrome yellow, emerald green, Prussian blue, and magenta) they can be used to impart colour to other bodies. In this way " colour " is used in a concrete sense to indicate 1 INTRODUCTORY. tangible bodies which have the power of causing other bodies to which they may be applied to create, so to speak, the sensation of colour. Although the subject is one of some importance to users of colours, it is not intended to enter here into a long dis- cussion of colour from an abstract point of view, inasmuch as space does not admit of doing so in any adequate manner, the reader must, therefore, be referred to other works specially devoted to the consideration of the subject. The Spectrum- Colours. — When, then, a beam of white light is passed in a particular manner through the edge of a triangular prism, it undergoes two changes — (1) the direction of its course is altered, i.e., it becomes refracted ; and (2) the beam of white light is separated into a s divergent band of several emerges at d, where it is again refracted so as to take the new direction, d, f. As the amount of refraction differs for each ray according to its colour, the result is that the original white beam of light is differentiated into a long band of numerous distinct colours, known as the spectrum, which extends from e to f in the screen. The rainbow is a spectrum of this kind formed by the refraction of the sun's light during its passage through the drops of water in a shower of rain. In the latter case, however, the spectrum is seen in front of the drops, not behind them, as it is formed by the rays, which, falling on the drops, pass to the back, and are then reflected so as to emerge again on the side nearest the sun. Fig. 1. s differently- coloured light-rays. Fig. 1 repre- sents the path of a beam of light through a tri- angular prism ; a is a ray travelling in the direc- tion of the arrow which strikes the prism at c. If the prism had not been there, it would have passed on and would have fallen upon the screen, s, s, at b ; but the prism, bending it out of this course, refracts it as shown at c ; it then passes through the prism in the new direction until it LIGHT FROM COLOURED BODIES. 3 The colours of the spectrum are pure colours— i.e., they cannot be further split up; if, say. the red part of the spectrum be passed through a second prism, no new colours are produced ; the light which passes through the second prism is still red, although it is distributed over a wider surface. It, therefore, follows that there are really a very large number of simple colours in the spectrum, although, owing to the limitations of language, it is impossible to separate and name every one of these in a popular manner ; although scientists can do so in another manner which it is not necessary to describe here. It is, however, customary to follow the lead of Sir Isaac Newton, who discovered this property of white light, and to distinguish seven colours — viz., red, orange, yellow, green, blue, indigo, and violet; but it should be distinctly understood that in the spectrum there is no well-marked line of division between these seven so-called primary colours ; the red passes insensibly into the orange, the orange into the yellow, and so on through the other colours in the order given above. White from Coloured Light. — By passing the spectrum colours through a lens, or through another prism, in a particular manner, the seven colours can be recombined so as to form white light. It is not even necessary to use all the spectrum colours, as two or three will suffice if properly selected. Thus blue and yellow will when united form white light ; as also red, green, and blue, and many other combinations, particulars of which will be found in special books on Colour, such as those of Professor Church and Mr. W. Benson. The consideration of this property of a few of the spectrum colours combining together to form white light led Young, and, later, Helmholtz, to consider that there are only three primary colours, red, green, and blue, from which all the other colours can be obtained ; thus, by combining red and green, yellow is produced; or by combining red and blue violet is the result. Light from Coloured Bodies. — When the light which is reflected from the surface of a coloured body like vermilion is passed through a prism, it is found to yield a spectrum ; not, however, a complete one, such as is got from a ray of white light, but one more or less incomplete ; thus, vermilion gives a spec- trum containing some red, orange, and a little blue light ; chrome yellow again gives a spectrum showing a few red, some yellow, and some green rays ; in each case the eye distinguishes the effect due to the combined action of all these rays on the retina. No artificial colouring-matter is known which reflects rays of one colour only ; in every case the rays of the dominant colour are mingled with those of other colours. The light from some bodies 4 INTRODUCTORY. is of a very complex character, while that from others is com- paratively simple. It is this complexity in the composition of the light reflected that makes it so difficult to demonstrate the true laws and facts of colour with pigments or any artificial colouring-matters. Cause of Colour in Coloured Bodies. — The actual reasons why bodies such as vermilion, magenta, or emerald green are coloured, it is almost impossible to investigate in the present state of knowledge, since the cause, whatever it may be, must be due to the molecular construction of the different compounds about which very little is known ; still, we know something of some of the reasons why coloured bodies appear coloured. When light falls upon a substance, the light may be affected in one or two ways ; it may be reflected, that is, it may be thrown back from the body; or it may be transmitted, that is, it may pass through, or, in some cases, be absorbed by the body on which it has fallen. As a rule, there is never either complete reflection or complete transmission of light, the most perfectly reflecting body allowing some rays to pass into it. It is by reflected light that we see bodies ; when the reflection is complete, or as nearly so as is the case with mercury or a very highly polished plate of silver, the body is nearly invisible ; it is only rendered visible because it does not reflect all the light which falls upon it in a regular manner ; some is irregularly reflected and it is this light which enables us to see the body. Two kinds of reflection can, therefore, be distinguished— regular and irregular. Regular reflec- tion is that where the light is thrown back in a straight line from the reflecting surface ; if this is perfect, only the light that is reflected is seen, the reflector itself is invisible. Irregular reflec- tion is that where the light is thrown back from the reflector in every direction; it is this light which makes the body visible, and it is due to the fact that no matter how apparently even the sur- face may appear to be, yet it is not even ; it is sufficiently rough to cause the light which falls upon it to be irregularly reflected. Then bodies never reflect or absorb the whole of the light which falls upon them, some of it is absorbed; the most perfectly polished plate of silver (which is the most highly reflecting body known) does not reflect the whole of the light which falls upon it, while a piece of black cloth reflects only a little of the light that falls upon it. Upon the character of the reflected light thrown off from a body depends its colour, which is independent of the pro- portion of the light that falls upon and is reflected by the body. If all the rays of light falling upon it are reflected, then the body appears white ; if all the light rays are absorbed, then the body COLOUR THEORIES. 5 appears to be black. If, now, some of the spectral rays are absorbed and the rest reflected, then the body appears to be coloured, the colour depending upon the composition of the rays which are reflected ; thus the rays from a red body, such as ver- milion, are red, as are also those from Derby red and oxide of iron ; similarly, the rays from a yellow body, such as chrome yellow or yellow ochre, are yellow, but it does not follow that the rays from all red bodies or from all yellow bodies are identical in composition. If the rays from, say, vermilion, oxide of iron, and crimson lake are passed through a prism, and the spectra of the coloured light which is reflected from each examined, they will be found to be different; that from the crimson lake will con- tain more blue rays than that from the vermilion, while that from the oxide of iron will contain more of the dark red and indigo rays than either of the others ; and it is the same with the other classes of colours. There is no coloured body known which reflects what might be called a pure light, while the spectrum-colours are pure, as has been already stated. It is this compound nature of the light which is reflected from coloured bodies that makes it extremely difficult to demonstrate the true laws of light and colour by the use of pigments. In the same manner as the coloured light which is reflected from bodies is compound, so that which is transmitted is com- pound and, usually, the complement of that which is reflected, but this does not always happen. When it is the comple- ment of that which is retiected, then the bodies which give rise- to this phenomena are known as dichroic ; in other cases both the reflected and transmitted rays are of the same general colour, although there is usually some difference in the actual tint of the two colours. It is assumed that the coloured bodies have a selective action on the light which falls upon them, reflecting or transmitting,, as the case may be, those coloured rays to which they owe their colour, while they absorb all the other rays. White bodies reflect all the rays which fall upon them, black bodies absorb all and are, in consequence, often nearly invisible. As to the character of the rays reflected from red, orange, yellow, green, or other coloured bodies, these will have been inferred from what has been said above. Colour Theories. — Two theories of colour are in use to explain the coloured effects of light. The old theory, which is mostly due to Brewster, considers that there are three primary colours — viz., red, yellow, and blue ; by the proper admixture of which in various proportions all the other colours can be 6 INTRODUCTORY. obtained. The more modern theory, first broached by Young and more fully developed by Helmholtz, considers that there are three primary colours, red, green, and blue, although some authorities add a fourth. However, it must be confessed that while the modern theory accurately explains all the phe- nomena of colour producible by the use of the spectrum colours, yet the older theory of Brewster more easily explains the phenomena of colour as produced by the admixture of the various colouring-matters, pigments, and dyestuffs in common use; this arises not from any fault in the newer theory, but from the compound nature of the light which is reflected or transmitted from the colouring-matters in question. Of the newer theory it is not intended to deal, although it is advisable for colourists to make themselves acquainted with it ; as to the old theory, it will be sufficient to say that when any two of the primary colours are mixed together a so-called secondary colour is produced ; thus red and yellow produce orange, red and blue produce violet, while yellow and blue make green. When the secondaries are mixed together they produce what are called tertiary colours, of which there are six, known as buff, citrine, sage, slate, plum, and russet. The nomenclature of these tertiary colours is very indefinite, and different authorities give them different names. The common theory of red, blue, and yellow is not wholly satisfactory, as it does not account for all the shades which may be produced by the admixture of pigments ; thus a mixture of ultramarine, a blue, with yellow ochre, a yellow, does not produce a green, as the theory would expect, but a kind of greenish-grey ; this effect can, however, be explained by the blue-red-green theory when we know the kind of rays reflected by the two pigments in question. Reference must be made to text-books on colour for a further development of the subject. Colours. — It has been explained above that the term " colours" is used in two senses — first, to express the sensation which light of various kinds evolved from bodies excites on the retina of the eye, and which sensation is purely functional ; second, to denote those bodies which, having the property of selective absorption of coloured rays from the light which falls upon them, appear to be coloured and which have the property of imparting this colour to other bodies ; such bodies are known as colouring matters and may be divided into two groups, dyestuffs and pigments ; the former are mostly soluble in water and are used solely to dye cotton, wool, or other textile fibres, while the latter are insoluble, and are used in the preparation of paints. PAINT. 7 Besides these two classes of coloured bodies there is another group which are distinguished by the fact that while possessing colour yet they cannot impart this colour to other bodies ; such are bluestone (sulphate of copper), nitrate of cobalt, chrome alum, &c. Pigments. — These are a fairly numerous class of colouring matters which are used to give colour to paint. They are mostly derived from the mineral kingdom, although a few are obtained from organic sources. As a class they are distinguished by being insoluble in water, turpentine, and most other solvents with the exception of the strong acids ; they are opaque or nearly so \ and they should be perfectly inert bodies exercising no action of any kind on any other substance with which they may be mixed. As typical examples of pigments may be taken barytes, oxide of iron, yellow ochre, chrome green, and umber. In dealing with pigments in detail they will, as a rule, be considered under the divisions of white pigments, red pigments, yellow pigments, and so on ; but here and there, deviations from this rule will be made, as in the case of Derby red (which will be dealt with under the head of yellow pigments) and in the case of lakes, where it is thought that the composition and properties of the particular pigments can be more conveniently pointed out, if dealt with in one group. Paint. — Paint is the name given to a liquid composition which is used very extensively for two purposes — first, to act as a protective substance to preserve the body on which it has been applied from the destructive action of the weather ; second, as a decorative agent. The first object is brought about by making the paint with materials which are not acted upon by the various agents present in the atmosphere, such as water, acid vapours, light, oxygen, that exert a more or less destructive action on bodies which may be exposed to their action. The bodies which have been found to resist this destructive action of the atmospheric influences are the various so-called drying oils, resinous matters, and the pigments. A paint is a liquid composition which will remain liquid until it is applied to the body to be painted, and yet when so applied and afterwards exposed to the atmosphere will dry and leave behind it a firm, hard (yet elastic), and opaque coating, which may be more or less lustrous and be capable of resisting the weather. The opacity of the coating is obtained by using pigments of various kinds, which also tend to increase the resisting power of the paint, and these pigments are mixed with liquid bodies, such as oils and spirits, which are 8 INTRODUCTORY. used partly to obtain a composition that is easy of application T and partly to secure volatility, so that when alone or when mixed with resinous matters, they will evaporate away and leave behind a hard mass firmly binding the pigments to the body over which they have been painted. The liquid bodies which have been found to answer this purpose best are the drying oils, such as linseed oil, which when spread over a surface and exposed to the air absorb oxygen and dry into a hard mass; but as these oils, for various reasons (which will be more fully dealt with later on), cannot be used alone with satisfactory results it becomes necessary to mix them with some solvent, such as turpentine or shale naphtha, which is volatile. In some kinds of paints a little resinous matter is used, which dissolves in the solvent; on exposure the latter evaporates off, leaving the resin behind in the form of a dry coat on the surface to which it has been applied. Paint is always more or less coloured to add to the decorative effect. Its primary purpose, however, is to hide the character of the surface to which it is applied and, as has been pointed out, to protect this surface. Varnishes. — These bodies are very similar to paints in their properties and uses. They differ in giving a transparent lustrous coat of a very resistant character to the destructive action of the weather. They may be coloured ; but, if so, transparent colours are used and not, as in the case of paints, opaque pigments. They are composed of a resinous matter dissolved in various oils and solvents, the latter forming the vehicle by means of which the resin is transferred to the surface to be varnished. The special properties of varnishes will be dealt with later on. 9 CHAPTER II. WHITE PIGMENTS. The white pigments are a very important group of painters'" " colours/' probably the most important, as while the red, blue, green, &c, pigments are used simply or almost entirely as colouring pigments, the white pigments are used in two ways — 1st, as "body colours," i.e., to give body or covering power to- paint; 2nd, as "colouring pigments." Thus, in making a red paint, white lead or barytes is added to give the necessary body and vermilionette is used to colour the paint. On account of this dual feature of the white pigments they merit a more detailed account of each individual member of the group than is necessary for other pigments. The white pigments are a fairly numerous group of bodies derived entirely from inorganic sources. Many white bodies are- known which could be used as pigments, but are not so used on account of expense, &c. The following list comprises all that are- used either on a large or small scale : — White Lead, basic carbonate of lead, 2 Pb C 0 3 Pb H 2 0 2 ; this pigment is also sold under a variety of other names. Lead Sulphate, Pb S 0 4 ; many pigments sold under various fancy names consist essentially of this body combined with other white pigments. Lead Oxychloride, Pb 2 O Cl 2 , Pattinson's white lead. Zinc White, zinc oxide, Zn O. Zinc Sulphide, ZnS; this body combined with barytes, etc., is largely used as a white pigment. Barium Sulphate, Ba S 0 4 , barytes. Barium Carbonate, Ba C 0 3 . Calcium Sulphate, Ca S 0 4 , gypsum. Calcium Carbonate, Ca C 0 3 , whiting. Calcium Oxide, Ca O, quicklime. Strontium Sulphate, Sr S 0 4 . Strontium Carbonate, Sr C 0 3 . Magnesium Carbonate, Mg 0 0 3 , magnesite. China Clay, hydrated silicate of alumina. French Chalk, silicate of magnesia. 10 WHITE PIGMENTS. Of these, the most important are white lead, lead sulphate, zinc white, zinc sulphide, barytes, gypsum, calcium carbonate, and •china clay. WHITE LEAD. White lead has been known and used as a pigment for cen- turies ; the Romans and Greeks used the native carbonate of lead or " cerusse, " as it was then called, from which the mineralogical name cerussite has arisen. This natural pigment is found only in comparatively small quantities, and it is no wonder that a process for the artificial production of white lead was soon found out and adopted, with the result that natural cerussite is not now used as a pigment. It is not known when white lead was first made, who made it, or to what country it owes its birth. The oldest known method is that commonly called the " Dutch method," from the supposi- tion that it was invented in Holland ; it is described as the Dutch process in an English patent granted in 1787, and there is no doubt but that it is the process referred to in three earlier patents granted in 1622, 1635, and 1745, in which it is spoken •of as an old process. Evidently white lead has been made for several centuries. During all this period there has been but little change made in the Dutch process. But in the interval inventors have not been idle, for there is no other pigment which has attracted so much attention at their hands as white lead ; and the number of processes and modifications of processes which have been devised, is almost innumerable. With all this invention, the ancient Dutch process still retains its pre-emi- nence as the best process for the manufacture of white lead. MANUFACTURE OP WHITE LEAD. White lead, the basic carbonate of lead, is manufactured by a variety of methods. It is not easy to classify these processes into groups, as they not unfrequently pass one into the other imperceptibly. The author suggests the following classification, which is based on the principles which appear to underlie the various methods adopted, or which have been proposed and used •on a limited scale : — 1st Group. — Stack method. 2nd Group. — Chamber methods. 3rd Group. — Precipitation processes based on the action of carbonic acid gas on various lead salts. STACK METHOD. 11 4th Group. — Precipitation methods based on the action of alkaline carbonates on various lead salts. 5th Group. — Miscellaneous methods. Most of these are now obsolete and are only of historical interest. 1st GROUP.— STACK METHOD. Only one process is included in this group, the old Dutch or stack process. No process which is now in use or which has been proposed can claim the antiquity that this process can, and, notwithstanding all the labours of chemists and white lead Fig. 2. — Shed for making white lead. 12 WHITE PIGMENTS. makers to supersede it, for reasons which will be pointed out presently, it still remains the best process for the manufacture of white lead. Tradition assigns its discovery to the Dutch and to a person named Stratingh in particular. It must be at least 300 years old. Since 1787 this process has been carried on without much alteration in its details. The Dutch method is used in all parts of the world for the manufacture of white lead, and there is but little variation in the details of the process and in the construction of the plant used in different countries. The plant used in the stack process is shown in Figs. 2 to 5. A shed of brickwork, Fig. 2, is built, the size of which varies a little, but averages 16 feet long by 13 feet wide and 20 feet high ; this may have either a lean-to roof, as shown in the figure ; or, as in some works, two of these sheds are built back to back, with a single-ridge roof between them. In some places parts of the structure are built below the level of the ground, but there is no advan- tage to be gained by so doing. A large white-lead works will have a number of these sheds, so as to keep the workmen fully occupied with filling and emptying them. A number of earthenware pots are provided. These pots vary in size at different works, but an average size is 8 inches high by Fig. 3. Fig. 4. Fig. 5. 4 inches in diameter. In shape they resemble crucibles (see Fig. 3), but have a shelf inside, as shown. In the bottom of these pots is placed some weak acetic acid or vinegar; this diluted acid contains about 2 to 3 per cent, of actual acetic acid. On the- shelf inside the pot is placed a roll of thin sheet lead (Fig. 4), STACK METHOD. 13 made from a strip of lead 2 feet long by 4J inches broad. In a stack of ordinary dimensions some 11,000 to 12,000 of these pots will be used, and they will contain about 800 to 900 gallons of weak acid. The stack is built up as follows : — First, a layer of ashes, upon which is placed a layer of spent tan of about 3 feet in thickness. In the older Dutch method horse-dung was used, but this is open to some disadvantages which will be pointed out presently ; the use of tan was introduced in England so that this modifi- cation of the Dutch process is sometimes.spoken of as the English method. This layer of tan is pressed down very firmly and is evenly spread ; on it is placed a layer of the pots, which layer is kept at a distance of about 6 inches from the sides of the shed. In some works the outside rows of pots are made of larger size than the others, so as to act as supports for a layer of flooring boards. In other places the pots are all of one size and wooden supports for the boards are provided. On the top of the pots is placed a layer of lead buckles or gratings (Fig. 5). These are placed face to face in a layer of about 3 to 5 inches thick; above these comes the layer of flooring boards, a space of about 6 inches being left between them. On the top of the boards another layer of tan, then a layer of pots, then a layer of gratings, then another layer of boards, and so on until the stack is completely built up. The number of sets of layers varies from seven to eleven. The doorway through which the filling is done is closed as the work progresses by boarding, but a small space is left at the top through which the progress of the operation can be observed, and fresh additions of material made as required to allow for sinking of the tan, &c. The quantity of lead used varies considerably, or from about 3 tons to 7 tons in a layer of materials, so that in a large stack there may be something like 85 tons of lead. In stacks of very large area it is usual to construct chimneys throughout the mass, whereby the steam which is produced during the operation is carried off ; in stacks of small area, these ohimneys are not required, as the space around the side walls of the shed affords a sufficient outlet. When the stack is built up it is left for a period of about three months, During this period the stack gets quite hot (140° F.) through the fermentation of the tan which sets in ; large quantities of carbonic acid gas are given off, and the acetic acid is converted into vapour. The "blue lead" is gradually converted into " white lead." At the end of three months the stack is pulled to pieces. As the boards are removed it is found 14 WHITE PIGMENTS. that the lead which has been corroded still retains the form of the blue lead, but is more bulky in volume, is white or greyish in tint, and opaque. The corrosions are not of a uniform character throughout the whole of the stack ; in some places they are porcellaneous and flaky, are firm to handle, do not break up, and give the best quality of white lead ; in other parts of the stack the corrosions are soft, easily crumble to a fine powder or dust when handled, and do not give a good quality of white lead. In some places the lead may be dis- coloured owing to a variety of causes, such as the presence of tarry matter in the acid (especially when crude pyroligneous acid is used), by droppings of coloured water from the layer of tan on to the lead, &c. In chemical composition the cor- rosions will vary ; in some places they will approximate closely to the normal composition of white lead, 2 Pb C 0 3? Pb H 9 0 2 , in others more nearly to that of 3 Pb C 0 3 , Pb H 2 0 2 , while in others they consist of the normal carbonate, Pb C 0 3 . As the stack is being pulled to pieces the corrosions are carried to the grinding rooms. The method of treating the white lead varies in different works, but the following may be taken as a good example of the usual manner of working : — The corrosions are first passed through a pair of rolls \ these break up the masses, the white lead crumbles to powder, while the unchanged blue lead is flattened out into thin sheets. The crushed materials are then sieved, which separates the white from the blue lead ; the latter is sent to the melting pot where it is melted and re-cast for use in building another stack. The white lead is sent into tanks full of water, where it is thoroughly agitated, and the small traces of acetate of lead which the corroded lead contains washed out of it. While still wet the white lead is ground as fine as possible under edge runners or between rollers, and then dried, when it is ready for sale. As the grinding must be thorough, the lead is passed through several sets of grinding mills. Grinding white lead is a source of clanger to the workpeople, for the fine dust flies about the room in which it is done and is breathed by the workpeople, who, sooner or later, suffer from lead poisoning ; much of this danger is avoided by grinding the lead in a wet condition only, when the particles of lead are practically too heavy to fly about. The greatest risk now arises in the packing of the ground lead, as the workmen frequently get some on their hands and eat their meals without previously washing their hands. Lead poisoning may be prevented by drinking water acidulated with STACK METHOD. 15 sulphuric acid, whereby the lead absorbed into the system is converted into the harmless sulphate of lead. The great trouble is that the workmen will not take sufficient care to make use of these precautions. It was stated above that in the early or Dutch modification of this process, horse-dung was used as the source of the heat and carbonic acid necessary to carry on the process ; while with dung the process is quicker (only taking from 8 to 9 weeks), yet it is not so good as the English method with tan, the product is not quite so regular in composition, and it is more liable to discolouration owing to the evolution of sulphuretted hydrogen from the decomposing dung, and to its combination with lead to form the objectionable black sulphide. The theory of the process of white-lead making by the Dutch process, which at present is most favoured by chemists, and was substantiated by some experiments carried out by Hochstetter, is due to Liebig. The first action which goes on in the stack is to convert the blue lead into basic acetate of lead ; this is brought about by the heat of the fermenting tan, or dung, causing the evolution of acetic acid from the liquid in the pots, which, attacking the lead, causes the production of the normal acetate of lead, thus — 1. Pb + 2HC 2 H 3 0 2 = Pb2C 2 H 3 0 2 + H 2 Lead. Acetic acid. Normal lead acetate. Hydrogen* The normal lead acetate, under the influence of water and heat, parts with some of its acetic acid and passes into the basic acetate, thus — - 2. Pb2C 2 H 3 0 2 + 2H 2 0 = 2Pb2C 2 H 3 0 2 , PbH 2 0 2 + 2HC 2 H 3 0 2 Normal lead Water. Basic lead acetate. Acetic acid, acetate. The acetic acid is ready to attack a further quantity of blue lead. The basic acetate is now attacked by the carbonic acid evolved by the fermenting tan, the acetic acid it contains is liberated and its place taken by the carbonic acid, and white lead is formed, thus — 3. 2Pb2C 2 H 3 0 2 + 2C0 2 = 2 Pb C 0 8 , Pb H 2 0 2 + 4HC 2 H 3 0 2 Normal lead Carbonic White lead. Acetic acid, acetate. acid. Although the reactions shown in the above equations are those usually accepted as representing the formation of white lead from blue lead in the stack process, yet they are probably 16 WHITE PIGMENTS. not quite correct ; the evolution of hydrogen in the first step in the process is rather improbable ; a better explanation would be the following : — (1) By the action of moisture and oxygen on the lead there is formed lead hydroxide, thus — 1. Pb + H 2 0 + 0 = PbH 2 0 2 Lead hydroxide. Then this being acted on by the acetic acid forms the normal or neutral acetate and water, thus — 2. PbH 2 0 2 + 2HC 2 H 3 0 2 = Pb2C 2 H 3 0 2 + H 2 0 The normal acetate now combines with lead hydroxide to form basic lead acetate, thus — 3. Pb 2 C 2 H 3 0 2 + 2 Pb H 2 0 2 = Pb 2 C 2 H 3 0 2 , 2 Pb H 2 0 2 Basic lead acetate. This is now acted upon by the carbonic acid with the for- mation of white lead and normal acetate, thus — 4. 3[Pb2C 2 H 3 0 2 , 2PbH 2 0 2 ] + 4C0 2 = 3Pb2C 2 H 3 0 2 Normal acetate. + 2[2PbC0 3 , PbH 2 0 2 ] + 4H 2 0 White lead. The normal acetate thus reproduced then forms more tribasic acetate by the reaction shown in equation 3. It is again decom- posed by the carbonic acid, as shown in equation 4, so that a continuous cycle of changes is set up ; the lead being oxidised to lead hydroxide, and this converted into white lead, the basic carbonate, pari passu with its formation. As a rule, nearly all the blue lead is converted into white lead, one ton of lead producing one and a quarter ton of white lead, the amount varying from time to time according to the -degree of perfection with which the corrosion has proceeded. The great fault of the Dutch process is the great length" of time required (8 to 12 weeks), the great amount of capital it takes to construct a stack of lead, and the loss of interest which takes place on the capital while the lead is in process of making. Then there is always a risk, owning to some defect, of producing ••a useless and imperfectly corroded lead, which has to be sent to the smelting furnace and again reduced to blue lead. Hence it is that inventors have turned their attention to devising other methods of producing white lead which shall be free from the defects of the Dutch process ; so far, however, no such method has been discovered. CHAMBER METHODS. 17 White lead, as made by the process described above, is a basic carbonate of lead having the composition Lead carbonate, Pb C 0 3 , .... 68*95 per cent. Lead hydroxide, Pb H 2 0 2 , . . . 31 '05 „ 100-00 Lead monoxide, Pb O, 86*32 per cent. Carbonic acid, C O2, ..... 11*36 ,, Water, H 2 0, 2 *32 100*00 „ therefore having the formula 2 Pb C 0 3 , Pb H 2 0 2 . As will be seen hereafter, when the properties of white lead as a pigment come to be more fully considered, the carbonate is the substance to which white lead owes its colour and body ; while the hydroxide with which it is associated, by chemically com- bining with the oil used to convert the white lead into a paint, imparts to the white lead great covering properties. Numerous samples of white lead, of both high and low qualities, have been analysed by many chemists, some of which will be given later on. These analyses show that when the composition of any sample varies greatly from the figures above given, it is more or less defective. Generally, an increased proportion of carbonate, while causing the colour to be better, reduces the covering power; on the other hand, an increase in the amount of the hydroxide causes a loss of body and opacity. If the sample contain any monoxide then the tint becomes more yellow or greyish. Some of these points will be touched upon when con- sidering the other processes for the production of white lead. In the meantime the success or non-success of any process depends upon the approximation of the white lead produced to the composition above given. 2nd GROUP.— CHAMBER METHODS. In one sense the Dutch method just described is a chamber method, but it has been classed as a separate group because while being made in a chamber, it differs materially from those now to be described. In these processes the operation of white- lead making is carried on in large chambers in which metallic lead is placed, and into which currents of carbonic acid gas, acetic or other acid vapours, are passed, together with air and steam. The different methods are distinguished one from another by the construction of the chambers, the method of admitting the 2 18 WHITE PIGMENTS. acid gases, &c., and in other points. Although many such have been invented and will be found described in the Patent Records,, yet very few are in actual operation for the production of white lead; and of those which have become obsolete very little is. known beyond the, often very scanty, description which is to be found in the specification of the patent which protected the process. In all the accounts which have appeared describing these pro- cesses, and which are evidently copied from one source, the chamber process is given as "the German method but it is a matter of doubt whether the process was invented in Germany or not. The author is inclined to consider it to be of English origin, partly because it is described in a patent taken out in 1749 by Sir James Creed, who makes no mention of having taken it from a foreign source. This was the first patent to describe a chamber process, and since then many have been patented ; but the author must refer readers to the Patent Pecords for an account of these or to a series of articles which appeared in The Chemical Trade Journal in October and November, 1890. L Creed or German Process. — This process was the first in which chambers were used in conjunction with lead and acid gases. In detail it is carried out as follows : — A chamber of brickwork is built of any convenient size and with few openings, the usual ones are a door to enter into the chamber for the pur- pose of filling it, and an opening in the roof for ventilation ; sometimes a window or two for the purpose of observation i& added. The chamber has a number of shelves, on which are placed sheets or gratings of lead ; it is immaterial which are adopted, although the gratings expose more surface to the action of the various gases which are used. When all the lead is placed on the shelves, the doors are closed, and currents of aqueous- vapour, air, carbonic acid, and acetic acid at once admitted into- the chamber. After a period varying from four to five weeks^ the white lead will have been formed ; it is collected and treated as in the Dutch process. The chemical action which proceeds is. supposed to be the same as that which takes place in the old stack method. The acetic acid acts upon the lead, forming neutral acetate of lead ; this, under the action of the aqueous vapour, is transformed into basic acetate of lead, and this, in its. turn, is changed by the carbonic acid into basic carbonate of lead or white lead. The quality of the product is usually very good, not, perhaps, quite equal to that produced by the Dutch method, but better than that produced by the precipitation processes. It is, how- CREED OR GERMAN PROCESS. 19 ever, inclined to be very variable, and the process requires some experience to carry out in the best possible manner to ensuie a good product. It is desirable, as far as possible, to cause the white lead to approximate in composition to the formula 2 Pb C 0 3 , Pb H 2 0 2 , and to do this it is necessary that the gases should be sent into the chamber in the proper proportions. If excess of acetic acid is present, too much acetate of lead is formed, which is not decomposed by the aqueous vapour and the carbonic acid ; too much of the latter tends to cause the formation of an excess of lead carbonate, and the white lead loses its covering powers. On the other hand, too much steam will lead to the formation of oxide, especially if the temperature be allowed to get high ; the oxide so formed being of a yellow tint spoils the colour of the white lead. The same result is brought about by a deficiency of acetic acid. Experience is the only factor which can guide the white-lead maker in adjusting the various gases in the proper proportions. The following are some analyses of white leads made by this process quoted by Weise : — 1. 2. 3. 4. 5. Lead monoxide, 86*80 86*24 86*03 84*69 83*47 Carbonic acid, 11*16 11*68 12*28 14*10 16*15 Water, 2*00 1*61 1*68 0*93 0*25 1. Firsts, of the best quality ; good both in colour and body. 2. Seconds, not so good as No. 1, but still very serviceable as a pigment. 3. Thirds is only just usable as a pigment. 4. Is not usable except for very common purposes. 5. Not usable at all ; it contains too much carbonate, and is sent to the smelting furnace. Various alterations in the details of this method have been made from time to time by various inventors, some of which may be briefly noted. Burton placed the lead in coils on the shelves of the chamber, and passed the current of steam through perforated pipes, thereby converting the lead into oxide ; when a sufficient amount of this has been formed the current of steam is stopped, and a current of acetic acid vapour sent in; this, acting on the basic acetate converts it into the basic acetate ; when this action is finished the acetic acid current is stopped, and carbonic acid gas sent in, which acts on the basic acetate and changes it into basic carbonate or white lead. These currents of steam, acetic acid vapour, and carbonic acid gas are sent in successively until all the lead is converted into white lead, then the currents are 20 WHITE PIGMENTS. stopped, and the white lead is collected and finished in the usual way. In Richardson's method the lead is soaked in a solution of acetate of lead, the action, both of the acetate and the gaseous bodies, to which the soaked lead is subjected being facilitated by the lead being cast into a granular form. After being soaked, the lead is placed on shelves in the chamber, and then subjected to the combined action of steam and carbonic acid gas, the chamber being maintained at a temperature of about 100° F. ; the process is continued until all the lead is converted into white lead. 2. Hatfield Process. — This process resembles those just described to some extent, but differs in a few minor particulars. The chamber is built with a double wall, and the bottom is hopper-shaped. The lead is cast into gratings of the same shape as used in the Dutch process, and placed in trays on shelves in the chamber. Into the chamber is sent water and acetic acid in the form of spray, at the same time the chamber is maintained at a suitable temperature by means of steam pipes. The action of the water and acid is to convert the lead into basic acetate of lead ; when this has been properly formed the water and acetic acid spray is stopped, and a current of carbonic acid gas sent in to form white lead as described above. 3. Thompson's method, patented in 1873 and 1877, was worked by the Innocuous White Lead Co., of London. In this process the chamber is built of brick, with a large door and one or two windows, so that the progress of the operation can be observed. The bottom of the chamber is made like a trough and acid proof, glass being recommended as a material for its con- struction. The roof is built double, so that any liquid which is condensed will flow down to the sides and not drop on to the corroded lead below. The lead used is cast into gratings which are placed on open trucks fitted with shelves ; the lead is soaked in a solution of acetate of lead and then wheeled into the chamber. A quantity of acetic acid is placed in the bottom of the chamber and vapourised by means of steam pipes passing through it, by which means the lead is converted into the basic acetate ; when this action is complete, carbonic acid gas is sent in to change it into white lead. This was said to be of good quality. In the practical application of this process, much depends upon the temperature at which the chamber is main- tained during the operation. If too high, then there is a tendency to form oxide of lead which does not readily change into white lead ; if too low, then the action of the acid on the Gardner's electric process. 21 lead is not energetic enough ; if too much acetic acid is used, then the tendency is to form normal acetate of lead which reduces the yield of white lead, and at the same time tends to cause this to have too much carbonate in its composition. A very similar process to this was patented by Morris. In this the lead was used in the form of sponge or wire so as to expose as much surface as possible to the action of the various gases. The acetic acid was placed in vessels on the floor of the chamber and steam and carbonic acid gas passed in, a constant current of these gases being maintained. The white lead was gradually formed, and when complete was collected and finished in the usual way. 4. Gardner's Electric Process.— In 1882, Prof. E. V. Gardner patented a process for making white lead which, as he considers that electricity plays a part, he calls an " electric process." The specification of this patent (No. 731, of 1882) is very full, and is well worth reading by white-lead makers. In the specification the conditions most favourable to a successful production of white lead are fully stated, and from it the following is abstracted : — As has been previously pointed out, in making white lead by the chamber methods there are several factors which require attention, if the product is to be a good one. Prof. Gardner states these to be as follows : — The proper formation of what he calls the sub-acetate or sub-nitrate of lead ; these basic salts are the compounds of the normal salts with the hydroxide of lead, and, therefore, have the formula? Pb 2 C 2 H 3 0 2 , Pb H 2 0 2 for the sub-acetate, and Pb 2 N 0 3 , Pb H 2 0 2 for the sub-nitrate. It is usual to consider the so-called subsalts of lead as com- pounds of the normal salts with the monoxide ; probably both kinds of salts exist — that is, there are compounds both of the monoxide and of the hydroxide of lead with the normal salts of lead. In white-lead making it is reasonable to suppose that better results would be obtained if the hydroxide compounds were formed than if the monoxide compounds were obtained in the process of making. Hence the conditions most favourable for the formation of the hydroxide should be carefully ascer- tained. Temperature is an important factor ; this should be from 120° to 130° P. A lower temperature increases the length of time required for the formation of the subsalts, and so increases the cost of the process, while the quality of the white is deteriorated, owing to its deficiency in hydroxide. Too high a temperature 22 WHITE PIGMENTS. must be avoided, for although a high temperature increases the rapidity with which the subsalts are formed, yet it is liable to cause them to lose their water of hydration and to pass into the monoxide subsalts ; the presence of these in the white lead makes it of bad colour and hence deteriorates the quality. Too little air, acetic acid, and aqueous vapour also tends to prevent the proper formation of the subsalts and, consequently, of the white lead of the best quality ; too much acetic acid converts the subsalts into the normal salts and, as is well known, these do not produce white lead of good quality ; besides which, being soluble in water, they are washed off the surface of the lead by the aqueous vapour which condenses on the lead, and are thus lost for the purpose of making white lead. These are a few of the principal conditions which Prof. Gardner points out as being necessary for the proper production of white lead of good quality, and, although given in connection with his own process, yet there is no doubt but that they are applicable to all chamber- processes and also to some other methods of making white lead. The electric process is carried out in a chamber made of any convenient form and material ; it is necessary, however, that it should be so constructed that the progress of the operation is readily visible. In this chamber are arranged a number of shelves covered with tin, a metal which is electro-negative to lead. Carbon, or any metal which is electro-negative to lead, may be used, but the inventor prefers tin. These shelves are connected together in succession by means of strips of tin, so that when lead is placed on them they form an electric couple. Instead of having the shelves a fixture in the chamber, they may be constructed on an open framework fitted with wheels ; on this, while outside the chamber, the lead gratings are arranged, and when the shelves are full the frame and its contents are run into the chamber, but before doing so the lead is soaked in a solution of acetate or nitrate of lead. The temperature of the chamber is maintained at 120° F. by means of steam which is sent into it for that purpose. At the same time currents of acetic or nitric acid vapours, made by boiling dilute solutions of those acids, are conveyed into the chamber, or dioxide of nitrogen with acetic acid may be used. The atmosphere of the chamber must be in a misty condition, and this is brought about by regulating the current of acid vapours and steam ; this state of affairs is kept up for 48 hours, when a current of pure carbonic acid gas is sent in for 2 hours, then stopped, and the acid gases sent in by themselves for 4 hours, when the admission of carbonic acid is again resumed for PRECIPITATION PROCESSES. 23 2 hours ; these alternation of currents of acid gases and steam for 4 hours, and acid gases and steam and carbonic acid for 2 hours, is carried on for 14 to 15 days, when the whole of the lead will be found to be converted into white lead. During all this time the temperature must be kept at about 120° F., and the atmosphere of the chamber misty ; it is for the purpose of closely watching the progress of the process that the chamber is fitted with windows. The carbonic acid gas may be prepared by any well-known method, but the inventor prefers to use a petroleum lamp as the source of it. After the operation of making the white lead is finished, the material is not immediately removed from the chamber, but the acid gases are stopped, and steam only sent in, which serves to wash the product ; then, after a time, the current of steam is stopped, and air only admitted, when the white lead becomes dry. It is now taken out of the chamber and finished in the usual way. The product obtained by Gardner's process is of good colou" and body, and closely approaches, if, indeed, it is not equal to, Dutch white lead in its properties. One advantage said to be possessed by this process over the stack method, is that while in the latter it is essential to work with the purest lead which can be made, in the new process ordinary commercial lead gives excellent results. 3rd GROUP. — PRECIPITATION PROCESSES. When a current of carbonic acid gas is passed through a solu- tion of a basic salt of lead, such as the basic acetate or the basic nitrate, a white precipitate will be obtained, which is due to the combination of the carbonic acid with the excess of lead oxide contained in the basic salt ; this precipitate consists of a more or less basic carbonate of lead. At the same time, a solu- tion of the normal salt is obtained, because carbonic acid is too weak to displace any other acid from its combination with lead. This action of the carbonic acid gas is shown in the following equation : — 3 [Pb 2 C 2 H 3 0 2 , 2 Pb H 2 0 2 ] + 4 C 0 2 = 2 [2 Pb C 0 3 , Pb H 2 0 2 ] Basic acetate of lead. White lead. + Pb2C 2 H 3 0 2 + 4H 2 0 Normal acetate of lead. 24 WHITE PIGMENTS. although this may not accurately represent the action which goes on in the majority of cases. Various salts of lead are used. The differences between the various processes based on the principle just described, depend upon the kind of salt used, and the method of carrying out the operation. These processes were introduced in the early part of the century, the first patent being dated 1808, and granted to E. Noble. The process described consisted in passing a current of carbonic acid through a solution of lead acetate. A very similar method is known as Thenard's, or the French, process, and will be found described below ; while another precipitation- method is known as the Kremnitz process, having been largely used there for the preparation of white lead. The precipitation-processes based on the action of carbonic acid gas upon lead salts may be divided into two sub-groups : — 3a. Dry methods, in which the lead salt is used in the dry state, or, at the most, simply moistened. 3b. "Wet methods, in which, the lead is used in the form of a solution. 3a.— DRY PRECIPITATION PROCESSES. 1. Kremnitz Process. — This process owes its name to having been worked at Kremnitz in Germany. It is carried on in a chamber built of brick or wood, having a number of shelves, on which is placed trays containing a paste made of litharge and either acetic acid or lead acetate, usually in the proportions of 100 lbs. of litharge to 18 pints of acetic acid, or an equivalent quantity of lead acetate solution. When the chamber is filled carbonic acid gas is sent into it, this becomes absorbed by the lead oxide present in the paste, the absorption of the gas being facilitated by raking over the paste from time to time, the mass being kept moist, as this increases the absorption of the gas. The mass originally has a yellowish-grey colour, but as the operation progresses it gradually changes into a white ; and when all traces of yellow have disappeared, the operation is stopped, and the white lead which is made is first washed with water, then ground and dried. Care is taken not to pass the carbonic acid in too long, because this would induce the formation of the normal, instead of the basic, carbonate, which means poor white lead. When carefully worked, good results can be obtained by this process. The following analysis, presumably of a Kremnitz white lead, is given in Wagners Technologie : — THENARD FROCKSS. 25 or Lead oxide, . . . . S3 '77 per cent. Carbonic acid, . . . . 15*06 ,, Water, 101 Lead hydroxide, . . . 8 21 per cent. Lead carbonate, . . . 91*21 ,, Moisture, ..... 0*42 ,, which shows that this sample did not approach Dutch white- lead in composition, but contained more carbonate. 2. Mullin Process. — In this process, which is not now in use, litharge was ground into a paste with water ; the paste was then placed in shallow lead-lined boxes, in layers of about an inch and a quarter thick, the boxes were closed by a lid, and then into them was sent currents of carbonic acid and acetic acid gases ; the litharge was gradually converted into- white lead. The process was, presumably, not a successful one,, or it would not have gone out of use. 3b. — WET PRECIPITATION PROCESSES. In this group of processes for the preparation of white lead r the lead is used in the form of solution, and the precipitation i& effected by means of a current of carbonic acid gas. There are a large number of these processes, and many are still in use on the large scale. The differences between the various, processes belonging to this group depend upon a variety of circumstances, such as the method of preparing the solution of lead, and the form of apparatus used, on which to a large extent depends the subsidiary, but not unimportant point, the- method of applying the carbonic acid to the lead solution. 1. Thenard Process. — This process, from having been worked on a large scale at Clichy, in France, is known as the French process ; it is also described in the patent granted to E. Noble in 1808. The principle of the Thenard process, which is also applicable to many others of this group, is that when a solution of normal lead acetate is boiled with litharge, some of the latter is dis- solved, and a solution of basic lead acetate, known as "Goulard's Extract," "Extract of Saturn," &c., is obtained. The reaction is expressed in the following equation : — Pb 2 C 2 H 3 0 2 + 2 Pb 0 + 2 H 2 0 = Pb 2 C 2 H 3 0 2 , 2 Pb H 2 0* Normal acetate Litharge. Water. Basic acetate of lead, of lead. 26 WHITE PIGMENTS. If a current of carbonic acid is passed through this solution of basic acetate of lead, the lead hydroxide it contains is precipitated as a more or less basic carbonate, thus — 3 [Pb 2 C 2 H 3 0, 2 Pb H 2 0 2 ] + 4C0 2 = 2 [Pb C 0 2 , Pb H 2 0 2 ] Basic acetate of lead. Carbonic acid. White lead. + 3Pb2C 2 H 3 0 2 4- 4H 2 0 Normal acetate of lead. The normal acetate which is thus re-formed can be used again for preparing a fresh solution of basic acetate of lead ; of course, while, theoretically, a very little normal acetate is sufficient for the preparation of a large quantity of white lead, and there should be no loss, practically, a small quantity of new acetate has to be added from time to time to make up for the little loss which does occur. The apparatus used in carrying out the French process at Clichy is shown in Fig. 6. In a vessel, A, of convenient size, litharge is dissolved in a solution of lead acetate, the solution being accelerated by heating the solution by means of the steam pipe, B ; from this vessel the liquor in A runs into another vessel, C, in which all insoluble matter settles out. The clear solution is now run into a trough-shaped vessel, D, into which dip a number of pipes connected with the large main pipe, E, through which a stream of carbonic acid gas from the generating system, F G, flows. This system consists of an oven, F, in which is burnt a mixture of chalk and coke, from which a large quantity of carbonic acid gas is evolved • this gas is washed in the apparatus, G, by passing it through water, after which it passes into the solution of lead in the vessel, E, precipitating white lead from it in so doing ; the length of time of treating depends upon the quantity and basicity of the lead solution, but usually it takes from 12 to 14 hours. At the end of this time the current of gas is stopped and the white lead allowed to settle ; the clear liquor, which is a solution of the neutral acetate, is run into a vessel, H, from which it is pumped up by the pump, I, into the vessel, A, to dissolve more litharge for a fresh operation. The mass of white lead which settles at the bottom of the vessel, E, is run into another vessel, K, from whence it passes on to filters to be washed, and then it is finished in the usual way. The product given by this process is fairly good, but liable to vary in composition from time to time, according to the strength of the solution of basic lead acetate, and to the basicity or pro- 28 WHITE PIGMENTS. portion of lead oxide the lead acetate has dissolved. These are points to which reference will be made in describing other processes. 2. Cory Process. — The same materials are used in this pro- cess as in the last, viz., basic lead acetate and carbonic acid gas, but it differs in the form of apparatus used. The process has been worked on a large scale for a long period. It was first Y Sect/on oftJw tfuemtepm wlitcli Fig. 7. — Cory's process for making white lead. patented in 1838, and the white lead produced by it is favour- ably spoken of by users. The author believes that the process is still in use. The plant used is shown in Fig. 7. A chamber is built of brickwork ; the bottom is made watertight and sloping towards one end so that any liquor which may fall upon it drains away into a tank ; this chamber is divided by a number of vertical MILNER PROCESS. 29 partitions into compartments ; the partitions are so constructed that each alternate one does not quite reach the top while the others do not quite reach the bottom, as shown in the figure ; the object of this is to make the carbonic acid gas, which is sent into the chamber at one end, take a circuitous course before it passes out at the other end. Above the chamber is a tank, the bottom of which forms the roof of the chamber, which bottom is perforated with a large number of fine holes, so that any liquor which may be run into the tank flows through into the chamber below, in a fine stream like rain. In another tank a solution of basic acetate of lead is prepared in the usual way, this flows into the chamber tank and from thence into the chamber ; here it comes into contact with carbonic acid gas which is sent into the chamber, the action between the lead solution and the gas being facilitated by the liquor being in such a finely divided form. The lead solution falls down to the bottom of the chamber, and thence into settling tanks, where the white ]ead which is formed settles ; it is collected, washed, dried, and finished in the usual way, while the solution of neutral acetate of lead, which is also obtained, is used over again. 3. Milner Process. — Milner does not use the basic acetate of lead in his process, but prepares his lead solution by taking 4 lbs. of finely-ground litharge, and mixing it with 1 lb. of salt dis- solved in 16 lbs. of water, the mixture being made in wooden tanks. The patentee states that these should be made of yellow pine ; oak-wood tanks will not do. In the tanks the mixture is well agitated for about 4| hours, at the end of which time it will have been converted into the basic chloride of lead. When the basic chloride has been fully formed, it is run into covered wooden tanks fitted with agitators ; through these tanks a current of carbonic acid gas passes, which, acting on the basic chloride, converts the latter into white lead. Instead of this procedure, the basic chloride may be mixed in lead-lined tanks with caustic soda, and gas is passed into the tanks, as before, until the liquor ceases to be alkaline. This point is ascertained by the workmen taking a little of the mixture out of the tanks from time to time ; if it appear viscid, forming a homogeneous mass and an even layer on the sides of the glass, then sufficient gas has not been passed in; if, however, it forms a sort of arborescent pattern on the sides of the glass, the operation is finished ; the current of gas is then stopped, and the white lead sent to be finished in the usual way. The process is said to yield a white lead of good colour and body, and very heavy, weighing about 200 lbs. to a cubic foot. 30 WHITE PIGMENTS. It was worked by the Sankey White Lead Co., but has been disc- ontinued for some time. 4. Martin Process. — Martin's process for the preparation of white lead is based on the action of carbonic acid on solutions of basic acetate of lead ; whether this process was ever used on the large scale the author has no knowledge. One great fault of all precipitation-processes for the manufacture of white lead is that they are apt to give a product which is more or less crystal- line, a condition fatal to its being of good quality ; the colour may be good, but the body is always deficient. The patentee states that this depends upon the proportion of acid solvent of the litharge to the water which is used in the process ; if the water be in excess, then too much basic salt is formed, and the carbonic acid, acting too energetically upon this, causes the formation of a crystalline product; therefore the acid solvent must be in excess. Martin prepares a solution of the neutral acetate of lead in one and a-half times its weight of water, or litharge may be dissolved in acetic acid in such a way as to produce a similar solution. 3,600 gallons of this solution are placed in a tank fitted with an agitator; there is then added 4 to 6 tons of granulated lead, and half a ton of litharge. After thoroughly mixing the materials together, carbonic acid gas is passed in for an hour, when all the litharge will have been converted into white lead, then half a ton more litharge is added, and more carbonic acid ; in about an hour this second lot of litharge will be converted into white lead, then more litharge is added, carbonic acid being mean- while sent in ; these additions of litharge are continued hourly until sufficient white lead has been formed, when it is collected and finished in the usual way. If thought desirable, instead of adding the litharge in lots every hour it may be run in in a constant stream. During the operation the temperature is- maintained at about 100° F. The distinctive feature of this process is using the litharge in an undissolved form, and strong solutions of lead acetate. In the absence of practical experience of the process it is not easy to speak definitely on the effect of using such strong solutions : but, judging from the known effects of using strong solutions on the character of precipitates obtained in other cases, one would naturally imagine that the white lead formed would have a crystalline character, and not that amorphous condition which is required in good white lead, still the patentee states that such is not the case. When the principles which underlie these precipitation- processes are considered, it becomes evident that the character FOURMENTIN PROCESS. 31 of the white lead, both chemically and physically, materially affects its value as a pigment. This character will depend upon the character of the solution of lead which is used, the temperature at which the reaction between the carbonic acid gas and the lead salt takes place, and the strength of the solutions used ; on these points information is scanty, and very few of the inventors of white-lead processes have mentioned the influence of any of them. The character and basicity of the lead salt will have some influence on the .result ; the basicity should be due to the presence of lead hydroxide, and not to lead oxide, or, at all events, the latter should be present in only small quantities. To ensure the production of lead hydroxide, water seems to be necessary, and therefore should be used in sufficient quantity. The quantity of carbonic acid should be so regulated that not more than two-thirds of the base present is converted into carbonate ; if too much gas is used, then all the base will be liable to be converted into carbonate, and the white lead has a tendency to become crystalline ; the difficulty is to ascertain when sufficient gas has been used. The strength of the solution of lead will also have some influence, but the diversity of opinion among white- lead makers as to the proper strength is great ; some prefer strong solutions, others weak ones. As a rule, weak solutions give the finest precipitates, and strong solutions give the coarsest. The temperature at which the operation is conducted will have some little influence; cold solutions will cause the formation of fine precipitates, while hot solutions tend to give rise to crys- talline precipitates, clue to the fact that the reaction between the carbonic acid and the lead salt takes place too readily; still it is not desirable to work with solutions that are too cold: the best temperature is from 100° to 120° F. 5. Fcairmentin Process. — This was proposed many years, ago, and somewhat resembles Milner's process. Litharge is taken and treated with salt in such proportions as to convert it into oxychloride of lead ; this body is placed along with water in a number of cylindrical vessels fitted with radial beaters. Carbonic acid is sent in, while the temperature is maintained at the boiling point. When the reaction between the acid and the lead has finished, the current of gas is stopped and the product run into a boiler, in which it is boiled with a quantity of finely-powdered carbonate of lime, equivalent to the amount of salt used in the preparation of the oxychloride, this boiling being continued until, on taking out a sample, and filtering off and testing the clear liquor with ammonia and 32 WHITE PIGMENTS. ammonium sulphide, no precipitate forms ; the period of boiling varies from two to four hours. When the boiling with the •carbonate of lime has been continued long enough, the opera- tion is stopped, and the white lead allowed to settle out, collected and finished in the usual way. 6. Spence Process. — The principle of this process consists in boiling a salt of lead (the oxide or carbonate gives the best results, but the sulphate or other salt which can be dissolved by caustic soda may be used) with a solution of caustic soda until the alkali is saturated with lead ; then a current of carbonic acid is passed through the liquor, and white lead is precipitated, while carbonate of soda is formed. The latter can be causticised by means of lime, and used over again. The white lead which is precipitated is collected, washed, and finished in the ordinary way. This process has not been used on a commercial scale. 7. Maclvor Process. — The principle of this process depends upon the fact that when litharge is acted upon by acetate of ammonia under the combined influence of heat and pressure, it is converted into basic acetate of ammonia and lead, while ammonia is liberated in the free condition, and dissolves in the water which is present to form the liquor ammonia of commerce. Then, when a current of carbonic acid gas is passed through the mixture of basic acetate of lead and ammonia, the lead is precipi- tated as basic carbonate or white lead, of good colour and covering power ; while acetate of ammonia is re-formed and can be used again for dissolving a fresh batch of litharge. The pro- cess is carried out somewhat in the following manner : — Into a digestor made of strong iron plate lined with lead, is placed a solution of acetate of ammonia of not less than 5 per cent, strength and a quantity of litharge which has been previously very finely ground. The proportions of the two will depend upon the strength of the solution of ammonia acetate which is used ; for that given, 1 ton of litharge is used for 1,200 gallons of liquor. The digestor is closed. The acetate solution is sent through a heater so that it may have a temperature of from 60° to 100° C, and then into the digestor, passing into it from a pipe fitted with a conical spreader at its lower end ; the acetate solu- tion flows upwards through the litharge, effectually agitating the mass and so assisting its solution ; from the digestor the liquor is drawn off from the upper portion by means of a pump and passed through the heater and again into the digestor, this cycle of flow being continued until all, or nearly all, the litharge is dissolved. 'The solution of basic acetate of lead and ammonia is now passed PRECIPITATION PROCESSES WITH ALKALINE CARBONATES. 33 through a filter-press into a cooler, from which it flows into a carbonator. The cooling of the liquor causes the separation of much of the basic acetate of lead in the form of fine crystals, so that in the carbonator a fine magma is presented to the action of the carbonic acid gas, which is sent into it from any convenient source. A circulation of the mass in the carbonator is kept up by drawing off from the upper portion of the carbonator and forcing it by means of a pump through a pipe, with a conical spreader at its end, to the bottom of the carbonator ; in this way every part of the mass of liquor and crystals is made to come in contact with carbonic acid gas. The white lead is rapidly formed as a fine white precipitate. When it is considered that the carbonation is finished the whole mass is passed through a filter- press, so as to separate the white lead formed, while the liquor, which consists of a solution of acetate of ammonia, together with unchanged basic acetate of lead and free ammonia, is sent to the digestor to be used again. The process is a rapid one ; the solu- tion of the litharge in the acetate of ammonia does not take long, while the conversion of the basic acetate of lead into white lead in the carbonator is almost instantaneous. The process is being worked by a limited company. 4th GROUP. PRECIPITATION PROCESSES WITH ALKALINE CARBONATES. When a solution of sodium carbonate, or other alkaline car- bonate, is added to a solution of lead, a white precipitate of a more or less basic carbonate of lead is obtained ; insoluble basic or neutral salts of lead, such as the oxychloride or the sulphate, are also acted upon by alkaline carbonates, and basic lead carbonate is formed ; these reactions are formulated in the following equations : — Pb 2 C 2 H 3 0 2 , 2 Pb H 2 0 2 + 2 Na 2 C 0 3 = 2 Pb C 0 3 , Pb H 2 0 2 Basic acetate of lead. Sodium carbonate. White lead. + 2NaC 2 H 3 0 2 + 2 Na 0 H Sodium acetate. Pb 2 OCl 2 + 2Na 2 C0 8 + H 2 0 = 2 Pb C 0 3 + 2NaCl + 2NaOH Lead Sodium Water. Lead Sodium Sodium oxychloride. carbonate. carbonate. chloride. hydroxide. PbS0 4 + Na 2 C0 3 = PbC0 3 + Na 2 S0 4 Lead sulphate. Sodium Lead Sodium carbonate. carbonate. sulphate. The chief difficulties met with in carrying out the processes 3 34 WHITE PIGMENTS. depending upon the action illustrated in the above equations, are to prevent the formation of a highly crystalline neutral carbonate and to ensure that the precipitate shall have the necessary amount of basicity ; for, as will be seen from the above equations, the tendency is to form the normal carbonate of lead instead of the basic carbonate. These methods of preparing white lead early attracted atten- tion from white-lead makers, and many and various have been the processes which have been patented and tried for the manu- facture of the pigment by such methods. While there is no doubt that good white lead can be made by them, yet the results seem to be so variable that from a commercial point of view these processes have always been failures. The first patented process belonging to this group dates from 1797 when the Earl of Dundonald secured a patent for making white lead from the oxychloride of lead. 1. Dundonald Process. — Litharge is taken and is treated with sufficient salt and water as to convert it into the oxy- chloride of lead, in the manner which will be found more fully detailed on p. 29. The insoluble oxychloride is collected, washed to free it from alkaline salts, then boiled in a solution of potash (potassium carbonate), when it is converted into white lead, which is collected, and, after washing, dried ; it is then ready for use. A very similar process was patented some years later by James Kier. No record exists as to whether this process was much, if at all, used on the large scale. 2. Pattinson Process. — Mr. Hugh Lee Pattinson, a large lead-smelter of Newcastle, has prepared white lead by many processes ; " Pattinson's white lead " (which see) is the oxy- chloride of lead. The process, which comes under the present group, has for its object the preparation of ordinary white lead. Chloride of lead prepared by any convenient process is mixed with carbonate of lime in the proportion of their chemical equivalents, 278 to 100, and the mixture is ground with water for several hours, then allowed to stand all night, the clear liquor (which consists principally of a solution of chloride of calcium) run off, more water added, and the grinding resumed for a few hours ; then it is again allowed to stand all night and the clear liquid again drawn off. These operations are continued until the effluent water is tasteless. The white lead, after being finished in the usual manner, is ready for use. Instead of the process just described a solution of the carbonate of lime or of carbonate of magnesia, made by means of carbonic acid, is used to act on the lead chloride. LOWE PROCESS. 35 In another modification of the process, chloride of lead and carbonate of calcium are placed in a revolving cylinder and a current of carbonic acid gas sent into the mixture, preferably the gas is used at a pressure of four or five atmospheres ; after four days the aqueous liquor, which is, as before, a solution of calcium chloride, is drawn off, more water run in and the gas again passed in for two days longer, when the reaction is completed, and the white lead only requires finishing to be ready for use. This process does not seem to have been much used. 3. Dale and Milner Process. — The inventors take litharge or a basic salt of lead and grind it with water and bicarbonate of soda for some time, when white lead is formed. This process was worked on a large scale for a short time, but it was super- seded by Milner's process described above. A process patented by Isham Baggs was almost identical with this. The results were not very satisfactory as the white lead obtained was rather too crystalline in structure. 4. Watt and Tebbutt Process. — This consisted in treating sulphate of lead, first with lime, then with potash. Cooper uses 25 lbs. of sulphate of lead to 10 lbs. of potash. The action of alkaline carbonates upon lead sulphate is, at the best, but im- perfect, and a complete change into carbonate is never obtained. 5. Delafield Process. — Delafield uses nitrate of lead, which he prepares by dissolving one cwt. of litharge in one cwt. of nitric acid and just enough water to form a saturated solution. This is heated by steam to a temperature of about 200° F. When a hot solution of 70 lbs. of potash is run in, white lead is precipitated, which is collected and, after washing, dried. The product is liable to contain too much carbonate and, therefore, to be deficient in body. 6. Rowan Process. — This resembles the Watt and Tebbutt process, only the action between the lead salts and the alkaline carbonate is effected under a pressure of from 30 to 40 lbs. 7. Lowe Process. — In Patent No. 9,122 of 1887, a process for making white lead is described, which consists of the following operations : — 50 lbs. of lead acetate, or 43*6 lbs. of lead nitrate, are dissolved in 25 to 30 gallons of water \ to this solution is then added 23 lbs. of solid bicarbonate of soda or 26*4 lbs. of solid bicarbonate of potash, when a precipitate of a more or less basic carbonate of lead will be obtained. In another vessel 25 lbs. of lead acetate and 15 lbs. of litharge are digested with 12J- gallons of water for 8 to 10 hours, when the product which is obtained is mixed with the precipitate obtained in the first 36 WHITE PIGMENTS. instance. White lead is formed and is collected and finished in the common way. An analysis of a sample of white lead made by this process is given in the patent, as follows : — Lead monoxide, Pb 0, .... 86*185 per cent. Carbonic acid, C 0.>, .... 11*270 ,, Water, H 2 0, 2*545 which differs but little from that of the Dutch process white lead. 8. Condy Process. — This process was patented in 1881, and has been worked on a large scale ; but whether the process is now in use or not the author is unaware. In this process, acetic acid of 1 *045 specific gravity is diluted with about five times its volume of w r ater, and allowed to act on granulated lead until a solution of lead acetate of 1 *2 specific gravity is obtained ; this solution is evaporated to dryness, when the bibasic lead acetate is obtained. 275 lbs. of bibasic acetate of lead, 112 lbs. of litharge and 5 gallons of water are ground together into a paste. Instead of preparing the bibasic acetate the neutral acetate may be used ; in this case, 189 lbs. are ground with 229 lbs. of litharge and 21 lbs. of water for a few hours, and then left for 48 hours. In either case there is formed the tribasic acetate of lead. The mass is dissolved in 10 times its weight of water, and, then, for every 112 lbs. of litharge in the mass 84 lbs. of solid bicarbonate of soda is added ; this precipitates the white lead, which is finished in the usual way. A modified process was described in a later patent. One part of acetic acid of specific gravity 1*045 is mixed with 12 J times its weight of water, and the dilute acid so obtained allowed to act upon granulated lead until a solution of specific gravity 1 *040 is obtained ; this is mixed with water, and, then, for every 60 lbs. of acid used in preparing the solution, 30 lbs. of solid bicarbonate of soda are added, and the white lead is precipitated. The white lead prepared by this process has been favourably spoken of; it has a good colour and covering power. In chemical composition it resembles white lead, but the process appears to be somewhat variable in its results, and, therefore, not com- mercially practicable. 5th GROUP. MISCELLANEOUS PROCESSES. Besides the processes described above, others have been proposed or patented from time to time which are perhaps just worth mentioning, as showing what has been done by MISCELLANEOUS PROCESSES. 37 inventors towards the preparation of white lead by other means than the old Dutch process. Some of these processes do not come within the groups of processes described above, others fall into one or other of them ; but as they are only of small importance, and as, in some cases, it is doubtful whether they were ever worked on a large scale, they have been relegated to this division of white-lead processes for description. The pro- cesses are rather numerous, and will only be given in outline; for further details the reader is referred to the records of the Patent Office. Torassa proposed a curious process, which is of interest on account of its novelty only, not from any practical value it may possess. Lead is granulated, and then placed, with a small quantity of water, in a revolving box, or a box fitted with agitators ; in this it is worked until it forms a very fine mud, which is again agitated with air until it has been converted into white lead. The process must have been a slow one, as the amount of carbonic acid in the air is small, and can only convert in a given time but a small quantity of fine lead into carbonate. Wood, a more recent inventor than Torassa, proposed to use the same process, but to hasten the preparation of the white lead by agitating the fine lead mud with carbonic acid ; but even this was not sufficient to make the process a practical success. Mullins proposed to make white lead by an ingenious but, from a practical point of view, unsuccessful method. Sponges, saturated with a solution of basic acetate of lead, were suspended by porous strings in a chamber into which carbonic acid was. passed ; this, of course, transformed the basic acetate into basic carbonate of lead. The sponges were kept saturated with a solution of acetate by connecting the porous strings with a tank containing the solution, which, by capillary attraction, passed along the strings to the sponges. The process was not used on a large scale. Martin prepares carbonate of lead so that it shall contain a slight excess of carbonic acid. Hydroxide of lead is prepared by thoroughly agitating granulated lead with air and water. The two bodies are mixed together in the proportion of 8 lbs. of hydroxide to one ton of carbonate, the mixture being made by grinding with water into a paste. Lewis prepares what he calls white lead from lead or lead ores, by mixing these with anthracite coal, and heating the mixture in a Wetherill zinc furnace with a powerful blast of air; the white lead sublimes, and is collected (see p. 18). 38 WHITE PIGMENTS. Although the product is spoken of as white lead, it is probably the sulphate of lead. Button and Dyar treat the basic nitrate of lead with carbonic acid gas. Brown and Young take the lead nitrate, and pass a current of carbonic acid until the liquor becomes saturated with the gas, when caustic soda is added in slight excess ; white lead is pre- cipitated ; it is allowed to settle, and, after pouring off the supernatant liquor, is digested with lime water, and then washed and dried. Maxwell-Lyte proposes to use spongy lead in the ordinary •chamber process, with the view of facilitating the action of the gases on the lead, and so making the process more rapid. Woolrich used a process not unlike that of Torassa ; he provides a revolving box, into which he places granulated lead ; by the attrition, which occurs during the revolution of the box, the lead is gradually converted into a fine powder ; a solution of basic acetate of lead is also placed in the box, and this, to some extent, by chemical action facilitates the operation. Every twelve hours the action is stopped, and the lead mud formed is washed out by means of a current of basic lead acetate liquor, through which is afterwards passed carbonic acid gas to transform the lead into white lead. Ozouf uses a solution of the tribasic acetate of lead, places this in a closed vessel fitted with agitators, and then sends in a current of carbonic acid gas to precipitate the white lead. Cookson has a process not unlike that of Cory ; he constructs large chambers, into which he throws a solution of basic acetate of lead in the form of a spray ; the spraying being done by means of a jet of carbonic acid gas. Other methods have been proposed, but they are all modifi- cations of those which have already been described. COMPOSITION AND PROPERTIES OP WHITE LEAD. White lead is sold commercially in two forms. One is a heavy white powder, having a specific gravity of about 6*47, and weighing about 180 lbs. to the cubic foot; it is stated that some processes yield white lead weighing as much as 200 lbs. to the cubic foot. The other form is that of a paste containing about 8 per cent, of linseed oil. The chemical composition of white lead has already (p. 17) been pointed out. It is a basic carbonate of lead formed by the union of two molecules of lead carbonate, Pb C 0 3 , with one COMPOSITION AND PROPERTIES OF WHITE LEAD. 39 molecule of lead hydroxide, Pb H 2 0 2 ; tins is the composi- tion of the best make of Dutch white lead, which has all the good properties of white lead in the highest degree of perfection. It is scarcely necessary to point out that as white lead is made by many processes it must necessarily vary in composition ; indeed the white leads yielded by the same process do nob always have the same composition, as is evinced by the analyses given here and on p. 19; these have been collected from a variety of sources. ANALYSES OF WHITE LEADS. 1. 2. 3. 4. 5. 6. 7. Lead monoxide, Pb 0, .86*35 85 93 83*77 85*93 84*42 86*5 86*24 Carbonic acid, C 0 2 , .10*44 11*89 15*06 11*89 14*45 11*3 11*68 Water, Ho 0, . . . 2 95 2*01 1*01 2 01 1*36 2*2 1*61 99*74 99*83 99*84 99*83 100*23 100*0 99*53 from which the composition of the white leads can be calculated to be — Lead carbonate, Pb O 0 3 , 63*35 72*15 91*21 71*93 87*42 6S*36 70*87 Lead hydroxide, PbH 2 0 2 , 36*14 27*68 8*21 27*88 12*33 31*64 28*66 Moisture, . . . 0*25 ... 0*42 0*02 0*48 No. 1. English make. Made by the Dutch process; of very good quality. No. 2. English make. Made by the Dutch process ; of good quality. No. 3. Krems white. Made by precipitation with carbonic acid gas ; this sample is deficient in body although of good colour. No. 4. German make. Precipitated by sodium carbonate ; it is only of medium quality. No. 5. German make. Precipitated by carbonic acid gas ; of good colour, but deficient in body. No. 6. German make. Made by the Dutch process ; a good white. No. 7. German make. Made by precipitation with carbonic acid gas ; quality fair. The second form in which lead is sold is that of a paste with linseed oil. To make this, the dry white lead, above described, is first mixed in a mixing mill, with about 8 to 9 per cent, of its weight of raw linseed oil ; then it is run through a grinding mill several times, to ensure a thorough mixture of the oil and white lead. This form is much favoured by painters, as it is more readily miscible with oil and turps to make into paint. The following are two analyses of ground white lead : — Lead hydroxide, Pb H 2 0 2 , . . 65*96 71 *14 per cent. Lead carbonate, Pb C 0 3 , . . 25*19 20*45 Insoluble, 0*70 ... ,, Oil, 8*34 8 34 40 WHITE PIGMENTS. In making this ground white lead only the best raw linseed oil should be used ; boiled oil is not admissible, as there would be too much tendency for the lead to become a hard dry mass before it could be used. It is customary to keep ground white lead under water to prevent it drying up too rapidly. Besides its use by painters, this form of white lead is also largely used for other purposes, as a cement for gas-piping, &c. White lead is soluble in dilute nitric acid, and in acetic acid with effervescence, due to the evolution of carbonic acid gas. It is also soluble in boiling dilute hydrochloric acid with effer- vescence ; on cooling the solution fine transparent needle-shaped crystals of lead chloride separate out. Boiling with sulphuric acid decomposes the white lead, insoluble lead sulphate being formed. Solutions of white lead in acids give white precipitates of lead sulphate with sulphuric acid ; of lead chloride with hydrochloric acid, soluble on boiling in water ; and of lead carbonate with sodium carbonate. Neutral solutions of white lead give a yellow precipitate of lead chromate with potassium bichromate, and a black precipitate of lead sulphide with sulphuretted hydrogen and solutions of sulphides. As a pigment white lead possesses all the good qualities desired by a painter — viz., good colour, body or covering power, and permanency. It is distinguished from all other pigments by the ease with which it mixes with oil and by forming a paint which readily flows from the brush, whereas most pigments, as for instance, barytes, tend to work what the painter calls slimy or livery, and streaky; white lead does not exhibit this property, but flows freely and evenly from the brush. This feature is due to the lead hydroxide in the white lead combining with some of the oil arid forming a lead soap which, dissolving in the rest of the oil used in the preparation of the paint, forms a kind of varnish ; this varnish takes up the lead carbonate to which is due the body or covering power of the pigment. Sometimes this chemical combination between the lead hydroxide and the oil extends to the lead carbonate and then the white lead loses its opacity and becomes more or less transparent or horny ; the conditions most favourable to the production of this change, which is of rare occurrence, are not properly known. This fact of the white lead forming a chemical combination with the oil is w r ell known to colour makers, who have endeavoured, by the addition of basic bodies, to bring about a similar action in the case of other white pigments, such as zinc white and barytes, but so far without any great success. COMPOSITION AND PROPERTIES OF WHITE LEAD. 41 When exposed to light and air white lead is fairly permanent and will resist exposure to normal conditions for a great length of time ; on the other hand, when exposed to the fumes of sulphuretted hydrogen and other sulphureous gases, white lead turns brown or black through the formation of the black sulphide or lead. The production of this body is more likely to occur in large towns, such as London and Manchester, where large quantities of gas are used for lighting and other purposes, which usually contains some sulphuretted hydrogen or other sulphur compounds. By oxidation this black sulphide can be transformed into the white sulphate of lead ; the only agent which can be safely used for this purpose in restoring paintings which have become discoloured is peroxide of hydrogen, but the action of this body is very slow and is much interfered w T ith by the oil which is present. White lead can be mixed with all pigments except those which, like cadmium yellow, ultramarine or king's yellow, contain sulphur ; such pigments sooner or later cause the formation of the black sulphide and thus bring about the discolouration of the pigment or paint. White lead is frequently adulterated, the pigment most used for this purpose being barytes, because it more nearly approaches white lead in specific gravity, and is, on that account, not so readily detected ; w T hereas the use of whiting or gypsum would soon be detected on account of the great difference in weight between genuine white lead and white lead adulterated with them. This adulteration of white lead is exceedingly common and is well understood by makers and dealers ; in fact it is the custom for makers to send out several qualities of commercial white lead distinguished as "genuine/' "No. 1," "No. 2," and so on; the degree of adulteration being regulated by the price which is paid for the product. The question whether this is adulteration is a matter of opinion ; if by adulteration one means the admixture of cheap products with dear products with a view of deceiving purchasers of the latter, then the admixture of barytes with white lead under the conditions named is not adulteration, for the purchaser knows what he is buying, and only pays a fair price for such mixed white leads. The author is, however, of opinion that this custom of mixing barytes with white lead would be much better honoured by the breach than the observance of it. 42 WHITE PIGMENTS. ASSAY AND ANALYSIS OF WHITE LEAD. White lead may be assayed for colour and covering power by the usual methods (see Chapter X.). Dry White Lead. — The purity of this pigment is ascertained by dissolving some of the lead in pure dilute nitric acid (1 acid, 2 water) ; strong nitric acid does not dissolve white lead owing to the insolubility of the lead nitrate which is formed in the acid ; the ordinary commercial nitric acid contains sulphuric acid, which would lead to the formation of the insoluble sulphate of lead, the production of which might lead to the -condemnation of a pure sample. On adding dilute sulphuric acid to the solution, after diluting it with water and filtering off the precipitate of lead sulphate thus obtained, no further precipitate should be obtained on suc- cessively adding ammonia, ammonium sulphide,, and ammonium oxalate to the filtrate. A white precipitate with ammonium sulphide would indicate the presence of zinc white, which is a rare thing to find with white lead ; a white precipitate with ammonium oxalate would indicate the presence of whiting. The insoluble residue, if any, will consist most probably of barytes, as other adulterants (for reasons already pointed out) are rarely used ; still any lead sulphate, china clay, gypsum, or strontium sulphate which may be used would also be left as an insoluble residue on treating w r hite lead with dilute nitric acid. To distinguish these bodies, boil the residue in hydrochloric acid and place the solution on one side to cool ; if crystals of lead chloride separate out and the solution gives a white precipitate with barium chloride, then lead sulphate is present. The hydrochloric acid solution should be diluted with water and sulphuretted hydrogen passed through it ; the black pre- cipitate of lead sulphide which may be obtained can be dis- regarded ; this is filtered off and the filtrate boiled for some time to concentrate it and to drive off the sulphuretted hygrogen it contains. Then ammonia is added, when a white precipitate of alumina may be obtained indicating the presence of china clay ; this is filtered off, and to the filtrate is added ammonium carbonate, which will precipitate any calcium that may have been added, in the form of gypsum or whiting. A little of the insoluble residue from the hydrochloric acid should be held on a piece of platinum wire in the lower part of a Bunsen flame when, if it contains barytes, the flame will be ANALYSIS OF WHITE LEAD. 43 coloured green ; if strontium sulphate is present a crimson flame will be obtained. This test is not always easy to carry out, but with a little care the coloured flames can be obtained, and they are good proof of the presence of the pigments named. A quantitative analysis of white lead may be made as follows : — Weigh out 2 grammes and dissolve them in a beaker with the smallest possible quantity of pure dilute nitric acid, remove the insoluble matter by filtering, wash the residue well with warm water, adding the first wash waters to the filtrate, then dry the residue, place the filter paper and its contents in a weighed crucible and burn the paper ; when completely burnt allow the crucible to cool in a desiccator and then weigh it. From the weight so obtained deduct the weight of the crucible and of the filter-paper ash, the difference is the weight of the insoluble residue. To the filtrate add dilute sulphuric acid and a little alcohol, filter off the precipitate of lead sulphate which is obtained, wash it dry and burn it in a crucible as before. By multiplying the weight of the lead sulphate so obtained by 0*73554 the weight of lead oxide in the white lead can be found. The carbonic acid can be ascertained by treating 2 grammes of the white lead with nitric acid in a Schrotter's or other form of apparatus for the estimation of carbonic acid. The water may be determined by taking the difference between the amounts of lead oxide and carbonic acid thus found and 100. Hygroscopic ivater can be ascertained by heating 2 grammes in an oven at 110° to 120° C. until no further loss of weight occurs. If the white lead be adulterated with barytes, lead sulphate, china clay or some of the other insoluble white pigments, these will be left behind as an insoluble residue on treatment with nitric acid ; their amount is ascertained by filtering off, washing, drying, and burning the residue in a weighed crucible in the usual way. Soluble adulterants like whiting, strontium carbonate, barium carbonate, and magnesite will be dissolved ; if the presence of these is suspected, to the filtrate should be added more dilute sulphuric acid, which will precipitate the lead and barium \ this precipitate can be filtered off, the two can be separated by boil- ing with hydrochloric acid, which dissolves the lead sulphate but not the barium sulphate. To the filtrate ammonia and ammonium oxalate are added ; this precipitates the calcium and the strontium, while the magnesite (if present) will remain in solution, and can be precipitated by sodium phosphate. It is not necessary to describe in detail the methods of separating these 44 WHITE PIGMENTS. adulterants any further ; some notes bearing on this point will be found in the descriptions of each individual pigment, while reference should be made to works on quantitative chemical analysis, such as that of Prof. Sexton, for fuller details. By multiplying the weight of the carbonic acid by 5*05 the amount of lead oxide with which it is combined can be calculated; the two amounts added together give the quantity of lead carbon- ate in the white lead. Deducting the amount of lead oxide combined with the carbonic oxide from the total present, and multiplying this difference by 0*077, gives the amount of water combined with it to form lead hydroxide, and the two amounts added together gives the amount of the latter body present. Paste White Lead can be quantitatively examined as follows : — Two grammes are treated with strong nitric acid at a gentle heat ; this converts all the oil into an insoluble greasy matter, of which no account need be taken ; the process then becomes identical with that for dry white lead. Should it be desired to ascertain the amount of the oil, then 10 grammes must be weighed into a filter paper and placed in a Soxhlet or other form of fat-extractor and the oil extracted by means of petroleum ether ; the ethereal solution is then run into a weighed glass, the ether evaporated off and the oil weighed. If a fat- extractor is not available it will suffice to agitate the white lead with some petroleum ether in a beaker, allowing the pigment to settle, pouring off* the liquid into a weighed glass, again pouring on more ether, and again allowing the white lead to settle ; the ether is poured off into the glass, or the mass may be filtered. Finally, the ether is evaporated off as before. It is sometimes recommended to burn off the oil from the white lead in a crucible, but this course is not so satisfactory as treating it with petroleum ether, as nob only is the oil burnt off, but the white lead is decomposed ; whereas in the ether method the lead is left in its original form for further examination if necessary. SULPHATE OF LEAD PIGMENTS. Sulphate of lead, Pb S 0 4 , forms the basis of a number of white pigments which are made on a large scale and sold under a variety of names, such as Patent White Lead, Non-poisonous White Lead, Sublimed White Lead, &c. These do not consist entirely of lead sulphate but contain other bodies, such as zinc oxide, barytes, magnesia, &c, in varying quantities ; they are SULPHATE OF LEAD PIGMENTS. 45 made by different methods, and most of those now sold are pro- duced by patented processes. Lead sulphate can be made by dissolving lead in strong sul- phuric acid, the action is, however, but slight and does not form a commercial method of manufacture. It is mostly made by adding sulphuric acid to solutions of either lead acetate or lead nitrate ; perhaps the best method is that where lead acetate is used. Metallic lead is granulated by melting and pouring the molten lead into cold water ; the object of granulating is to obtain the metal in such a form as to expose a large surface to the action of acids and air. The granulated lead is placed in a large tub fitted with a closed steam coil so that the action of the acid may be facilitated by heat, if necessary. Acetic acid diluted with its own volume of water is poured on to the lead ; the action of the acid is at first rather sluggish, but by allowing the action to go on for about 12 hours, then running off the acid and leaving the lead in the tub without any liquor, a certain amount of oxidation goes on, resulting in the formation of a deposit of oxide on the lead, so that when the acid is again admitted to the lead, the acid acts more rapidly, and a strong solution of lead acetate is soon obtained ; this action is facilitated by a gentle heating of the contents of the tub. From the tub the solution is run into a large wooden vessel and to it is added strong sulphuric acid in small quantities at a time with constant stirring ; lead sulphate is thereby precipitated. Care is taken that the amount of sul- phuric acid used is not sufficient to throw down all the lead, but that some of the latter is left in solution. The lead sulphate is allowed to settle, and then the clear supernatant liquor is pumped back again into the lead tub, for it contains all the acetic acid ; for, as will be seen from the equation — acetic acid is reproduced as the result of the reaction which goes on between the lead acetate and the sulphuric acid, and this acetic acid can be used over again for preparing fresh solution of lead for precipitation ; thus a comparatively small quantity of acetic acid may be used to prepare a large quantity of lead sul- phate ; theoretically speaking, beyond the first charge no more acetic acid is required, but practically, there is a small loss which requires to be restored by new additions of acid from time to time. Pb 2 C 2 H 3 0 ; Lead acetate. 46 WHITE PIGMENTS. The sulphate of lead which is precipitated is washed with water, drained on a filter, and dried ; after which it is ready for use as a pigment, or can be combined with other white pigments if desired. The acetic acid used should be as pure as possible ; the usual commercial variety of a strength of about 1*050 = 10° to 12* Twaddell is sufficiently good for the purpose ; this contains about 35 per cent, of actual acetic acid, although some makes contain more. 100 lbs. of it will dissolve about 60 ]bs. of lead supposing all the acetic acid exerts its solvent power, but in practice this never or but rarely happens, nor indeed is it necessary that it should. From 20 to 28 lbs. of sulphuric acid will be required to precipitate the lead dissolved by this quantity of acetic acid. These figures are only approximate and are given simply as guides to actual practice. Lead sulphate has the formula Pb S 0 4 , and contains Lead oxide, Pb O, . . . 73*55 per cent. Sulphuric anhydride, S 0 3 , . 26*45 ,, and of metallic lead 68*31 per cent. It is a white, somewhat crystalline, and very heavy powder, its specific gravity being about 6*3. It is only slightly soluble in water, insoluble in dilute acids and in alcohol, but soluble in solutions of ammoniacal salts, and in strong sulphuric acid ; from the latter solution it is precipitated on the addition of water. Boiling concentrated hydrochloric acid dissolves it, and crystals of lead chloride fall down as the solution cools. As a pigment it is not satisfactory, its crystalline character reduces its body and covering power, causing it often to work streaky or livery under the brush ; this defect can be remedied to some extent by grinding it. It is not readily acted upon by sulphuretted hydrogen, and is, therefore, more permanent than white lead under exposure to air. Owing to its solubility being less it is free from the poisonous character of white lead, and, therefore, white pigments containing it are often sold as " non- poisonous white leads." Its colour or hue is a good white, but slightly yellower in tone than white lead and about equal to "barytes. It is used as a diluent in the manufacture of pale chromes. Many have been the attempts to make lead sulphate the base of commercial white leads, the records of these are to be found in the publications of the Patent Office, where they lie buried in an almost unknown condition, and it would really be most instructive for colour-makers and would-be inventors if they SUBLIMED WHITE LEAD. 47 would peruse these records and see what has been done in the past. A few of these inventions may be briefly noticed here,, and a fuller description given of such as are at present used on a large scale. Richardson, in 1839, patented the use of the sulphate only as a pigment; in 1853, Carter & Marriott prepared a chloro- sulphate of lead made by treating 100 lbs. of litharge with 25 lbs. of salt, and the product so obtained with 5 lbs. of sulphuric acid. Woods, in 1866, took out a patent for the preparation of a white pigment from lead fume, which is a mixture of lead, lead oxide, and lead sulphate ; this he treated with hydrochloric acid thus forming a chloro-sulphate ; or he calcined the fume in a furnace, whereby it was converted into a white mixture of lead oxide and lead sulphate, which was then treated with hydro- chloric acid as before. Groves, in 1826, treated galena with potassium nitrate and sulphuric acid, whereby the lead sulphide was converted into lead sulphate, which was dried and sold as a> pigment. In 1866, Messrs. Bell generally known as "launders;" these are long and wide; across these at intervals are placed a number of pieces of wood, called the u drags," which serve to impede the flow of the water, and cause it to form a series of pools, in which the heavier particles of sand can collect. These launders are emptied of the sand which accumulates in them from time to time, so as to give Fig. 12. — China-clay works. plenty of room for the sand to deposit. From the launders the water, with the clay and mica, passes on to what are called the " micas," an ingenious arrangement for promoting the deposition of the light and flaky pieces of mica. The micas are a series of troughs 20 feet long, 2 feet wide, and 6 inches deep, placed side by side in a peculiar manner. The clayey water from the launders MANUFACTURE OF CHINA CLAY. 8 r > first passes into two of these, then from these into three, then again into four, and, finally, into six of these mica troughs ; thus as it passes to the exit end the flow of water is spread over a larger surface, and becomes more feeble, a condition which facilitates the deposition of the mica. The micas get filled in about eight hours, when they are flushed of their contents with water, which carries the deposited mica through suitable channels into the waste mica pits. Next in order to the micas is a set of settling pits. These are usually three in number, sometimes more, according to the quantity of clay which is being worked. These pits may be of any shape, but, as a rule, they are made circular (or rather cylindrical) in form, 7 feet in diameter, and 40 feet deep. Into one of these pits the clayey water from the micas is run until it is full, when the current is changed and the water run into the second one until it is full. While the filling of the second pit is proceeding, the clay in the first one is settling, and, probably, by the time that the second is quite full, has completely settled. The current of clayey water from the micas is now diverted into the third pit, while the clay in the second one is settling. The water in the first pit is now run or pumped off, and is generally used over again for washing the clay from the stopes. From a pit full of clayey water there will usually be obtained a deposit of clay about 5 feet in depth, which still contains a large pro- portion of water ; in such a thickness of clay, and 7 feet in diameter, there will be something like 285 tons of dry clay. The clay in the first pit is dug out and thrown into what are called clay tanks, where a further settling takes place ; when all the clay has been dug out the pit is ready to be filled again with water from the micas. This alternation of filling, settling, and emptying is carried out with the three pits in succession, so that it will be seen that for continuous working a series of not less than three pits is required ; if more pits are used, then the time of settling can be lengthened, which would have the advantage of giving a drier clay and shortening the subsequent operations. From the settling pits the still wet clay passes to the settling or clay tanks ; these are, at least, three in number, corresponding with the three settling pits; in some works there are more; much, of course, depends upon the quantity of clay it is desired to turn out. These settling tanks are usually rectangular in shape, about 60 feet long by 7 feet wide and 6 feet deep, and they will hold about 1000 tons of clay; in these tanks settling occurs, and the clay begins to assume the consistency of lard ; when this happens no more clay is sent into it from the pits, and the clay in the 86 WHITE PIGMENTS. tank is allowed to settle. The water is then run off, and the clay transferred to the drying place, where it is dried ready for sale. The clay in the clay tanks contains about 50 per cent, of water, most of which must be driven off before the clay is marketable. This drying operation is done on a series of flues, technically known as the "dry;" a usual size is 60 feet long by 13 J feet wide. In this kiln or dry there will be three fireplaces, two at one end, and one at the other, each fireplace having three flues about 9 inches wide ; the sides are formed of brickwork, but the bottom is usually made of sand, partly because sand is a bad conductor of heat, and partly because any water which may drain through from the top of the flue readily sinks into it and drains away. The tops of the flues forming the bed of the dry is made of fireclay bricks about 18 inches wide. On these fire- clay bricks the wet clay from the tanks is thrown, and it remains until it is dry. It takes about 1 ton of coal to dry 10 tons of clay. After being dried on the dry the clay is thrown on the floor of the clay linhay, which is a storage place for the dry clay, from whence it is sent out as required. The dry and the linhay are parts of one large room, being covered over with a roof, as is seen in the drawing (Fig. 12). COMPOSITION AND PROPERTIES OF CHINA CLAY. — China clay is essentially a hydrated silicate of alu- mina, as has been already stated ; but there are some minor differences in the composition of samples from various localities, as will be seen on examining the table given on p. 81 ; these are, of course, primarily due to differences in the composition of the granite from which the china clay has been formed, and, secondarily, to the degree with which the decomposition has proceeded. China clay, or kaolin, is a fine, white, amorphous powder having slight adhesive properties and adhering to the fingers when moist. It is light, its specific gravity being about 2*2; so that it is the lightest of all the white pigments. The best qualities have a very soft unctuous feel ; the common qualities are rather rougher, but none have the slightest trace of grittiness about them. The best qualities have a pure white tint, others a more or less yellowish tint, which the china-clay makers are accus- tomed to disguise by adding a small quantity of ultramarine. It is quite insoluble in water, dilute acids, and alkalies. Boiling in strong sulphuric acid for some time decomposes it Wilkinson's and pattinson's white lead. 87 with the formation of a gelatinous residue of silica and a solu- tion of alumina sulphate. Hydrochloric acid has little action on it. As a pigment it is quite permanent, resisting perfectly exposure to the abmosphere and to light for any length of time. As a pigment it is not, however, much used. In oil it loses its body and becomes more or less transparent. It can be used in water-colours and in distemper work with good results, and it is used in paper-making and paper-staining. It also finds a use in the preparation of the aniline lakes, especially when these are to be used in paper-staining. Its principal uses are for making pottery, ultramarine, finish- ing cotton cloths, making paper, &c. ASSAY AND ANALYSIS OF CHINA CLAY. — China clay can be assayed for colour or tint, covering power, &c, by the methods given below. An analysis is rarely wanted, since it is never adulterated, while for all pigment purposes absolute chemical purity is not required. WILKINSON'S WHITE LEAD AND PATTINSON'S WHITE LEAD are oxychlorides of lead prepared in various ways; neither pigment is now used. Wilkinson's white was patented in 1799, and is made by digesting litharge with a solution of salt until it acquires a pure white colour. Unfortunately, as it is the product of various operations, there is a lack of uniformity in its composition, which is much against its use as a commercial article. Another method of making it is to precipitate acetate of lead with hydrochloric acid, and to digest the precipitate of lead chloride obtained with basic lead acetate. Pattinson's lead was made by treating chloride of lead with lime, when it forms the basic chloride, a white insoluble body having a fair body, but wanting in uniformity of composition. 88 CHAPTER III. RED PIGMENTS. This is a fairly numerous and important class of painters' colours. They are derived from both inorganic and organic sources, and include some of the most highly valued and most used of the pigments at the disposal of the painter and artist. VERMILION. Vermilion has been used for a long time as a pigment. It is a compound of mercury and sulphur in the proportion of 200 parts of the former to 32 of the latter ; its chemical name is mercuric sulphide, and it has the formula, Hg S. It is found naturally in large quantities as the mineral cinnabar, especially in Spain ; but it rarely occurs naturally of sufficient brightness to be used as a pigment, and is, therefore, mostly made artificially. When a current of sulphuretted hydrogen is passed through a solution of a mercuric salt a black precipitate of the mercuric sulphide, identical in composition with vermilion, is obtained ; this precipitate is characterised by being insoluble in most single acids, but soluble in a mixture of hydrochloric and nitric acids. By heat it is volatilised, and the sulphide sublimes in the form of a red powder ; this transformation from black to red can also be brought about by boiling it for some time with aqueous solu- tions of the caustic alkalies or of alkaline sulphides. What the cause of the change may be is rather uncertain ; probably there has been a re-arrangement of the atoms in the molecule of mer- curic sulphide ; there are many cases known of similar differences in the colour of inorganic compounds, as, for example, cadmium sulphide and basic chromate of lead. Although it is generally considered that the molecule of each of these poly-coloured bodies is always made up of the same number of atoms, yet there is no direct evidence on that point ; and it is quite possible that in the different modifications of these bodies the number of atoms may vary and, therefore, be arranged dif- MANUFACTURE OF VERMILION. 89 ferently. This subject requires further investigation before the point can be definitely decided. MANUFACTURE OP VERMILION.— Yermilion can be made both by dry and wet methods ; the former are those mostly used as they give the best product ; the latter are employed in some places but not to the extent of the dry methods. The product is not quite equal, although very little inferior to that made by the dry methods. The Chinese have long been renowned as makers of vermilion ; their product is finer and more brilliant in tone than that made in Europe. Until lately, the process by which Chinese vermilion was made was not known with certainty, although it was conjectured that the wet method was used, and, consequently, this method is usually described in text-books as " The Chinese method ; " but this is now known to be erroneous, and that Chinese vermilion is made by a process very little different from that used in Europe. The difference in quality almost entirely arises from the greater care the Chinaman takes in making it. DRY METHODS— 1st. Dutch Process. — This is the method commonly used for making vermilion. It is conducted in two stages. In the first stage 108 lbs. of mercury are mixed with 1 5 lbs. of sulphur in a shallow iron pot ; this is usually placed over a furnace so that a gentle heat may be applied ; the two bodies gradually combine together to form a black sulphide of mercury or " ethiops " as it is called, the union being pro- moted by a continual stirring with an iron spatula. When the combination is considered by the workman to be complete, the iron pot is emptied of its contents into a store pot and a fresh mixing is made. The " ethiops " contains some free mercury, free sulphur, as well as sulphide ; the proportions will vary according to the length of time the operation has been continued, the heat applied, &c. The second stage consists in heating black ethiops in a suitable furnace, whereby it is converted into the red vermilion. A number of simple furnaces or fireplaces are built side by side to form a range ; in each of these fireplaces is placed a cylin- drical earthenware pot, so arranged that the lower two-thirds of the pot are in, while the upper third is outside the furnace. The pots are fitted with a closely-fitting iron lid, in the centre of which is a small charging hole. The fire in the fireplace is lighted, and, when the pot has been heated to a red heat, a small quantity of the black ethiops obtained in the first stage is charged into the pot ; much of the sulphur in the ethiops burns off; when there is no further appearance of sulphur fumes 90 RED PIGMENTS. from the pot more ethiops is added ; these additions are con- tinued at intervals for thirty-six hours, the cover being kept on during the whole of the operation ; then the pots are allowed to cool down ; when cold the cover is removed and the vermilion is found as a crust on the under side of the cover and around the sides of the upper portion of the pot. This crust is carefully removed, the red portions being placed on one side for further treatment, while any black, unchanged portions are mixed with some fresh ethiops to be again heated. The red vermilion is now ground up as fine as possible with water; if not of sufficiently brilliant colour it may be treated either with acids or alkalies as is described below, well washed with water, allowed to settle out of the wash waters, dried at a gentle heat and sent into the market ready for use. 2nd. Chinese Method. — A few years ago a description* of the process used by the Chinese for the preparation of vermilion appeared in several journals, and at the Colonial and Indian Exhibition held in 1886 there was shown in the Hong Kong Court a Model of a Chinese vermilion factory. Like the Dutch method, the Chinese process is in two stages, and is carried out as follows : — An iron pan, measuring 25 inches in diameter and 6 inches deep, is placed over a charcoal fire; into this pan is placed 17 J lbs. of sulphur and 37 J lbs. of mercury; heat is applied, and the mixture stirred until the materials melt and become amal- gamated together ; then 37^ lbs. more mercury are added, and the heating and stirring continued until the two bodies have become united. The pot is now removed from the fire and water added in sufficient quantity to form a paste, which has a blood-red colour ; the first stage of the process is now com- plete. Second Stage. — The crude vermilion obtained in the first stage is broken up into small pieces and placed in iron pans measuring 29 \ inches in diameter and 8| inches deep ; on the top of the vermilion is placed a number of broken pieces of porcelain plates arranged in the form of a dome ; over all is placed the pan used in the first stage, the two pans being luted together with clay, and a few vent holes left in the luting. The pans are placed on a furnace, which is constructed in a simple manner ; usually a number are built side by side. The pans are heated for 18 hours at a dull red heat, after which they are allowed to cool down; when cold, the pans are opened, when * Oil and Colourman's Journal, 1883, p. 86. Chemical News, 1884, vol. 50, p. 77. MANUFACTURE OF VERMILION. 91 the vermilion is found as a red sublimate on the under side of the porcelain plates and the upper pan ; this red mass is collected and transferred to another place for the finishing operation. The crude vermilion which has been scraped off the porcelain plates is now ground as finely as possible with water in a mortar ; the ground colour is next mixed with water in which alum and glue in the proportions of 1 oz. of each in a gallon of water have been dissolved, and allowed to stand for a day ; it settles down and is found as a cake at the bottom of the vessel, which is made of earthenware, and has a capacity of 6 gallons. The top of the cake is of fine quality * this is separated from the bottom portion, which is re-ground up with the next batch ; sometimes the top portion is re-ground. After being washed well with clean water, the finely-ground vermilion is dried and then packed up ready for sale. WET METHODS — 1st. Common Method.— In making vermilion by this method 68 lbs. of sulphur and 300 lbs. of mercury are mixed and ground together until they are thoroughly incorporated ; they are then added to a solution of 160 lbs. of caustic potash in water, placed in iron pots and heated to a temperature of 45° C, which is maintained for some hours. For the first two hours the water lost by evaporation is made good, but after this no further addition is made, and the mass is kept constantly stirred. After some time the mass, which has at first a blackish appearance, turns brown and then gradually passes into red ; when it is considered that the colour is fully developed the mixture is removed from the fire, well washed in water and dried. This process requires careful watching. 2nd. Firmenieh Process. — The process described by Fir- menich * consists in taking 10 parts of mercury and agitating them with 2 parts of sulphur and 4J parts of potassium penta- sulphide (prepared by heating potassium sulphate with charcoal) and boiling the residue with excess of sulphur for three to four hours, when it takes a brown colour ; it is then kept at a temperature of 45° to 50° C. for three to four days, being agitated at intervals during that period ; it is next treated with water, then with a weak lye of caustic soda (to free it from excess of sulphur), washed thoroughly and dried. In these wet processes it is important that care be taken not to heat the mixtures of mercury, sulphur, and alkali to too high a temperature ; from 45° to 50° C. is high enough. Time, not * Chemical News, vol. v., 1862, p. 247. 92 RED PIGMENTS. heat, seems to be the most important element to consider in these processes ; too great a heat turns the vermilion brownish. The brilliancy or fire, as it is sometimes called, of the vermilion may be increased during manufacture by 1st, Grinding very fine and levigating ; 2nd, By warming with a caustic soda lye ; 3rd, By treatment with nitric acid ; 4th, By treatment at about 50° C. with a mixture of the caustic and sulphide of potash ; and 5th, By treatment with hydrochloric acid. Any of these, or a combination of them, may be, and are, used for this purpose. PROPERTIES OP VERMILION. — Vermilion is a very bright scarlet powder. It is the heaviest pigment known, its specific gravity being 8*2, which causes it to settle readily out of paints, &c, in which it is used, and renders its application somewhat troublesome. It is very opaque, and, consequently, has great covering power or body. It is quite insoluble in water, alkalies, and any single acid, but a mixture of nitric and hydrochloric acids dissolves it with the formation of a colourless solution of mercuric chloride ; as a rule, very few substances are capable of acting on vermilion. Heated in a tube out of contact with air, vermilion first turns brown, then sublimes in the form of a red sublimate. Heated in contact with air, vermilion burns with a pale blue, lambent flame, giving off vapours of sulphur dioxide and mercury oxide ; if pure, there will be but a trace of ash left ; thus a sample of good vermilion analysed by the author contained Sulphide of mercury, . . . . 99 '63 Ash, -37 100 00 This forms a reliable test for the adulteration of mercury, for any adulterant which may be used will be left behind on heating. The usual adulterants employed are red lead, oxide of iron, red lakes, vermilionettes, &c. The presence of any of these is easily ascertained by the application of the characteristic tests, which will be found described under each particular pigment. When used as an oil-colour vermilion is permanent ; when used as a water-colour it is generally considered to be permanent, but the experiments recently made by Capt. Abney and Dr. Russell throw some doubt on this point ; they found that vermilion used as a water-colour turned brown after two years exposure to light MANUFACTURE OF RED LEAD. 03 and air, probably owing to an intermolecular change ; much appears to depend on the care with which the vermilion has been made. RED LEAD. This valuable red pigment has been known and used for a very long time. Pliny, in his writings, describes this body, which in his time was known as minium, under which name it is also frequently referred to in later writings. Pliny also mentions its use for adulterating vermilion. Davy, who had an opportunity of examining the contents of some pots of colour found in the remains of Roman and Greek cities, frequently found red lead among them. How it was made by the ancients is not definitely recorded. MANUFACTURE OF RED LEAD.— There is only one process for making red lead, which consists of two stages — the Fig. 13. first stage has for its object the conversion of metallic lead into monoxide of lead; in the second stage this oxide is converted into red lead. 1st Stage, Drossing. — This is conducted in what is called the " drossing oven," a kind of reverberatory furnace of which Figs. 13 94 RED PIGMENTS. and 14 show respectively the longitudinal and vertical sections. From these drawings it will be seen that it is a low oven, open only in front, over which is constructed a hood and chimney to carry off the products of combustion, &c, from the furnace. The bed usually measures about 11 feet by 12 feet 4 inches, and is divided (as shown) into three divisions — the central one measures 8 feet 4 inches wide, and constitutes the bed or hearth of the furnace, while the two side divisions measure about 2 feet each, and form the fireplaces of the oven, as a Fig. 14. rule, they are not fitted with firebars ; the partitions between the fireplaces and hearth are formed of firebrick ; the bed of the furnace is made to slope from the back to the front, usually the back is about 7 inches higher than the front, while it also slopes slightly from the side to the centre. In the front of the furnace are three doors — the two side ones are for feeding the fires, while the centre one serves for introducing and extracting the material, and for working the charge while in the furnace ; it is placed a little higher than the two side doors so that a draught is generated through the latter and out of the centre door ; in the top of this door an opening is left so that the products of combustion, &c, can pass out and up the chimney. This furnace is open to improvement, and an improved form is shown in Fig. 15, from which it will be seen that this form of furnace has firebars fitted to the fireplaces. The operation of drossing is carried out in the following manner : — 22 cwts. of lead, which is the quantity usually dealt with in one charge, are placed in the furnace, which is now MANUFACTURE OF RED LEAD. 95 raised to a dull red heat, just enough to melt the lead, the molten lead being prevented from flowing out of the furnace by^ the construction of a dam, formed of pieces of dross or " leanings" from previous workings, across the front of the hearth ; the melted lead rapidly becomes coated with a layer of Fig. 15. oxide, the formation of which is hastened by rabbling the lead and pushing the oxide as it is formed to the back of the furnace, the object being to always have a fresh surface of lead exposed to the oxidising action of the air which passes through the furnace. The workmen by a peculiar splashing action while rabbling expedite this oxidation very much ; at intervals pigs of lead are thrown into the furnace. This drossing takes about 10 to 12 hours, at the end of which time the dam across the front of the furnace is broken down, and the unmelted lead allowed to run out, while the " dross " or " casing," as it is called, is taken out to be worked for the next stage. The furnace is now ready for another charge. The " dross " or " casing " has a rather bright yellow colour, and is coarse in texture ; it consists essentially of the monoxide of lead, Pb O, but still contains some unoxidised lead. It is 96 RED PIGMENTS. now ground and levigated with water ; the oxide grinds to the form of a fine powder, while the lead is simply flattened out, and by sieving can easily be removed ; it is sent back again into the furnace, while the ground oxide is washed by a stream of water into settling tanks, where it settles out in the form of a paste, which is ready for use in the next stage. One point of importance in the drossing stage is to see that the temperature is carefully regulated, so that, while it is above the melting point of the lead and therefore in a molten state, yet it is below the melting point of the casing ; as the margin is not great, considerable care has to be taken to avoid over- stepping the limit. If the casing is allowed to melt it passes into litharge and this cannot be converted into red lead. The dross or casing is also known as massicot. 2nd Stage, Colouring. — The next operation consists in heating the dross obtained in the first stage, either in the same oven or in another, which only differs from the drossing oven in a few minor details. The colouring oven is heated to a low red heat, care being taken to ensure a large supply of air. The operation takes about 48 hours, and the mass is frequently rabbled during that period; after it has been in about 12 hours a sample is taken out and its colour examined ; this sampling is repeated at the end of each twelfth hour and near the end of the operation more frequently. When the red lead has attained the correct colour, the fires are drawn and the furnace allowed to cool down; when cold, the red lead is drawn from the oven, ground as finely as possible, and sent into the market. The change which takes place in the transformation of the metallic lead into red lead is shown in the following equations — 1st Stage. Pb + 0 = PbO Lead plus oxygen forms lead monoxide. 2nd Stage. 3 Pb 0 + 0 = Pb 3 0 4 Lead monoxide plus oxygen forms lead peroxide. Theoretically, 100 lbs. of lead should yield 110*36 lbs. of red lead ; practically, about 108 lbs. of red lead are obtained, which is a very near approach to the theoretical amount. The best red lead for painters' use is made from pure lead, as the presence of impurities in the metal has a material and injurious influence on the colour of the product ; iron, in particular, causes the colour to be dark. For glass-makers' red lead a pure product is absolutely necessary, as an impure lead causes the glass to be coloured, not white as it should be. Burton's Process. — Although the only process at present PROPERTIES AND COMPOSITION OF RED LEAD. 97 worked for the preparation of red lead is the one described above, yet in 1862 Burton patented a process for making red lead from sulphate of lead, in which 1 equivalent or 1*894 parts of lead sulphate are mixed with 1 equivalent or 0-665 part of sodium carbonate and 1 equivalent or 0*143 part of sodium nitrate. The mixture is heated to a dull red heat with an excess of nitre ; the fused mass is lixiviated with water, whereby the red lead formed is separated from the alkaline salts, and this is washed and dried. PROPERTIES AND COMPOSITION OF RED LEAD. — Red lead is a heavy, bright red powder of an orange hue, its specific gravity being 8*53. Heat turns it to a dark brownish- red, but the colour is restored on cooling. Acids act on red lead. Nitric acid and glacial acetic acid first dissolve out the monoxide, leaving the dark puce oxide ; on further boiling, this gradually dissolves and colourless solutions of the nitrate or acetate are formed. Hydrochloric acid when heated with red lead decomposes it with the evolution of chlorine and the forma- tion of the chloride, which settles as the solution cools in the form of transparent needles, a very characteristic reaction of lead. Sulphuric acid boiled with red lead forms the sulphate, with the evolution of oxygen. Red lead is a combination of the two oxides of lead, the monoxide, Pb O, and the puce or dioxide, Pb 0 2 ; it is generally considered that they are present in the proportion of two equiva- lents of the first to one of the second, red lead, therefore, having the formula Pb 3 0 4 , the percentage composition being — Lead monoxide, Pb O, . . . 64*5 Lead dioxide, Pb 0 2 . . . 35*5 lOO'O There is reason for believing that Pb 3 0 4 does not accurately represent the true composition of red lead ; for, although the proportions of the two oxides is about that given in the above analysis, it is probable that the whole of the monoxide present is not combined with the dioxide as red lead, but that some of it is in the free condition ; this free oxide is not distinguishable from the combined oxides by treatment with acids, but, by treating with a 10 to 12 per cent, solution of lead nitrate, it is quite possible to extract 16 to 31 per cent, of free oxide, while the purified red lead thus obtained contains 25*4 to 25*7 per cent, of dioxide. # * Lowe, Dingl. Polytech. Journ., vol. 271, pp. 472-477. 7 98 RED PIGMENTS. The formula of red lead would then be Pb 4 0 5 , which is that assigned to it by Phillips and other authorities. Percy* gives the following analysis of red lead : — Lead monoxide, Pb 0, . . . . 80*54 per cent. Lead dioxide, Pb 0 2 , .... 18*89 Ferric oxide, Fe 2 0 3 , . . . . *19 ,, Copper and silver, .... trace 99*62 which corresponds to the formula 4 Pb 0, Pb O, or Pb 5 0 6 . Both Pb 3 0 4 and Pb 4 0 5 are known ; the former is much easier to prepare than the latter, and the latter can only be made by repeated oxidation of the monoxide. A little free monoxide is desirable in red lead, as then the colour is not so readily liable to spoil by over-oxidation. As a pigment red lead is very useful, it mixes very well with linseed oil, and takes from 8 to 9 per cent, of it to grind into a stiff paste. It exerts a powerful drying action on the oil ; hence, paint containing red lead dries very quickly ; on this account, also, red lead mixed with linseed oil is largely used as a lute and packing for steam pipes and joints of all kinds. It possesses good covering and colouring power, and is capable of resisting all ordinary atmospheric influences, although it is liable to be discoloured by sulphuretted hydrogen as is the case with all lead pigments. It may be mixed with nearly all pigments, the only exceptions being those containing sulphur, such as ultramarine, cadmium yellow, &c. ASSAY AND ANALYSIS OF RED LEAD. — Red lead should be assayed for colour, fineness, and body in the usual way. It is rarely adulterated ; but if so, it is usually by the oxide of iron reds. The quantity of red lead in such an adulterated sample can be ascertained by taking 2 grammes and boiling with nitric acid until it is thoroughly decomposed ; the insoluble matter can be filtered off and its amount ascertained by weighing it ; to the solution, which is colourless if the red lead be pure, but yellow if there is any iron present, a little dilute sulphuric acid is added, and a precipitate of sulphate of lead obtained ; this is filtered off, washed, dried, and weighed in the usual manner. The weight multiplied by 0*955 gives the amount of red lead in the sample. The solution from the lead sulphate can be tested for iron, &c., by the usual tests. * Percy, Metallurgy of Lead. OXIDE REDS. 99 ORANGE LEAD. This pigment is identical in composition with red lead, but is rather paler in colour and lighter in weight. To make it, white lead is placed in a furnace similar to a red lead furnace, and heated to a low red heat for from 24 to 48 hours, or until the mass has acquired the desired red tint. During this operation the white lead loses its carbonic acid and water, while it takes up oxygen from the air which passes through the furnace. The change is shown in the following equation : — 2 Pb C 0 3 Pb H 2 0 2 + 0 = Pb 3 0 4 + H 2 0 + 2 C 0 2 . In washing white lead a scum collects on the top of the washing waters; this is collected and made into orange lead, and gives a brighter and more bulky product than dry white lead. Orange lead has a slightly paler colour than red lead, is more voluminous and of lower specific gravity, which is about 6-95. In its composition and properties it is identical with red lead, and it is used for very similar purposes. RED OXIDE, INDIAN RED, AND IRON REDS. Ferric oxide, Fe 2 0 3 , the red oxide of iron, is the basis of a very large number of red pigments which are sold under the names of rouge, light red, Indian red, red oxide, Venetian red, purple oxide, scarlet red, &c., which are all red pigments of varying shades of colour. In the hydrated form, ferric oxide also forms the colouring principle of the ochres, siennas, and umbers. The red oxides are valued very highly as pigments, on account of their generally fine colour and their permanence. Ferric oxide occurs naturally in a variety of forms in the minerals haematite, specular iron ore, limonite, &c, in which it is nearly chemically pure. As a rule, they are too dark in colour, and too hard to be used as pigments ; but, occasionally, deposits of oxide of iron are found of sufficient brilliance to be used as a pigment — e.g., the Warton oxide named below. Indian red was originally found native, but is now mostly of artificial manufacture. Such substances as red ochre, yellow ochre, red raddle, umber, CO 00 CO CO • Oi TtH * »G rf • »-g £ tjQ l> o 8 is ^ " (-i 500 o ^ ¥ . s a? §^ * s • C5 jh CS m HH GO I— I P> c3 « a " 0) 2 o '■P H — * 0 U p U CD 5 sin ^ H H ^ HH 110 RED PIGMENTS. add carbonate of soda until the solution is neutral, then ammonium acetate ; boil, filter, wash and dry, and weigh the precipitate ; this consists of oxide of iron, alumina, and, in some rare cases, phos- phoric acid, but this may be neglected, as a rule, in iron reds. The iron may be estimated in another portion of the original solution by a volumetric test and the amount of alumina calculated from the two results. The filtrate from the precipitate is mixed with a small quantity of ammonium sulphide to pre- cipitate any manganese, this precipitate being collected, dried, and weighed. To the filtrate is added ammonium oxalate to precipitate the calcium, which is filtered off, dried, and weighed. To the filtrate from this, sodium, phosphate is added to precipitate the magnesium, if present. For the sulphate which is present 100 cc. of the original solution are taken and some barium chloride is added ; the pre- cipitate of barium sulphate is filtered off, dried, and weighed. For fuller details as to the method of carrying out this scheme, works on quantitative analysis, such as that of Professor Sexton, published by Griffin & Co., should be consulted. The analyses on p. 109, which, with the exception of Nos. 3 and 7. have been made by the author, will show the composition of the iron reds in common use as pigments. A pigment must possess two properties, good colouring power and body. As the iron reds are so variable in their composition it follows that in these two particulars they will vary also ; for these two properties they should be assayed, the methods of doing which will be found detailed in the chapter on assaying pigments. ANTIMONY VERMILION. This pigment, also known as antimony orange, is very largely used for colouring india-rubber; for other purposes it is not used as extensively as it might be. It is the sulphide of the metal antimony and has the formula Sb 2 S 3 . This body occurs naturally as the mineral stibnite or antimony glance of a lustrous black colour ; when ground up it is known as black antimony, and is used for various purposes in the arts, one being as a source for the manufacture of antimony vermilion. PREPARATION OF ANTIMONY VERMILION.— Murdoch, in 1847, patented a process for the preparation of anti- mony vermilion and a similar process was the subject of a subse- quent patent taken out by Clark. The pigment can be made in SCARLET ANTIMONY. Ill two modifications, orange and red ; the former by precipitation with sulphuretted hydrogen, the latter by other agents. (a) Orange Antimony. — Murdoch dissolves the black anti- mony in hydrochloric acid; during the operation some sulphu- retted hydrogen is evolved and may be used for precipitating another solution previously made ; so as to form a solution of antimony chloride, which is concentrated till it has a strength of 19° Tw. ; through it is then passed a current of sulphuretted hydrogen gas which precipitates the sulphide of antimony as an orange powder, which, after being well washed and dried, is ready for use. Fig. 17 shows a convenient form of apparatus for precipitating antimony vermilion. A is a vessel made of wood, lined with lead, and fitted with a lid which, while being removable to admit of the sulphide of iron used for the preparation of the gas, can A C D D D Fig. 17. — Apparatus for making antimony vermilion. yet, by means of india-rubber joints, be made gastight ; B is a funnel for the admission of acid ; C is a two-necked bottle filled with water to wash the gas as it comes over ; D,D,D, are three- necked bottles (whose construction is shown in the drawing), con- taining the solution of antimony through which the gas is passed. The most convenient method of preparing the gas is by the action of dilute sulphuric acid on sulphide of iron. By varying the strength of the antimony solution the shade of the resulting pigment can be varied to some extent ; thus, a solu- tion of 19° Tw. gives an orange-red; one of 40° Tw., a reddish- orange; while one of 52° Tw. gives an orange colour. When strong solutions are used the precipitate is, however, liable to contain free sulphur, which is sometimes objectionable as it may cause decolourisation of the pigment. (b) Scarlet Antimony. — 1. Maihieu Plessy Process. — This con- sists in precipitating a solution of chloride of antimony with a solution of sodium thiosulphate (hyposulphite of soda) under 112 RED PIGMENTS. certain conditions. A solution of chloride of antimony of 40* Tw. is prepared. The ordinary commercial chloride is a liquid of about 1*26 (52° Tw.) specific gravity; if this is diluted with water in the proportion of 5 vols, of the chloride to 2 vols, of water, a solution of about 40° Tw. will be obtained. A solution of sodium thiosulphate of 40° Tw. is also prepared ; this will take about 14£ ozs. weight of the salt to 25 ozs. measure of water. 7 1 gallons of the thiosulphate solution are taken, and into them are poured 3 gallons of the antimony solution ; a black precipitate forms at first, but this disappears rapidly. The liquid mixture is now gently heated ; when the temperature reaches about 78° to 90° F. a yellow precipitate begins to form ; as the temperature increases the colour changes, passing through various shades of orange till at about 130° to 140° F. it acquires a scarlet-red colour ; the operation is now stopped and the mass allowed to cool down; when cold, the clear supernatant liquor is poured off, water containing a trace of hydrochloric acid is poured on to the precipitate, which is stirred up ; after which the mass is allowed to stand to settle, the top liquor poured off, and the colour washed with water two or three times ; it is now dried at a low temperature (about 140° F.), and when dry is ready for use. During the process of precipitation a very considerable evolu- tion of sulphur dioxide gas takes place ; therefore, the operation should be carried on in a place and under conditions where the gas cannot cause inconvenience. 2. Wag?ier's Process. — 4 lbs. of tartar emetic and 3 lbs. of tartaric acid are dissolved in 18 lbs. of water, and the solution heated to 140° F., a solution of sodium thiosulphate of 40° Tw. added thereto, and the mixture heated to 180° F. The red is gradually precipitated, and when fully formed is washed with water and dried. In drying antimony vermilion it is important that the temperature be kept low, and not be allowed to rise above 160° F. The colour of the scarlet antimony powder is rather dull, but it becomes bright when mixed with oil or water. PROPERTIES OF ANTIMONY VERMILION.— Anti- mony orange is a light, bulky, orange-coloured powder, while antimony vermilion is a scarlet powder rather heavier than the orange variety. Both pigments have practically the same properties ; they are not attacked by dilute acids, but strong nitric acid gradually decomposes them, with the formation of white antimonic oxide and sulphuric acid. Strong hydro- chloric acid has little action in the cold, but when boiling COMPOSITION OF ANTIMONY VERMILION. 113 gradually dissolves them with the formation of chloride of anti- mony and evolution of sulphuretted hydrogen. Caustic soda and potash when boiled dissolves the colour, which is re-precipitated as an orange precipitate on the addition of an acid ; ammonia has little action ; lime has a similar action to soda. Both the orange and red forms are rather dull in the pulveru- lent state, but when mixed with oil or water they become bright; being opaque they have a good body or covering power, and mix well with oil, but cannot be used with alkaline vehicles, like lime or silicate of soda, which have a decolourising action. They are unalterable by air or light, or by deleterious atmospheric agents. They can be mixed with all those pigments which are unaffected by sulphur. COMPOSITION OP ANTIMONY VERMILION. — Pure sulphide of antimony, Sb 2 S 3 , has the following percentage composition : — Antimony, 71*42 Sulphur, 28*28 100-00 The antimony pigments are liable to contain free sulphur, especially those which are made with sulphuretted hydrogen. The following is an analysis of an orange antimony : — Moisture, 2*200 per cent. Sulphur, 40-557 Antimony, 56*990 99*747 from which it will be seen that the sulphur is greatly in excess of that required by theory, so that some of it must be in the free condition. The red antimony vermilions approach more closely to the theoretical composition, as will be evident from the following- analyses : — a. b. Water, 11 4*227 Sulphur, 26*7 27 103 Antimony, 72*2 68*670 100 0 100*000 the latter analysis by the author is of an English made sample, the other analyses are taken from a foreign work on pigment- making. 8 114 RED PIGMENTS. The presence of free sulphur in these pigments is likely to be a cause of want of permanence when used as a pigment. ANALYSIS AND ASSAY. — For practical purposes it is not necessary to make a complete analysis of antimony vermilions. They are liable to adulteration by red lead, oxide of iron, or chrome orange. Red lead would be shown by the colour dark- ening when treated with hydrochloric acid, and, after solution, by crystals of chloride of lead separating out on cooling, and the application of the usual tests for lead. Oxide of iron can be distinguished by the solution in hydrochloric acid having a yellow colour and giving the characteristic tests for iron ; chrome orange can be detected by the colour of the acid solution being green and giving the tests for lead and chromium. Antimony vermilions should be assayed for colour, both in the form of powder and when mixed with oil, and for covering power by the usual methods. BRILLIANT SCARLET is the name given to the iodide of mercury, Hg I 2 , prepared by carefully precipitating a solution of mercuric chloride with a solution of potassium iodide ; it has a brilliant scarlet colour, but is very fugitive. It is rarely used as a pigment. DERBY RED is the basic chromate of lead ; its preparation and properties will be found on p. 116, et seq. The CHROMATE OP MERCURY has been used as a red pigment and is prepared by precipitating a solution of mercuric chloride with potassium chromate ; its cost and want of per- manence has caused it to become obsolete as a pigment. The CHROMATE OF SILVER has been proposed as a pig- ment ; it has a dark red colour and is prepared by precipitating a solution of silver nitrate with a solution of potassium chromate. It is costly and fugitive. The CHROMATE OP COPPER, a dark red coloured body prepared by precipitating solutions of copper with potassium chromate, has also been suggested, and probably used on a small scale, as a pigment ; but it is fugitive, and, therefore, cannot be recommended for this purpose. MAGNESIA PINK, prepared by calcining a mixture of magnesia and cobalt nitrate, has been little used ; it has but a pale colour and little body, although probably permanent. 115 CHAPTER IV. YELLOW AND ORANGE PIGMENTS. There is a fairly large list of yellow and orange pigments derived from the vegetable, animal, and mineral kingdoms, the most important being the chromes and ochres ; the others are only used in small quantities. THE CHROMES. The chromes, as they are generally called, are a very important group of pigments varying in colour from a pale yellow through deep yellow, orange to bright red, and sold under a variety of names — primrose-chrome, lemon-chrome, chrome-yellow, orange- chrome, scarlet -chrome, chrome -red, Derby -red, American - vermilion, &c. The base of all these chrome pigments is the chromate of lead, Pb Cr 0 4 , and its basic derivative Pb O, PbCr 0 4 . Chromate of lead is capable of existing in the form of acid, normal and basic modifications ; to the colour maker, only the last two are of any interest. The normal chromate of lead is a deep-yellow coloured body having the composition Lead, . . .' . . 63*99 per cent. Chromium, 16 "23 „ Oxygen, 19*78 100-00 or Lead oxide, Pb 0, . . . 68 93 per cent. Chromium oxide, Cr Os, . . . 31*07 ,, 100 00 It is obtained as a bright yellow precipitate by adding a solution of bichromate of potash to one of acetate of lead ; the reaction is shown in the following equation : — Pb 2 C 2 H 3 0 2 + K 2 Cr 2 0 7 + H 2 O = 2 Pb Cr 0 4 + 2 K C 2 H 3 0 2 + 2HC 2 H 3 0 2 . 116 YELLOW AND ORANGE PIGMENTS. Nitrate of lead may be used instead of the acetate, when the reaction becomes 2Pb2N0 3 + K 2 Cr 2 0 7 + H 2 0 = 2PbCr0 4 + 2KN0 3 + 2HN0 3 . From the above equations the equivalent quantities of the two compounds can be calculated, and are as follows : — 650 parts of lead acetate or 662 parts of lead nitrate are equal to 295 parts of potassium bichromate, or 100 parts of lead nitrate require 44*5 parts of potassium bichromate, while 100 parts of lead acetate require 38*9 parts of potassium bichromate to precipitate them. Lead chromate is insoluble in acetic acid and water, but soluble in moderately concentrated nitric or hydrochloric acids. When treated with a large excess of caustic potash or soda it dissolves, but when heated with a small quantity of the alkalies it is converted into the basic chromate, Pb 2 Cr 0 5 . Heated alone it first turns reddish-brown, and, finally, becomes greenish-grey with evolution of oxygen, while a mixture of the oxides of lead and chromium are left behind. It has a specific gravity of 5*653. The formation of the basic chromate from the normal chromate is shown in the equation 2Pb Cr0 4 + 2NaOH = Na 2 Cr0 4 + 2Pb. PbOCr0 4 . It is a scarlet-red powder of somewhat crystalline structure, with a specific gravity of 6*266 ; by friction it loses its crystalline- form and changes colour, becoming orange ; in other properties it resembles the normal chromate. It has the following com- position : — Lead, 75*75 per cent. Chromium, 9*61 ,, Oxygen, 14*64 100*00 or Lead oxide, Pb O, . . . . 81 *61 per cent. Chromium trioxide, Cr 0 3 , . . 18*39 ,, 100 00 Both the normal and basic chromates when boiled with strong sulphuric acid are decomposed, lead sulphate and chromium sul- phate being formed and oxygen evolved. Strong hydrochloric MANUFACTURE OF LEAD CHROMES. 117 acid on boiling dissolves them, forming a green solution of lead and chromium chlorides, from which, on cooling, the lead chloride separates out, and the chlorine is evolved. When boiled with solutions of the alkaline carbonates, the chromates are decom- posed, white carbonate of lead being formed and a solution of the alkaline chromate obtained. When boiled with solutions of caustic soda or potash the lead chromates dissolve ; on adding acetic acid to this solution a yellow precipitate of the normal chromate is obtained. MANUFACTURE OF LEAD CHROMES.— In commerce the pigments having chromate of lead as the base are met with in a very great variety of shades from a very pale primrose-yellow to a deep red ; as a rule, the deep shades are almost chemically pure, but the pale shades are obtained by mixing the pure chromate with the requisite quantity of a white base, such as sulphate of lead, barytes, gypsum, SIENNAS.— So far as their properties as pigments are concerned, the ochres and siennas rank among the most permanent pigments at the disposal of the painter. They are unaffected by admixture with any other pigments, do not act injuriously upon other pigments, and are scarcely affected by exposure to the atmosphere and its de- structive influences. They work well with all kinds of vehicles, and can, therefore, be used in any kind of painting — oil, water, distemper, fresco, &c. Ochres and siennas vary very much in tint, brightness of colour, and strength. Oxford ochre is the brightest of the ochres and is of a fairly bright brownish-yellow colour. Siennas are of a brownish-yellow colour varying much in depth of tint or shade. Welsh ochres are rather duller than Oxford ochres ; French ochres are moderately bright ; Derbyshire ochres are reddish in tone and are darker than other varieties of ochre. They vary very much in texture ; Oxford ochre and the siennas are of a soft texture ; some are gritty in feel, while others have a clayey feel. In body or opacity these pigments vary much. The Oxford ochre and the siennas are rather transparent, and are commonly used as glazing colours ; the other ochres are more opaque and have good body ; hence, they are largely used as body colours, especially in house painting. The colour of ochres is due to the presence of hydrated per- oxide of iron, while siennas also contain small quantities of manganese ; the shade or tint depends mainly upon the propor- tion of iron and manganese present, and also, but to a less extent, upon the degree of hydration of the oxide of iron ; in proportion as the iron oxide is less and the hydration greater, the yellower and brighter the shade of colour ; when the proportion of non- hydrated oxide of iron is large the shade becomes redder. When ochres are treated with hydrochloric acid, the iron they contain is nearly all dissolved out, and yields a yellow solution which will give the characteristic tests for iron, while a more or less insoluble residue is left behind. Heat turns ochres a red colour, the shade of which depends upon the temperature and length of time the heating is carried on ; these red colours are sold as Yenetian red, light red, Indian red, &c. ; their preparation and properties have already been described (see p. 105). Siennas are converted by heat into a COMPOSITION OF OCHRES AND SIENNAS. 137 reddish-orange pigment, known as burnt sienna (see p. 141 ). This change of colour is due to the passage of the iron oxide from the hydrated to the anhydrous condition, but the reason why ochres should give reds and the siennas orange is not known. ASSAY AND ANALYSIS OF OCHKES AND SIEN- NAS. — (a) Crude Ochres and Siennas. — These should be assayed for, first, the actual quantity of colour present and, second, for the tint or shade of the colour it gives. This last can be done in the usual way; the first can be ascertained as follows : — A tall glass of a conical shape is provided ; a glass funnel with a long stem passes down to the bottom of the glass into which is put about 25 to 30 grammes of the crude ochre ; into the glass is now passed a gentle current of water sufficiently strong to carry out of the glass all the finer particles of colour while leaving the heavier and more gritty particles behind, which are collected by filtering and, after drying, are weighed in the usual way ; from the weight is calculated the proportion of colour and grit. Thus, the sample of crude Irish ochre (an analysis of which is given below) assayed in this way, was found to contain — (b) Prepared Ochres. — These only need assaying for colour and covering power by the usual methods. It is rarely that an analysis of ochres and siennas is required ; but analyses of several varieties are given below, which show their constituents and what to look for in analysing them. Ochres are rarely, if ever, adulterated. Ochres which are naturally poor in colour sometimes have a little chrome-yellow added to them to bring up the tint ; such an addition may be recognised by treating the ochre with hydrochloric acid and alcohol, when a green-coloured solution containing chromium will be obtained, and the chromium in which may be detected by the usual tests. COMPOSITION OF OCHRES AND SIENNAS.— Most of the following analyses have been made by the author; others are quoted from various published analyses. The notes appended to some of them will be found useful and of interest as showing some indications of the origin of these pigments. 1. Oxford Ochre. — The ochres from Oxfordshire have long had a reputation for their quality, exceeding, as they do, all other ochres in the brightness of their colour and depth of covering power. Most of the ochre is found in pits at Shotover, near Grit, , Colour, 30*24 per cent. 69 76 138 YELLOW AND ORANGE PIGMENTS. Oxford, of which the following section is given in Dictionary " : — Ur 1. Summit of hill, highly ferruginous grit, 2. Grey sand, 3. Ferruginous concretions, 4. Yellow sand, . 5. Cream-coloured loam, 6. Ochre, .... 6 feet. 3 „ 1 „ 6 „ 6 inches. Oxford ochre contains Water, hygroscopic, Water, combined, Calcium oxide, Ca O, Sulphur trioxide, S Oo, Alumina, Al 2 O3, . Ferric oxide, Fe 2 0 3 , Silica, Si 0 2 , . 6*887 per cent. 8*150 0- 998 1- 321 6-475 12-812 63 478 100-121 The layer of ferruginous concretions is, probably, the source of the colouring matter of this ochre, while the clay which under- lies the layer of ochre is the source of the base of the pig- ment. 2. Welsh Crude Ochre. — The exact locality from whence this sample was derived is not known ; it has a fairly good colour and covering power. It contains Water, hygroscopic, .... 2*000 per cent. Water, combined, .... 12*500 ,, Sulphur trioxide, S 0 3 , . . . 1*315 ,, Silica, Si 0 2 , 29*725 Alumina, Al 2 0 3 , .... 33'315 Ferric oxide, Fe 2 0 3 , .... 20*705 ,, Copper sulphide, Cu S, . . . '515 ,, 100-075 Some of the iron oxide, 0*765 per cent., exists in a soluble form, probably as sulphate, for there is 0*555 per cent, of soluble sulphur trioxide. This, and the fact that there is copper sulphide present, indicate that this ochre has been formed by the de- composition of a cupreous pyrites, which supposition is further strengthened by the fact that small pieces of pyrites may be picked out of the crude ochre. This ochre requires well levigating to get rid of the pyrites as this body would introduce COMPOSITION OF OCHRES AND SIENNAS. 139 an element of change which may exercise an injurious influence upon the permanent properties of the ochre. 3. Irish Crude Ochre. — This sample contains Water, hygroscopic, . Water, combined, Insoluble matter, Sulphur trioxide, S O3, Alumina, Al 2 O3, Ferric oxide, Fe 2 0 3 , . Calcium oxide, Ca O, . Copper oxide, Cu 0, . 9*050 per cent. 12 000 32-502 2-685 16-770 26 381 •258 •630 100-260 The insoluble matter consists of silica and some gritty matter. Of these constituents a portion (0*33 per cent, of sulphur trioxide, 0*191 of ferric oxide, 0118 of calcium oxide, and 0*228 of copper oxide) was soluble in water in the form of sulphates, the iron being in the ferrous form ; this points to the fact that this ochre was formed by the oxidation of the cupreous pyrites existing, disseminated through a siliceous mineral matrix which was broken up by the oxidation, and part of which went to form the base of the ochre, while much of it exists in the form of gritty angular pieces which must be separated before the ochre is fit to use as a pigment. 4. Derbyshire Crude Ochre. — This is of a reddish colour and is found incrusting masses of pyritous minerals and barytes, from which the ochre is separated by levigation. This ochre, like the others just noticed, has been formed by the oxidation of pyrites, small fragments of which, in an unchanged condition, are disseminated through the crude ochre. This sample contained more iron than any other examined by the author, as the following analysis shows : — Water, hygroscopic, . Water, combined, Alumina, Al 2 0 3 , Ferric oxide, Fe 2 0 3 , . Calcium oxide, Ca O, . Sulphur trioxide, S 0 3 , Pyrites, Fe S, . Silica, Si 0 2 , 1 Barytes, Ba S 0 4 , / 5*400 per cent. 6*000 1040 76-081 0- 561 1- 744 „ 4-783 4*394 100 000 5. Derbyshire Prepared Ochre. — This sample is of a reddish 140 YELLOW AND ORANGE PIGMENTS. shade, not so dark as the last. The covering power of this ochre is good. Water, combined, Barium sulphate, Ba S 0 4 , . Silica, Si0 2 , Calcium sulphate, Ca S O4, . Calcium carbonate, Ca C 0 3 , Alumina, Al 2 O3, Ferric oxide, Fe 2 O3, . Magnesia, MgO, 6*100 per cent. 20 946 4*530 2-516 21-755 10 655 33-498 trace 100-000 6. Cornwall Prepared Ochre.— This ochre is of a pale brownish-yellow shade and has not much covering power. It contains Water, hygroscopic, Water, combined, Silica, Si 0 2 , Alumina, Al 2 O3, Ferric oxide, Fe 2 0 3 , Calcium oxide, Ca O, 1 *40 per cent. 10-00 59-67 9*72 18 54 0-23 99-56 7. French Prepared Ochre. — The exact locality from which this was obtained is not known to the author, but it is usually of a bright brownish-yellow colour with a good covering power. Water, hygroscopic, Water, combined, Silica, Si 0 2 , Alumina, Al 2 0 3 , Ferric oxide, Fe 2 0 3 , Calcium oxide, Ca O, 1 "80 per cent. 9-20 54 00 13-75 20-73 0-19 99-67 8. South Australian Ochres. — The following analyses of South Australian ochres are quoted in the Journ. Socy. Ghem. Ind., 1889, p. 313 :— Water, hygroscopic, Water, combined, Silica, Si0 2 , Ferric oxide, Fe 2 0 3 , Alumina, Al 2 0 3 , . Per cent. Per cent. Per cent. 1-82 1-92 0-21 6-48 7-60 4-00 41*20 56-60 65 20 38-40 11-68 5-76 12-56 1912 21-74 100-46 96-92 96-91 BURNT SIENNA. 141 9. Siennas. — The following are some analyses of raw siennas, which are of Italian origin — the first and second from the neighbourhood of Rome ; the locality from whence the third came is unknown. The Roman siennas are found in hollows on hill sides, which hollows are now filled up with deposits of sienna ; but, at one time, were the site of small ponds into which flowed streams highly charged with iron and manganese, from deposits of those materials situated above the ponds. The rocks of the district consist chiefly of trachyte and granite, charged with ferriferous and manganiferous minerals, which are the source of the colouring matter of the sienna. Dark. Pale. Per cent. Per cent. Per cent. Water, hygroscopic, 17*550 8*250 12-400 Water, combined and organic matter, .... 9-000 11-000 9-400 Silica, Si 0 2 , .... 22 656 17-406 5-016 Calcium, carbonate, CaC O3, . 0-960 1-075 4-460 Alumina, Al 2 O3, 2-840 5*177 7-265 Manganese, Mn 0 2 , 1-190 0-627 1-465 Ferric oxide, Fe 2 O3, 45-820 57*032 59-965 Magnesia, MgO, trace. 100 016 100-567 99-701 It may be assumed that the shade of the siennas varies with the amount of manganese it contains, as is shown by the analyses given above. The commercial value of an ochre depends upon its colour and body ; those which excel in these points naturally commanding the best prices. The following is approximately the order of the various ochres as regards price : — Oxford, 100 ; French, 33 ; Derbyshire and Welsh, 25 ; Irish and Devon, 20. BURNT SIENNA. The siennas are sold in two forms, raw and burnt ; the first has already been dealt with and the latter will now be described. Burnt sienna is prepared by calcining the raw sienna at a moderate red heat until it has acquired the desired shade. The tint of the burnt sienna depends not only upon the temperature used and the length of time it is exposed to heat, but, also, upon the shade of the raw sienna used. Burnt sienna is a pigment of a reddish-orange shade, very similar to that of the coal-tar colour known as Bismarck-brown. It is very transparent, and is, there- fore, mostly used as a glazing or tinting colour by painters and 142 YELLOW AND ORANGE PIGMENTS. artists. It is sold in the form of small pieces, and of a paste ground up with water or oil. The former variety is very difficult to grind. The composition of burnt sienna naturally resembles that of raw siennas, only that the heat has driven off most, if not all, the water the latter contains. The following analysis will serve to show the composition of burnt sienna : — Water, hygroscopic, . Water, combined, Silica, Si 0 2 , Calcium carbonate, Ca C O3, Manganese, Mn 0 2 , . Alumina, Al 2 0 3 , Ferric oxide, Fe 2 0 3 , . 9 '450 per cent. 3 275 36912 1-233 traces. 3-480 45 650 100-000 The whole of the water in this sample had not been driven off by the burning. Why raw siennas should give an orange-red pigment on calcining and ochres a red is somewhat uncertain ; probably the fact that siennas contain organic matter and that the iron is in both the ferrous and ferric conditions may have some influence. MARS COLOURS. Under the generic name of Mars colours the late George Field, a noted colour manufacturer, introduced a series of yellows, oranges, reds, and violets, owing their colour to ferric oxide. Field did not publish any account of the method by which he produced these colours ; but descriptions of similar products have been given by various French and German writers on pigments. These colours present no advantage over ochres and iron-reds as regards permanency or brightness of tone, but have disadvantages as regards cost. Mars yellow is made by taking equal weights of ferrous sulphate and alum, and adding a solution of carbonate of soda, thereby precipitating the iron and alumina ; the precipitate is collected, washed well with water, and dried slowly. Mars orange is made by slightly calcining the yellow. Mars red is made by calcining the yellow at a red heat. Mars violet is made by calcining the yellow at a white heat. By using milk of lime instead of the soda salt the colours could be made cheaper, a plan wdiich is in use for making some forms of iron-reds (see p. 106). Mars brown was made in a similar manner from a mixture of ferrous sulphate, alum, and manganese chloride. NAPLES YELLOW. 143 Mars colours can be distinguished from the ochres and ochre- reds by being soluble in strong hydrochloric acid, and by containing a large proportion of alumina, but no silica. TURNER'S YELLOW. Turner's yellow (so named after the inventor, James Turner), or patent yellow (from its having been patented in 1781) was at one time largely used ; but since the introduction of the chrome- yellows it has been gradually, and, perhaps, entirely abandoned. It has been known as Montpelier yellow, Cassel yellow, Kassler yellow, Verona yellow, mineral yellow, and, probably, by other names. It is essentially an oxy chloride or basic chloride of lead. It is made by mixing two parts of litharge and one part of salt with water to a thin paste and allowing the mixture to stand for 24 hours, stirring at intervals ; at the end of this time it will, as a rule, have a white colour ; if it has not, more water must be added and the mixture again allowed to stand for another 24 hours or until it becomes white ; it is now washed (to free it from alkaline salts), dried, then put into a crucible, and calcined at a gentle heat sufficient to melt the mass. The shade of colour- depends upon the temperature and duration of the heating; usually small samples are taken out of the crucible from time to time, and when the right shade has been obtained the contents of the crucible are allowed to cool, after which they are ready for use. Sal ammoniac may be used in the place of salt. Another method of preparation consists in precipitating a solution of lead with hydrochloric acid, collecting the precipitate, and washing and calcining the lead chloride so obtained ; but the result is not so good as that obtained by the process above described. Turner's yellow is met with in many shades of yellow, from a fairly bright yellow to a dark orange-yellow ; usually it is in the form of heavy, glassy-looking masses, which are rather difficult to grind. It has a good body or covering power, and can be used either as an oil- or water-colour. It is not a permanent colour, being affected by exposure to light and air and to sulphureous gases, which turn it brownish-black. NAPLES YELLOW. Like the last this is a lead colour and has been superseded by the chromes. Naples yellow is a compound of the oxides of 144 YELLOW AND ORANGE PIGMENTS. antimony and lead, and can be prepared of various shades and from different materials. (a) 1 part of tartar emetic, 2 parts of lead nitrate, and 4 parts of salt are intimately mixed together, and the mixture placed in a crucible and heated to fusion, at which point it is kept for 2 hours ; after which the fused mass is treated with water to wash out the soluble alkaline salts present in it, and the pigment is dried at a gentle heat. (b) 1 part of tartar emetic, 2 parts of red lead, and 4 parts of salt are treated as above. (c) 3 parts of antimony, 1 part of zinc oxide, and 2 parts of red lead are heated to fusion in a covered crucible for 4 hours; after which they are ground under water and the pigment dried at a gentle heat. (d) A process of preparing an antimony-lead yellow from the dross of lead refining was patented in 1858, by Dick, which con- sisted in mixing this dross (which is a mixture of the oxides of lead and antimony with some small quantities of other impurities) with salt, fusing the mass for 2 to 3 hours, then washing it well with water and drying the pigment. (e) A yellow not unlike Naples yellow has been made from the three oxides of tin, lead, and antimony, by calcining for 3 to 4 hours in a crucible a mixture of 2 parts of levigated crude anti- mony, 2 parts of tin ashes, and 5 parts of white lead ; or 1 part each of tin ashes, litharge, and antimony may be used. (/) 1 part of type-metal, 2 parts of potassium nitrate, and 4 parts of salt are fused together, and the fused mass treated as in process a. (g) Processes for preparing antimony yellows were patented by Hallet and Stenhoitse, in 1861, as follows : — 1. Antimony ore was calcined and then mixed with oxide of zinc and litharge, the mixture being fused. 2. A mixture of type-metal and zinc oxide are fused together. The yellows made by the above methods have been sold under various names — Naples yellow, Jaime, solid yellow, antimony yellow, &c. They were rather favourite colours at one time with artists, but their use has become nearly obsolete. They are bright colours, although not equal to the chromes in this respect; are fairly fast to light, but, like all lead colours, are affected by sulphureous gases ; iron has an injurious effect upon these colours so that they cannot be ground in iron mills with safety. They are equally useful as oil- or water-colours, and are of good body or covering power. king's yellow. 145 KING'S YELLOW. This yellow, which at one time was in extensive use, is the trisulphide of arsenic, As 2 S 3 . It is found native as the mineral orpiment, which is sometimes ground up and used us a pigment. The artificial colour is usually made by precipitation, but it can also be made by sublimation. 1. By Precipitation. — (a) Arsenic is dissolved in hydrochloric acid and a current of sulphuretted hydrogen gas passed through the solution ; a fine yellow precipitate of the colour is obtained, which is collected and dried at a gentle heat. (6) A fine yellow pigment, formerly sold under the name of Royal yellow, is made by mixing 2 parts of barium sulphate with 1 part of charcoal and calcining the mixture, when barium sulphide is formed ; this is ground with orpiment and water into a fine paste, and by boiling with water a sulpho-arsenite of barium is obtained ; to this is added dilute sulphuric acid which precipitates a mixture of barium sulphate and sulphide of arsenic as a fine yellow colour, which is collected, washed, and dried. 2. By Sublimation. — 1 lb. of sublimed sulphur and 2 lbs. of white arsenic are thoroughly mixed together and placed in a crucible ; this is covered with another crucible or with a special condenser, and heated in a furnace. The arsenic and sulphur react and form the sulphide, which, being volatile, sublimes into the cover, and is collected, washed, and dried ; it varies a little from time to time in shade. King's or arsenic yellow is a very bright pigment, almost rivalling the chromes in beauty. It has good body and works well either in oil or water, but is not a durable colour, as exposure to light causes it to fade, while air and moisture have no action on it. It does not mix well with other pigments, since, when associated with lead pigments, or with verdigris, emerald green, or other copper pigments, it gradually acquires a dark brownish tint owing to the formation of the black sulphides of lead and copper. It can be mixed with ultramarine, cadmium yellow, or oxide of iron without change. Being an arsenic colour it is very poisonous, and, therefore, its use is not to be recommended. Partly in consequence of this objection it has become nearly, if not entirely, obsolete. King's yellow forms a colourless solution with strong hydrochloric acid ; as also with caustic soda, from which, on adding acid, the pigment is reprecipitated. The presence of arsenic may be tested for by means of Marsh's test, described in the section on emerald green (p. 171). 10 146 YELLOW AND ORANGE PIGMENTS. REALGAR, ARSENIC ORANGE. Realgar is a native arsenic disulphide found in small quantities in various localities. It is prepared artificially by a process of sublimation as follows : — (a) A mixture of 8 lbs. of white arsenic (arsenious oxide) and 4 lbs. of flowers of sulphur are heated in a crucible as in making orpiment. (6) A mixture of 30 lbs. of arsenic, 20 lbs. of flowers of sulphur, and 40 lbs. of charcoal is made ; a charge of 60 lbs. of this mixture is heated at a time in earthenware crucibles so arranged that the product which sub- limes can be collected. This sublimate is then remelted to form the colour. Realgar has the same properties as orpiment. INDIAN YELLOW. Indian yellow or Purree is a most curious product. It has long been used in India, but is of comparatively recent introduction in this country, where its use is limited. It is made exclusively at Monghyr in Bengal by a caste of people known as the Gwalas. It is made from the urine of cows fed upon the leaves of the mango tree, which food increases the secretion of bile and the excess passes into the urine to which it imparts a strong yellow colour. The flow of the urine is stimulated by the Gwalas gently rubbing the urinary organs two or three times a day ; indeed, the cows are so habituated to this that they are unable to pass the urine themselves ; the feeding with mango leaves is so injurious that its long continuance causes the death of the cows, and grass, &c, is occasionally substituted for them ; the average life of these cows is from six to seven years. The urine as it comes from the cows is collected, and each evening it is boiled down in earthen vessels when the yellow is deposited ; it is gathered on calico, made into balls and sent into the market for sale. The annual production is said to be about 100 to 120 cwts. Indian yellow is a fairly bright yellow pigment, and is sold in the form of small round balls ; it is non-poisonous and has a good colouring power ; unfortunately it is not durable, as exposure to light soon causes it to fade. Authorities differ somewhat upon the composition of Indian yellow, but most agree that it is a compound of a peculiar acid known as euxanthic acid (which exists in the purree) combined with magnesia ; there is also present potassium benzoate and other bodies. The acid itself generally crystallises in small needles of a pale yellow colour ; it is slightly soluble in cold water, more readily in boiling CADMIUM YELLOW. 147 water, and freely in ether and alcohol. On being sub- jected to dry distillation a yellow body, called euxanthone, sublimes. The salts of euxanthic acid are all yellow-coloured bodies ; those of the alkalies are soluble in water, while those of most of the metals are insoluble and may, therefore, be used as pigments. This important yellow pigment, so much used by artists on account of the brilliance of its colour and its permanence, is the sulphide of the metal cadmium and is composed of and has the formula Cd S. It is made by passing a current of sulphuretted hydrogen gas through solutions of cadmium salts, as shown in the equation : — G. Buchner has investigated the properties of cadmium yellow more thoroughly than any other chemist. He describes * four modifications of cadmium sulphide which he distinguishes as 1. A-modification, obtained by passing a current of sulphur- etted hydrogen gas through a slightly acid solution of a cadmium salt. The colour is a very bright and pure citron-yellow, has a good body, and works well as an oil-colour. By various means it can be converted into the B-modification. When used as an oil-colour it is quite permanent, but when used in water, or kept in a moist atmosphere, it gradually undergoes oxidation, passing into sulphate, this change being accompanied by a loss of colour. 2. B-modification. — This has a bright vermilion-red colour, and is obtained by passing sulphuretted hydrogen gas through a strongly acid solution of a cadmium salt. It is the most permanent form of cadmium sulphide and is unaffected by exposure to light and air. The author has been unable to prepare this red variety of cadmium yellow. Although Buchner does not give any clear description of the method of obtaining it, yet from the remarks as to the conditions under which this red variety is formed, it is evident that it cannot be obtained by precipitation free from the yellow variety, and that the process of separation consists in * Chemiker Zeitung quoted in Journal Socy. Ghem. Ind., VI. 665. CADMIUM YELLOW. Cadmium, Sulphur, 77*78 percent. 2222 148 YELLOW AND ORANGE PIGMENTS. exposing the mixture of yellow and red sulphides to light and air for some considerable period. It is not practicable to prepare this variety as a commercial article. 3. C-modiflcation. — A variety soluble in water, of no practical interest, and prepared by a process of dialysis. 4. D-modification. — This variety has a pale yellowish colour and little or no body. It is prepared by passing sulphuretted hydrogen gas through an ammoniacal solution of a cadmium salt. It is of no practical use. Cadmium yellow is made commercially in various shades of yellow and orange, the processes for the production of which are described below. Preparation of Yellow Cadmium. — This is prepared in several ways. 1. A slightly acid solution of any cadmium salt is pre- pared and through it is passed a current of sulphuretted hydro- gen gas; the apparatus shown in Eig. 17, p. Ill may be used. This has a pure chrome-yellow shade. 2. A lemon-yellow shade is obtained by dissolving 1 lb. of cadmium sulphate in 4 gallons of water and adding 1 \ gallons of the ordinary yellow ammonium sulphide. 3. Or a solution of cadmium sulphate is made, to which is added a solution of sodium thiosulphate and a little sulphuric acid and the mixture boiled for an hour. This variety contains much free sulphur, and is, hence, liable to undergo oxidation to sulphuric acid, which destroys the yellow. Preparation of Orange Cadmium. — 1. A solution of cadmium sulphate or chloride is prepared. It is made strongly acid by the addition of excess of hydrochloric acid, and a current of sul- phuretted hydrogen gas is passed through it. 2. 1 lb. of cadmium sulphate is dissolved in 4 gallons of water; the solution is boiled, and while boiling, yellow ammonium sulphide is added. All the precipitates of yellow obtained in the various ways just described must be well washed in water, especially those obtained with the ammonium sulphide, to free them from any trace of acid or sulphide which, if left in, would ultimately lead to the destruction of the colour. After being washed they should be thoroughly dried at as low a temperature as possible, not exceeding about 150° to 160° F. ; too high a temperature causes the shade to become brown while hot and although the colour comes back on cooling, yet it never quite regains the original brilliancy. PROPERTIES OP CADMIUM YELLOW. — Pure cad- mium yellow is one of the most permanent pigments known ; it is unaffected by exposure to light and air. It mixes with any vehicle used in painting. When heated strongly the colour darkens, changing to a dark violet-red; on cooliig, the original AUREOLIN. 149 colour comes back, not, however, always in its original brilliance, but with a brownish tone. The impure yellows, those which are made with yellow ammonium sulphide or sodium thiosulphate, are not permanent pigments. When they are exposed to the combined action of air and moisture, the free sulphur they con- tain becomes oxidised to sulphuric acid, and this, acting on the yellow cadmium, changes it to sulphate, which change is shown by a bleaching of the colour, and occurs whether the pigment be ground or used in oil or water. Cadmium yellow can be mixed with almost all the other pig- ments without affecting them or being affected by them ; the only exceptions are those pigments which, like white lead, emerald- green, and the chrome-yellows, contain lead or copper as their basis. When such pigments are mixed with cadmium yellow double decomposition sets in, resulting in the formation of black sulphide of lead or copper as the case may be ; the production of either compounds causes the mixture to acquire a greyish or brownish tint. ASSAY AND ANALYSIS OF CADMIUM YELLOW. — Besides the usual tests for colour and body, cadmium yellow should satisfy the following tests : — Strong hydrochloric acid should completely dissolve the yellow with evolution of sul- phuretted hydrogen to a clear colourless solution, from which, on dilution with water and passing sulphuretted hydrogen gas, a yellow precipitate only should be obtained. The filtrate from this precipitate should give no precipitates on adding ammonia and ammonium sulphide. The addition of barium chloride to the solution should produce no turbidity. On boiling with caus- tic soda, filtering off the residue and adding hydrochloric acid to the filtrate no yellow precipitate, indicating the presence of arsenic yellow, should be obtained. Carbon bisulphide should extract no sulphur from it. Samples should not yield anything to water when boiled with it. The aqueous liquor should not give any precipitates with silver nitrate or barium chloride nor any acid or alkaline reactions to test papers. Cadmium yellow is rarely adulterated ; the common adulter- ants are arsenic yellow, zinc chrome, and the chrome-yellows, the presence of which can be distinguished by the application of the characteristic tests, which are given under the respective pigments. AUREOLIN. This pigment is a double nitrite of potassium and cobalt pre- pared by precipitating cobalt nitrate with sodium carbonate, 150 YELLOW AND ORANGE PIGMENTS. dissolving the precipitate in acetic acid and adding a strong solu- tion of potassium nitrite. On allowing the mixture to stand for some time the colour is gradually precipitated, and is collected and washed ; after being dried it is ready for use. Aureolin is of a bright yellow colour, but is not permanent, being affected by exposure to light and air; acids dissolve it, while alkalies have no action. 151 CHAPTER Y. GREEN PIGMENTS. The green pigments in common use by the painter and the artist are derived from both natural and artificial sources, but usually from the latter. The green pigments are valuable, largely made and used, and are fairly numerous. The commonest are those known as Brunswick greens, which are made in very large quantities for common painting ; next is emerald-green, although this colour, owing to its poisonous nature, is gradually being displaced by substitutes made from the coal-tar greens ; then comes the true chrome-green ; then some of the other copper-greens ; while the rest are only used on a limited scale. BRUNSWICK GREEN. Under the names of " pale Brunswick green," " middle Bruns- wick green," " deep Brunswick green," are sent out several green pigments, varying in shade or tint from a pale yellowish- green to a very deep blue-green. These pigments are made in very large quantities, and are mixtures, in various proportions, of chrome- yellow, Prussian blue, and barytes. They must not be con- founded with the pigment originally known under the same name, which was a compound of copper, and which has almost completely gone out of use. Brunswick greens can be made in various ways, almost every colour maker having his own favourite manner of mixing the various ingredients together. DRY METHOD. — In this method the materials composing the green are thrown into the pan of an edge-runner grinding mill or into a mixing mill ; the former is preferable, as the materials are ground as well as mixed, and this has some influence in developing the tint of the green. The main advantage in this method is considered to be that the shade of green which is being produced is visible while in the mill, and that if too 152 GREEN PIGMENTS. much yellowing is being produced as compared with a standard sample, then, by throwing a little more blue into the mill, the fault can be remedied at once ; but against this advantage must be set off the disadvantage that the tints of green are not so fine as those which are obtainable by wet methods. The following proportions can be taken as guides in making the various shades of Brunswick green by this dry method : — Pale Brunswick Green. — 1 cwt. of barytes, 1J lbs. of Prussian blue, and 35 lbs. of chrome-yellow. Middle Brunswick Green. — 1 cwt. of barytes, 2\ lbs. of Prussian blue, and 35 lbs. of chrome-yellow. Deep Brunswick Green. — 1 cwt. of barytes, 5 lbs. of Prussian blue, and 35 lbs. of chrome-yellow. Extra Deep Brunswick Green. — 1 cwt. of barytes, 8 lbs. of Prussian blue, and 35 lbs. of chrome-yellow. The quality and tint of the chrome-yellow used will be found to have some influence upon the tint of the green produced ; for the pale shades lemon-chromes should be used ; while for the deep shades the middle shades of chrome can be used. Before making up a large batch of green with a new batch of either chrome-yellow or Prussian blue a small trial lot should be made to see if the two pigments will produce the required shade, as experience shows that different makes of chrome-yellows, even if of the same shade, do not always give the same results in making these greens ; while the difference between two makes of Prussian blue as green-producers is greater than that between two makes of chrome-yellow. For making the palest shades of Brunswick greens, only the best and brightest Prussian blues should be used ; for the darker shades of green the quality of the blue is not of so much con- sequence. The barytes may be replaced with gypsum, if thought fit ; less gypsum is required to produce a given tint of green than barytes, the proportion being about 1 cwt. of gypsum to '2\ cwts. of barytes. WET METHODS. — The wet methods are those commonly adopted by makers of Brunswick greens, partly because they are the oldest (for colour makers are so conservative that it is difficult to induce them to alter their methods), and, partly, because the wet methods produce the finest greens; but they are very much more troublesome to carry out and require no little practical experience on the part of a colour maker to produce the best results. From the recipes which have been given above it will be seen that barytes is the principal ingredient in these greens ; hence the principle which should underlie all wet pro- BRUNSWICK GREENS. 153 cesses is to precipitate the yellow and the blue constituents on the barytes simultaneously. This is by no means easy to do, and yet much of the brilliancy of the green depends upon this being done as successfully as possible. The best way to ensure this result would be to mix solutions of the acetates of lead and iron or of the nitrates of those metals together and to add the barytes and precipitate with a mixture of the bichromate and ferrocyanide of potash, but, unfortunately, this course is not at present available, for the reason that while the acetate or nitrate of lead can be purchased on a commercial scale of sufficient purity, the acetate or nitrate of iron is not so purchasable ; the common iron liquor is too impure for use in making greens, while the nitrate of iron so-called is of too variable a composition to be recommended for use in colour making. The materials which are used in the preparation of the greens are copperas (ferrous sulphate), which should be used as pure and as fresh as possible, the variety known as green copperas is the one required, acetate of lead, bichromate of potash, ferrocy- anide of potassium (yellow prussiate of potash), and barytes. The red prussiate of potash would give rather better results than the yellow, only that its extra cost is against its use for making greens for common use ; but when a good price is obtainable its use is to be recommended, as the green is much easier to make with it than with the yellow prussiate. For producing the various shades of Brunswick greens the following proportions may be used which, as well as those given above, may be varied so as to suit the special requirements of each individual maker. The following points are, however, well worth attending to in making alterations in the proportions. 1st. That equal weights of prussiate and copperas must be used. 2nd. That the proportion of acetate of lead to the bichromate of potash should be, as nearly as possible, 10 to 3J. Pale Brunswick Green. — 1 cwt. of barytes, 13 lbs. of acetate of lead, 1 lb. of copperas, 1 lb. of yellow prussiate of potash, and 4 lbs. of bichromate of potash. Middle Brunswick Green. — 1 cwt. of barytes, 13 J lbs. of acetate of lead, 1J lbs. of copperas, 1J lbs. of yellow prussiate of potash, and 4J lbs. of bichromate of potash. Deep Brunswick Green. — 1 cwt. of barytes, 14 lbs. of acetate of lead, 2 lbs. of copperas, 2 lbs. of yellow prussiate of potash, and 4J lbs. of bichromate of potash. Extra Deep Brunswick Green. — 1 cwt. of barytes, 16 lbs. of acetate of lead, 4 lbs. of copperas, 4 lbs. of yellow prussiate of potash, and 5 lbs. of bichromate of potash. 154 GREEN PIGMENTS. Instead of the bichromate of potash the bichromate of soda may be used with some advantage, on account of its greater solubility and less cost. Instead of barytes, any white pigment may be used, when it will be found that a smaller quantity will be required to give a green of the same depth of colour ; or, to put it in another way, an equal weight of another white base will require a greater quantity of the other ingredients to produce the same shade of colour ; with gypsum the proportion will be about 2| times as much, with china clay about 3 or 4 times as much, and with zinc white from 4 to 5 times as much ; it is very rarely that any other pigment than barytes is used in making these greens. By varying the proportions a great variety of green shades can be produced. It is mainly owing to the variable proportions used that the middle green, say, of one maker so seldom has exactly the same tint or depth of colour as that of another maker. The following is the best method of working, in order to obtain good bright pigments with the ingredients given above. The iron salt is dissolved in a tank of cold water, the lead salt is similarly dissolved in another tank, while the two potash salts can be dissolved together in one tank. The barytes is thoroughly mixed with water in another tank, and, when properly mixed r the iron solution is run in with constant stirring; and then the lead salt is run in. Some of the lead will be precipitated as the sulphate, owing to double decomposition taking place between the two salts, but this cannot be avoided, so that allowance should be made for it in all recipes for making Brunswick green by increasing the amount of acetate of lead as the quantity of copperas is increased ; every extra pound of the latter will require 1 lb. 3 oz. of acetate of lead to be added in addition to- that required to form chrome-yellow with the bichromate. After the lead has been run in and mixed with the rest of the in- gredients, the whole is kept stirred while the potash salts are run in ; the green soon forms and is allowed to settle ; the clear liquor at the top is run off and the pigment washed by running in water and stirring well, again allowing to settle and running off the wash waters ; this washing should be repeated once or twice. Then the colour is taken out, thrown on filters to drain, and, finally, dried at a gentle heat. Various other methods of manipulating the preparations of these greens are in use among the various makers, but it is not necessary to describe them. A method of working, which has been used by the author with very good results as to ease and quality of the colour produced,. COMPOSITION AND PROPERTIES OF BRUNSWICK GREENS. 155 is to grind all the ingredients together, in the proportions given above, in an edge-runner mill, and when they are properly mixed, to put them into the tub and run water on to them, with constant stirring ; the green is rapidly developed, and is allowed to settle ; the clear top liquor is then run off, and fresh water run on to wash the pigment, after w T hich it is finished as usual. These greens are sold under a variety of names — Brunswick green (which is the commonest), Chrome-green, Victoria green, Prussian green, &c. COMPOSITION AND PROPERTIES OF BRUNSWICK GREENS. These pigments are compounds of barytes, chrome-yellow, Prussian blues, with occasionally small quantities of lead sulphate, gypsum, and other bodies. The following analyses made by the author of different shades of these greens, of a good make, will show the average composition of the pigments : — ANALYSES OF BRUNSWICK GREENS. Pale. Middle. Deep. Extra deep. Water, .... 1-000 0 800 1-000 0-800 Barytes, .... 72744 71944 72-764 71-404 Gypsum, .... 2-341 traces. traces. Prussian blue, . 0-929 0-442 4 543 6-910 Chrome-yellow, 14-266 25-734 17*660 17*867 Lead sulphate, 8 073 1039 3-280 3152 99*353 99-959 99-247 100-130 They are good pigments, and work well both in oil and water, especially the former ; their opacity is good, and, therefore, they have good body or covering power, in this respect surpassing all other green pigments. They can be mixed with other pigments, with but few exceptions, without any change being brought about by interaction \ these exceptions being those pigments containing sulphur, which would act upon the chrome-yellow and darken the green, by the production of black lead sulphide and highly basic colours, like whiting or lime, which would act 156 GREEN PIGMENTS. both upon the chrome-yellow and the blue, turning the green into a red. They are fairly permanent when exposed to light and air, for, although not quite permanent, they are so for all practical purposes ; exposure to light causes the yellow constituent to fade first, as a rule, so that, especially in the dark shades, the green has a tendency to turn blue, but in this respect the blue is very variable ; in some makes the yellow goes first, in others the blue, much probably depends upon the composition of the particular green and the circumstances under which it is placed. Acids turn the colour bluer, owing to their dissolving out the chrome-yellow ; on the other hand, alkalies turn it orange, owing to their combined action both on the blue (turning this of a reddish-brown) and on the yellow (which they turn orange), as is noted in describing the blue and the yellow in their respective places. Sulphuretted hydrogen darkens the tint considerably. ASSAY AND ANALYSIS OF BRUNSWICK GREENS. — Brunswick greens require assaying for colour or tint, covering power or body, brilliance, &c, by the usual methods. Since, as already stated, the pale shade of one maker may not exactly agree with the pale shade of another maker, the different makes should always be compared together for the various properties just named ; as a rule, it will be found that different batches of the same maker s green will match one another very closely. It is rarely that an analysis of these greens is required ; but, if so, the following method, described by Brown,* may be followed : — 1. For Chrome Green. — Weigh out 2 grammes of the green, treat with 28 to 30 cc. of strong hydrochloric acid, at the boil, for about 10 minutes, then filter while still hot, and wash well with boiling water, adding the wash waters to the filtrate. The residue, consisting of barytes and Prussian blue, is strongly heated until the blue is decomposed, and, after cooling, the residue is weighed; this gives the weight of oxide of iron and barytes in the green. Treat the residue with a mixture of nitric and hydrochloric acids, boil well, then dilute with water and filter ; dry, ignite, and weigh the residue, which is the barytes ; deduct this weight from the original weight of barytes plus oxide of iron ; the difference is the amount of oxide of iron, which, multiplied by 2*212, gives the amount of Prussian blue in the green. Filtrate. — Nearly neutralise by the addition of ammonia, then * Brown, Chemical ISFeivs, December 31, 1886. ASSAY AND ANALYSIS OF BRUNSWICK GREENS. 157 pass a current of sulphuretted hydrogen gas through the solution, which will precipitate the lead as lead sulphide ; filter this off and wash the precipitate, adding the wash water to the filtrate. Lead Sulphide. — Treat the precipitate of lead sulphide with hot nitric acid, boil down to a small bulk, then add a little strong sulphuric acid, heat until acid fumes begin to appear, then allow to cool, add water and a little alcohol, filter, wash, and, after drying and igniting the precipitate in the usual way, weigh as lead sulphate. This gives the total amount of lead in the green, which may be in the condition of sulphate as well as of chromate (see the analyses given above). Filtrate from the Lead Sulphide. — This contains the chromium and occasionally a little iron. Boil down to a small bulk, then test the solution for iron by taking a drop out with a glass rod and placing it on a piece of paper moistened with potassium ferrocyanide; if a blue spot appears, then iron is present, and the solution is treated according to method No. 1 ; if iron is absent it is treated according to method No. 2. Method No. 1. — Boil the solution with nitric acid and potassium chlorate until a clear yellow solution is obtained, then add sufficient ammonia to precipitate the iron, filter off, wash, dry, and weigh the precipitate, which weight is to be added to the weight of the iron previously found. Take the filtrate from the iron precipitate, boil down to a small bulk, add some strong hydrochloric acid and a little alcohol and boil until the colour of the solution becomes a clear green ; this is effected by cautious addition of more acid and alcohol, if required, but too great excess of either must be avoided ; to the solution is added ammonia in excess and the mixture is boiled until it gives on filtering a colourless filtrate ; the precipitate consists of chromium hydroxide, which is filtered off, dried, ignited, and weighed ; the weight multiplied by 4*241 gives the weight of chrome-yellow (pure chromate of lead) in the green. Method No. 2. — The filtrate from the lead sulphide in which no iron is present is boiled aud ammonia added in excess ; the mix- ture is then treated as described under method No. 1, from the same point. 2. For Lead Sulphate. — Weigh out 2 grammes of the green, boil with hydrochloric acid, filter while still hot, and wash with boiling water ; evaporate the filtrate down and, while still boiling, add barium chloride in slight excess ; filter, wash well with boiling water, and treat the precipitate of barium sulphate in the usual way. The weight of barium sulphate multiplied by 1*3 gives the weight of lead sulphate in the green. The difference 158 GREEN PIGMENTS. between this amount and that found in the first instance repre- sents the lead which is present in the green in other forms. A qualitative analysis should always precede a quantitative one ; and the above scheme should be modified if such an analysis shows that other constituents, besides those mentioned above, are present in the greens. The notes on the analysis of chrome-yellows (p. 127) will be found useful. BRUNSWICK GREEN. The modern Brunswick greens must not be confused with the pigment which was made and sold at one time under this name and which has now become quite obsolete. This old Brunswick green was a basic chloride of copper, sometimes called an oxy- chloride, Cu 2 O Cl 2 . It can be prepared by several methods. 1. 20 lbs. of copper turnings are placed in a vessel capable of being closed ; over them is poured a solution of 30 lbs. of ammonium chloride in 6 gallons of water ; the vessel is closed up and the contents well mixed by shaking ; the vessel is kept in a warm place for about two months, and at intervals the contents are mixed by shaking up ; at the end of the time the vessel is opened, when it will be found that most of the copper has been converted into the green oxychloride. 2. Into a wooden tub is placed about 1 cwt. of old sheet copper cut into small pieces ; over them is poured a solution of 105 lbs. of sulphate of potash and 1 cwt. of common salt; the mass is allowed to react together for some time, the length of which depends upon the temperature, and is longer in winter than in summer. The green gradually forms, and when it is seen that most of the copper has been converted, the green is separated from the undecomposed copper by sieving and washing. 3. 1 cwt. of copper is mixed with 67 lbs. of salt and 34 lbs. of sulphuric acid mixed with 3 times its volume of water; after standing some time the green is formed, when it is treated as before. 4. Metallic copper is taken and just covered with a strong solution of chloride of copper and left until it is changed into the basic chloride, when it is finished as described under method 2. The preparation of Brunswick green is a very slow operation, extending over 2 to 4 months as a rule ; in all cases the green is collected by washing it with water to free it from any alkaline bodies, sieving to free it from unchanged copper, drying slowly at a low temperature, since high temperatures tend to decompose it ; necessarily it is somewhat costly. CHROME-GREEN. 159 As a pigment it is fairly good, working well both in oil and water, and having a good covering power ; in tint it has some- what of a bluish-green cast of no great depth of colour. It is not quite permanent, although it resists some considerable amount of exposure to light and air. In its general properties it closely resembles the Bremen blues and the Bremen greens, which see. CHROME-GREEN. True chrome-green is a most valuable pigment, not only on account of the brilliance of its colour, but also on account of its great permanence, being in fact the most permanent green pigment known. In its chemical composition chrome-green varies somewhat according to the method of making; in some cases it consists entirely of the oxide of chrome, Cr 2 0 3 ; in others of the phosphate of chrome, 0r 2 2 P0 4 ; while in yet others it is a mixture of these bodies. It should not be confused with the mixture of chrome- yellow and Prussian blue which is sometimes sold as chrome- green. Various methods are in use for preparing chrome-green. 1. Guignet' s Process. — Guignet was one of the first to prepare chrome-green, if not the very first; hence the pigment is frequently sold under the name of "Guignet's green." Guignet uses boric acid (boracic acid) and bichromate of potash. As a rule, the commercial articles are of sufficient purity to prepare good pigments with. But if very good results are required it is advisable to purify them by re-crystallisation. 88 lbs. of potassium bichromate and 33 lbs. of boracic acid are ground into a stiff paste with water; the mixture is then put into a furnace where it is heated to a dark red heat for 4 hours. A form of reverberatory furnace is the best that can be used. The fused mass is thrown into water and repeatedly washed by decantation ; the washed pigment is ground whilst still wet under an edge-runner mill, again washed, filtered, and dried. The first wash-waters contain a good deal of the boracic acid in the form of potassium borate ; this acid may be recovered and used over again ; the waters are boiled down a little and to the liquor is added hydrochloric acid; this throws out the boracic acid, which gradually collects in the form of crystals on standing; these crystals can be collected and used for making another batch of green ; in this way at least 70 to 75 per cent, of the boracic acid originally used is recovered. 160 GREEN PIGMENTS. The reaction which takes place between the boracic acid and the bichromate is expressed in the following equation : — 3 K 2 Cr 2 0 7 + 2 H 3 B 0 3 = 3 Cr 2 0 3 + 2 K 3 B 0 3 + 3 H 2 0 + 9 0 from which it can be calculated that a considerable excess of boracic acid has been used in the process of making the colour ; this excess is not wasted, since part is recovered, and, moreover, an excess is necessary for the production of a fine quality of the pigment. Borax cannot be substituted for the boracic acid. Chrome-green made by this process has a fine yellow-green tint. 2. 3 lbs. of bichromate of potash and 2 lbs. of ammonium chloride are thoroughly mixed together into a paste with water ; this is dried and then calcined at a red heat in a furnace ; the calcined mass is well washed in water, and the pigment thus obtained is ground. This process gives a fine quality of green, but it is not quite equal to that obtained by the last process. 3. When solutions of ammonia or caustic soda or carbonate of soda are added to solutions of the basic chromium salts a pre- cipitate of the hydroxide, Cr 2 H 6 0 6 , is obtained ; when this is heated to redness it loses water and passes into the oxide, Cr 2 0 3 ; the tint of green obtained in this way is not good, being of a greyish hue; by mixing with the precipitate some salt before calcining, and afterwards thoroughly washing with water, the tint of the green is materially improved. All the above processes yield the green in the form of oxide, Cr 2 0 3 , which is perfectly permanent when used as a pigment. 4. A solution of chromium chloride is prepared by heating a strong solution of potassium bichromate with hydrochloric acid and a little methylated spirit ; to this solution is added, first, sufficient soda to neutralise the acid, and, then, a solution of sodium phosphate ; the precipitate of chromium phosphate is collected, dried, and calcined; the green is then finished by washing and grinding in the usual way. 5. A cheaper method of producing the phosphate consists in preparing a solution of 10 lbs. of potassium bichromate and 18 lbs. of sodium phosphate; the mixture is boiled, and, while boiling, a solution of 10 lbs. of sodium thiosulphate is added, and then a little hydrochloric acid. On continuing the boiling the chromium phosphate is slowly precipitated; when the precipi- tation is complete, the green is treated as in the last process. The pigment obtained by this process is apt to contain a trace of sulphur, which introduces into it an element of change. The phosphate-of-chrome greens are by no means equal to the oxide-of-chrome greens for brilliancy of tint. COPPER GREENS. 161 PROPERTIES OP CHROME - GREEN. — Chrome-green forms a fine green pigment of a slightly yellowish tone ; it mixes well with either oil or water, has good body or covering power, and is quite permanent, being one of the best pigments which the painter can use, on which account it is much used by artists. It mixes with all other pigments without being affected by them or altering them in any way. When properly made it is quite insoluble in either acids or alkalies. The solubility of oxide of chromium depends upon the temperature and length of time to which it has been heated; the greater these two factors are the more insoluble becomes the oxide, so that well prepared oxides are very insoluble owing to the fact that they have to be heated to a high temperature for some time. Yery nearly the same property is found in the phosphate greens. ASSAY AND ANALYSIS OP CHROME-GREENS. — Chrome-greens should be assayed for colour, brilliance, covering power, and similar properties in the usual way. When pure, chrome-green should not impart a yellow colour to dilute hydrochloric acid when boiled with that reagent, such yellow colour would indicate adulteration with chrome-yellow. When boiled with caustic soda chrome-green should remain unacted upon. The liquor should be divided into two portions — to the one acetic acid should be added, when no yellow precipitate indicating chrome-yellow should be obtained ; to the other hydrochloric acid and ferric chloride should be added, when no blue precipitate should be obtained, such precipitate would indicate the presence of Prussian blue. Chrome-greens are usually adulterated with the Brunswick greens, which adultera- tion is detected by the application of the two tests just given. For use by calico-printers, Guignet's green is supplied in the form of a paste, containing usually 30 per cent, of actual colour. COPPER GREENS. Copper forms the base of a number of greens, some of which are of value, although the bulk are of but minor importance, and their use is gradually decreasing. The variety of names under which the copper-greens have been offered from time to time is very great ; very few are now in use, and it is rather difficult to know exactly to what copper compound any particular name one •neets with in old books and papers belongs. One of these greens has already been described. 11 162 GREEN PIGMENTS. Three of these copper-greens — verdigris, Scheele's green, and emerald green — are very closely related to one another, and form, as it were, a group of colours. Verdigris is the basic acetate of copper. Scheele's green is the arseniate of copper, while emerald green is the aceto-arsenite of copper, and may be viewed as a compound of the other two colours. VERDIGRIS. The chemical composition of verdigris has been already stated. It is made in two forms, known as " distilled" and "common" verdigris. The first, being somewhat of a crystalline nature, is rarely used as a pigment, and finds its chief use in medicine ; the latter is of the most importance from a painter's point of view. Preparation of Verdigris. — Distilled verdigris is prepared by dissolving copper or oxide of copper in the acid obtained during the distillation of wood, from which circumstance arises the name "distilled verdigris;" for the product itself is not distilled. Another method of manufacturing this variety is to mix together solutions of sulphate of copper and acetate of lime, or of the acetate of lead ; the sulphate of lime or lead, as the case may be, is precipitated, and a solution of acetate of copper is obtained. From the solution of acetate of copper, obtained by either of the above methods, the verdigris is obtained by con- centrating down to the crystallising point, and allowing the salt to crystallise out \ this gives the best product. Or, the solution may be cautiously evaporated to dryness ; this is costly and there is a risk of decomposing the green, causing it to lose its brilliancy of tint. Distilled verdigris occurs in the form of dark green crystals, soluble in water and in acetic acid ; as a pigment it is of little use, being too transparent; then, again, its solubility is against its being a good pigment. Common verdigris is prepared in several ways. 1. French Process. — The skins and marc of grapes, left after the juice has been pressed out for making wine, are used in France for making verdigris ; the material is placed in large tubs, loosely covered over with netting, in which it remains for a few days, when acetic fermentation sets in ; when this has commenced sheets of copper (averaging about 8 inches by 4 inches) are thrown in among the fermenting mass — generally old scrap copper is used. They are left in the tub among the grape skins for from 18 to 20 days, the period varying according to the weather; COMPOSITION AND PROPERTIES OF VERDIGRIS. 163 in summer it may be only about 12 to 14 days, but in winter the longer period named is always required. At the end of the time the tubs are emptied and the grape refuse thrown away ; the copper sheets are dried, then dipped into water, or, what is better, into bad wine (if that is obtainable), and again dried ; by this means a coat of verdigris is formed on the plates, which is scraped off and placed on one side ; the plates are redipped and again dried, when another coating of verdigris is formed, and scraped off as before ; the process is repeated until all the copper has been converted into verdigris. The green is washed with water and then dried, when it is ready for use. At one time almost every vineyard in France and Belgium made verdigris somewhat on the above lines, although there were some variations in the minor details ; but, as the consump- tion of verdigris has decreased considerably, its manufacture has not been so generally followed of late years. 2. English Process, — In England verdigris is made by packing plates of copper between cloths soaked in the crude pyroligneous acid obtained in the distillation of wood ; this is done in casks ; every four or five days the casks are unpacked and the cloths redipped in the acid, and the operation repeated until the sheets of copper begin to have a coat of verdigris ; they are then dipped in water and dried ; the verdigris on them is then scraped off and the copper is again packed with the cloths ; and the process repeated until all the copper has been converted into verdigris. The refuse from the manufacture of cider has been used in making this pigment. The verdigris is finished for use by washing and drying. The latter has to be done very carefully, as too high a temperature would affect the brilliancy of the tint. Common verdigris is not quite so pure as distilled verdigris ; but as it is more insoluble in water and more opaque, it can be used as a pigment. COMPOSITION AND PROPERTIES OP VERDIGRIS. — Phillips gives the following analysis of distilled verdigris : — Copper oxide, CuO, . . . . 43*25 percent. A cetic anhydride, C 2 H 3 0 . 0 . C 2 H 3 0, 28 '30 „ Water, H 2 0, 28 45 which corresponds to the formula Cu 2 C 2 H 3 0 2 , Cu H 2 0 2 , 5 H 2 0. Distilled verdigris is very constant in its composition. It forms dark green crystals somewhat soluble in water and in 164 GREEN PIGMENTS. acetic acid ; heated in the air they lose their water of crystallis- ation and their acetic acid, and a black residue of copper oxide, Cu O, is left behind. Common verdigris is very variable in its composition and usually contains some impurities ; as a rule, it is generally allowable for commercial verdigris to contain about 2 per cent. The following analyses of some samples of common verdigris will show the average composition of this pigment : — 1 is by Phillips of an English-made sample. 2 and 3 are by the author ; the water in these was present both as hygroscopic and combined water — in No. 2 the amounts were respectively 7*95 and 10 "05, in No. 3 they were 4*49 and 8*75. 4 and 5 are by Berzelius of French-made samples. Verdigris has a greenish-blue colour. It makes but a poor pigment, being the most fugitive of the copper-greens ; in water it soon fades, in oil it is rather more permanent, if kept free from moisture, which causes it to effloresce. While being almost insoluble in water, it is readily soluble in all acids, without effervescence, to a blue solution, which gives the characteristic tests for copper. Heated with strong sulphuric acid it evolves acetic acid. Heated alone it loses its water and acid and turns black, from the production of oxide of copper. ASSAY AND ANALYSIS OP VERDIGRIS.— Verdigris may be assayed for tint, covering power, &c, in the usual way. The following impurities in verdigris should be tested for ; Insoluble matter, carbonates, sulphates, and metals : — Insoluble Matter, which may consist of barytes, sand, &c, is readily tested for. A weighed quantity is treated with hot dilute hydrochloric acid for a short time and the insoluble matter collected on a filter, dried, and weighed. The amount of in- soluble matter should not exceed 3 per cent., and even this is an excessive allowance, as can be inferred from the analyses already quoted \ 2 per cent, is, in the opinion of the author, quite a sufficient allowance, and anything above this ought to be considered as an adulteration. If, when treating with the acid, effervescence occurs it may be taken as an indication of the presence of carbonates either of copper or calcium, or both. 1. 2. 3. 4. 5. Copper oxide, 44*25 4379 40*79 43*24 Acetic anhydride, 29*62 38*49 45*97 27*57 Water, . . 23*51 18*00 13*04 29 19 Impurities, . 2*62 43*5 29*3 25*2 2 0 scheele's green. 165 Sulphates can be tested for in the hydrochloric acid solution by adding barium chloride, and their amount may be determined by filtering off, drying, and weighing the precipitate of barium sulphate, which will be formed if they are present. The presence of other metals than copper is best ascertained by working according to the following scheme. The pigment is heated to a red heat to decompose it, and the residue is dissolved in hydrochloric acid and any insoluble matter filtered off ; through the solution a current of sulphuretted hydrogen is passed, to precipitate out the copper as the black sulphide ; the filtrate is boiled down to a small bulk, a little strong nitric acid is added, and the mixture is boiled for a few minutes ; ammonia is then added in slight excess ; if any iron is present a reddish- brown precipitate will be obtained ; this is filtered off and to the filtrate ammonium sulphide is added, which, if zinc is present, will throw down a white precipitate of the sulphide of zinc ; after filtering this off, ammonium oxalate is added to the filtrate when, if calcium is present, a white precipitate of calcium oxalate will be obtained. Calcium may be present either in the form of carbonate or sulphate, which will be inferred from the results of other tests. It is rare for any other metal to be present. Adulteration of verdigris with Prussian blue can be detected by the blue residue left after the treatment with acid giving all the reactions of Prussian blue. The addition of ultramarine is readily detected by the action of acid on the sample. Green verditer is sometimes sold under the name of " British verdigris." SCHEELE'S G-REEN. This pigment was discovered by Scheele, the eminent Swedish chemist, who communicated his method of making it to the Academy of Sciences of Stockholm in 1778. At first it was very much used, being at that time one of the best greens known, but the introduction of emerald-green in 1814 soon brought about its gradual disuse, and now it is doubtful whether it is ever used as a pigment ; this is partly due to the fact that it is but a dull colour, while much brighter and better greens are now known ; then, again, the fact of its being an arsenical colour has always been much against its use as a pigment. PREPARATION OF SCHEELE'S GREEN.— 1. Scheele gave the following instructions for preparing this green : — 1 part of powdered white arsenic (arsenious oxide) and 2 parts of potash (carbonate of potassium) are dissolved, by boiling, in 35 parts of V 166 GREEN PIGMENTS. water; the solution is filtered and then poured into a solution of 2 parts of copper sulphate as long as a precipitate falls. The precipitate is collected on a filter, washed with water, and dried at a gentle heat. 2. Parker patented, in 1812, a process of making Scheele's green — Two solutions were made in boiling water, one containing 16 ozs. of sulphate of copper, the other 14 drms. of arsenic and 14 ozs. of potash. The precipitate obtained on mixing the solu- tions was washed and dried. As the alkali is greatly in excess in this process, the precipitate must consist largely of carbonate of copper. 3. Sharpies* prepares Scheele's green by dissolving 2 parts of arsenious oxide (white arsenic) in 8 parts of soda crystals by boiling with 10 parts of water; when dissolved, the arsenite of soda formed is poured into a solution of 6 parts of copper sul- phate in 40 parts of water. Both solutions are mixed while boiling, and the mixture itself boiled for a few minutes ; it is then allowed to stand until the next day, when the green super- natant liquor is poured off and the green washed two or three times with hot water, dried, and filtered. This is stated to be the most economical process of making the green. 4. Berzelius describes a process of preparing a green by boiling copper carbonate with white arsenic ; the green has a fine tint. COMPOSITION AND PROPERTIES OP SCHEELE'S GREEN. — Scheele's green is essentially an arsenite of copper. Sharpies, who has made a very exhaustive examination of this pigment, states that it is a basic arsenite of copper, usually con- taining small traces of carbonate and sulphate of copper. He gives the following as the composition of a pure Scheele's green : Copper oxide, Cu 0, . . . 50 '00 per cent. Arsenious oxide, As 2 O s , . . 42 00 ,, Water, H 2 O, .... 8 00 which corresponds to the formula Cu As 0 3 Cu O . 2 H 2 O. The pigment as prepared on a commercial scale differs some- what from this, as might be expected ; but the variation is not very great when properly made. The following analyses given by Sharpies of samples made by Scheele's and Berzelius' pro- cesses will show the average composition of this pigment : — * Sharpies, Chemical News, vol. 35, p. 89 et seq. EMERALD-GREEN. 167 Berzelius 1 Process. Scheele's Process. L 2. 3. Copper oxide, Cu 0, . Arsenious oxide, As2 0 3 , Sulphur trioxide, S O3, Water, H 2 0, . Carbonic acid, C O2, 66-02 8*32 10-33 15 26 50-76 40-82 1-63 6-41 49-25 42-66 0-42 671 99 93 99 62 99 04 Sample 2 was made according to the original directions ; but sample 3 was washed until the wash-waters were free from arsenic. Scheele's green is of a pale yellowish-green colour, but not very bright ; it is quite insoluble in water, but soluble in dilute acids, in dilute solutions of the caustic alkalies, and in ammonia with a blue colour; when boiled with solutions either of the caustic alkalies or of their carbonates it is decomposed, black oxide of copper being deposited ; boiling with ammonia does not decompose it. When heated it decomposes, a residue of black oxide of copper being left behind and the arsenic being volatilised. As a pigment it is not satisfactory ; its covering power is small, although it can be used either for oil- or water-colours ; it is not permanent and fades on exposure to light and air ; in this respect it is rather better than any of the other copper-greens previously described. As a pigment it has gone out of use. EMERALD-GREEN. Emerald-green was discovered in 1814, but by whom has not been recorded ; from the place at which it was supposed to have been first made it is also known as " Schweinfurth green " ; while in America it is largely known as " Paris green," under which name it is mostly consumed as an insecticide on fruit farms. Owing to the brilliancy of its tint, the ease with which it works, and its comparative permanence, it has been extensively used as a pigment. In composition it is an aceto-arsenite of copper, and may be regarded as a compound of verdigris and Scheele's green. PREPARATION OF EMERALD- GREEN. — 1. 125 lbs. of copper sulphate are dissolved in boiling water ; 50 lbs. of white arsenic (arsenious oxide) are boiled with a solution of 130 lbs. of 168 GREEN PIGMENTS. soda crystals until the arsenic is dissolved ; this solution while still hot is poured into the copper solution, when a precipitate of copper arsenite will be obtained, and a little carbonate of copper also thrown down; sufficient acetic acid is now added to neutralise all the carbonate and leave a little in excess ; the mixture is now allowed to stand for some time for the emerald-green to fully develop ; in summer this may take from a week to ten days, in winter it will take about three or four weeks ; when formed the green is filtered off, washed, and dried. 2. 8 lbs. of white arsenic is thoroughly mixed with water and then 8 lbs. of verdigris is stirred in ; on standing for some time in a warm place emerald-green begins to form ; when fully developed it is filtered, washed, and dried. 3. 50 lbs. of copper sulphate are dissolved in water, and to the solution is added 10 lbs. of lime dissolved in 20 gallons of vinegar ; to the mixture is added 50 lbs. of white arsenic previously mixed into a paste with water ; the mass is allowed to stand in a warm place until the emerald-green has formed, when it is finished as above. 4. Galloway's Process. — In the course of an article on emerald- green in the Journal of Science, a few years ago, Prof. Galloway described a process for the preparation of emerald-green on rather more scientific lines than either of the above processes, and which gives very good results. This process is carried out in the following manner: — A quantity (100 lbs.) of copper sulphate is dissolved in water, and sufficient sodium carbonate (28f lbs. of soda crystals or 12 \ lbs. of crystal carbonate) is added to pre- cipitate one-fourth of the copper sulphate used in the form of copper carbonate ; then acetic acid is added in sufficient quantity to dissolve this copper carbonate. There is thus obtained a solution containing copper acetate and copper sulphate in about the proportions 3 Cu S 0 4 + Cu 2 C 2 H 3 0 2 . The copper sulphate has now to be converted into copper arsenite ; to do this the requisite amount of arsenic (60 lbs.) is dissolved by boiling in sodium carbonate (38 lbs. of crystal carbonate or 87 1 lbs. of soda crystals), which is rather less than is required to completely precipitate the copper sulphate in the first solution ; the two solutions are heated to the boil and then the arsenic solution is run into the copper solution ; the green is formed immediately and only requires filtering, washing, and drying, for use as a pig- ment. The quantities given above have been added by the author, and are not given in the original instructions. When carefully carried out this process gives excellent results. The fineness of the pigment can be regulated by altering the EMERALD-GREEN. 169 strength of the solutions used ; the weaker these are the finer is the precipitate and the more beautiful is the tint of the green produced. If during the precipitation of the green any tendency to form the yellow-green arsenite be noticed, the addition of the arsenic solution is stopped, and the mixture is boiled until all the yellow-green arsenite is converted into the blue-green emerald- green. 5. M. Camille Koechlin, in 1886, described in the Bulletin of the Industrial Society of Mulhouse, a process for the preparation of emerald-green. 100 grammes of copper sulphate are dissolved in 500 cc. of water ; to this is added 187^ cc. of a solution of arsenite of soda; this solution contains 500 grammes of the salt in 1 litre of water. A precipitate of arsenite of copper is obtained, and is treated for one hour at from 104° to 122° F. with either 62 cc. of acetic acid of 11° to 12° Tw. or 31 cc. of pure formic acid; in either case a fine emerald-green is obtained. By using only half the quantity of formic acid a fine blue is obtained, a result which is not got with acetic acid. 6. Liebig's Process. — 1 part of verdigris is dissolved by heat in acetic acid, then 1 part of arsenious acid, mixed with water, is added, and a yellow-green precipitate is obtained. The mixture is boiled for some time, and the green gradually forms ; if necessary, a little acetic acid should be added from time to time to ensure that all the arsenite is converted into the aceto-arsenite ; too great an excess of acid, however, should be avoided, as it would decrease the yield of emerald-green. As soon as the green is fully developed it is filtered off, washed, and dried. The drying of emerald-green must be done at as low a tem- perature as possible, as heat causes the tint to deteriorate. Emerald-green is by no means a difficult colour to make ; the first process described takes some time, but the last three are quick processes and give good results. COMPOSITION AND PROPERTIES OF EMERALD- GREEN. — Emerald-green is an aceto-arsenite of copper of somewhat variable composition, according to the process by which it has been made. The following analysis of a sample of English-made emerald-green will serve to show the average composition of this pigment : — Copper oxide, Cu O, . 32*55 per cent. Arsenious oxide, As 2 0 3 , . 57-51 Acetic anhydride, O 2 C 2 H s O2, 6-63 Sulphur trioxide, S 0 3 , 1-67 „ 0-90 99-26 170 GREEN PIGMENTS. Leaving out of consideration the impurities, the formula for emerald-green deducible from the above analysis is — 7Cu2C 2 H 3 0 2 , 3CuAs 2 0 4 . Emerald-green is a bluish-green of a very fine tint, quite different from any other known pigment, and very difficult to imitate ; it is very opaque, and hence has good covering power ; it works well in both oil and water, but best in the latter ; kept in a dry place it is fairly permanent, and resists exposure to light and air, but in a damp place it turns brownish. It is soluble in acids to blue solutions ; in ammonia it also dissolves with the characteristic copper-ammonia colour ; in solutions of caustic soda and potash it is also soluble ; on boiling, a red precipitate of cupreous oxide falls down, a characteristic reaction of emerald-green. Emerald-green cannot be mixed with pigments — such as cadmium-yellow, ultramarine, &c. — which contain sulphur, as this causes its discolouration, owing to the formation of black copper sulphide. With other pigments it can be mixed without any alteration. The use of emerald-green has been on the decrease of late years, partly owing to its poisonous character, due to its containing arsenic, although one authority states that there is no foundation for the statement that emerald-green is poisonous, and says that it has no poisonous properties whatever. The accounts of the poisonous action of emerald-green are very conflicting; some persons are much affected by emerald-green; even going into a room covered with paper printed with this pigment is sufficient to produce poisonous symptoms in them, while others are not affected at all ; arsenic seems to be very peculiar in its toxic action, and much depends upon the physiological idiosyncrasies of the person. ASSAY AND ANALYSIS OF EMERALD- GREEN.— The colour or tint, body, and colouring power of emerald-green should be assayed for in the usual way. To test for the purity or otherwise of a sample of emerald- green the following tests can be applied: — In hydrochloric acid it should completely dissolve with a yellow-green colour, and on diluting with water this colour should turn bluish. It dissolves in ammonia with a deep blue colour. In caustic soda it dissolves to a pale blue solution, from which, on boiling, a red precipitate of cupreous oxide falls down. The solution in hydrochloric acid should not give more than a faint precipitate with barium chloride, showing the absence of sulphates ; through the solution a current of sulphuretted hydrogen should be passed for some MINERAL GREEN. 171 time and the precipitate of copper and arsenic sulphides obtained filtered off ; the filtrate after boiling, to free it from excess of sulphuretted hydrogen, should give no further precipitate on the successive addition of ammonia, amonium sulphate, and am- monium oxalate, showing the absence of metals, such as iron, zinc, and calcium. The presence of arsenic in emerald-green or other pigments is best detected by Marsh's test. This is carried out as follows : — Provide a wide-mouthed bottle and fit it with a tight-fitting cork through which a piece of glass tube drawn out to a point is passed. Into the bottle, water, zinc, and sulphuric acid are placed. It is necessary that the two latter bodies be free from arsenic, as the ordinary commercial articles are very liable to contain arsenic which would interfere with the proper testing of any pigment for arsenic. By the action of the acid on the zinc hydrogen is evolved ; this may be lighted as it issues from the glass jet, and will burn with a non-luminous flame. On pressing a piece of white porcelain down on the flame no brownish-black spot should be produced. The gas must not be lighted im- mediately it begins to issue from the jet, but a few minutes should be allowed to elapse before doing so, to allow the air in the bottle to be completely driven out ; otherwise an explosion may ensue. If, after it has been proved that the gas flame produces no spot on a porcelain plate, the sample to be tested for arsenic be introduced into the bottle and the gas re-lighted, it will now be found to burn with a faintly luminous flame and will give a blackish-brown metallic looking spot on a piece of white porcelain pressed down on the flame ; this stain is soluble in a solution of bleaching powder. Very small traces of arsenic can be detected by this test. Another test for arsenic is Reinsch's, which consists in heating the sample with hydrochloric acid and a clean copper plate ; if arsenic is present the latter becomes covered with a grey deposit. MINERAL QUEEN. Under this name and that of Mountain green is offered for use as a pigment the natural green mineral known as Malachite. There is also an artificial green pigment made under the name of mineral green. Mineral green, Mountain green, Malachite is a natural basic carbonate of copper found in many places — Cornwall, Siberia, Persia, Australia, &c. The ordinary commercial product comes from Siberia where it is found in the greatest abundance and in 172 GREEN PIGMENTS. large masses of a very fine colour. For use as a pigment the natural mineral is simply ground as finely as possible. The mineral exists in two forms — in the one it forms compact masses of a fine yellowish-green tint ; in the other form it is rather paler and more porous or powdery in character ; the former is the most valuable as a pigment. In composition it is a basic carbonate, containing on the average Copper oxide, Cu 0, . . 71 '9 per cent. Carbonic acid j C O2, . . . 20 0 ,, Water, H 2 0, . . . . 8*1 lOO'O and having the formula Cu C 0 3 . Cu H 2 0 2 . It makes a good pigment, is fairly permanent, and works well both in oil and water. Its faults are those common to all copper pigments. It dissolves in acids with effervescence and evolution of car- bonic acid, and the solution gives all the characteristic tests for copper. Mineral green has been made artificially by several processes, of which the following are two examples : — (a) For preparing mineral green by this method the following materials are required :— 2 cwts. of soda crystals, 1J cwts. of blue stone (copper sulphate), 70 lbs. of quicklime, 12| lbs. of white arsenic, and 4 ozs. of tartaric acid. Boil the arsenic and soda together until the former is dissolved ; dissolve the copper in water • and slake the lime in water. Add the lime to the copper solution ; then the arsenic and soda ; and, finally, the tartaric acid. Keep the whole at a temperature of about 150° to 160° F. for some time until the colour is properly developed, then wash the pigment with clean water, filter and dry at a low temperature. (b) 14 ozs. of potash and 14 drms. of arsenic are dissolved in water ; 1 lb. of sulphate of copper is dissolved separately in 2 gallons of water, and into this solution is poured the arsenic and potash solution ; the green is precipitated and is collected and finished in the usual way. No process of making mineral green artificially produces it of the same deep green tint as the natural variety ; such prepared greens have a pale yellowish-green tint and are not so permanent as the natural variety. In their general features they resemble the verditer greens, but are rather brighter in tint and deeper in colour. GREEN VERDITER. — This pigment is a basic carbonate TERRE VERTE. 173 of copper, prepared by precipitating solutions of copper with the carbonates of potash or soda in the same way as blue verditer is made. It has a pale tint of a somewhat yellow tone of green. As a pigment it has no great use, and has become nearly obsolete ; it is not permanent, either as an oil or a water colour. Green verditer has also been known as "British verdigris," and its preparation under this name has been patented. BREMEN GREEN. — This is of a pale green tint, prepared in the same way as Bremen blue, except that the final blueing is omitted. Like green verditer it is essentially a basic car- bonate of copper. Its use as a pigment has become obsolete. PROPERTIES OF THE COPPER GREENS. — With one or two exceptions, the copper greens are by no means satis- factory pigments ; their colour is but pale and not brilliant, except emerald-green and the natural mineral green. Their covering power is also deficient; they mix with either oil or water. They are not permanent, as exposure to light and air causes them to fade, while sulphuretted hydrogen and sulphur compounds cause them to go black, owing to the formation of the black sulphide of copper ; hence they cannot be mixed with any pigments containing sulphur — such as cadmium yellow, King's yellow, ultramarine, &c. Strongly alkaline bodies, such as lime, change their colour to a blue. Heat decomposes all the copper greens, the acid portion — carbonic, acetic or arsenious — being volatilised, and a black residue of oxide of copper being left behind. They are all soluble in acids, some with effervescence, indicat- ing the presence of a carbonate; the solutions have a blue colour, to which ammonia imparts a characteristic deep blue tint ; on adding caustic soda a blue precipitate is obtained, which, on boiling, turns black. Sulphuretted hydrogen gas passed through the solutions throws down a black precipitate of the sulphide, which is soluble in nitric acid to a blue solution. TERRE VERTE. Terre verte is the name given to green pigments of an earthy character, found naturally in various places ; in some cases, the pigment has been named after the place where it was found, as " Verona green," " Verona earth," &c. These natural greens are usually of a pale greyish-green tint, and are only useful on account of their permanence. Deposits of green earth are found in many places, but only the deep bright samples are usable as pigments. The places where the best qualities of terre verte are found are 174 GREEN PIGMENTS. the Mendip Hills, as also many localities in France, Italy, and Cyprus. Terre verte is found in masses of a more or less compact character; some varieties are soft and easily powdered, others v are harder and more vitreous in appearance. For use as a pigment, the mineral is ground up as fine as possible; sometimes it is levigated. Although varying somewhat, as might be expected in earthy pigments from various sources, yet there is a certain amount of resemblance between different samples of terre verte, as shown by the few analyses of this pigment available. Berthier gives the following analysis of terre verte, the source of which is not stated : — Silica, Si 0 2 , Ferrous oxide, Fe 0, Soda, Na 2 0, Alumina, Al 2 0 3 , Magnesia, MgO, Water, H 2 0, . Manganese, Mn0 2 , 51*21 per cent. 20-72 6- 21 7- 25 „ 6*16 4-49 „ trace. 96-04 This is not a very satisfactory analysis ; of what does the 4 per cent, of unaccounted for material consist ? A sample of terre verte from Cyprus, analysed by Klaproth, gave the following figures : — Silica, Si 0 2 , ..... 51 *5 per cent. Ferrous oxide, Fe 0, Potash, K 2 O, . Magnesia, MgO, Water, H 2 0, 51*5 20-5 18-0 1-5 8-0 99'5 A sample of terre verte from the neighbourhood of Rome was examined by the author, and found to have the following composition : — Water, hygroscopic, H 2 0, Water, combined, H 2 0, Ferrous oxide, Fe O, Alumina, Al 2 0 3 , Manganese, Mn 0 2 , Calcium oxide, Ca 0, Silica, Si 0 2 , Magnesium oxide, Mg 0, 1*450 per cent. 3-650 26-870 3-165 trace. 2-065 52-120 10665 99 985 COBALT GREEN. 175 This sample was very hard, and had a conchoidal fracture, a waxy lustre, and a soapy feel. Acids had but slight action on it. It was evidently a specimen of the mineral bronzite, which is essentially a ferrous magnesium silicate. Terre verte is of a pale bluish-grey tint, and has no great colouring power or body, being somewhat transparent; it mixes well with either oil or water, and is perfectly permanent, being unaffected by any length of exposure to light and air ; it is not altered by sulphur or sulphureous gases in any way. As a pigment terre verte has been used from very early times, being one of the best greens available to the early painters. Heat turns the colour of terre verte to a reddish-brown, the change being similar in nature to that which takes place when the ochres are heated. Sometimes greens having a copper base are offered as terre vertes; these are not permanent. COBALT GREEN. Cobalt green, Rinman's green, Zinc green is a compound of the oxides of zinc and cobalt, having an analogous composition to cobalt blue and being prepared in a similar manner. Preparation of Cobalt Green. — Cobalt green can be prepared in several ways. (a) Sulphate of cobalt in solution is mixed with zinc oxide into a paste ; this is then dried and exposed to a red heat in a muffle furnace until the desired green tint has been developed, which will take from three to four hours. The tint of colour will depend upon the proportions of the cobalt salt and zinc oxide used ; 1 lb. of cobalt sulphate to 5 lbs. of zinc oxide will give a deep green ; with twice as much zinc oxide a grass green is obtained ; while if the proportions are 1 lb. of cobalt sulphate to 20 lbs. of zinc oxide a fine bluish-green is obtained, which forms a fair substitute for emerald-green. (b) Instead of the sulphate, the nitrate of cobalt may be used. 1 lb. of cobalt nitrate is mixed with 2 to 5 lbs. of zinc oxide according to the depth of colour required ; the mixture is kept at a bright red heat in a muffle furnace for a few hours until the green has been fully developed ; it is then ground with water, and dried. The cobalt salt must be free from metallic impurities, such as iron, alumina, or tin. The principal difficulty in these two processes is that of ensuring a thorough mixture of the cobalt and zinc compounds ; if this is not properly done the green which is formed will not be 176 GREEN PIGMENTS. of a uniform tint throughout the mass ; there will be dark and light places. The following processes avoid this difficulty by mixing solutions of the two metallic salts, thereby ensuring perfect admixture, then precipitating the oxides from the solution and finishing as in the above processes : — (c) 1 lb. of nitrate of cobalt or J lb. of the chloride of cobalt and 6 lbs. of sulphate of zinc are dissolved in 7 gallons of water ; a solution of carbonate of soda is then added as long as a precipitate falls ; the mixture filtered, and the precipitate of hydroxides of cobalt and zinc so obtained washed, dried, and heated as before. By varying the proportions between the zinc and cobalt salts the depth of colour of the resulting green can be varied to a great extent. (d) Instead of using the carbonate of soda to precipitate the solution of zinc and cobalt, there may be used either the phosphate or the arseniate of soda. Wagner states that the resulting green is purer, brighter, and less dense. The greens made in this way will contain phosphoric acid, which will give the greens a rather bluer tint. Cobalt green has a bright green colour of a slightly yellow hue. It is perfectly permanent when exposed to light and air, and is on that account a useful pigment. It can be mixed with all other pigments without being affected by them or altering them in any way. It is unacted upon by acids in the dilute state ; but strong acids decompose it, forming a blue solution. Alkalies have no action on it. Wagner gives the following analyses of cobalt green made by various processes : — Zinc oxide, Zn 0, Cobalt oxide, Co 0, Phosphoric oxide, P 2 1 Ferric oxide, Fe 2 O3, Soda, Na 2 0, 1. 2. 3. Per cent. Per cent. Per cent. 88-040 71*93 71 '68 11-622 19-15 18-93 8-22 8-29 0-298 100-000 99 99 98-90 Owing to its cost cobalt green is not much used. Should it be possible to find a cheap source of cobalt this green might come into more general use, as its permanent qualities would give it superiority over many of the other greens. Besides the greens described above many others have been used on a small scale ; some are still used for special purposes, and others have been described by various chemists, but whether they CHINESE GREEN OR LOKAO. 177 have ever been used on a practical scale is doubtful ; these will be briefly described. BRIGHTON GREEN is the name given to a pigment made by grinding together in the dry condition 7 lbs. of copper sulphate, 3 lbs. of acetate of lead, and 24 lbs. of whiting ; during the grinding chemical decomposition sets in, resulting in the formation of a basic acetate of lead. It was a pigment of no great depth of colour or permanency. DOUGLAS GREEN. — Mr. Thomas Douglas has described in the Chemical News, vol. xl., p. 59, a green prepared from barium chromate. The latter compound, prepared in the usual way by mixing solutions of barium chloride and potassium chromate, is mixed with 20 per cent, of its weight of strong sulphuric acid, which partially decomposes it, forming a mixture of barium chromate, chromic acid, and barium sulphate ; the mixture is dried and then calcined at a bright red heat in a crucible ; the chromic acid is thereby decomposed into the green oxide of chromium, which, being disseminated throughout the mass of barium sulphate and chromate, colours it green, forming a pigment possessing considerable body and permanency. Nothing definite is known as to its having been used as a pigment. CHINESE GREEN or LOKAO. — This is a green pigment, as yet but little used, made from the juice of various Chinese species of buckthorn trees by extracting the juice from the berries by pressure, mixing this with alum, &c, and drying. It comes into commerce in the form of bluish-green slabs, which are easy to break, but somewhat difficult to powder. Chinese green con- tains from 27 to 47 per cent, of mineral matter, principally lime and alumina, and, probably, consists of the lake formed by the combination of those bases with the colouring principle of the juice from the buckthorn berries, named by Kayser lokaonic acid, C 42 H 48 0 27 . According to the same authority, the colouring principle consists of a glucose, which he calls lokaose, to which he assigns the formula C 6 H 12 0 6 , and lokanic acid, a body having the composition C 33 H 36 0 21 . The colouring principle has also received the name lokain and the formula C 28 H 34 0 1T . SAP GREEN. — This pigment is prepared from buckthorn berries. Two methods are adopted in its preparation. In one the berries are allowed to ferment slightly by placing them in a warm place for a few days ; they are then pressed, the juice collected, and alum added in the proportion of from ± an ounce to 1 ounce per pound ; the mixture is then boiled down and evaporated to dryness at the boiling heat. Another plan is to 12 178 GREEN PIGMENTS. boil the berries in water for two or three hours with constant stirring ; the liquors are then strained through cloths in order to separate the woody and other insoluble particles ; the clear liquor is boiled down to a syrup, 5 ozs. of alum per gallon added to the syrup, and the mixture carefully evaporated to dryness. For some purposes the mass is left in the pulpy condition. Sap green is a dark yellowish-green pigment; when dry it breaks with a glossy fracture ; it is very transparent, and hence is not used as a body colour, but chiefly as a glazing colour ; another use for it is in colouring confectionery and beverages. It works well as a water-colour, but not as an oil-colour ; and fades on exposure to light. An analysis of sap green made by the author shows it to have the following composition : — Water, ...... 12*95 per cent. Mineral constituents, . . . . 10 '69 ,, Organic constituents, .... 76*36 ,, 100-00 Of the organic constituents a quantity equal to about 29*34 per cent, of the original colour are soluble in alcohol. Its com- position and general properties somewhat resemble those of a lake. MANGANESE GREEN. — This pigment was patented in 1864 by Schad, who prepared it by the following process : — 14 parts of oxide of manganese, 80 parts of nitrate of barium, and 6 parts of sulphate of barium, are intimately mixed together. The mixture is heated in a crucible in a suitable furnace to a bright red heat until it has assumed a green colour ; it is then ground in a mill with water to a fine powder ; a small quantity of gum arabic, dextrine, or similar substance, amounting to about 5 per cent, of the original material, is added, and the mass is dried at from 190° to 212° F. ; or it may be used in the form of a paste. Instead of the above mixture there may be used one of 24 parts of nitrate of manganese, 46 parts of nitrate of barium, and 30 parts of sulphate of barium. The addition of the gum or dextrine is said to be essential for its stability, a factor which cannot but have an adverse influence on its value as a pigment, for which purpose it has probably not been used. It consists principally of manganate of barium. TITANIUM GBEEN. — This pigment is the ferrocyanide of titanium, prepared by mixing solutions of potassium ferrocyanide and of a titanium salt ; the pigment must be dried at a low temperature, as decomposition sets in above 100° C. It has a pale ZINC GREEN. 179 green colour, and was proposed as a substitute for the arsenical greens • owing to its cost it has never come into use. ZINC GBJSEN. — Under this name there is frequently sold greens made in a similar way to the Brunswick greens, by mixing together zinc chrome, Prussian blue, and barytes ; such greens possess the advantage that they are not affected by sulphur as much as the Brunswick greens. They are best made in the dry way (see p. 151). The green lakes and the pigments made from coal-tar colouring- matters will be found described in the chapter on Lakes. 180 CHAPTER VI. BLUE PIGMENTS. Blue, as a colour, enters very largely into the decoration of objects, both alone and in combination with other colours, to form a large and very useful series of tints and shades. Although so important as a colour, yet there are few blue pigments ; but these possess the merit of being more permanent, and, therefore, more useful than any other group of pigments. The list of blue pigments includes ultramarine (a curious compound of silica, alumina, and soda, which was at one time obtained exclusively from natural sources, but is now mostly prepared artificially), and Prussian blue with its varieties (a most valuable blue, whose base is iron), which are the most predominant blue pigments used. Cobalt is the base of cobalt blue and smalts, while copper forms the basis of several unimportant pigments. ULTRAMARINE. Ultramarine is one of the most important pigments possessed by the painter ; being used in painting, in printing of all kinds (letterpress, wall-papers, calico), and in bleaching ; it is un- doubtedly the best blue for the laundry, and in soapmaking it is used to produce the blue mottled soap. Ultramarine has been known for centuries, but its extended use has only been possible during the last half century. Prior to about 1820 the natural supplies were small, and the processes so expensive that it could only be used by artists who did not find the cost prohibitive ; but about the year named, discoveries were made by several chemists, which resulted in ultramarine being made artificially at such a cheap rate that it is the cheapest blue pigment known ; consequently, its consumption is now measured by tons. Natural Ultramarine. — The source of natural ultramarine is a blue mineral, lapis lazuli, found in small quantities in Persia, China, Siberia, and a few other places. This mineral is found in streaks and small patches distributed through an earthy matrix ULTRAMARINE. 181 or gangue, from which it has to be separated by mechanical means. The production of natural ultramarine has declined very much during the last fifty years, it having been displaced by artificial ultramarine ; but the mineral is still sought for in fair quantities, for use in the production of inlaid ornamental work, as the peculiar blue colour of the mineral cannot be obtained by other means. The process of extraction of the pigment from the mineral consists in grinding the mineral to a fine powder, after separating as much of the gangue as possible ; it is then mixed with a com- pound of resin, wax, and linseed oil, and the mixture put into cloths and kneaded under hot water ; the colour comes through the cloth into the water, several waters being used ; after the working, the waters are placed on one side for the colour to settle. The blue thus obtained varies in shade in the different waters ; that which settles out of the first water is the deepest in colour, and the brightest, and is sold as ultramarine ; that which comes from the last waters has a blue-grey colour, and is sold as ultramarine ash. After the colour has settled, it is usual to grind it still finer, so that the beauty of the pigment shall be developed as much as possible. No better process for extracting ultramarine has been devised, although it is so tedious and gives such poor results. Yery little is now so produced, as the natural variety has been almost replaced in European and other countries by the artificial variety. The chemical composition and constitution of ultramarine early became the subject of research by chemists, which researches were partly undertaken with a view to its artificial production ; for it was recognised that, from the beauty of its colour and its permanent qualities, ultramarine would, if it could be produced cheap enough, have a wide field of use. Several analyses were made by different chemists, but these vary very much, owing, as is probable, to the difficulty of obtaining the pigment quite free from its matrix. Those by Clement and Desormes and by Gmelin, which are, perhaps, the most typical, are here given. Clement and Desormes. Gmelin. Silica, Si 0 2 , .... 35 -8 47*306 Alumina, Al 2 0 3 , . . . 34 '8 22*000 Soda, Na 2 0, . . . . 23*2 12*063 Sulphur, S, 3-1 0-188 Calcium carbonate, Ca C 0 3 , . 3*1 Calcium oxide, Ca O, . . ... 1 *546 Sulphuric acid, H 2 S 0 4 , 4*679 Water and loss, 12-218 100*0 100*000 182 BLUE PIGMENTS. It is evident that with such discrepancies in the analyses nothing could be satisfactorily inferred as to the chemical com- position and constitution of ultramarine, and it is no wonder that none of them led to its artificial production. Artificial Ultramarine. — Early in the present century, soon after soda began to be produced on the large scale from salt by the Leblanc process, many persons noticed the formation of a substance resembling ultramarine in colour; Tessart and Kuhlmann recorded, in 1814, that they had seen this blue colour in a soda furnace. Yauquelin, on examining it, found it to be a compound of silica, alumina, lime, soda, and sulphur, and showed that it had a similar composition to ultramarine. It was recorded that it was formed only when sandstone was used in the construction of the furnace; when bricks were used it was not formed. Guimet, an eminent French manufacturing chemist, studied the production of ultramarine. In 1828 he succeeded in making it on a large scale, and obtained a prize of 6,000 francs, offered by the Societe d'Encouragement of France to any one who made ultramarine in a wholesale way. Guiniet's process is still used by his successors, but has not been published. Gmelin also interested himself in the production of ultra- marine, and in 1828 he published an elaborate description of his method of making it. Kottig, the director of the Miessen Porcelain Works also, about the same time, observed the production of ultramarine in his furnaces, and, as the result of his researches, succeeded in making the pigment on a large scale ; the Miessen ultramarine was for many years one of the leading brands; the works are now closed. About 1834 Dr. Leverkus, working by Gmelin's process, started its manufacture in Germany at works which are still in existence. The great bulk of the ultramarine used is made in Germany ; there are two or three works in England, a few in France, and one in America. Several writers have given descriptions, more or less complete, of the process of making ultramarine ; but the best and most complete description is that by J. G. Gentele,* and more recently one by Rawlins, t Varieties of Ultramarine. — There are two principal varieties of artificial ultramarine — 1st, sulphate ultramarine, which is of * Technologist, vol. xviii., pp. 389-411. fJourn. Soc. Chem. hid., 1887, p. 791. VARIETIES OF ULTRAMARINE. 183 a pale greenish-blue colour ; 2nd, soda ultramarine, which has a violet-blue colour. Of the latter there are two varieties — one contains more silica than the other, and is mostly used by paper-makers, owing to its resisting the action of acids and alum better, while the variety poor in silica is used for all other purposes. The materials used are nearly the same for both kinds, and comprise kaolin or china clay, sodium sulphate (Na 2 S 0 4 ), sodium carbonate (Na 2 C 0 3 ), sulphur, coal or charcoal, rosin, quartz, and infusorial earth. All these are not used in the same operation; some makers using one kind of mixture, others another. The quality of the materials is a matter of very great importance. The kaolin or china clay should be as free as possible from any earthy matrix; a trace of lime has no injurious influence, but the clay must be free from iron, which has a tendency to dull the colour of the ultramarine. It has been found from experience that every sample of china clay does not give equally good results, although all may be pure and of good quality. It has been found that the relative proportions in which the silica, Si 0 2 , are combined with the alumina, Al 2 0 3 , is a matter of some importance, and in china clays from different localities there is wide differences in this respect ; then, again, a china clay which will work well for sulphate ultramarine will not do for soda ultramarine. For making sulphate ultramarine the china clay should contain the silica and alumina in the proportion of 2 silica to 1 alumina, 2 Si 0 2 , Al 2 0 3 ; if the proportions much exceed these the shade will be poor, while if they reach those indicated by the formula, 3 Si 0 2 , Al 2 0 3 , the clay will not make sulphate ultramarine. On the other hand, while almost all clays will make soda ultramarine, yet the best results are obtained with clays containing from 2^ to 3 parts of silica to 1 part of alumina ; the larger the proportion of silica, the redder the shade of the ultramarine made from it, and the more resisting power it has to the action of acids and alum. The china clay is prepared for use by a process of grinding and levigating, so as to obtain it in as fine a form and as free from impurity as possible. The sodium salts are used in the anhydrous state ; both should be as pure as possible, especially should they be free from iron, which has a most deleterious influence upon the shade of the ultra- marine made from the salts. Although other impurities are of small moment, still, where first-class ultramarine is required, it is best to purify the commercial products. 184 BLUE PIGMENTS. The sulphur is the ordinary roll sulphur or brimstone. The coal and charcoal are the ordinary commercial varieties ; but the coal used must be free from pyrites. Both articles are ground before using. The quartz should be as free from impurities as possible ; the better the quality of the quartz, the better the quality of the ultramarine made from it. The infusorial earth or kieselguhr is the well-known com- mercial article. The rosin used is the best commercial variety obtainable. The manner in which these are mixed together depends upon the variety of ultramarine to be made, and it also varies in different works, each of which has its own formula, although there is not much variation in the essential points. Gentele lays down the following rules : — 1st, That the soda used be sufficient to neutralise half the silica present in the kaolin or clay and silica used ; 2nd, that the proportions of soda and sulphur le such as to produce a polysulphide of soda. ULTRAMARINE MANUFACTURE. — There are two pro- cesses in use for the manufacture of ultramarine; the oldest, called the indirect process, is used for making both sulphide and soda ultramarines, and is the only process by which the former can be made. Indirect Process of Making Ultramarine. — This consists of the two stages or operations, viz. : — (a) The calcining operation. (b) The colouring operation. (a) Calcining Operation — Manufacture of Ultramarine Green. — A mixture of the ingredients named above is made ; if sulphate ultramarine is required, sulphate of soda is used ; if soda ultramarine is to be made, then soda carbonate is used. Some works use a mixture of the two soda salts. The various ingredients are ground together with water into a very fine paste ; the finer the grinding, the better will be the quality of the ultramarine ; after the grinding, the paste is dried. In some works the water is omitted, it being considered unnecessary, while the subsequent drying adds to the expense of making. The following are examples of the mixings used in different works : — For sulphate ultramarine. ULTRAMARINE MANUFACTURE. 185 Kaolin, Sodium sulphate, Coal, . Sodium carbonate, Sulphur, Rosin, . 1. Parts. 100 83 17 Parts. Parts. 100 100 41 17 41 13 220 30 35 When sodium sulphate is used less sulphur is required ; in proportion as the latter is decreased so the proportion of the former must be increased. For soda ultramarine poor in silica — Kaolin, ...... Sodium carbonate, .... Coal, Sulphur, For soda tdtramarine rich in silica — 100 parts. 100 „ 12 „ 60 „ Kaolin, Quartz, Sodium carbonate, Sulphur, . Coal, Rosin, 1. 2. Parts. Parts. 90 10 100 60 12 100 90 100 4 6 The second recipe gives a dark ultramarine ; the more sulphur there is used in making soda ultramarines, the deeper is the shade of the blue produced; on the other hand, by re- ducing the quantity of sul- phur and silica the blue obtained is not so deep, but is rather more brilliant in hue. The mixture is then placed in crucibles, about 6 inches by 4 inches in size, and fitted with lids, which are somewhat saucer- shaped, so that the crucibles can be piled one above an- other in a furnace. Fig. 19 shows the shape of the crucible and its lid. The mixture is packed rather tightly into these; sometimes what are called seggars are used, but the crucible form is better, 186 BLUE PIGMENTS. a firmer pile when placed in the furnace. In some works open, flat, round capsules are used of such a size that they hold when | full, about 9 ozs. of the material, which forms a layer 1^ to If inches thick; these are piled one above the other in a furnace capable of holding about 216 arranged in 9 layers of 24 capsules, each formed of lots of 6 by 4. The furnace in which these pots of material are placed varies in form in different works. Fig. 20 shows one form of ultra- marine furnace. The furnace chamber, B, is an almost exact cube in form ; the back is completely closed in, and the front, C, is open, but is made up with firebricks when the furnace has been filled with the crucibles. The fireplace, A, is under the furnace chamber, the flames and heat in it pass through openings, e e e, in the floor of the chamber; similar openings, ff 9 in the roof, D, of the chamber serve as outlets for the waste heat and gases ULTRAMARINE MANUFACTURE. 187 of the furnace into the flue, EE. A number of these furnaces are built side by side and back to back, forming a range or bench of furnaces, but are not all worked together, for while some are being filled, others are being emptied, and others, again, are being heated. Tn some works a kind of muffle furnace, similar to Fig. 21, is used. After the furnace has been charged with the crucibles, the front is made up with bricks, and the interstices between these filled with a mixture of sand and clay, a small sight-hole being left so that the temperature of the furnace can be observed; if necessary, this sight-hole is stopped with an easily removable plug of clay. The temperature of the furnace is then slowly raised to a bright red heat, at which it is maintained for from 7 to 10 hours, the time varying with the nature of the composition and determin- able only by actual practice. Sulphate ultramarine requires a higher temperature than soda ultramarine ; if a muffle furnace is used, the temperature is often raised to a bright yellow for from 2£ to 3 hours only. When the calcination is considered to be complete the fire is drawn and the furnace allowed to cool ; this must be done as slowly as possible, and care must be taken that no air enters into the furnace during the cooling, because while hot the crude ultra- marine is very susceptible to the action of the oxygen of the air, and the yield as well as shade of the colour would be injured. When cold the crucibles are removed and the furnace is ready for another charge. This first burning of the ultramarine is a most important operation, and great care must be exercised in carrying it out; access of air to the contents of the crucible must be carefully avoided; the temperature should not be too high nor too prolonged, as then the material would be overburnt and will not give a satisfactory blue; on the other hand, under- burning is just as bad, for then the colour will not be homogeneous. A furnace such as that shown can be charged three times per week. The colour of the burnt mass varies somewhat; usually it is of a green colour, mostly of a bluish tone (which is generally indicative of good burning), but sometimes it is of a yellowish- green shade, and at others it passes more into a blue, while, if not properly burnt, it will have a brown shade. The crude green ultramarine, which is somewhat cindery in appearance, is now thrown into water for the purpose of washing out all the soluble soda salts; the last washings of one batch are often used as the first wash-waters of another batch for the purpose of economising the water. While still wet the ultra- 188 BLUE PIGMENTS. marine is ground up in mills into as fine a form as possible, in order to effect the completest practicable extraction of the soluble matter. The ground-up green ultramarine is then dried, when it is ready for the next operation. In this form it is sold under the name of green ultramarine for use as a pigment. The wash-waters contain a large proportion of sodium salts, chiefly in the form of sodium sulphide. In many works it is customary to evaporate the liquors to dryness by means of the waste heat of the furnaces, and to use the dry residue for another mixing. (b) Colouring Operation — Manufacture of Ultramarine Blue. — The green ultramarine obtained in the first stage has now to be converted into the blue, which is done by heating it with sulphur in a furnace at a low temperature. There are three ways of carrying out this colouring operation. (1) On trays, (2) in a cylinder, and (3) in a muffle. 1. Tray Method. — A form of muffle furnace is built in which the muffle is filled with a number of trays or shelves. On these trays the green ultramarine is spread in layers of about an inch thick, and over them is sprinkled some sulphur; the muffle door is closed, the furnace is lighted, and the heat continued until the sulphur takes fire ; then the fires are drawn and the sulphur allowed to burn itself out, after which the crude pigment is taken from the muffle and finished in the manner described further on. 2. Cylinder Method. — It is also known as the German method. Small cast-iron cylindrical vessels are imbedded in brickwork over an ordinary fireplace ; these cylinders are closed at the back end, but open in front, which is fitted with a door made of wrought iron ; in this door are two apertures for the purpose of charging the cylinder with sulphur, while a pipe from the top of the cylinder carries off the gases produced by the burning of the sulphur. An agitator is fitted to the cylinder, by means of which its contents can be kept well mixed during the progress of the operation. From 27 to 34 lbs. of the ground green ultramarine is charged into the furnace, the door closed and the fire lighted. When the temperature is sufficient to ignite sulphur, 1 lb. of sulphur is thrown into the cylinder ; when this has burned away and fumes have ceased to issue from the cylinder, another pound of sulphur is thrown in and allowed to burn ; a small sample is now drawn from the furnace and its colour noted ; if not blue enough, more sulphur is thrown in at intervals until a sample taken out of the cylinder shows that the blue has properly formed ; after the cylinder has cooled down the pigment is scraped into a box, and is ready for the finishing operation. ULTRAMARINE MANUFACTURE. 189 During the whole of this operation the temperature of the fur- nace is kept at the proper heat, viz., that at which sulphur will burn, and the agitator is kept at work. 3. Muffle Method. — It is also known as the French method. In this method the green ultramarine is coloured by heating with sulphur in a muffle furnace. Such a furnace is shown in Fig. 21, which represents a longitudinal section of an ultramarine muffle furnace. The fireplace is shown at A, and is separated from the Fig. 21. — Ultramarine muffle furnace. muffle-chamber by a flued arch. The muffle, B, is made of earthenware, and is completely closed at one end, while the other is fitted with a door, D, so built into the furnace that none of the furnace gases can get into the interior of the muffle ; C is the flue of the furnace ; G- is a kind of hood which collects all the vapours of the burning sulphur, and passes them into the flue, C, and so up the general chimney of the works. The green ultramarine is spread in a layer of about \\ to 2 inches thick on the floor of the muffle, the door closed and the fire lighted ; when the temperature is high enough for sulphur to burn, a shovelful of that substance is thrown into the muffle and stirred with an iron rod ; when the first shovelful has burnt out, more sulphur is added from time to time, until a sample of the colour taken out of the furnace shows that it has acquired the 190 BLUE PIGMENTS. desired blue colour. The muffle is more rapid in working than the cylinder. The blue is raked out of the furnace and is finished in the usual manner. The indirect process, while yielding a very good quality of ultramarine, labours under the disadvantages of making it in small quantities only at a time, and of being attended with a large loss of material in the operations. The Direct Process of Ultramarine-making. — The disad- vantages of the indirect method induced the manufacturers to seek a new method, by which larger quantities could be made at one time, and in which the loss of material would not be so great ; the labours of the chemists who have been engaged on this object were rewarded with success by the discovery of a direct method having the desired advantages. The direct method can, however, only be used for making the soda ultramarine, but as this happens to be the principal variety, the one disadvantage attending the process is practically of no moment. The direct process can be carried out either in (1) muffle furnaces, or (2) in crucibles. 1. Muffle Method. — A mixture of kaolin, sodium carbonate, sodium sulphate, sulphur, sand, rosin, or charcoal is made, the proportions varying in different works, but approximating to those already given. The mixture, reduced to the finest state so as to ensure the most intimate union of the ingredients, is placed in layers of from 2 \ to 3 inches thick, and firmly pressed down on the floor of the muffle ; a charge weighs about 45 lbs. The surface of the charge is covered with fireclay tiles, and the spaces between these luted with mortar ; at the front of the muffle one of the tiles is left loose, so that, when required, it can be raised to admit of samples being withdrawn for testing. The front opening of the muffle is now made up, a small aperture being left for the purpose of observing the temperature of the muffle and for drawing out samples from time to time. The furnace is now heated, at first slowly, towards the last more strongly, so that in about 8 or 9 hours it has attained a dull red heat, at which temperature it is maintained for 24 hours, and then raised to a bright red heat until the end of the operation. A sample is now withdrawn from the furnace through the hole in the door and a corresponding hole in the tiles ; this sample is placed between two tiles as quickly as possible, and a second sample is taken out and placed on the top of the tiles ; when the samples have cooled, the colour of the samples are compared. If the operation is finished, the colour of the second ULTRAMARINE MANUFACTURE. 191 exposed sample will be of the blue colour, while that of the first sample (the covered one) will be of a blue-green colour ; when this is found to be the case, the fires are drawn and the furnace and its contents allowed to cool down, care being taken that no air enters into the furnace ; or, to make quite sure, the heat is usually maintained for another hour. Should the trial samples have a brown colour, the mass has been insufficiently heated, and the temperature of the furnace is raised a little higher. When the furnace is opened the ultramarine is found to be in two layers — an upper one of a bright blue colour, and a lower one of a bluish-green ; these are separated and finished in the usual way, the upper layer forming the best and the lower layer an inferior quality of ultramarine. Although the quality of the blue produced by this method is good, yet the quantity capable of being produced is small ; therefore it is not much used, and the crucible method described below has replaced it to a large extent. 2. Crucible Method, — The method most largely employed for the production of ultramarine is that known as the crucible method, and is carried out as follows : — A mixture is made of Kaolin, 100 parts. Sodium carbonate, . . . . . 90 ,, A variable quantity of infusorial earth (from 20 to 30 parts) are added, according as the ultramarine has to be rich or poor in silica ; in some works 6 parts of rosin are added. All these ingredients are ground together into a homogeneous mass ; this is a point of great importance. The mixture is loosely packed into crucibles fitted with flat lids, which are luted on by means of mortar. When the mortar luting is dry the crucibles are piled in ovens large enough to hold from 400 to 500 crucibles, and, in shape, not unlike that described for the first stage of the indirect process. After all doors and openings into the oven arc made up it is fired to a bright red heat for several hours, the length of time varying considerably and depending upon a number of factors, such as the state of the weather, the com- position of the mixture, &c. Experience is the only school in which an ultramarine-maker can learn how to regulate the time required. After the heating, all apertures are carefully closed, so as to exclude air, and the furnace allowed to cool for four or five days; the oven is then opened, the crucibles withdrawn and opened, Sulphur, Charcoal, 110 20 192 BLUE PIGMENTS. the contents turned out, and the badly-burnt pieces carefully separated \ the good portions are ready to be finished. The changes which go on during the heating of the mixture are both curious and interesting. The mixture when first put into the crucibles is of a greyish colour, but during the process of burning it passes through quite a series of colour-changes — brown, green, blue, violet, red, and white. The brown appears with the blue flames, due to the burning of the sulphur ; it is a fine chocolate brown, but is very unstable ; on exposure to the air it enters into combustion. Many efforts have been made to preserve it, but these have been fruitless. The green, which is the next change, begins to form when the sulphur has ceased to burn ; like the brown it is unstable, as the substance burns on exposure to the air. Following the green comes the blue, which is formed when the temperature has reached about 700° C, or a bright red heat ; when the temperature gets higher the colour changes to a violet. With still higher temperatures, first a red, then a white variety is formed. These changes are due to oxidation ; when the white ultramarine is heated with reducing agents, such as carbon, the colours are re-formed in the reverse order to that in which they first appeared. The form of furnace to be used in burning the ultramarine is not a matter of importance, the operation can be effected in a reverberatory furnace, in a muffle furnace, in earthenware pots, in ovens, or in any convenient apparatus. Finishing Ultramarine. — By whatever process the pigment is prepared it comes from the furnaces in the form of a gritty, somewhat cindery-looking, blue mass containing a large quantity of soluble sodium salts, and in this condition is unserviceable for use as a pigment. To fit it for this purpose the crude ultra- marine has to undergo a finishing process, which has for its object to purify the colour and to develop the hidden beauty of the pigment. The process of finishing is essentially one of washing and levigation. The crude ultramarine is thrown into grinding mills where it is ground with water, this grinding being done as thoroughly as possible, as on it depends to a very large extent the excellence of the pigment as regards colouring power and fineness. After the grinding, the wet ultramarine is run into large tubs, where it is treated with hot water, or even boiled with water, so as to make sure that all the soluble contents of the crude ultramarine are dissolved out. The ultramarine is now allowed to settle and the liquor run off ; this contains sodium sulphate which may be recovered by evaporation and used in making new batches of ultramarine. Then clean water is again ULTRAMARINE MANUFACTURE. 193 run on to the pigment, which, after being thoroughly stirred up, is again allowed to settle and the water again poured off ; this washing is repeated several times. The wet ultramarine is now ground in grinding mills specially constructed for grinding wet materials very finely ; such mills will be found described in another Chapter. This grinding is import- ant and takes several hours; the length of time depends upon the use to which the ultramarine is to be put. The finer qualities, which are used in calico-printing and letterpress and lithographic printing, and must be very fine, require the longer grinding; they are sold under the name of calico-printers' ultramarine ; painters do not require so fine a quality, and for this the wet ultramarine is not subjected to lengthy grinding. Another method of separating the different qualities of ultramarine is by levigation, which forms an essential part of the process. The wet ultramarine as it comes from the grinding mills is run into large tubs of water, in which it is thoroughly stirred and then allowed to settle for two hours ; this allows the coarser particles to subside, while the finer particles still remain in suspension, and are run into other tubs, where they are allowed to settle. The coarse particles in the first tub are run into the mills again to be re-ground with another batch of crude ultra- marine. The particles which settle in the second tubs are collected, dried at a gentle heat, and sent into the market. In the water of the second tub there still remains some fine ultramarine ; this is run into a third tub, where it is allowed to settle, and, after drying, is sold as a fine quality. Frequently, there still remains in the last waters some very fine ultramarine, even when the tubs have been allowed to stand for a month to settle ; by adding a little lime water, which causes an aggregation of the particles, this can be collected by filtering. Before being sold the dry ultramarine is, in many works, subjected to a process of sieving, which separates the coarser particles and yields the pigment in the form of an impalpable powder ; the finer qualities should have a buttery feel when rubbed between the fingers. The shade of the finished ultramarine depends upon several factors, such as the proportions of the constituents used in the mixings, the perfection of the burning operations, and the fineness to which the pigment has been ground ; as it is impos- sible to regulate each of these factors with mathematical accuracy, it follows that the shade of the finished colour must vary from time to time; and as this variation is objectionable the makers overcome it by having a number of standard or type colours or 13 194 BLUE PIGMENTS. shades, to which standard they bring up all batches by a process of blending and mixing different shades together so as to obtain the marketable brands. Wet Methods of Making Ultramarine. — Many attempts have been made to prepare ultramarine by wet processes, but mostly without any success. Knapp's process, given in the Jour, far Praktisch. Chem., 1885, p. 375, consists in first roasting a mixture of kaolin, sodium carbonate, and sulphur to such a temperature that the roasted mass has a brown colour, at which point it is maintained until the kaolin is completely decomposed ; after cooling, the mass is digested in a solution of sodium persulphide. The defect of this process consists in the small margin there is between success and failure ; if the colour of the roasted mass be allowed to pass beyond the brown, the colour of the finished ultramarine begins to deteriorate ; and if it becomes red, then no blue is produced when the mass is digested with the persulphide under these conditions the process can hardly become a com- mercial success. The colour of the finished product is not quite equal to that of ultramarine made by the dry methods ; it is, however, not much inferior. PROPERTIES OF ULTRAMARINE. — Ultramarine is one of the most important pigments at the command of the painter. As a pigment it is perfectly permanent when exposed under all ordinary conditions, being perfectly fast to light and air, the only destructive agents being acid vapours which rapidly decolorise it. It can be mixed with all the ordinary vehicles used by painters and with most other pigments without being changed thereby or itself causing any change. The only exceptions are those pigments containing lead or copper, which, owing to their forming black sulphides with sulphur, are liable to become dis- coloured when mixed with ultramarine ; the rate of change of such mixtures as ultramarine with chrome-yellow or emerald- green is very variable ; sometimes the mixture will change colour very soon, at other times the mixture will keep its colour for a considerable time ; much depends upon the quality of the pig- ments and the care with which they have been made. Ultramarine is distinguished by its pale but pure tone and by its tint of blue being quite different from that of all other blue pigments. The soda ultramarines are of a violet-blue shade, the variety rich in silica having the darkest and deepest tint ; the sulphate ultramarine is of a pale greenish-blue tint and is the palest blue pigment made, resembling blue verditer in tint. The most characteristic property of ultramarines is their being COMPOSITION OF ULTRAMARINE. 195 readily acted upon by acids ; the colour is discharged and the pigment decomposed, sulphuretted hydrogen being evolved and sulphur deposited. All acids have this property, even weak organic acids, such as acetic acid, tartaric acid, &c., this distin- guishes ultramarine from all other blue pigments ; on the other hand, it prevents the use of ultramarine wherever there is the least chance of its coming into contact with acid influences, which are, sooner or later, sure to destroy the colour. Of the varieties of ultramarine, the sulphate is the most readily decom- posed, while the highly silicated soda variety is the most stable of the soda ultramarines. Boiled in strong nitric acid, there is, first, a decoloration, and then a deposition of sulphur; afterwards the sulphur is dissolved and a residue of gelatinous silica is left behind. Alkalies have no action on ultramarine. When boiled in alum, ultramarines take a more violet tone ; the sulphate variety is the most readily changed, while the highly silicated soda ultramarine resists the action most; the latter variety is therefore used by papermakers, because, owing to their having to use alum or sulphate of alumina in sizing their papers, they require an ultramarine which will not change much, if anything, under the influence of those bodies. Heat has no action on ultramarine. COMPOSITION OP ULTRAMARINE. — Ultramarines are compounds of silica, Si 0 2 , alumina, Al 2 0 3 , soda, Na 2 O, sulphur, S, and sulphur oxide, S 0 3 . The last, although present in almost every sample of ultramarine, is not an essential con- stituent of the colour. The following are some analyses of ultramarines, mostly by the author, which will show the average composition of these im- portant pigments : — ANALYSES OF ULTRAMARINES. Sulphate. Soap- makers' Calico- Printers'. Paper- makers'. Grieen. Silica, Si 0 2 , Alumina, Al 2 0 3 , . . . Sulphur trioxide, S 0 3 , . . Soda, Na 2 0, Water, H 2 0, 49-685 23 000 9-234 2464 12 492 3*125 40 647 25-047 12-953 4-814 14-264 2-275 40-885 24110 13-740 3 047 15-618 2-600 45 420 21-147 11-624 5- 578 9-906 6- 325 38*52 28*94 8-30 23 : 68 100-000 100-000 100-000 100-000 99-44 196 BLUE PIGMENTS. The soap-makers', calico-printers', and paper-makers' ultra- marines are of English make, the others of Continental make. The analyses of paper-makers' and soap-makers' ultramarines show the difference between the two varieties of soda ultra- marines ; the first named is rich in silica, while the other is poor in silica ; the soap-makers' and calico-printers' samples are evidently identical in composition, but the latter is much finer than the former. The analysis of green ultramarine shows the difference between the green and blue ultramarines. CONSTITUTION OF ULTRAMARINE.— One of the pro- blems chemists have endeavoured to solve has been how the various constituents of ultramarine are combined together, but it is still unsolved, and will probably remain so for some time to come; the difficulty of solving it seems to be the inability to effect the substitution of particular groups of elements in it in the same manner as can be done in organic chemistry, where, even in complex molecules, the power of replacing one group by another enables one to form some conception as to the actual constitution of the compound. It is true that the sodium in ultramarine can be replaced by silver and other metals so as to form varieties of ultramarine, and that the sulphur can be re- placed by selenium or tellurium; but these replacements throw no light on the problem, for they are simply replacements of one element by another, not of groups of elements. Many chemists, e.g., Wilkins, Hofmann, Unger, Endeman, and Elmer, have worked on this question. Hofmann's theory of the constitution of ultramarine is, perhaps, nearest the truth. Hofmann was head of the Marienberg Ultra- marine Works and did much to throw light on this subject ; he considered ultramarine to be a double silicate of alumina and soda combined with bisulphide of sodium. The formula assigned to the soda ultramarine poor in silica was 4 (Al 2 Na 2 Si 2 0 8 ) + Na 2 S 4 , and to that rich in silica, 2 (Al 2 Na 2 Si 3 O 10 ) 4- Na 2 S 4 . Endeman considers that the ultramarines contain a colour- nucleus (an oxysulphide of alumina and soda) disseminated through a double silicate of alumina and soda. The colour-nucleus of white ultramarine (which may be re- garded as the parent body) has the formula Al Na 4 0 2 S 2 ; the action of sulphur upon this is to remove soda and to form green ultramarine, which contains the nucleus, Al 2 Na 2 S 2 0 3 ; this, by oxidation, can be converted into Al 2 Na 4 S 3 0 3 which has a jet green colour ; by burning with sulphur, this is converted into the nucleus of the blue variety, which has the formula Al 2 Na 2 S 3 0 3 . The base through which the colour-nucleus is distributed is of CONSTITUTION OF ULTRAMARINE. 197 variable composition ; Endeman gives the formula of one variety as 16 Si 0 2 , 3 Al 2 0 3 , 5 Na 2 O. But all this is open to great doubt. The most competent authorities consider that the green ultra- marine is not a true chemical compound, but a combination of the blue with sodium salts; because, by simply boiling with water, it is converted into the blue ultramarine, while soluble sodium salts are found in the water ; on the other hand, by heating the blue ultramarine with sodium sulphate and charcoal, it is con- verted into the green ultramarine. Gueckelberger, one of the most recent writers on the subject, confirms the figures given by Hofmann : but considers that the ultramarines are derived from a typical compound containing Si 18 • thus, for the variety rich in silica he proposes the formula Si 18 Al 12 Na 20 S 6 0 62 , while the variety poor in silica has the formula Si 18 Al 18 Na 20 S 6 0 71 . It is doubtful whether ultramarines have the complex composition here assigned to them and, more- over, no light is thrown on their constitution. ASSAY AND ANALYSIS.— As there is so much differ- ence between various makes of ultramarines, it is necessary to assay for colour, fineness, body, &c. Those ultramarines which are to be used by paper-makers should be tested for their power of resisting the action of alum ; this can be done by taking about 5 grammes and boiling in a solution of alum of about 5 per cent, strength. To see what change of colour may have taken place, 5 grammes of the colour should be shaken up with clean watei; and the two wet samples compared together ; any change brought about by the alum can then be readily detected. It is rarely that a complete analysis of ultramarine is required; in such an event, the following scheme can be adopted : — For Water. — Heat 2 grammes in a weighed crucible for about half an hour over the Bunsen flame; the loss in weight is the amount of water present. For Silica, Si 0 2 . — Treat 2 grammes with hydrochloric acid until the colour is completely destroyed ; evaporate the mixture to dryness and gently ignite the residue ; treat the dry mass with hydrochloric acid, filter off the insoluble silica, well wash it, then dry, and burn in a weighed crucible; the increase in weight minus the weight of the filter-paper ash is the weight of the silica. For Alumina, Al 2 0 3 . — To the filtrate from the silica add am- monia in slight excess, boil gently, then filter, and treat the precipitate of alumina as the silica. For Soda, Ka 2 O. — To the ammoniacal filtrate from the alumina add sufficient sulphuric acid to neutralise the ammonia, then evaporate to dryness in a weighed basin and ignite the residue 198 BLUE PIGMENTS. until all ammoniacal fumes have been given off; weigh the residue of sodium sulphate, and multiply this weight by 0*4366 to ascer- tain the weight of the soda, Na 2 O. Total Sulphur, — Treat 2 grammes of the ultramarine with a mixture of 2 parts of nitric acid and 1 part of hydrochloric acid, until the colour is completely decomposed and only a transparent mass of silicate is left ; filter this off, and to the filtrate add a solution of barium chloride in excess, boil and filter, wash the precipitate well, dry, burn, and weigh it in a crucible. To find the weight of sulphur, multiply the weight of barium sulphate so found by 0*13734 ; from this deduct the weight of sulphur present as sulphuric acid to find the quantity of sulphur present as sulphide. For Sulphur as Sulphuric Acid. — Weigh out 2 grammes of ultramarine, treat with dilute hydrochloric acid, filter off the precipitated sulphur and silica, and precipitate the filtrate with barium chloride, treat the precipitate as in the last. To find the amount of sulphur trioxide present, multiply the weight of the barium sulphate so found by 0*34335. ULTRAMARINE DERIVATIVES. — It has been stated above that some of the constituents of the blue and green ultra- marines can be substituted by other analogous bodies, such as selenium for the sulphur or the sodium by silver ; in this way other ultramarines can be prepared, but, as a rule, they are only H of scientific interest, as their colour is of no technical moment, as will be seen later on ; hence they are not made on a large scale ; still their production may ultimately throw light upon the question of the chemical constitution of ultramarine, and the fact of their formation must be faced by all chemists who essay to deal with this question. There are one or two coloured derivatives of ultramarine which are used to a limited extent ; these are the violet and red ultramarines. VIOLET ULTRAMARINE. — This product can be made from either the green or blue ultramarines, from which it differs , by containing less sulphur and more alumina. It can be made in several ways. Zeltner makes violet ultramarine by submitting either the blue or green varieties to the temperature of about 300° C, and passing chlorine gas over them ; at first the colour is, if the blue is used, turned green, then this becomes dark red; at this point the operation is stopped and the red product is boiled in an alkaline solution until it turns violet; after which it is washed. Instead of using dry chlorine at a temperature of 300° C, the green or blue varieties may be heated in a mixed current of steam and chlorine at a temperature of 160° to 180° C, VIOLET AND RED ULTRAMARINES. 199 until the colour is developed ; after which it is washed with water to free it from the sodium chloride formed, and dried. In another method, devised by Hofmann, blue ultramarine mixed with about 2*5 per cent, of ammonium chloride; the mixture heated to a temperature of 200° 0., exposed to the air until the violet colour is properly developed, and the mass allowed to cool slowly ; when cold, it is washed thoroughly and dried. Yiolet ultramarine has very similar properties to the blue variety, and is similarly decomposed by acids ; boiling in alkalies changes the colour to blue. The shade of the pigment is a very pale reddish-violet. This pigment is not used to any great extent owing to its want of colouring power. In composition it resembles the blue varieties ; a sample analysed by the author contained : — Silica, Si 0 2 42*010 per cent. Alumina, Al 2 O s , .... 26 '360 Sulphur, S, 9-235 „ Sulphur trioxide, S O s , . . . 3'140 Soda, Na 2 0, 17 '905 Water, H 2 O, 1-350 100-000 RED ULTRAMARINE.— Zeltner prepares the red ultra- marine by exposing the blue variety at a temperature of 130° to 150° 0. to the action of the vapours of nitric acid, when he obtains deep or dark red, or light rose or pink shades, according as the acid vapours are dilute or strong. Hofmann prepares the red ultramarine by passing dry hydrochloric acid gas over either the blue or violet varieties until the proper colour is developed, when the mass is washed and dried. OTHER ULTRAMARINES. — By using boracic acid instead of silica a boron ultramarine of a blue colour is obtained. Yellow ultramarine is made by heating the blue ultramarine with a solution of silver nitrate, in sealed tubes, at a temperature of 120° C. for 15 hours; the sodium is replaced by silver, and the new pigment contains 46*5 per cent, of silver. The blue ultramarine heated with silver chloride turns green, taking up silver in the process. The yellow silver ultramarine heated with sodium chloride loses some of its silver, turning green ; if the sodium chloride is replaced by potassium chloride a bluish-green potassium ultramarine is formed. If barium chloride is used a yellowish-brown barium ultramarine is obtained ; in the same way zinc chloride yields a violet zinc ultramarine, and 200 BLUE PIGMENTS. magnesium chloride a grey ultramarine. None of these products have any technical value. The sulphur can be replaced by selenium, when brown and purple ultramarines are obtained. PRUSSIAN BLUE. Prussian blue, or Berlin blue, or Chinese blue, is, next to ultramarine, the most valuable blue pigment in use for painting and other purposes for which pigments are used. It was dis- covered in the early part of the last century (about 1704) by a Berlin colour-maker, named Diesbach, by accident, as many such discoveries have been made. Diesbach was making Florentine lake, and for this purpose he used a solution of cochineal, which he mixed with alum and copperas (ferrous sulphate) and pre- cipitated with an alkali ; in the particular instance which led to the discovery of Prussian blue he used an alkaline solution (which had been used to purify some Dippel's oil made by dis- tilling ox blood), and instead of getting a red lake he got a blue. Diesbach followed up this discovery and found that the blue could be got by calcining blood with alkali, and, after lixiviating the mass, precipitating the liquor with a solution of copperas. The technical manufacture of this pigment was further de- veloped by a London colour-maker, named Wilkinson, from whom the colour was named "Wilkinson's blue/' a name which is now obsolete. Wilkinson prepared the colour by first de- flagrating a mixture of tartar and saltpetre, and calcining the residue with dried blood ; the fused mass was lixiviated with water, and to the lye so obtained a solution of alum and copperas was added ; the resulting pale blue precipitate was treated with hydrochloric acid to develop the blue. Since the time of Diesbach and Wilkinson the composition of Prussian blue has been the subject of numerous researches by chemists, so that very little remains to be learnt as to the com- position and constitution of this blue. Prussian blue is a compound of iron, carbon, and nitrogen \ the carbon and nitrogen are combined together in the form of the radicle cyanogen, C N, which is the characteristic element of a group of compounds, of which Prussian blue is a member, known as cyanides. The iron exists in the blue in two forms; one in combination with the cyanogen in an acid condition, the other in the basic condition. When Prussian blue is boiled with a solution of potash it yields oxide of iron (which remains as an insoluble red mass), and a yellow solution, which, on being PRUSSIAN BLUE. 201 allowed to crystallise, deposits yellow tabular crystals of a com- pound of potassium, iron, carbon, and nitrogen, originally known as yellow prussiate of potash ; the iron in this exists in com- bination with the carbon and nitrogen in an acid state, forming the radicle known as ferrocyanogen, Fe C G N 6 . The chemical name of the yellow prussiate of potash is potassium ferrocyanide, K 4 Fe C 6 N 6 . Besides the yellow prussiate there is another, the red prussiate or potassium ferricyanide, K 3 Fe C 6 N 6 . These two compounds differ from one another in their colour and in the reactions which they give with iron salts. With ferrous salts the ferrocyanide gives a bluish-white precipitate of ferrous ferro- cyanide ; while with ferric salts a deep blue (Prussian blue) pre- cipitate of ferric ferrocyanide is obtained. With ferrous salts the ferricyanide gives a deep blue precipitate of ferrous ferri- cyanide (Turnbull's blue); with ferric salts no precipitate is obtained, but the colour of the solution becomes a little darker. The production of Prussian blue is a most characteristic reaction of iron, no other metal is capable of producing it, and very minute traces of iron in a solution can be detected by adding a few drops of a solution of potassium ferrocyanide. It has been ascertained by various observers that the precipi- tates obtained by adding solutions of the prussiates to solutions of iron salts contain potassium as an essential part of their composition, and it is difficult, although possible, to rid them of this potassium. Thus the bluish-white precipitate is really potassium ferrous ferrocyanide, K 2 Fe C 6 N 6 ; Prussian blue is potassium ferric ferrocyanide, K 2 Fe 2 2 Fe C 6 N 6 ; Turnbull's blue is potassium ferro-ferricyanide, K Fe Fe C 6 N 6 . Prussian blue and Turnbull's blue have exactly the same composition, but their constitution is different \ the one being a ferrocyanide and the other a ferricyanide. Skraup,* Reindel,t Kekule,J and other authorities consider that they are identical; but that they are different is proved by the fact that when the alkali is eliminated from them the residual blues are of different composition, Prussian blue having the composition Fe r C 18 N 18 , and Turnbull's blue the composition Fe 3 C 12 N 12 § ; then the shade of the two blues is different, Prussian blue is a greenish- blue, while Turnbull's blue is a violet-blue. COMMERCIAL PRUSSIAN BLUE. — In commerce several varieties of Prussian blue are sold under the names of — * Skraup, Liebig's Annalen, clxxi., p. 371. tReindel, Jour. Prak. Chemie, cii., p. 38. + Kekule, Lehrbuch. Organ. Chem. § Williamson, Mem. Soc. Chem., hi., p. 125; Reynolds, Jour. Chem. Soc, li., p. 644. 202 BLUE PIGMENTS. 1, Chinese blue ; 2, Prussian blue ; 3, soluble blue ; 4, Antwerp blue ; 5, Brunswick blue, and others of less importance. There are several synonyms, such as Berlin blue and Paris blue. Turnbull's blue is a name rarely met with now. No special distinction is made between a blue obtained from the red prus- siate or a blue obtained from the yellow prussiate of potash. CHINESE BLUE. — This is the name given to the best quali- ties of Prussian blue, and in the manufacture of which every care is taken to obtain a product of good colour. Chinese blue is especially characterised by being in pieces or powder having a fine bronze lustre, the pieces break with a peculiar conchoidal fracture, and the fractured surfaces show the lustre or bloom ; it is completely soluble in oxalic acid. Chinese blue is largely used by calico-printers and dyers. It is a blue of a greenish shade. Chinese blue is made as follows : — 1 cwt. of ferrous sulphate (green copperas), as free from insoluble oxide as possible, is dissolved in cold water, and to it is added 10 lbs. of sulphuric acid. This solution must be made as required, as it soon begins to oxidise, and to deposit oxide of iron, while the liquor will then not make good Chinese blue. One cwt. of yellow prussiate of potash is dissolved in water. The solutions should be made as dilute as possible, not less than 30 to 35 gallons of water for each cwt. of material ; even weaker solutions are preferable, as these yield finer precipitates than strong solutions, and so facilitate the production of the lustre on the finished blue. On mixing the solutions a bluish-white precipitate is obtained, which is allowed to settle, and the clear liquor poured off; to the residual blue is added — first, a thin cream of 20 lbs. of bleaching powder with water, which is thoroughly mixed with the precipitate, and then some hydrochloric acid, and the blue colour gradually develops. It is allowed to settle, the top liquor run off, and the blue well washed with water, and drained on a filter; the wet mass is then pressed into drying-pans, and slowly dried in the dark, at a temperature not exceeding 120° to 130° F. It is important that the oxidation of the precipitate first obtained be done by purely chemical means, and not by the agency of the oxygen of the air; in the former case a pure blue colour is obtained, while in the latter case oxide of iron is mixed with the blue, and materially influences the tint of the pigment. The best oxidiser and the cheapest is bleaching powder ; nitric acid may be used, but it is more costly and not more efficient than bleaching powder. It will be found best to PRUSSIAN BLUE. 203 add the oxidiser in small quantities, as when used all at once there is generally an escape of chlorine, owing to this body being evolved rather more rapidly than the blue can take it up, which not only increases the cost of production, but also deleteriously affects the workmen's lungs. No part of the blue should be allowed to come in contact with the atmosphere before it is iully oxidised. The slower the colour is dried, the better and finer is the lustre of the finished product. Instead of adding the bleaching powder after precipitating, it may be added to the iron solution to oxidise that to the ferric condition ; the blue obtained is not of so green a tint, being, if anything, a little more violet. If a blue with a violet tint is required it may be made by dissolving 1 cwt. of copperas in water and adding first 10 lbs. of sulphuric acid, and then a solution of 1 cwt. of the red prussiate of potash. The precipitate is collected, washed with water, and dried as before. Chinese blue is mostly sold in the form of small cubical lumps, about 1 to 2 inches in diameter, but it is also sold in the form of fine powder. In grinding the blue great precaution is required to exclude particles of iron, as the production of a spark will ignite the dry powdered Chinese or Prussian blue and reduce it to a mass of red oxide of iron. A sample of Chinese blue examined by the author had the following composition : — Water, ..... 4 # 487 per cent. Oxide of iron, .... 52*055 Cyanogen, .... 43*508 100-000 PRUSSIAN BLUE. — The commoner makes of blue are sold under the names of Prussian blue, Berlin blue, paste blue, - Captain Abney, named by him a ' c colour patch" apparatus. It is described in his book on Colour Measurement and Mixture, This method, while of some interest from a scientific point of view, is, however, scarcely one which will come into practical use in a colour shop, owing to its rather complex construction and to its requiring powers of experimenting beyond those of colour-makers generally. How- ever, a brief description of the process and apparatus may be use- ful. The colour patch apparatus consists, first, of a spectroscope, with which a spectrum of the light from an arc electric lamp can be formed on the screen of a camera ; by substituting a slide having a narrow slit in it for this screen and passing the light which comes through this slit through a lens, a patch of coloured light can be obtained on a screen placed behind the camera ; the colour of this patch will depend upon the position of the slit in relation to the spectrum which falls upon it and will necessarily be monochromatic. The arc electric lamp is preferred as the source of light, because it can be more depended upon than any other kind of light for uniformity in amount and quality, which feature is of importance where light measurements have to be made. Another part of the apparatus consists of an arrangement by which a disc of coloured card can be rotated ; the same apparatus also carries a larger pair of overlapping black and white discs, the amount of overlapping being capable of variation at will. This is placed so that the colour patch falls partly upon the coloured disc and partly upon the black and white discs ; these are rotated, and the slit of the colour patch apparatus moved along the spec- trum until a point is reached when the luminosity of the patch on the two discs is equal. Then a note is made of the position of the slit, as given on a scale attached to the colour patch apparatus, COLOURING POWER. 285 and also of the relative quantity of black and white exposed in the black and white discs ; then another trial is made in the same way, only that the black and white discs are altered so that a different proportion of the two colours are exposed: It will now be found that equal luminosity of the colour patch on the discs occurs when the slit is in a different part of the spectrum to what it was before the two measurements were made. These measurements are repeated for various proportional exposures of the black and white discs. Then on a chart is drawn two sets of lines, viz., a horizontal set, to show the proportions of black and white ; and a vertical set, to show the position of the slit on the spectrum. Then by drawing a line through the points given by the various readings, a curve is obtained indicating the reflecting power of the particular pigment experimented with in each part of the spectrum. By carrying out this system with different pigments we are able to see how one compares with another. By a modification of the experiment the light reflected from a surface painted with a pigment can be compared with that which is reflected from a surface painted with a standard sample of the pigment. If instead of laying down the curve on a chart, it is drawn on a sector of a circle, so that the scale of the spectrum is measured off along a radius and the relative intensities on concentric circles, then a curve of somewhat different shape is obtained. If this be cut out it forms a colour template which, when revolved in front of the spectrum formed in the apparatus, cuts off just enough light that the remainder forms a colour patch of the same hue as the colour pigment ; by causing a patch of colour to be formed side by side with the colour patch, the relative hues may be accurately compared together. This second patch of colour is obtained by reflecting from a surface painted with the pigment. By making templates in this way and using the colour patch apparatus in the manner indicated, there is a certain method of comparing hues of pigments. BRILLIANCY or LUMINOSITY. — This is an important feature of pigments, and one in which different makes of the same pigments are rather liable to vary. As with hue, brilliancy is assayed by comparison with a standard sample, and this can be done in precisely the same manner as described for hue. COLOURING- POWER. — Colouring power is that property of pigments which enables them to give colour to surfaces and to other pigments. As explained in another place, pigments possess two properties available for paint making, viz., colour and body, or covering power (see below). Some pigments are 286 ASSAY AND ANALYSIS OF PIGMENTS. used almost solely on account of their colour, as, for instance, carmine, Prussian blue, ultramarine, vermilionettes ; hence with these strength of colour or colouring power is an important feature. Other pigments are used solely on account of their covering power, and then colour is immaterial. Colouring power is tested also by comparison with a standard sample. In principle it is done by ascertaining how much of another pigment it will colour to a given depth. Supposing it is a sample of vermilionette whose colouring power is to be determined, then 10 grms. of the sample are weighed out and mixed with 30 grms. of china clay; the mixing must be thoroughly done. 10 grms. of the standard sample are mixed in the same way with 30 grms. of the same sample of china clay. The two mixtures are now compared together for depth of colour as described above ; if the two samples are equal in colouring power, the depth of colour of the two mixtures will be the same ; if one is stronger than the other, then one of the mixtures will be darker than the other. Some idea of the relative strength of colouring power may be obtained by adding small and known weights of china clay to the darkest sample until the tint of the mixtures are equal to one another ; then the samples have a colouring power proportional to the amount of china clay used ; thus, if one sample took 30 grms. of china clay and the other sample 37'5 grms., then the relative colouring power is as 30 to 37*5 ; or, if the strongest sample be taken at 100, then the colouring power may be expressed in percentages thus, 37*5 : 30 : : 100 : 80 ; the weakest colour has only 80 per cent, of the colouring power of the strongest. As the toning colour for all pigments except whites a good sample of china clay may be used ; gypsum also makes a good toning colour ; barytes and white lead are a little too heavy. For whites a good animal black makes a good toning colour. When a large number of assays for colouring power have to be made a standard tint should be made by taking, say, 50 grammes of the standard sample, and mixing with about twice its weight of the toning colour; this tint may be used in subsequent tests, and will save some time in the preparation of a standard tint. It is important, however, that the same sample of toning colour be used to mix with the samples, whose colouring power is being tested, as has been used in making the standard tint. COVERING POWER or BODY. — This is a most impor- tant property of pigment — perhaps the most important, for those which possess it in the greatest degree are universally considered to be the best pigments. It may be denned to be the power of COVERING POWER OR BODY. 287 covering over or hiding the surface of any body on which it may be spread when mixed into a paint. Some pigments, such as crimson lake, Prussian blue, and barytes, are deficient in this property ; others, such as white lead or the chromes, possess it in great degree. In the same pigment, however, the covering power is liable to vary to a greater or less extent. To some extent the covering power is dependent upon the condition of the pigment: if this is of an amorphous character, without any definite form of its own, and is opaque, it will, as a rule, be found to have good covering power or body ; on the other hand, if a pigment is of a transparent character, and is crystalline in its structure, then its covering power is liable to be small. Sometimes a pigment may be obtained in both conditions, according to the particular circumstances under which it is made; thus, lead chloride may be obtained as a white amorphous powder, or in small transparent crystals. In the former condition it may be used as a pigment, as it has some covering power; on the other hand, the crystalline variety is useless as a pigment, as it has no covering power at all. Unfortunately it is by no means an easy matter to devise a method of assaying the body of pigments; it cannot be expressed in absolute figures as can chemical composition, at the most it can only be assayed in a comparative manner as is colouring power. The best plan for assaying the covering power or body in pig- ments is to place 2 grammes of the standard sample and of the pigment to be compared with it on a black porcelain tile, and to add 3 grammes of oil to each; the oil and pigment are thoroughly incorporated by means of a palette knife, and then each is spread over the plate in a layer, making each layer of paint of as uniform a thickness as possible. That sample which, when thus made into a paint and spread over the tile, most completely obliterates the surface of the latter has the most covering power. It is possible to obtain some idea of the comparative covering power of two samples by this method. To that sample which has the most body a small additional quantity of oil is added, and the body of the mixture again compared with the other sample ; if it is still the best more oil is added, and the process repeated until both samples appear to have the same covering power. Now, it may be assumed, without much error, that the covering power or body of the two samples is in proportion to the quantities of oil used to mix with them; that sample taking the most oil having the most body. Thus, of two samples of barytes one took 3 grammes of oil, and the other 3*25 grammes; 288 ASSAY AND ANALYSIS OF PIGMENTS. taking the last as the standard or 100, the former had only 3*25 : 3 : : 100 = 92*3, or 7*7 per cent, less body than the stan- dard sample. Or, to put it in another way, 3 : 3*25 : : 100 = 108*3; that is, 100 lbs. of the standard sample will cover as much surface as 108*3 lbs. of the other, weaker sample. DURABILITY or PERMANENCE.— Durability is one of the most important properties a pigment can possess, for upon it depends the fact whether it will ever come into extensive use as a pigment, especially for artists' use, where permanence is one of the most essential things a picture must possess. Until recently our knowledge regarding the permanence of pigments, or, what is the same thing, their power of resisting exposure to light and air, was empirical and unreliable; but recent researches on the subject have quite altered its character. Of those colours which have been shown to be permanent, the mineral pigments — ochres, umbers, sienna, Vandyke brown, barytes, Chinese white, siennas, ultramarine, vermilion, Prussian blue, and some others — are the most important, and can safely be used on work which is required to have great permanence. What is frequently sold under the names given to old pig- ments is, however, not always what it ought to be, being more or less adulterated with an inferior pigment ; it is, therefore, advisable to test a sample, not only for purity, but also for durability, as the adulterants are frequently anything but permanent. Of late years many new pigments have been placed on the market, which have been made from coal-tar colours ; unfortun- ately, many of these, although of brilliant hue, are far from being permanent, and will not resist any lengthened exposure to light and air. It is desirable that a user of these pigments should make some experiments as to their durability. There is a great deal of difference in this respect among these coal-tar pigments. Some are as permanent as can be wished; others, again, are very fugitive. Then, again, the method of using has more influence on the durability of these pigments than it has on that of the older pigments ; for some which are rather fugitive, when used as water-colours, will resist a fair amount of exposure when used as oil-colours. Probably the simplest method (which is a very good one) of testing the durability of colours, is to provide a sheet of unglazed cardboard ; that known as Bristol board will do very well, It must have so slight an absorbent property that if any coat of paint is placed on the surface it will remain there, and not soak into the substance of the cardboard. This sheet of board DURABILITY OR PERMANENCE. 289 is ruled into squares or rectangles, measuring about 3 x 2 or 2x2 inches. A little of the colour to be tested is ground up with a little gum water into a smooth paste, and a portion of one of the ruled spaces on the cardboard painted with it. It is advisable to rule and prepare two sheets at the same time. The name of the colour can be written either underneath the patch of colour in the square, or in a corresponding position on the back of the card. It is also advisable to grind a little of the pigment with oil, so that the relative durability as a water-colour and as an oil-colour can be tested. One of the prepared cards is hung in a place where it is exposed to as much sunlight and air as possible, while the other card is placed in a drawer away from any such influence. After a week or two of exposure the cards can be compared to see if any changes have occurred; they can then be replaced in their respective positions, and from time to time are compared together. Any change which may have been brought about by the action of sunlight and air on the exposed card will be observable ; some colours will be changed in a few weeks' exposure, other colours require months of exposure to produce any effect. By placing a card painted in the manner described with dif- ferent pigments in a closed cupboard, in which is placed a vessel containing some ferrous sulphide and dilute sulphuric acid, the action of sulphuretted hydrogen on the colours can be tested ; if any are affected by this test it is certain that they will be similarly affected when exposed to the action of impure air. Testing pigments for durability is a very long operation, and it is no wonder that there have been few systematic researches on this subject. The most exhaustive and systematic experi- ments on the permanence of pigments which have ever been made are those made by Captain Abney and Dr. W. J. Russell, at the request of the Science and Art Department ; these extended over a period of two years, and the results were published in the form of a Blue Book, entitled — "Report on the Action of Light on Water Colours." This report must be consulted for details as to the method of testing adopted, &c. ; but the following will give some idea of the methods and results of these researches. The experiments were carried out as follows : — A sheet of Whatman paper of good quality was covered with washes of the pigment to be tested in such a manner as to form a series of eight tints, varying from pale to dark. From this sheet, strips, 8 inches long by 2 inches wide, and containing all the tints, 19 290 ASSAY AND ANALYSIS OF PIGMENTS. 2 >» £ bo be «2 a? 43 S3 o be 2 5 5 5 S o o bo O e3 bo ^ a> S3 £ £ I tc 'o ft bO to 3 %to « 0) 43 c Tj o3 H is O o bo c o3 o o g^ g 1 * 1 l^fe * QJ 03 O g _ ' 7>>S3 ® !i » g o 2 S 2 3 C^i3 ^ 03 03^ > d.S-»a"^ g g qj go^slga^ S3 •*§•§ ••^"S " ^ „r^? S3 e S3 MIXABILITY. 291 were cut, and two of them placed in a glass tube, 2 feet long and | inch in diameter, open at both ends, and bent over at the upper end in the form of a hook, in order to prevent the admission of dirt ; the tube was then hung in such a place as to receive as much sunlight as possible. It may be mentioned that one of the strips of paper was covered with oil-cloth so that it could not be acted on by light, although otherwise it was subjected to the same influences as the other strip. Similar tubes were filled with dry air, moist air, moist hydrogen, while one set had the air withdrawn so that the strips were exposed in a vacuum. The experiments lasted from May, 1886, to March, 1888. The results of these experiments are given in the table opposite, which has been compiled from the tables given in the Report. Prussian blue fades when exposed to light, but on placing the faded colour in a dark place the colour comes back again. It is evident from the above experiments that moisture has a material influence on the durability of pigments ; colours which fade in moist air are permanent in dry air ; then, again, colours are more permanent in an atmosphere of moist hydrogen gas and in a vacuum than in air ; it is evident, therefore, that the three elements of destruction which cause colours to fade are light (which may be called the determining cause), oxygen, and water. But light alone has little effect. Hence it may be concluded that when a colour is kept under conditions where moisture and air can have little action it will be permanent ; so that it should always be kept under such conditions if possible. MIXABILITY. — This is a term which the author has intro- duced in connection with pigments to express the power of pig- ments to mix more or less readily with oil or other vehicles or other pigments. It is a most important property, and much of the value of a body when used as a pigment depends on it; some pigments do not readily mix with oil, while some of the modern pigments made from coal-tar colours seem to have the property of retarding the drying of the oil, therefore they cannot be said to mix well with it. Then, again, in mixing pigments together to produce compound tints or shades, there are some pigments which can be mixed with all others without any ill effect being observed ; on the other hand, some pigments can be mixed with a few others without any change occurring, but when mixed with others some action of a deteriorative character will take place. To test pigments for the property of mixability, the best method is to provide a sheet of cardboard of not too porous a character, and to rule this into squares of about 2 inches each 292 ASSAY AND ANALYSIS OF PIGMENTS. way. A little of a pigment is rubbed with a small quantity of linseed oil in a white basin, during which operation its behaviour with the vehicle will be noticed ; it should be observed whether it mixes freely with oil or shows a tendency to separate out ; this latter effect may be due in most cases to the pigment not being thoroughly dry. A little of the mixed pigment is now rubbed in one of the squares of the card. Then prepare mixtures of the pigment with other pigments — white lead, whiting, Prussian blue, emerald green, yellow chrome, vermilion, lamp-black — rubbing the pigments together with a little gum-water ; these mixtures are rubbed on the card. When all the mixtures are ready the card is exposed to the air, and to diffused daylight only, for some time, say two or three weeks. After two or three weeks' exposure, the card may be examined and any effect of change of colour noted ; during the interval observations should be made as to the drying of the oil in the first square to see whether the pigment has any influence of any kind, either in retarding or in facilitating the drying; the former case will show that the pigment is not suitable to be used as an oil-colour, although it may be per- factly suitable as a water-colour. The other squares will show the action of one pigment on another ; those which exhibit no alteration in shade or tint beyond, perhaps, a little fading will show the pigments which may be mixed together without any effect upon one another ; while those which have altered will show the observer what mixtures to avoid. FINENESS. — The quality of a pigment is a feature which is more or less dependent upon the size of its particles, the smaller these are (or, in other words, the finer the pigment has been ground or produced in the process of manufacture) the better will it be as a pigment; its body or covering power will be increased, its colouring power will also be improved and its tone brightened very considerably ; therefore, it may be laid down that the finer a pigment is the better it will be for use in painting. It is by no means easy to make a practical test for the fineness of a sample of pigment. By rubbing between the fingers it is possible to make a rough comparative examination, but no accurate results can be arrived at by this means ; when the quality of two samples is very similar this rough test cannot be relied on. By spreading a little on a plain microscope slide and examining it through a powerful microscope, using, say, a ^-inch objective, some idea of the relative fineness of two samples may be obtained. Another method of testing which will give more reliable information and better comparative figures than the tests just noted is the following : — Weigh out about 5 grammes into a FINENESS. 293 mortar, and grind, without much rubbing action, into a smooth paste with water ; then transfer this to a tall cylindrical graduated measuring glass, and rinse out the mortar with water, so as to get the whole of the material into the glass ; fill this up with water to the top mark, and, putting in the stopper, shake well for a few minutes; then place on one side; the particles will gradually settle, and the time it takes for the water to become clear up to, say, the half-mark should be noted. If this be done with a number of samples a series of figures will be obtained which may be taken as showing the comparative fineness of the various samples, for the rate at which the material settles depends upon the fineness of its particles ; the finer these are the slower is the action, while the best samples are characterised by subsiding most slowly. Then this test will also show whether the sample is of uniform quality or whether it contains both coarse and fine particles ; in the former case the rate of deposition will be uni- form, while in the latter case the larger particles will settle out very rapidly, leaving the finer particles to subside more slowly. ' For example, the author tested three samples of china clay by this method. Sample A took 60 minutes to settle out; sample B 44 minutes ; while the coarse part, forming the great bulk of sample 0, settled out in 30 minutes, and the finer remainder in 90 minutes. A and B were uniform in quality, but sample A was superior to B, being composed of the finer particles. Sample C was of mixed quality, containing both coarse and fine particles; the former settling out rapidly, the latter more slowly. C would not be so good as B for many purposes. This test cannot be used for examining the fineness of samples of different pigments : thus, for example, the comparative fineness of a sample of white lead and of barytes cannot be ascertained by its means ; only different samples of the same pigment can be compared together. This arises from the fact that the specific gravity of a body has a great influence on the speed with which its particles will fall when placed in water, and as there is a great difference in this respect between different pigments, it follows that a sample of white lead will settle out much quicker than a sample of china clay, although both samples may be equal in fineness of powder. The method of testing the fineness of the particles of a pigment here given is possibly not a perfectly accurate one ; but still some very useful information as to the quality of a pigment may be obtained by its means, and it does not need any elaborate apparatus to carry it out. 294 CHAPTER XL COLOUR AND PAINT MACHINERY. In making colours or pigments, and in preparing them for use in painting, and in making paint, there are a good many mechani- cal operations which are common to all pigments and all paints. Upon the care with which the various mechanical operations, such as precipitating, drying, grinding, &c, are carried out, depends much of the quality of the pigment or paint, especially as regards its brilliance and covering power. The machinery for carrying out these various mechanical operations forms an important part of the outfit of a colour- or paint-shop. In the present chapter it is proposed to discuss the various machines which are required to carry out the various processes involved in making a pigment and its conversion into a paint. LEVIGATION. In the preparation of such natural pigments as the ochres, siennas, umbers, china clay, and barytes for use in paint-making, levigation plays an important part. These bodies as they are found in nature contain a good deal of gritty matter and other impurities, from which they must be freed before they are of use in paint-making ; there is no better process for this purpose than levigation. The principle of the process of levigation depends upon the fact that when fine particles of a comparatively light material mixed with coarser particles of the same material or with par- ticles of a heavier material are agitated with water and then allowed to stand, the coarser and heavier particles will fall first, while the lighter particles will form a layer on the top of the coarse particles, which can thus be separated from the fine par- ticles. A modification involving the same principle is where the mass of material is subjected to the sifting action of a current ^of water, the strength of which suffices to carry the fine particles only into a tank, where they are allowed to subside. China clay LEVIGATION. 295 is an example of the preparation of a pigment in this way (see p. 84). Should the raw material be made up of several distinct kinds of particles — very fine, fine, medium, and coarse, as in the case of some ochres — it is possible, by means of levigation, to separate them into their various constituents. By so arranging the current of water that it runs through a number of tanks with varying rates of speed, the coarse particles will be left in the first tank, the medium particles in the second tank, the fine particles in the third tank, and the very fine in the fourth or last tank. It will be seen that levigation, while effective, is a very cheap process ; for it only requires a cheap material, water, and the cheapest kind of colour plant, tanks, for carrying it out. The details of the plant required or used in levigating at any particular works depends upon many factors, such as the position of the works, whether situated in the centre of a town, in a wide valley, or on a hill side. The facilities for obtaining the requisite supply of water is also a factor in determining the arrangement of the plant. In Eig. 28 is shown in plan and elevation a plant suitable for Fig. 28. — Levigating plant. levigating ochres, umbers, &c. It consists of 9 tanks, 8 of which are arranged in 2 series of 4, while the ninth is an odd tank. Another good arrangement would be one of 10 tanks, 9 arranged in -3 sets of 3, the tenth being an odd one. In the odd tank, A, the crude material is thoroughly mixed with water ; in Cornwall, Derbyshire, and a few other mining 296 COLOUR AND PAINT MACHINERY. districts, this tank is known as the "buddle;" in this the very heavy stuff remains while the current of water, which is con- tinually passing through, washes away the finer particles. From the buddle the water flows into the first settling tank 1 ; this being large, the current becomes retarded, and some of the material it contains settles out ; from tank No. 1 the water flows into tank No. 2 ; this is made, or should be made, rather larger than tank No. 1, so that the current being spread over a larger surface becomes slower, and, therefore, has less force, thus allowing the finer particles to settle out. From No. 2 the water flows to No. 3 tank, which is larger still ; and, finally, to No. 4 tank, which is yet larger, so that very fine particles of pigment can settle out ; when tank No. 1 is full the current from the buddle is diverted into the second series of tanks, while the colour or pigment in the tanks of the first series is settling out ; when this is completed the water in these tanks is run off*, and the pigment dug out, when the tanks are ready to be filled again. By having a set of four settling tanks, four qualities of ochres, or siennas, or umbers, may be obtained. When the second series of tanks are full, the current is again sent through the first series. By having three series a more perfect system can be adopted ; the current of water is sent through the first series until these are full, then through the second series while the material in the first is settling out ; when the second is full, the current is diverted to the third series ; by this time the colour in the first will have settled out, and can, as explained above, be collected ; when the first lot of tanks are emptied of their contents they are ready to be refilled by diverting the current from the third set of tanks. Thus the three operations of filling, settling, and emptying can go on concurrently in a complete manner. The tanks should be arranged, as shown in the drawing, one above the other, so that the water can run from one to the other ; and the last tank of the series should be of such a size that it will take a day to fill it. In ultramarine-making, where levigation forms an important part of the finishing process, the last tank is either made very large or a large number of small ones are provided, as the fine ultramarine takes a week to settle. When space is available it is a good plan to have a set of large storage tanks ; into these is thrown the wet pigment taken out of the settling tanks, and here it remains for some time ; a further settling takes place, and the pigment becomes drier ; this effects an economy of both time and fuel in the complete drying daring the final stage. This saving of fuel is a matter of some import- DRYING OF PIGMENTS. 297 ance in dealing with such cheap natural pigments as china clay, umber, &c. The strength of the current of water is a matter that requires attention ; it* too strong, it will carry over some of the coarse material from the buddle to the settling tanks, and will prevent the fine material from settling in the end tanks ; on the other hand, too gentle a current will not extract the whole of the valuable material from the crude stuff in the buddle ; this is a detail which the operator can easily arrange. If only small tanks are required they may be made of wood ; large tanks may be built of stone flags, or of brick, if flagstones of sufficient size are not available. If bricks are used the inside of the tank should have a smooth surface, so as to facilitate the ready removal of the colour which has settled out. In any case arrangements should be provided for running off the clear top liquor from the settled pigment ; this may be done by pro- viding in each tank a set of holes kept stopped by plugs, which are removed when it is desired to run the water away. Or the water may be syphoned off by means of syphons provided for that purpose. The amount of water required to levigate a pigment is a variable amount, depending on the nature of the sample of colour under treatment and on the plant used, so that no definite rules can be laid down. The size of the tanks can be varied to suit the required out- put of colour, and is a point which every colour-maker must settle for himself, remembering, first, the deposited colour will contain about half its weight of water, and will therefore be heavier than the dry material ; second, that the total volume of the tanks must be much larger than that of the material which settles out from them. Another point is to make the tanks sufficiently strong to bear the pressure of the water, &c, they contain, which is great ; thus a tank, 20 feet long x 5 feet broad and 4 feet deep will hold 20 x 5 x 4 = 400 cubic feet of water, or 400 x 62-35 lbs. = 11*13 tons, which is the pressure exerted by the water on the bottom of the tank. In some cases, before levigating, the material is ground, and in such cases the grinding is usually done under water ; for this purpose special mills are made, descriptions of which are given further on. DRYING OF PIGMENTS. After a colour has been prepared for use as a pigment by the process of levigation, as just described, or by that of precipitation, 298 COLOUR AND PAINT MACHINERY. described below, and also by other processes, it is in a wet con- dition, probably containing from 25 to 50 per cent, of water, according to its nature. If required in what is known as the pulp state, in which condition it is used by paper-makers and stainers, no further treatment is necessary ; but, if required to be used in the preparation of paint, it is absolutely necessary that it be dried, otherwise it will not mix with the oil used in the manufacture of the paint. The drying of pigments is carried on in what are called " drying-stoves these are usually nothing more than brick chambers with solid walls on three sides, and a door on the other, covered with a roof ; round the bottom of three sides runs a horizontal flue belonging to a furnace which can be fed from the outside. The wet colours are usually placed in shallow, flat, earthenware pans, which are placed in piles one above another, and then left in until they are dry. This is by no means a satis- factory method, the piling of the pans, one above another, and the absence of any system of ventilation beyond accidental cracks in the door and walls, tend to keep the atmosphere of the stove saturated with steam, and to check the drying operation. A better plan is shown in Fig. 29 ; it consists of a brick chamber built of any convenient size ; as before, the flue, F, of a furnace runs round the bottom ; the sides of the flue are built of brick, the top of flagstone, and the fireplace, E, is placed outside the chamber. Instead of such a flue, steam pipes may be used for heating it. Above the flue or steam pipes, is a staging, s, forming a false floor, on which is erected a framework, C, C, 0, 0, of iron or wood forming skeleton shelves on which the pans of wet colour are placed. These shelves support the pans a small distance apart from one another, and so allow free egress for the water- vapour which comes from the colour. A constant current of warm air, generated by a fan or air propeller, is continually flowing over the pans of colour and out through the ventilator, V, in the roof of the stove, thereby carrying off the water- vapour as fast as it is given off from the wet colour. It should be borne in mind that the colour, just as it comes from the filters or presses, may contain from 25 to 50 per cent, of water ; if, by any means, this water is prevented from escaping from the colour, then the drying is retarded ; or if it is prevented from readily escaping from the stove, it is liable to condense on the inside of the roof, and to fall down in drops on to the colour below. In some cases, e.g., chrome-yellows, these drops are apt to produce spots on, and discolouration of, the pigment which is being dried. The more freely the water- vapour can escape into the atmosphere DRYING OF PIGMENTS. 299 the less chance there is of such mishaps occurring. D is a door for filling the stove, and G, G skylights. Fig. 29.— Drying stove for pigments. In dealing with barytes and china clay, special forms of drying stoves have been described. 300 COLOUR AND PAINT MACHINERY. Some pigments, like the two just mentioned, the oxide reds, burnt umbers, burnt siennas, ultramarine, Guignet's green, are capable of standing a high temperature without being altered in shade; these may be dried in a stove heated to a high tempera- ture, in which case the drying is done quickly. On the other hand, certain colours, such as the chromes, Prussian blue, emerald green, &c, must be dried slowly; for such colours the stove shown in Fig. 30 would be very useful. The two sides not represented in the drawing are of brick, and support the roof. Fig. 30. — Drying stove for pigments. Stretching from side to side are a number of iron shelves just far enough apart to take an earthenware pan and leave a little space between it and the shelf above. These shelves do not stretch completely from back to front, but, as shown in the drawing, they are arranged to come alternately flush with the front and back, the side of the shelf nearest the front and back of each shelf being turned up to form a flange. The front and back of the stove are made of a number of iron plates, which form a series of doors to the shelves, the top of the plates being bent over to catch on the flange of the shelf above, as shown in the PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. 301 drawing ; it is not necessary that the doors should fit air-tight. A fan at the top of the stove creates a current of air through it, a chamber at the bottom is kept hot by steam pipes, or flue from a furnace ; through this chamber passes all the air that is allowed to go into the stove ; this hot air passing over and under the colours dries them, and, being hot, absorbs and carries away the water vapour liberated from the wet colours. This stove is effective and economical, and is so constructed that the pans of colour can be readily removed and the shelves quickly refilled. A drying stove has been constructed in the following manner. A cylindrical vessel was constructed of iron plates of any con- venient size. This was divided into three chambers by two perforated iron plates ; in the central chamber, which is the largest, is placed the material to be dried ; the bottom chamber is kept hot by means of steam pipes, and is provided with an opening to admit air. The upper chamber is fitted with an exhaust fan, so arranged as to draw the air out of the central chamber ; the perforations in the plate dividing the central from the top chamber are larger than those in the plate dividing the bottom from the central chamber, the consequence being that the air is drawn away from the central chamber faster than it enters from the bottom hot air chamber, so that a partial vacuum is created in the central chamber which is beneficial to effective drying. In any stove the colours are best placed in earthenware pans of about 12 to 16 inches in diameter, and 3 to 6 inches in depth ; smaller pans may be used, but it is not advisable to exceed the sizes just given. Pans made of galvanised iron have been used, but these are liable to rust and so lead to discolouration of the pigments dried in them ; enamelled iron pans, which can now be bought at a reasonable figure, are well worth a trial as being lighter and less liable to break than earthenware pans. PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. Many colours — the chrome-yellows, Prussian blues, Brunswick greens, lakes, &c. — are prepared by a process of precipitation, the principle of which is that when two or more substances in the state of solution are mixed together a reaction sets in — what the chemist calls double decomposition occurs — and new products are formed ; one of these being insoluble in the liquid used is thrown down or precipitated out of the solution, usually in the form of 302 COLOUR AND PAINT MACHINERY. a fine powder. Thus when to a solution of nitrate of lead, one of chromate of potash is added, a yellow powder falls down; this on examination is found to be chromate of lead, while the liquor contains nitrate of potash in solution ; thus there has been an ex- change of constituents, the chromic acid has left the potash to form lead chromate, while the potash has combined with the nitric acid of the lead nitrate to form nitrate of potash ; the chromate of lead forms a precipitate because it is insoluble in water. Put into the form of a chemical equation this reaction is expressed as : — Pb2N0 3 + K 2 Cr0 4 = PbCr0 4 + 2KN0 3 . Lead nitrate. Potassium Lead Potassium chromate. chromate. nitrate. Another example of precipitation met with in colour making is that of zinc sulphide, from solutions of zinc chloride and sodium sulphide, which is represented in the following equation : — ZnCl 2 + Na 2 S = Zn S + 2NaCL Zinc Sodium Zinc Sodium chloride. sulphide. sulphide. chloride. Here, again, there has been an interchange of constituents, and the zinc sulphide being insoluble is thrown down as a pre- cipitate. As precipitation is a chemical reaction it always takes place in fixed and definite proportions ; thus in the preparation of chrome- yellow it is found that 331 parts of lead nitrate interact with 194 parts of potassium chromate, the result being that 323 parts of lead chromate are precipitated while 202 parts of potassium nitrate are left in solution; should the salts be mixed in any other proportion, then one or the other must be in excess, and this excess will be wasted ; thus, suppose 1 50 lbs. of lead nitrate and 95 lbs. of potassium chromate are used; the latter quantity is not sufficient to precipitate all the lead from solution; con- sequently, the excess, which is 7 lbs., remains, and is practically wasted. The necessity of using equivalent proportions of the materials is a matter of importance as regards economy in making colours by precipitation. Every case of precipitation is a case of double decomposition, so that the main product is always associated with bye -products, which are sometimes worth recovering, or which may be utilised in other ways. Thus, in making chrome-yellow by the process mentioned above, potassium nitrate in solution is a bye-product; PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. 303 in places where fuel is cheap it might pay to boil down this solution and recover the salt. Then again, by paying attention to the bye-products, and their probable use in other ways, it is possible to effect economies in the production of colours. Thus, supposing lead sulphate is to be made, this can be done by precipitating a solution of lead acetate, with either sodium sul- phate or sulphuric acid, as shown in the following equations : — Pb2C 2 H 3 0 2 + Na 2 S0 4 = Pb S 0 4 + 2NaC 2 H 3 0 2 . In the first case, sodium acetate is formed, and although this has cost money, yet it must be thrown away, because it cannot be economically recovered by the colour-maker. In the second case, the acetic acid formed may be utilised in producing a fresh stock of acetate of lead from metallic lead; in this case there are no waste products, and the manufacture of the lead sulphate is conducted most economically. The character of the precipitate formed is modified in regard to its tint, its consistence, and in other ways by the conditions under which it is obtained. Thus, if barium sulphate be pre- cipitated from cold solutions, it falls down as a very fine, rather light powder, which is difficult to filter; but if thrown down from hot solutions, it is not so fine, and can be filtered more readily. Again, in making chromes, the conditions under which the operation is performed have a wonderful influence on the result; a difference in the lead salt influences the tint, the nitrate giving a finer product than the acetate. The temperature also modifies the result considerably. Thus, the chrome which falls down from cold solutions is much paler and more voluminous than that obtained from hot solutions. The acid or neutral state of the solution also has some influence, while the presence of such bodies as alum or sulphate of soda has a material influence. In making colours by precipitation, the following conditions affect the character of the resulting pigment: — (1) Strength of solution. Generally, weak solutions yield finer and more volu- minous precipitates than strong ones. (2) Temperature. From cold solutions the product is usually finer and more volu- minous than from hot solutions. (3) Proportion between the inter- changing bodies. This also has some influence on the result. Thus, in making chromes it is preferable to keep the lead in excess ; if a soluble Prussian blue is required, then the potassium Lead Sodium acetate. sulphate. Lead Sodium sulphate. acetate. Pb2C 2 H 3 0 2 + H 2 S0 4 Pb S 0 4 + 2 H C 2 H 3 0 2 . Sulphuric acid. Acetic acid. 304 COLOUR AND PAINT MACHINERY. ferrocyanide must be in excess. Other examples might be given illustrative of this point. (4) The order of mixing is important. If the lead salt, in making chromes, were added to the bichromate of potash, the pigment obtained would not be so fine as in adding the bichromate of potash to the lead salt. In making soluble Prussian blue it is important to add the iron salt to the potassium salt, not vice versd. In making emerald green, the acetic acid should be added to the copper before adding the arsenic preparation, if a good result is to be obtained. There is another feature in precipitation worth mentioning here. When a solution of a metallic salt is added to another solution con- taining two other salts, both capable of precipitating the first, it may happen that a kind of selective action may take place; the metallic salt will at first form a precipitate with one of the mixed salts only, and not until this action has ceased will it precipitate the other. An excellent example is the action of silver nitrate on a mixture of sodium chloride and potassium chromate ; with the first it will give a white precipitate of silver chloride, with the second a dark red precipitate of silver chromate. When the two salts are mixed together, the silver nitrate will not precipitate the chromate until all the chloride has been thrown down, which point is shown by a change in colour of the precipitate from white to red. Another example is the precipitation of a mixture of sodium chromate and carbonate by zinc sulphate; in this case zinc carbonate is first thrown down. In precipitating a mixture of potassium bichromate and sulphuric acid with lead acetate, lead sulphate is thrown down before the lead chromate. This is a very interesting feature in precipitation, and is often taken advantage of by chemists for the separation of substances, one from the other; they know it as fractional precipitation. The plant required for making colours by precipitation is comparatively simple. There are required, 1st, vessels where- in to dissolve the various ingredients used ; and 2nd, vessels in which the ingredients are mixed together, precipitation vessels, which are preferably of wood, as those of earthenware would be too easily broken, and could not be made so large as required. The two classes of tanks or tubs should be kept distinct ; a dissolving tub should not be used for precipitating in. Then each colour that is made should have its own set of tubs. If a set of tubs were first used for making a chrome-yellow, and then for making a Prussian blue, the results in the second case would not be very satisfactory ; the rule of a colour shop should be that every distinct colour has its own set of tubs. The reason PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. 305 for such a course is that it is practically impossible to clean the tubs so as to avoid the liability of the remnants of one batch spoiling the next batch. Fig. 31 shows a good arrangement of plant for preparing pig- ments by precipitation; a set of five tubs is shown; two of these, CC, are placed on the floor, and used for the actual precipi- tation ; three, DDD, used for making solutions of the ingredients, and which may be smaller than C, are placed on a platform, P, above C. A steam pipe, S, with branch pipes, carry steam to all the tubs for the purpose of heating the contents, if that be necessary; s Fig. 31. — Plant for preparing pigments by precipitation. arrangements may be made for conveying water to these tubs. In the bottom of the tubs are plug holes or tap holes, which allow of the contents, when ready, flowing into C, through troughs. C has a number of holes, h, h, h, fitted with plugs, by the removal of which the supernatant liquors are easily 20 306 COLOUR AND PAINT MACHINERY. run off after the pigment has settled. Or the liquors may be siphoned off. Although a set of five are shown in the drawing as being an economical number, yet three only are, as a rule, required to be used at one time for preparing a pigment, but with five two batches of a pigment requiring the same materials may be more readily prepared. The size of these tubs must be proportioned to that of the quantity of pigment required to be turned out. A convenient size for the dissolving-tubs is 3 ft. 6 in. high, by 2 ft. 6 in. diameter; the capacity being about 110 gallons; the precipitating tub may be 3 ft. 6 in. high by 3 ft. 6 in. diameter, and will hold 320 gallons; batches of 7 to 10 cwts. of colour can easily be made in such tubs. The usual method of procedure in making pigments by precipi- tation is as follows. The materials, after being weighed out, are placed in the dissolving-tubs with the requisite quantity of water ; then, by means of the steam pipes, they are heated until complete solution has been effected. The liquors are now run into the precipitation tank, if they are to be used hot, or they may be allowed to cool before running into the precipitation tub. While running into the tub, it is desirable that the liquors be thoroughly mixed by stirring together. When all the liquors have been run into the precipitating tub, the mass is allowed to stand for the precipitate to settle ; when this has occurred, the clear liquor is run off, fresh clean water run in, the precipitate stirred up and again allowed to settle out, and the water run off ; if necessary, this washing is repeated once or twice ; finally, the precipitate is allowed to settle, the clear top water run off as much as possible, the precipitate thrown on to a filter for the rest of the water to drain away, and the still wet precipitate placed in the drying stove to dry. In making some pigments, such as rose pink, the lake-pigments from the coal-tar colours, vermilionettes, &c, solid bodies such as barytes, whiting and orange-lead are added. This is best done by running one of the solutions into the precipitating tub, adding the barytes, Gaeidic, . . ) Oleic, . . . . \ Elaidic, ... J Doeglic, . . . Brassic, . . . ) Erucic, . . . . \ H Cie H 2 g 0 2 H Cig H33 0 2 H C19 H35 0 2 H C22 H41 0 2 These acids are very characteristic of fats and oils ; oleic is by far the commonest of all fat acids, as when combined with glyceryl it forms olein, the fluid constituent of almost all oils. The lower members are more or less soluble in water and vola- tile by heat without decomposition ; the higher members are insoluble, and are decomposed by heat. 3. LINOLEIC SERIES OF FAT ACIDS. Name. Formula. Linoleic, ..... Homolinoleic, .... H C 16 H 27 0 2 H C 18 H 3 i 0 2 H Cj8 H33 O3 334 PAINT VEHICLES. Linoleic and homolinoleic acids are characteristic of linseed and other drying oils, while ricinoleic acid, which has properties very different from other acids, is found only in castor oil. Both the oleic and linoleic series of acids are monobasic, like the stearic series, and combine with potash and soda to form soaps which are rather more soluble in water than the soaps made from the stearic acids. Glycerine, the sweet spirit of oils, is a water-white, very viscid liquid, quite odourless, but possessing a sweet, though metallic, sort of taste. It is heavy, having, when pure, a specific gravity of 1*270. When heated it volatilises with difficulty, being slightly decomposed during the operation. It will only burn when heated, and then with a smoky flame having a small amount of luminosity. It has a great affinity for water, with which it mixes in all proportions, and which it absorbs from the atmosphere in no small proportions, being strongly hygroscopic. On this account glycerine gradually becomes weaker when exposed to the air. It is soluble in alcohol. Glycerine is a compound of the basic organic radicle glyceryl (C 3 H 5 ), with three equivalents of hydroxyl, O H. and has the (OH formula C 3 H 5 < OH It possesses alcoholic properties, and is (OH capable of combining with acids ; with monobasic acids it requires three equivalents to form saturated salts, and hence is capable of forming three different compounds with such acids ; thus, with oleic acid it forms — (OH (OH Monolein, C 3 H 5 OH Diolein, C 3 H 5 j C 18 H 33 0 2 I Gig H33 0 2 Triolein, C 3 H 5 |C 18 H 33 0 2 ' H 33 0 2 in which one, two, or three equivalents of hydroxyl are replaced by one, two, or three equivalents of oleic acid. These compounds can be formed by the direct union of oleic acid and glycerine, and it is of interest to note that the triolein so made is indis- tinguishable from the olein which is present in oils. When oils are boiled with solutions of caustic soda or potash they are decomposed. A compound of alkali and fat acid is formed while glycerine is liberated. This reaction is shown in the following equation : — THE DRYING OILS. 335 C 3 H 5 ] C 16 H 27 0 2 + 3 Na 0 H = ( H27 O2 Linolein. Sodium Hydroxide. (HO 3NaC ]6 H 27 0 2 + C 3 H 5 HO ( HO Sodium Glycerine. Linoleate. This reaction is known as saponification, because the alkaline compound obtained forms the familiar article, soap. As stated above, the oils are divisible into two groups — non- drying and drying oils. The former group is by far the larger of the two, but the oils in it are of no use to the painter. The latter group, which is the one which will be dealt with in this chapter, is small in number, but it contains oils of great im- portance to the painter. THE DRYING- OILS. There are but few oils, compared with the great number known, that can be used for painting on account of their pos- sessing the essential property of becoming hard or drying when exposed to the air. The best drying oils are obtained from vegetable sources, although one or two are obtained from animal sources. The following list includes all that can be included within this group : — Linseed oil. Hempseed oil. Poppy seed oil. Tobacco seed oil. Weld seed oil. Walnut oil. Firseed oil. Japanese wood oil. Menhaden oil. A few other oils, such as niger seed oil, cress seed oil, grape seed oil, cotton seed oil, possess weak drying properties, but these are of much too weak a character to permit of their being used as paint oils. Rosin oil is also offered as a paint oil, but it is not a good drying oil ; it will be dealt with in detail further on. The most important of this group of oils is linseed oil, which is the painter's oil par excellence ; the others are only used in painting on a comparatively small scale, and mostly by artists, not because they are any better drying oils than linseed, but because they have a paler colour, and, therefore, do not affect the tints of the colours quite so much — a matter of some im- portance when delicate tints have to be painted. For house- decorating purposes no other oil is used, because no other is so cheap or abundant. 336 PAINT VEHICLES. LINSEED OIL. — This oil is obtained from linseed— i.e., the seeds of the flax plant, Linum usitatissimum — which is culti- vated both for its seed and for its fibre, which latter is spun and woven into linen. Ireland, England, Holland, Germany, Russia, America, Canada, and India are noted for the cultivation of the flax plant, which will grow anywhere where the climate is not too hot, but a cold or temperate climate suits the plant best. Linseed is a small seed of a flat oval shape, somewhat pointed at one end ; it is lustrous, and of a pale brown colour. It varies a little in shape and colour, according to the locality in which it is grown, and an expert can tell by inspection without much error from whence a particular parcel of seed has come. It is exported in large quantities from Riga, Libau, Taganrog, and other Russian ports, and from Calcutta in India. Smaller quan- tities come from other places, but these occupy only a minor position compared with the seed coming from the places named above. Three qualities of linseed are recognised in the trade — Baltic seed, which comes from Riga and other ports on the Baltic coast of Russia, and is the seed of flax grown in the north of Russia ; Black Sea seed, coming from Libau, Odessa, and other ports on the Black Sea, and which is the seed of flax grown in Southern Russia ; and East India seed, which is exported from Calcutta. The Baltic seed yields the best and most valuable oil, that from Black Sea seed is next in quality, while East India seed gives oil of inferior quality. The oil obtained from seed grown in America, Canada, and other places, is mostly used locally, and very little finds its way to the English market. The seed imported is rarely free from other seeds such as those of hemp and rape. Extraction of Linseed Oil. — The oil is obtained from linseed and other seeds by a process of pressing ; but, before being pressed, the seeds undergo some preliminary treatment, with the object of facilitating the process of oil extraction. Naturally the process of extracting oil has undergone many changes during the last fifty years, and for the purpose of this work, it will be sufficient if the modern systems only are described in outline; for further details, the companion work on Lubricating Oils in this series of text books may be consulted. Two systems of oil extraction are used — viz., the English and the Anglo-American; these do not differ very much from one another. The English system includes five operations: — 1st, crushing; 2nd, grinding; 3rd, heating; 4th, pressing; 5th, refining. 1st, Crushing. — Prior to this, however, the seed is sifted EXTRACTION OF LINSEED OIL. 337 through sieves, with the object of separating out as much dirt and foreign seed as possible; this is a preparatory operation common and essential to all systems of oil pressing, as the presence of much oil from other seeds is apt to spoil the linseed oil. As the majority of the foreign seeds found in the crude linseed are rather smaller than linseed itself, nearly all are separable by using sieves of a certain gauge of mesh. Fig. 55. — Oil seed crushing mill. The sifted seed is now passed through a crushing machine which consists of a pair of heavy wheels placed horizontally in a strong frame (Fig. 55). One of these wheels is 4 feet in diameter, and is driven by the one to which power is applied; the other wheel, one foot in diameter, is generally driven by friction from the larger wheel, but in some makes of the mills by gearing ; by means of a system of screws and springs in 22 338 PAINT VEHICLES. connection with the bearings of the small wheel, a regulated pressure can be brought to bear on any seed which passes between the two wheels. The large wheel is driven at about fifty-six revolutions per minute. A hopper placed above the wheels supplies the seed in a regular manner, which is crushed in passing between the wheels, and falls into the receptacle Fig. 56. — Oil seed crushing mill. placed] underneath the machine for the purpose of collecting the crushed seed. Attached to each wheel is a scraper for keeping the surface of the wheel free from crushed seed, p 2nd, Grinding. — From the crushing machine the seed passes to the grinding mills, which are of the edge-runner type (Fig. 56), and consists of a pair of large stones, about 7 feet in diameter, EXTRACTION OF LINSEED OIL. 339 and 16 inches thick, revolving in a rather flat-shaped pan. The stones weigh about 7 tons, and have a speed of about 17 revolutions per minute. In this mill, the seed is ground for about 20 minutes, a little water being usually added. After it has passed through the edge-runner mill, the seed is in the form of a fine pasty mass. 3rd, Heating. — After being ground under the edge-runners, the seed is placed in a kettle, a cylindrical vessel made of copper, and jacketted, so that the inner portion and the seed it contains can be heated; agitators worked by suitable means are also provided to ensure that every part of the seed is heated. In this kettle the seed is kept for 20 minutes at a temperature of about 150° to 160° F. This heating of the seed serves two useful objects; it liquefies the oil, and so causes it to flow more freely from the seed while being pressed; and it causes the coagulation of the albuminous and mucilaginous matters contained in the seed, and so prevents these from flowing out of the seed during the pressing. 4th, Pressing. — The hot seed from the kettle is placed in woollen bags, large enough to hold about 8 lbs. of seed ; these are first placed in horse-hair cloths, generally known as "hairs"; and then between the press-plates of an hydraulic press, where they are subjected to a pressure of about 740 lbs. on the square inch for 20 minutes, followed by a pressure of about 2 tons for 7 minutes; during the whole period, oil flows out from the seed and into suitable vessels which have been placed ready to receive it. 5th, defining. — The oil as it flows from the press is far from being pure and bright; to ensure these qualities it undergoes a process of refining, the details of which vary in different works. First, it is usually placed in large tanks and heated by means of a steam coil to about 160° or 170° F.; this makes the oil more fluid, and causes the coagulation of any albumen which may have passed out of the seed; it is allowed to stand for some time, until all the solid impurities have settled, leaving the oil fairly bright; occasionally it is sold in this condition, but generally it is subjected to a further treatment, which consists in mixing it with 3 per cent of sulphuric acid, previously mixed with an equal quantity of water; the two bodies are thoroughly mixed together and then allowed to stand for 24 hours, when the acid will have settled to the bottom, leaving the oil at the top; the acid layer is run off, and the oil washed free from any traces of acid by means of warm water; it is then ready for use. The sulphuric acid has the property of acting upon any albuminous 340 PAINT VEHICLES. or mucilaginous matter contained in the oil, charring and dissolving it, thus carrying it out of the oil, leaving the latter in a purer condition, and, by removal of bodies which are prone to decomposition, rendering the oil less liable to grow rancid ; while in other respects its properties are materially improved. The oil as thus made is sold as "Raw Linseed Oil." The Anglo-American system is comparatively of recent intro- duction, but already it is most extensively adopted and will in all large oil mills be used exclusively. Like the English system it includes five operations, some of which resemble those of the system just described, while others are different: — 1st, crushing ; 2nd, heating ; 3rd, moulding ; 4th, pressing ; and 5th, refining. 1st, Crushing. — This operation is done by means of a crushing Fig. 57. — Oil seed crushing rolls. mill, shown in Fig. 57, which consists of a number (usually five) of chilled iron rolls placed one above another in a strong iron frame ; the size of the rolls varies according to the required capa- city of the machine, but a usual size is two feet six inches long, and one foot in diameter. The seed is fed by means of a hopper between the first and second rolls, and passing between the nip of these receives a first crushing ; then, passing round the second roll and between the second and third rolls, it receives a second crushing as it passes through the nip of these rolls ; the EXTRACTION OF LINSEED OIL. 341 crushing weight now exerted on it is greater than at first, the weight of the first roll in the first crushing being augmented by that of the second roll in the second crushing. As some of the crushed seed may adhere in the form of a cake on the surface of the rolls, scrapers are provided to scrape any such seed off the rolls and guide it through the various nips. In the same way as it has passed through the first, second, and third rolls, the seed passes through between the nips of the other rolls, being further crushed in so doing, the weight to which it is subjected increasing as it passes through the various nips ; in a roll machine having rolls of the size given above, each roll will weigh about 1J tons, so that if there are five rolls, the final pressure on the seed will be from h\ to 6 tons. The seed is crushed by such a set of rolls in a very perfect manner, much better than in the older system of a pair of rolls and an edge- runner grinding mill, and less labour is required. 2nd, Heating. — From the crushing mills the seed passes to the kettle, the construction of which is shown in Fig. 58 ; here the seed is heated to a tem- perature of 170° F. for 20 minutes. Steam is sent into the interior of the kettle to moisten the seed and to improve its con- dition for yielding oil dur- ing the pressing. 3rd, Moulding. — From the bottom of the kettle the hot seed is passed into a measuring box holding some 18 lbs. of the crushed seed ; thence it passes to the table of a moulding machine, Fig. 58, where it is moulded into a cake of a certain size depending upon the size of the press plates, but usually having a thickness of H inch, the machine exerting a small amount of pressure to bring the mass of meal to this thickness. The surplus seed is returned to the kettle or the crushing rolls to be passed through with fresh seed. 342 PAINT VEHICLES. 4th, Pressing. — After leaving the moulding machines, the cakes of meal are placed between two corrugated iron plates hinged together like the backs of a book. In the Anglo- American process the use of hairs and cloths are dispensed with, thus effecting no little economy in the process of oil extraction, for the hairs and cloths were an expensive item in the older processes, as fre- quent renewals were re- quired. The press plates, besides being corrugated, are often marked with various letters and other marks, which, being im- pressed upon the cakes of meal, serve to show what firm pressed the oil cake. A number of the press plates and their contents are placed in the hydraulic press and subjected to a pressure of from | to 1 ton on the square inch, for from 20 to 25 minutes, during which time most of the oil comes from the seed. It is a matter worth noting that the oil passes out from the seed cake along the edges and never from the sides of the cake. After being subjected to a pres- sure of about a ton for 25 minutes, the pressure is in many works increased to about 2 tons, at which it remains for 5 to 7 minutes, then, when all the oil has flowed out of the seed, the pressure is taken off, the press plates removed from the press and the oil-cakes taken out ; these are sold for feeding cattle, for which purpose they are of great value. The oil flows into tanks, from which it is pumped into large storage tanks, where it is stored until required for further treatment. It is interesting Fig. 59. — Hydraulic oil press. PROPERTIES AND COMPOSITION OF LINSEED OIL. 343 to note that an intermittent pressure, such as is given by the old stamper press and in the modern forms of hydraulic oil presses, gives far better results than a steady uniform pressure. Fig. 59 represents the form of hydraulic press made by Messrs. Rose, Downs & Thompson, Hull, to whom we are indebted for the various illustrations of the crushing of linseed. 5th, Refining. — This is done in exactly the same way as in the English system, which see. Properties and Composition of Linseed Oil. — Linseed oil is sold in two forms, known as " raw ,? and " boiled " linseed oil ; some text-books speak of a third form called " refined linseed oil," but the author considers this to be only a variety of the raw oil. Linseed oil as it comes from the press is rather turbid, of a dark colour, and contains some albuminous and mucilaginous matter. Before it is fit for use as a paint oil these must be removed, which is done by the methods detailed above ; then it forms the " raw linseed oil " of commerce. The '•' refined " linseed oil is mostly prepared for artists' use, and is obtained from the raw oil in various ways. In some cases the oil is allowed to stand for some months to settle, and then the clear top oil is exposed in closed glass vessels to sunlight to bleach it. Sometimes a small quantity of litharge or acetate of lead is mixed with the oil, and then, after standing some time, the oil, which is clear, is run off and bleached as before ; in other cases metallic lead is placed in the oil and left in contact with it for months ; the lead seems to exert a kind of bleaching action, and at the same time causes the albuminous matter in the oil to settle out. It may be laid down as a principle in treating oils that the simplest plan is always the best ; in any case the use of too much chemical action should be avoided. Raw linseed oil is a yellowish oil, having a brown hue, and possesses a characteristic odour and taste unlike those of any other oil. It is perfectly clear and limpid at all ordinary temperatures, but when subjected to moderate cold it thickens slightly, and solidifies at a temperature below — 27° 0. The vis- cosity of linseed oil measured in Hurst's standard viscosimeter at 70° F. is for the three principal varieties : — Baltic oil, .105 Black Sea oil, 108 East India oil, 112 The specific gravity of linseed oil at 60° F. (15°*5 0.) averages 0-935, but ranges from 0*932 to 0*937 ; Baltic oil is usually the heaviest. A sample of Baltic oil examined by the author had a 344 PAINT VEHICLES. specific gravity of 0*9378, a sample of Black Sea oil a specific gravity of 0*9326, while a sample of East Indian oil had a specific gravity of 0*9339. At 212° F. (100° C.) linseed oil usually has a specific gravity of 0*8801. Linseed oil is soluble in about 40 times its volume of alcohol at ordinary temperatures, and 5 times its volume at the boiling point. It readily dissolves in ether, petroleum spirit, shale naphtha, turpentine, chloroform, and similar solvents. Sulphuric acid has a strong action on linseed oil, causing it to become thick and of a dark colour ; large quantities of sulphur dioxide are evolved, while the temperature of the mixture is con- siderably increased, the amount varying somewhat in different kinds of linseed oil. Thus, with Baltic linseed oil the author obtained an increase of 120° C, with Black Sea oil an increase of 114° C.j and with East India oil an increase of 106° C. The action of nitric acid varies with the strength of the acid ; a moderately strong acid converts linseed oil into a viscid yellowish mass, which is insoluble, or nearly so, in petroleum spirit or benzol ; while strong, fuming nitric acid often causes linseed oil to take fire. Nitrous acid does not give a solid elaidin with linseed oil. In glacial acetic acid it is readily soluble on warming, while the turbidity temperature ranges from 36° 0. to 47° 0. # according to the quality of the oil and the strength of the acetic acid. Linseed oil combines very readily with bromine and iodine, absorbing a larger proportion of these bodies than any other oil ; there are slight differences between the various kinds of linseed oil in the quantities of iodine and bromine that they will com- bine with, but it may be laid down as a rule that the better the quality of the oil, the more iodine or bromine will it absorb. Of iodine the average amount taken up is 156 per cent, of the weight of the oil, while of bromine the average is 98 per cent. That is 100 parts of linseed oil will combine with 156 parts of iodine or with 98 parts of bromine. The property which gives linseed oil its special value as a paint oil is that when exposed to the air it gradually becomes hard, "dries up," in doing which it takes up from the atmosphere a large proportion of oxygen, forming a new compound of a resi- nous character, the properties of which have never been fully investigated. In this power of combining with oxygen, linseed is distinguished very markedly from other oils, which have little or no power of combining with oxygen. W. Foxf gives the * Hurst. On Valenta's test for oils. Journ. Soc. Chem. Ind., Jan., 18S7. t Oil and Colourmari's Journal, 1884, p 234. PROPERTIES AND COMPOSITION OF LINSEED OIL. 345 following as the number of cubic centimeters of oxygen absorbed by 1 gramme of various oils : — Baltic linseed oil, 191 Black Sea linseed oil, 186 East Indian Calcutta oil, 126 East Indian Bombay oil, 130 American linseed oil, 156 Brown rape oil, ....... 20 Colza oil, . 17*6 Cotton seed oil, 24*6 Olive oil, 8*2 Evidently, tbe quality of linseed oil depends very much upon its oxygen-absorbing powers ; thus, Baltic oil, which dries better than any other variety of linseed oil, takes up more oxygen than Black Sea or East Indian, which latter takes up the least, and is the worst variety of linseed oil known. American linseed oil is not equal to Black Sea oil in its drying properties, but it is much better than East Indian, owing, as is clear, to its greater absorb- ing power for oxygen. Baw linseed oil when exposed in the form of a thin film to the air, as it would be in painting, takes about two days to become thoroughly dry and hard ; the coat left is firm and elastic, but has not much lustre or gloss. When linseed oil is heated with caustic soda or caustic potash, it undergoes saponification, the amount of alkali required being from 13*4 to 14 per cent, of caustic soda, and 18*7 to 19*5 per •cent, of caustic potash, while about 9*4 to 10 per cent, of glycer- ine is liberated. When the soaps so produced are treated with dilute sulphuric acid they are decomposed, and the fatty acids of the linseed oil are liberated. These acids have a specific gravity of from 0-924 to 0*927 at 15° C, and from 0*861 to 0-864 at 100° C. They are solid acids, melting at 22° to 25° 0., and solidi- fying at from 20° to 18° 0., are insoluble in water, but readily soluble in alcohol, glacial acetic acid, ether and other similar solvents. Their combining equivalent is about 306, which points to the presence of acids of complex composition. Although the composition of linoleic acid (the characteristic acid of linseed oil) is usually given as 0 16 H 28 0 2 , recent researches have thrown some doubt on the correctness of this formula. It may be pointed out that the combining equivalent of the fatty acids is 306, that is more than is indicated by an acid having 16 atoms of carbon in its molecule. Allen has considered linseed oil to contain an acid which has 18 atoms of carbon; he has named it homolinoleic acid, but gives no further details. If linoleic 346 PAINT VEHICLES. acid contains 16 atoms of carbon, it is isologous with palmitic acid, and should on being hydrogenised, that is being acted upon in such a manner that it takes up hydrogen, yield palmitic acid ; while, as a matter of fact, it forms stearic acid, which is an acid containing 18 atoms of carbon. Then again linoleic acid when subjected to the action of alkaline permanganate of potash yields sativic acid, which is an hydroxy acid having the composition shown by the formula C 18 H 32 0 2 (HO) 4 . According to more recent researches linseed oil contains two acids. One is named linolic acid, and has the composition C 18 H 32 0 2 . It is a tetrolic acid, being capable of combining with 4 atoms of bromine. This acid yields sativic acid on oxidation with alkaline permanganate of potash. The other acid has received the name of linolenic acid, has the composition C 18 H 30 0 2 , and belongs to a series of acids having the general formula 0 n H 2n — 6 0 2 , a series not hitherto described. This acid has a high iodine value 245, a& might be expected from its being capable of combining with 6 atoms of iodine, or the same number of atoms of bromine. BOILED LINSEED OIL. — The property which gives linseed oil its peculiar value to the painter is that it absorbs oxygen from the atmosphere when it is exposed in thin films or even in large masses, thereby forming the body known as oxy linoleic acid ; this is capable of further oxidation to a body known as linoxyn, which has a composition indicated by the formula 0 32 H 54 O n , and has some valuable properties, being quite neutral in its reactions, more or less transparent, and somewhat elastic. It is quite insoluble in water, alcohol, or ether, but is slightly soluble in chloroform. By long boiling with caustic potash it is saponified, forming a red soap. This body is the final oxidation product of linseed oil when exposed to the air. Oxidised oil can, by means of solvents, be separated into two products ; one is insoluble, and the other is soluble. The insoluble body is, when freshly pre- pared, colourless, transparent, and gelatinous ; when dried it becomes a friable yet elastic solid. The soluble compound forms a coloured sticky mass resembling indiarubber, and for which it might act as a substitute. This property of absorbing oxygen is increased by heating the oil to a temperature of 400° or 500° F. for a few hours ; it is still further increased by adding to the oil while it is being heated certain bodies which are known as "driers" (see p. 384). Oil which has thus been heated is known as boiled oil, because it is usually heated in large boilers. Boiling Linseed Oil. — The boiling of linseed oil is done in several ways ; 1st, by fire heat ; 2nd, by steam. BOILING LINSEED OIL. 347 TO spa Fire boiling of Linseed Oil. — The most usual or common method of boiling linseed oil is by means of fire in ordinary shaped boilers of capacities varying from 100 to GOO gallons. In shape these boilers are similar to those used in laundries, Fig. 60, A. One fault of these boilers is that they do not last very long, owing to the oil, or the acid in the oil, attacking and corroding the boiler. This action is most energetic at the bottom where the flame impinges, and is a very serious matter for those who use large and expensive boilers. An attempt was made to remedy this evil by making the bottom of the boiler thicker than the rest as in Fig. 60, B; but these did not last any longer, while a special evil was introduced, owing to the thick metal re- taining the heat so long that simple withdrawal of the fire did not prevent the oil boiling over, as it does in the case of the thin-bottomed boilers. In all large oil boiling works, the boilers are made of wrought- iron boiler-plate of the form shown in Fig 60, C; in this form the boiler is made of a uniform thickness of plate, and the bottom, which is made separately, is rivetted to the sides. As the corroding action is confined to the bottom and to the rivets, these can easily be replaced when re- quired; so that these boilers will last longer than the other forms. The boilers are set in the furnace in the usual way. It is best, however, in all cases, to set the furnaces against the wall of the boiling shed, and to place the fireplaces for feeding the furnaces outside the shed, so that, should the oil boil over during the process, there is less risk of a conflagration Ficr. 60.— Oil boilers. 348 PAINT VEHICLES. from the oil finding its way into the fire. The fumes from the boiling oil should not be allowed to inconvenience the workmen, but should be conveyed to a hood placed over the boiler and connected with the chimney of the works. The boilers should be about one third larger than the quantity of oil which they are to boil, so as to allow room for expansion of the oil, and for any frothing or effervescence of the oil which may take place. The process of oil boiling is comparatively simple in principle, yet some difficulties crop up now and again in practice ; some of these are due to the quality of the oil, others are due to various obscure causes. The oil is placed in the boiler, which should never be more than two-thirds full, and the fire lighted. While the temperature of the oil is rising, the fluid should be closely watched as it is then that effervescence is likely to take place, and the oil to boil over with possibly disastrous results. Should there be any sign of the oil boiling over to too great an extent, then the fires should be withdrawn, and, by beating the oil, or ladling it out into an- other boiler, efforts should be made to keep down the effervescence. Much of this is due to the presence of small quantities of water and mucilaginous matter in the oil, and it more often occurs with fresh pressed oil than with oil which has been kept for some time after pressing, and from which, therefore, the water and mucilagi- nous matter has had a chance to settle out. After some time, dependent upon the quantity of oil being treated, and the size of the fire, the oil begins to enter into quiet ebullition ; this is said to be the boiling of the oil ; it occurs usually at a tempera- ture of about 500° F. The heating of the oil to this temperature should not be too rapid, so as to give the oil every chance of becoming oxidised, and it should not take less than two hours ; a longer time is preferable. When the oil has reached the boil, or better, after it has been boiling for about half an hour, a small quantity of driers is added ; other additions are made at short intervals during a period of three hours. The total amount of driers added varies a little in different works, but it averages about 5 lbs. to 1 ton of oil. After all the driers have been added, the oil is boiled for one hour longer ; then the fire is drawn and the oil allowed to stand over night to cool and settle. The clear oil at the top is sent into the warehouse, and sold as "boiled oil," while the turbid oil at the bottom is known as boiled oil foots, and is used in making putty or putting into cheap ready mixed paints. It is not advisable to add the whole of the driers, small as it is in proportion to the oil, to the latter all at BOILING LINSEED OIL. 349 once, as the action between the two might become too great, and the oil enter into rather violent ebullition, which could not be controlled readily ; by adding in small quantities at a time the action between the driers and the oil is less energetic and the boiling more under control; besides that, the combination between the driers and the oil is more complete. During the process of boiling oil, the latter undergoes some decomposition. Water is continually being given off, while large quantities of acrolein, C 3 H 4 O, a derivative of glycerine, which has a powerful action on the lachrymal glands of the eyes, acetic acid, formic acid, and other acids are also given off. As these products are somewhat obnoxious to the workmen, they should be conveyed by means of a collecting hood into the chimney of the works. The oil acquires a dark red colour, due to the presence of some of the products of the decomposition of the oil, although much depends upon the temperature at which the boiling is done. If this is kept below 400° F., a comparatively pale coloured oil can be produced; but if allowed to get over 500° F., then a dark coloured oil is sure to result. The demand is now for a pale oil, so that in boiling it is desirable to keep the temperature down as low as possible. The quantity and nature of the driers used have some influence on the colour of the oil. Manganese produces a darker oil than any other drier ; next to this is red lead and litharge. The acetates of lead and manganese, and the oxalate of the latter metal, produce the palest oils. It will be found best to give an hour's extra boil at a low temperature, say from 400° to 450° F., rather than to heat the oil to 500° F. and over, when darkening is sure to occur. What the character of the action is which goes on during the process of boiling linseed oil is somewhat uncertain. That oxidation occurs is certain, but that is all that is definitely known; probably linoxyn, which may be regarded as the resin of linseed oil, is formed to some extent. Then, when driers are used, there is formed a combination of linoleic acid with the base of the driers, which, dissolving in the rest of the oil, forms a kind of varnish, to which action some of the gloss of boiled oil is due. The function of driers in oil boiling is undoubtedly twofold. First, they act by increasing the oxidation of the oil, not by yielding up oxygen directly to the oil, because they are added in too small a quantity to have any appreciable action in this respect, but by acting in what is called a catalytic manner, causing by their presence a more ready and complete combination be- 350 PAINT VEHICLES. tween the oil and the oxygen of the air. How they accomplish this is not known with certainty, and only the barest assumptions can be made on this point. It is noticeable that the best driers are compounds of bases which can form more than one oxide or more than one series of salts. Lead forms four oxides, man- ganese forms three well-defined oxides, iron three oxides, while each forms more than one salt with an acid. When such is the case, it often happens that the salts can easily be transformed from one kind to another. Thus, ferrous salts are readily changed into ferric salts and vice versa; manganous salts into manganic salts, and so on. When iron or manganese is used as a drier in oil boiling, it can be conceived that these changes are continually going on, the iron or manganese and lead, if the latter be used, passing from one stage to the other, carrying oxygen from the air to the oil; at the most, however, this is only a theory, and without many facts to support it. The author is not inclined to lay much stress on this theory of the catalytic action of driers. The decomposition of linseed oil is not complete, because some eight or nine per cent, of glycerine can be extracted from boiled oil. 2nd. Steam Process of Oil Boiling. — Besides the ordinary process by means of fire-heat, oil can be boiled by means of steam-heat. Vincent* describes the following arrangement for carrying out this process. A copper pan (Fig. 61) of sufficient size is provided. This is fitted with a jacket, A, for steam to about two-thirds its depth. Above the pan, and forming a con- tinuation of it, is a dome, in which are three openings, one, B, in the centre for a vertical shaft, S, carrying agitators, C, for the purpose of keeping the oil in constant agitation during the operation. A large opening, D, in the front serves for the purpose of introducing the oil, and observing from time to time the progress of the operation, and for introducing, as required, the driers which are added to the oil. The third opening at the back is fitted with a flue, F, which passes into the chimney, and which serves to carry off any vapours which may be produced. Into the jacket a pipe, T, conveys steam at a pressure of 40 lbs., while another pipe, L, connected with a pump passes into the oil chamber, and is for passing air into the oil. A very convenient size for such a boiler is one holding about 2 tons of oil. * Journal Society of Arts, 1871. BOILING LINSEED OIL. 351 The plan of working is to first heat the oil in a tank with a closed steam coil for about two hours to a temperature of 95° to 97° C. (203° to 206° F.). This preliminary heating prevents a great deal of frothing in the boiling pan, which is a matter of consequence, especially with Indian oils. From the tank the warm oil is run into the boiler and the steam turned on in the jacket ; after some time the oil begins to emit a peculiar, somewhat sickly odour ; then the air is blown Fig. 61. — Steam oil boiler. through the oil, when the latter begins to froth a good deal and the odour increases and becomes more pungent. The agitators are kept at work during the whole of the time the oil is being treated. Driers are now added in small quantities at a time until \ lb. for every cwt. of oil in the pan has been added, this addition taking about two hours ; after which the oil is kept 352 PAINT VEHICLES. boiling for four hours longer. Then the steam and air is stopped and the oil run into the settling tanks, where it is allowed to remain for from three days to a week to settle. The clear oil is used for making paints, &c., and the foots for putty making. Another steam oil boiler is shown in Fig. 62.— Oil boiler, Fig. 62, and is made by Messrs. Rose, Downs & Thompson. It consists of a jacketted still-pan set on brickwork ; in this an agitating gear is fixed, which may either be driven from a separate engine (as shown in the drawing) or from the general shafting of the works. The oil is placed in the pan, the steam turned on and the agitator set in motion ; when all effervescence ADULTERATION OF LINSEED OIL. 353 of the oil has ceased driers are added in small quantities at a time, and the air is blown through by means of an air-pump. The advantages of using a steam process of oil boiling are that the risk of fire is greatly reduced, and the oil is paler in colour, which is a great consideration, although the character of the driers used will have some influence on this point, acetate of lead and other colourless driers giving the palest oil. For all purposes steam-boiled oil is as good as, if not better than, fire-boiled oil. Messrs. Hartley & Blenkinsop have patented the production of a heavy drying oil from linseed oil by the following process. A manganese linoleate is made by preparing a soap of linseed oil and adding this to a solution of manganese salt. The manganese linoleate is dissolved in twice its weight of turpentine, and from 2 to 5 volumes of this solution are added to 100 volumes of linseed oil. Insoluble matters are separated, the mass raised to a temperature of 212° F. in a special apparatus, and a current of air or oxygen passed through the oil until it has attained the desired degree of thickness ; for example, a clear, transparent oil of a pale amber colour having a specific gravity of 0*997 can be obtained from a linseed oil of specific gravity 0*937. This thick- ened oil may be used in painting, and in the manufacture of floor-cloth, linoleum, and similar materials. Properties of Boiled Oil. — Boiled oil is a slightly viscid oil of a reddish colour, varying a little in depth of colour according to the temperature, and the length of time it has been heated in the process of boiling. Its odour is peculiar, and its taste, which is rather acrid, somewhat characteristic. In specific gravity it varies a good deal, but the average is about 0*945 ; some samples will reach 0*950, while others may be as low as 0*940. Boiled oil is soluble in turpentine, petroleum spirit, shale spirit, benzene, carbon bisulphide and other similar solvents. When boiled with caustic soda or caustic potash it is saponified almost completely; there is usually a small trace of unsaponifiable hydrocarbon oil formed by the decomposition of the oil during the process of boiling. When exposed to the air in thin layers it dries much more rapidly than raw linseed oil, and leaves behind a hard, lustrous coat; it is this property which makes boiled oil of so much use to the painter ; yet it does not do to use boiled oil alone in the making of paints, because the coat which it leaves is too hard and rather liable to crack on exposure to the air; raw linseed oil is always added, as, by leaving a more elastic coat, it prevents this bad fault of boiled oil from showing itself. Adulteration of Linseed Oil. — Both the raw and boiled 23 354 PAINT VEHICLES. linseed oils are frequently adulterated ; (substitutes for boiled oil will be described more fully later on) ; the principal adul- terants used are mineral and rosin oils. Other fatty oils, such as cotton seed, niger seed, and whale oils, are sometimes used ; but, as linseed oil is cheap, the small gain arising from their use does not compensate for the probable loss of custom which must ensue if the adulteration be found out ; while the great difference in the cost of linseed and mineral oils is a strong inducement for adulterating with the latter. For the purpose of detecting adulteration the following tests may be applied : — 1. Specific Gravity. — For raw linseed oil this should be about 0*932 ; if less than 0*930, adulteration with fish, seed, or mineral oils would be indicated ; while if the gravity exceeds 0*937, then admixture with rosin oil is very likely. The specific gravity of boiled oil averages about 0*945 ; if much heavier than this it is quite probable that rosin oil has been mixed with the oil ; while if below 0*940, then other fatty and mineral oils may be looked for. 2. Flash Point. — Linseed oil, whether raw or boiled, flashes at about 470° F. Other fatty oils flash at about the same tem- perature. Rosin oil flashes at from 300° to 330° F., and during the process of testing a strong odour of rosin would be given off. Mineral oils, such as would be used to adulterate linseed oil, will flash at from 380° to 420° F., so that the flash point is one of the best tests for detecting the adulteration of linseed oil with mineral or rosin oils. 3. Proportion of Mineral or Rosin Oils in Linseed Oil, — To determine the proportion of mineral or rosin oils, in adulterated linseed oils, place 20 grammes in a beaker with a little water and alcohol ; then add some caustic soda and boil for some time, stirring at intervals ; the linseed oil becomes saponified, while the adulterants are not acted on ; after about an hour's boil, the mass is allowed to cool a little ; then it is poured into a separating funnel and some petroleum ether is added, which will take up the mineral oil and form a layer on the top of the aqueous layer ; after allowing the two layers to separate completely, the bottom layer is run off, and the top layer is washed quite free from all traces of the soap formed by the action of the alkali on the linseed oil, by several treatments with warm water. The ethereal layer is then run into a weighed glass, the ether evaporated off, and the residue of mineral oil weighed. Whether the residue is mineral or rosin oil must be judged from the nature of the residual oil after evaporatiug off the ether ; if this is heavy and viscid, and smells of rosin when heated, then rosin oil is present ; if the residual oil is light, then mineral oil is present. POPPY OIL. 355 4. Cotton and other Fat Oils in Linseed Oil. — The detection of cotton seed, niger seed, or other fat oils in linseed oil is much more difficult, but much valuable information on this point will be gained by noticing the behaviour of the oil with strong sul- phuric acid, the character of the mass formed, and the temperature which the mixture of acid and oil attains. The character of the soap formed on boiling the oil with caustic soda, the appearance, melting point, and combining equivalent of the fatty acids which may be obtained from the soap so formed are also valuable indi- cations of the character of the fatty oil adulterants. 5. Driers in Boiled Oil. — About 25 grammes are boiled with a little dilute hydrochloric acid, with constant stirring, for about half an hour ; the mass is allowed to stand to separate ; the bottom acid layer contains the driers added during the boiling of the oil, this is run off and tested in the usual way ; then the oil is boiled with caustic soda until it is saponified, then the mass is treated in the separating funnel, as described above, to separate the mineral or rosin oil used to adulterate the boiled oil. The aqueous layer which has been run off may be acidified, and the acids obtained tested for rosin by Gladding' s test. BOILED OIL SUBSTITUTES. — Many substitutes are offered for boiled oil, some of which have been patented. In composition they vary greatly, and it is not possible to do more than briefly indicate their general features. Some are mixtures of boiled oil, rosin, turpentine, rosin oil ; others more closely approach an oil varnish in composition, being made by melting rosin, then mixing it with hot oil and thinning down with rosin spirit. Some are made by preparing a compound of lime or alkali with rosin or other resinous products, and dissolving this in oil and rosin spirit or turpentine. The quality of these products varies very much. None of them are equal to good boiled oil, although one or two very nearly approach it ; others are but inferior substitutes, and cannot be recommended even for inferior work. It is not possible to deal more particularly with these boiled oil substi- tutes in this book. POPPY OIL. — This oil is obtained from the seeds of the poppy (Papaver somniferum) by pressure, or it may be extracted by means of solvents. This oil, although a very good drying oil, is not largely used, chiefly because its price does not allow it to compete with linseed oil ; artists make use of it on account of its paleness in colour not interfering so much with pale tints as linseed oil does, its price not being so much an object with them as it is with house painters. 356 PAINT VEHICLES. Poppy oil is usually of a pale straw colour, very limpid, has little or no odour when fresh, and a pleasant taste ; the oil is free from the narcotic properties for which the plant itself is famous. In specific gravity it ranges from 0*924 to 0*927. It solidifies at - 18° C. It is soluble in about four times its volume of boiling alcohol and twenty-five times its volume of cold alcohol. Mixed with strong sulphuric acid (Maumene's test), the rise in temperature is about 88° to 90° C. It takes about 19 per cent, of caustic potash (K OH) to saponify it, and it absorbs about 134 to 137 per cent, of iodine. HE MP SEED OIL. — The hemp plant [Cannabis sativa) yields a roundish greenish-grey seed, very familiar to lovers of canaries, from which, on expression, an oil is obtained that is used for painting. The yield of oil varies from 15 to 25 per cent. Hempseed oil when fresh has a greenish-yellow tint, but on keeping it slowly turns to a brownish-yellow • its odour and taste are rather unpleasant. Its specific gravity ranges from 0*925 to 0*931. It becomes turbid at a temperature of - 15° C, but does not set completely solid until a temperature of - 25° C. is attained. Strong sulphuric acid has a vigorous action on it, the increase in temperature being about 100° C. It absorbs from 143 to 144 per cent, of its weight of iodine, which indicates that it contains a large proportion of linoleic acid, and shows that its drying properties must be good. In this country hempseed oil is rarely used as a paint oil, its price being against it ; still, it has been mixed with linseed oil, and it is difficult to obtain the latter free from it, owing to the Russian linseed growers mixing hempseed with the linseed. In Russia, and other places where hempseed is grown, the oil is used rather largely for painting. WALNUT OIL. — The common walnut, the fruit of the walnut tree {Juglans regia) contains about 50 per cent, of its weight of an oil possessing drying properties. The process of extraction of this oil is as follows : — The nuts are collected and placed in heaps for a period of about three months, when they begin to decompose ; they are then crushed and pressed ; this gives " virgin nut oil," often used as a food oil as well as a paint oil. The nuts still contain some oil, which is extracted by grinding the cake with hot water and again subjecting it to pressure; the oil so got is known as " fire- drawn nut oil." Walnut oil is usually of a pale yellowish-green tint, but it can be prepared from fresh kernels almost colourless. The specific gravity varies from 0*925 to 0*927 ; it begins to be turbid at a ROSIN OIL. 357 temperature of - 15° C, but becomes solid only when at a temper- ature of - 27 '5° 0. Strong sulphuric acid causes the evolution of some heat, the increase in temperature being 101° to 103° C. It will absorb about 144 per cent, of iodine, pointing to its con- taining linoleic acid in large proportion. It is a powerful drying oil • some authorities say that it is superior to linseed oil in this respect ; at all events it is quite equal to it in drying power. It is chiefly used by artists, as it is pale in colour, and can, by bleaching, be obtained almost colour- less. Its greater cost prevents its coming into extensive use as a substitute for linseed oil in house painting. ROSIN OIL. — When rosin (the solid residue left by turpen- tine after all its volatile spirit has been distilled off by the aid of steam) (see p. 362) is distilled in large iron retorts it is decom- posed, and five principal products are obtained: 1st, gas; 2nd, pyrolic acid, a watery liquid containing 10 to 12 per cent, of acetic acid ; 3rd, rosin spirit ; 4th, rosin oil ; and 5th, a residue, rosin pitch, which is left behind in the still. The proportions of these bodies obtained depends partly on the nature of the rosin and partly on the method of distillation. There are two principal methods of distillation. In one, the most used, the rosin is placed in large vertical stills of cast iron capable of holding about 2,000 gallons, the usual charge being 70 barrels of 25 gallons capacity. The still is connected with worm condensers and receiving tanks. When heated by fire the various crude products named above pass over. The proportion of products to the rosin employed varies from time to time, but the following are the average quantities : — Gas, 5*4 per cent. Acid water, 2*5 Rosin spirit, 3*1 ,, Rosin oil, ." . . . . 85*1 Pitch, o . 3*9 „ Sometimes the distillation is carried to dryness, when coke is obtained instead of pitch as the final residue; but, pitch being the more valuable product of the two, this is seldom done. The operation of distilling takes from 50 to 60 hours, the usual length of time being 56 hours. The second method of distilling is similar to the first so far as the plant is concerned, but the distillation is carried out with the aid of superheated steam in addition to fire heat. The main products are the same, but there is a larger proportion of spirit and a smaller proportion of rosin oil; the spirit averages about 15 per cent, and the oil about 62 to 64 per cent, of the weight of 358 PAINT VEHICLES. the rosin employed, while the amount of acid water is necessarily largely increased by the condensation of the steam passed into the still. This process is not much used. The rosin spirit is described more fully on p. 372 et seq. The crude rosin oil which comes from the stills varies in appear- ance and quality at various periods of the process ; and also according to the manner in which the operation is conducted. Two varieties of crude rosin oil are recognised; " hard rosin oil," which is chiefly obtained when the distillation is conducted rapidly, and is the product which comes over during the first stages of the operation ; and " soft crude rosin oil," which is the product obtained when the process is conducted slowly, and during the middle period of the distillation ; sometimes a " medium crude rosin oil " is collected as the final part of the oil to come over. " Hard crude " is a thick turbid oil of a dark red colour, used for mixing with coal-tar and paraffin greases for making lubri- cating greases with. As it, when exposed to the air, absorbs oxygen and resinifies somewhat, it has been used as a paint oil, but its use, for reasons which will be pointed out presently, is unsatisfactory. " Soft crude" is rather thinner than the last; is, if anything, lighter in specific gravity, and is more acid in character. Its chief use is for mixing with lime in the preparation of wheel greases. It does not dry as well as " hard " rosin oil. The specific gravity of the crude rosin oil varies from 0*996 to 1*030; it has acid properties, due to the presence of an acid or acids, the nature of which is unknown. It takes 0*3 parts of sodium hydroxide to neutralise the acid in 100 parts of the oil. There is much, about 60 per cent., unsaponifiable oil in crude rosin oil. Crude rosin oil is refined by alternate treatments with sulphuric acid, caustic soda, and redistillation; the oftener these operations are repeated the purer becomes the refined rosin oil obtained; the refining can be carried so far as to yield a product of a very pale colour. Refined rosin oil is a viscid, oily liquid, varying in colour from dark red to pale yellow in the best refined samples ; its specific gravity ranges from 0*980 to 0*995; it has a peculiar odour. Its taste, especially if there is a strong after-taste, is peculiar and characteristic. The cruder varieties have a bluish or bluish- violet bloom or fluorescence, which is less marked in the more highly-refined samples. The cruder varieties of refined rosin oil contain small quantities TABLE OF DRYING OILS. 359 3 » o erg * St QQ o i is* °£3 9>o O c S3? or? »-3 K p o H •si ft . eg ■2 S Ifl 8 i ^3 ^ o *t3 S3 p£3 5 £ SI d> 5* -f3 oo 6S ""I o T3 ft- r3 _ o « ■as CO s w ft 360 PAINT VEHICLES. of free acid and some products capable of combining with caustic soda ; the more highly refined products are almost, if not entirely, pure hydrocarbons. The flash point of rosin oil is about 320° F., and the fire point about 390° F. The odour on heating is characteristic of rosin oil. The cruder varieties of rosin oil possess feeble drying powers, but the more refined oils have none at all. Rosin oil possesses one very objectionable feature in its drying properties; it dries, but in the course of a few weeks the coat becomes soft and tacky or gives again, as the painters say ; even if rosin oil paint has been used for a bottom coat and a good linseed oil paint for a top coat this defect will make itself apparent. No means are known by which this defect of rosin oil can be remedied or of increasing the drying power in any degree. The use of rosin oil in paints should, therefore, be avoided. The addition of rosin oil to linseed oil or other paint oils can be readily detected by the increase in specific gravity, the low flash point, and the odour of rosin on heating ; while the amount may be approximately ascertained from the amount of unsaponi- fiable oil left after boiling with caustic soda. The other oils are of but slight importance, and the preceding table will give all necessary information about them. TURPENTINE. — Turpentine is the term which was origi- nally given to some resinous exudations from various species of pine and other coniferous trees, but, of late years, this term has been used to distinguish a volatile liquid obtained from the crude turpentines by distillation; formerly this liquid was known as "oil" or "spirits of turpentine," and occasionally it is now so named; it is also known shortly as "turps." The crude turpen- tines are but of small value commercially, and some are only used in medicine. There are many varieties, such as Venice, Strasburg, Canadian, Chian, Aleppo, &c. Each of these has a soft resinous character and an aromatic odour ; when distilled with steam they are decomposed into a volatile spirit and a solid residue, rosin or colophony. As these bodies are not used in making varnishes no further mention will be made of them. Under the term turpentine will be described the liquid spirit used by painters and varnish makers. There are three varieties of turpentine met with in the English market — viz., American, French, and Russian turpentines. All these are derived from various species of pine trees. American Turpentine is derived from two or three species of pine, chiefly from the swamp or Georgia pine (Pinus australis), which grows in extensive forests in North and South Carolina, FRENCH TURPENTINE. 361 Georgia, and Alabama, the former State being the largest producer of turpentine. From the loblolly pine (Pinus taeda), turpentine is also obtained. In winter, which extends from November to March, gangs of men proceed to the forests for the purpose of collecting the resin; for this purpose the trees are boxed, that is, a cavity is cut into the side of the tree, about 1 foot from the ground; the boxes have a capacity of about 2 or 3 pints. Sometimes 3 boxes will be made in a tree, but care is taken not to touch the heartwood, as such a proceeding would certainly kill the tree. The upper part of the box is always kept free from resin, and is frequently chipped so as to expose fresh surfaces of wood, which causes the resin to flow more freely. About March, the sap begins to flow and to collect in the box and on the sides of the cut surface, that which collects in the box is called " dip," and that which collects on the sides is known as "scrape." That which collects the first year in a box is known as " virgin dip/' and is always collected separately. The crude resin is known commercially as "gum thus," and is exported for use in making varnishes. Most of the resin is, however, treated locally for turpentine and rosin. Turpentine is obtained from the crude resin by placing it in a still; into this still passes a steam pipe from a steam boiler, while out of it passes a pipe in connection with a worm condenser; a manhole on the top serves for the purpose of filling the still, while a large pipe at the bottom serves to run off the residual rosin. The still is built into a suitable furnace, so that it can be heated by fire. When sufficient resin has been placed in the still the fire is lighted, and when the temperature has attained a little above the boiling point of water, the current of steam from the boiler is sent in ; the turpentine passes over into the worm condenser and condenses along with water from the steam ; when no more turpentine comes over, the rosin left in the still is run off into barrels, when the still is ready for another charge. The turpentine is often purified by a second distillation. The appli- ances in use are generally of a crude description. The properties of American turpentine will be dealt with shortly. French Turpentine. — This variety is obtained from the maritime pine (Pijius maritima), which grows very extensively in the South-west of France, especially in the Departments of Landes and Gironde. The industry in these districts is con- ducted on a rather more scientific principle than in America. The trees are cut in February or March, and the sap is caused to flow into an earthenware vessel placed at the foot of the tree. 362 PAINT VEHICLES. The trees are tapped for five years in succession, when they are not touched for a few years, and then tapping commences again ; when the tree has got somewhat exhausted, the final tapping takes place, and a large yield of resin obtained, but the tree is killed. It is felled, and another planted in its place. The distillation of the crude resin is carried on in the plant shown in Fig. 63, which is a front view or section of, and Fig. 64, which is a side view or section of the plant. A is a boiler heated by means of a steam coil, or (as shown in the drawing) by means of a fire, the former method being preferable ; in this, the crude resin is heated to a temperature of 96° 0. (194° F.), when it becomes liquid. The boiler is fitted with a movable cover to prevent the easy escape of turpentine and the entrance of dirt. When the resin is melted, it is run into the tank, B, through the pipe, a ; the particles of woody tissue, dirt, &c, are deposited in this tank, as also in the boiler, A. From the tank, B, the melted resin is run into a tank, C, which holds the quantity usually treated at one time (about 66 gallons); this tank is, therefore, a measuring tank, from this it runs through the pipe, b, into the still, D, which has the form shown; into this passes a steam coil, by which steam from an ordinary steam boiler can be sent into the still. An opening near the bottom of the still (which is kept plugged during the time the turpen- tine is being distilled), permits of the rosin being run off. E is an ordinary worm condenser fitted into a tub through which cold water is continually passing; with this worm condenser the still is connected by means of a goose neck (shown in the drawing). The crude resin is placed in the still, 66 gallons being the usual charge; it is then heated by fire until a temperature of 135° C. (275° F.), is attained; when a current of steam is passed into the still, turpentine begins to come over and to condense along with the water from the steam in the worm condenser, the condensed products passing into a suitable receptacle, in which the water gradually settles to the bottom, while the turpentine rises to the surface; the latter is skimmed off and run into other narrow-mouthed vessels, in which it is allowed to stand for several days, during which the remaining water and other impurities settle out. The yield of turpentine is rather more than one-fifth that of the crude resin employed. When all the turpentine has been distilled over, the residue in the still is run first into a tank, F, and from thence into a revolving screen, G, through which it flows in a fairly clear condition free from dirt and grit of any kind. By this means, 363 364 PAINT VEHICLES. TURPENTINE, 365 too, the rosin is freed from any water it may contain. The quantity of rosin obtained is rather less than four-fifths of the resin used. French turpentine is almost entirely consumed in France; very little is now exported into England. Its properties will be discussed later on. Russian turpentine is obtained chiefly from the Scotch pine, Finns sylvestris. The method of obtaining it does not differ essentially from that adopted in extracting American or French turpentine, although there are some minor differences in the method of tapping the trees and collecting the crude resin, and in the manner of distilling the turpentine, which is usually done in a rather crude manner. Russian turpentine differs slightly in properties from American and French turpentines. Turpentine is a hydrocarbon having the formula C 10 H 16 ; there are, however, a number of isomeric compounds known which have the composition represented by the above formula. These bodies have been named the terpenes; they are derived, as well as the three varieties of turpentine already described, from natural resins or from various natural oils. They closely resemble one another in their chemical as well as in many of their physical properties. The terpenes have been investigated by Berthelot, Tilden, Wallach and other chemists, and a number of them are known. Berthelot pointed out that French turpentine had some different pro- perties from American turpentine, although their chemical composition was the same. He named the terpene of American turpentine, australene, and that from French turpentine, terebenthene ; while he gave to the characteristic hydrocarbon of Russian turpentine the name of sylvestrene. Armstrong considers that American turpentine is a compound of two terpenes, one of which is the same as found in the French turpentine and which rotates a ray of polarised light to the left ; this he names leGvoterebenthene. The other terpene has similar properties, only it rotates the ray of polarised light to the right ; this he names dextroterebentbene ; it is found in a very pure condition in the turpentine from Pinus Khasyana, a Burmese tree. Wallach describes nine terpenes which he names — 1. Pinene, the main constituent of French and American turpentine. 2. Camphene, which differs from all other terpenes in being solid ; it is not found naturally, but can be prepared by artificial means from pinene. 3. Fenchene, which is also obtained artificially. 4. Cimonene, found in the essential oils of various species of 366 PAINT VEHICLES. aurantiacece, oils of lemon, orange, bergamot, &c. 5. Dipentene, found in oil of camphor, Russian and Swedish turpentine, &c. 6. Sylvestrene, the characteristic terpene of Russian and Swedish turpentine. 7. Phellandrene, found in various essential oils. 8. Ter pinene, found in several oils. 9. Terpinonlene, a rare terpene. The two most important of these are Pinene and Sylvestrene, which are found in the chief commercial turpentines. Pinene is a colourless or water-white mobile liquid of a peculiar and characteristic odour, having a specific gravity of 0*8749 according to Tilden; Wallach gives it as 0*860. It boils at from 155° to 156° C. When dry hydrochloric acid gas is passed into it, combination ensues, and a crystalline body having the formula C 10 H- l6 HOl is formed; this closely resembles camphor in appearance and is known as artificial camphor ; by heating, under pressure, with caustic potash this body is decomposed and the solid terpene, camphene, is formed. When pinene is exposed to sunlight in the presence of water a crystalline compound is formed which has the composition O 10 H 18 0 2 , and is named by Armstrong sobrerol. Pinene in contact with water gradually combines with it, forming a crystalline hydrate, terpene hydrate, C ]0 H 16 3 H 2 O, which is soluble in alcohol, insoluble in turpen- tine, slightly soluble in cold water, a little more freely in hot water, and sparingly soluble in ether, chloroform and carbon bisulphide. There are two varieties of pinene, which differ from one another simply in their action on a ray of polarised light. One variety, that in French turpentine, turns the ray to the left, and is distinguished as lsevo-pinene, the other is found in American turpentine, and turns the ray to the right, and is named dextro- pinene. The air-oxidation and other products from the two terpenes differ from one another in the same manner. A mixture of the two pinenes, in equal proportions, would have no action on polarised light, and gives rise to inactive oxidation products. American turpentine contains both pinenes, the dextro variety predominating. Sylvestrene is the characteristic terpene of Russian and Swedish turpentine, derived from the Scotch pine, Pinus sylvestris. It is a colourless, or water- white limpid liquid, having a specific gravity of 0*846 at 20° C. ; it boils at 175° C. It has a dextro- rotatory action on polarised light; the lsevo-rotatory and inactive varieties are not known. Dry hydrochloric acid gas, when passed through sylvestrene, forms an hydrochloride, C 10 H 16 H 01, which is liquid. In this respect this terpene differs from pinene; it is also more easily oxidised when exposed to air and light. TURPENTINE. 367 The other terpenes are of no practical importance to the painter. Commercial French and American turpentine is a water- white, limpid liquid, with a peculiar and characteristic odour that dis- tinguishes it from all other bodies. The specific gravity ranges from 0*864 to 0'870, but usually is about 0*867. French turpentine is a little more uniform than American turpentine in this re- spect. It begins to boil at from 156° to 160° C, and is completely distilled at 170° C. If the sample be fresh, there is little or no residue left behind, but old samples generally leave a slight residue of resinous matter, which in any case does not amount to more than 1 per cent, of the turpentine. Turpentine is readily combustible, burning with a smoky name, a peculiar and characteristic odour being evolved. The flashing point of ordinary turpentine is 36° to 38° C. (97° to 100° F.). Turpentine is readily miscible with ether, carbon bisulphide, alcohol, benzene, petroleum spirit, but it is insoluble in water. It is a good solvent for oils, fats, resins, 200° C. 210° C. 215° C. COAL-TAR NAPHTHA. 377 the semi-purified naphtha, the latter is treated wtih caustic lime or caustic soda, which removes all the oxygen and sulphur com- pounds ; finally, the naphtha is well washed with water, and is then ready to be finally purified by a re-distillation. Once run naphtha has a specific gravity of 0*886 to 0*893, and is the raw material for the preparation of 90 per cent, benzol, " 50/90 per cent, benzol," 30 per cent, benzol, solvent naphtha, and burning naphtha, as the commercial products are named. The benzols are light products used in the manufacture of aniline dyes. Burning naphtha, which has a specific gravity of about 0*880 to 0*887, is sold for burning in out-door lamps, especially costermongers' lamps, although it has of late been largely displaced by the petroleum oils for this purpose. Solvent or coal-tar naphtha is largely used in the india-rubber industry, and for making varnish. It is a water-white liquid, having a peculiar and characteristic odour of coal-tar hydro- carbons ; in specific gravity it varies somewhat, the usual range being between 0*865 to 0*877. On being subjected to distilla- tion, it gives from 8 to 30 per cent, of distillate below 130° C, while as a rule 90 per cent, distils over below 160° 0. It burns with a very smoky flame, and is very inflammable, the flash point being about 1 20° F. It is miscible with alcohol, ether, turpentine, petroleum spirit, shale naphtha, and other similar solvents, while it is a good solvent for oils, fats, resins, and is almost the only solvent for coal-tar pitch, and other pitches. In composition it is very complex, but it consists chiefly of the three isomeric, para-, meta-, and ortho-xylenes, C 8 H 10 , cumenes, small quantities of paraffins and olefines, and occasionally traces of naphthalene. Sulphuric acid has little or no action on coal- tar naphtha, but nitric acid has a powerful action, and trans- forms the coal-tar hydrocarbons into the nitro derivatives, nitro- xylenes, 0 8 H 9 N0 2 ; nitro-cumenes, &c. Hydrochloric acid, caustic soda, and caustic potash have no action on it. It is used in making cheap quick-drying varnishes, rosin being the usual substance added to give the requisite coat ; it is more volatile than turpentine, although it does not leave any residue behind it. Commercial coal-tar naphtha is occasionally adulterated with petroleum or shale spirit, or with petroleum or paraffin burning oils ; in every case the specific gravity and flash points are reduced. The addition of the petroleum and shale spirits causes it to distil at lower temperatures and a little more regularly, while the burning oils raise the distillation temperatures rather considerably. 378 PAINT VEHICLES. Such additions may also be detected by treating the suspected sample with a well-cooled mixture of sulphuric and nitric acids, which converts all the coal-tar hydrocarbons into nitrocom- pounds, while the paraffin or petroleum oils are unaffected ; if now water is added, the nitro bodies, being heavy, sink to the bottom, while the petroleum hydrocarbons being light rise to the top, and may be collected and measured. It should be pointed out that finding a small amount of such unchanged hydrocarbons does not necessarily indicate adulteration, as coal-tar naphtha naturally contains small quantities of paraffin hydrocarbons. The best method of examining coal-tar naphtha for its quality is by distillation. The method commonly used is the fol- lowing : — 100 cc. of the naphtha is measured by means of an accurate glass measure into a tubulated retort of 200 cc. capacity; through the tubulure is inserted a thermometer, the bulb of which reaches within § of an inch of the bottom of the retort. The beak of the retort is connected with a long Liebig's condenser, and the distillation carried on by means of a Bunsen burner. It is best to insert the bulb of the retort into a deep sand bath, so that if the retort should crack, the naphtha would flow into and be absorbed by the sand, and no disastrous results ensue. The temperature at which the first drop flows from the end of the condenser is noted; with naphtha this occurs at about 110° C. Then the rate of distillation is noted; at 120° C. about 20 per cent, will come over, at 130° C. about 60 per cent., at 140° C. about 72 per cent., and 90 per cent, usually comes over below 150° C. Or, instead of taking the temperatures, as in the above example, and noting the quantity distilled at them, the tempera- ture at which each successive 10 cc, or 10 per cent., comes over may be noted ; the results will then be somewhat as follows : — 10 per cent, at about 128° 0., 20 per cent, at 130° C, 30 per cent, at 132° C, 40 per cent, at 135° C, 50 per cent, at 137° C, 60 per cent, at 140° 0., 70 per cent, at 145° C, 80 per cent, at 148° C, 90 per cent, at 158° C. Addition of petroleum or shale spirits increases the proportion distilled at the different temperatures, while petroleum or paraffin burning oils decreases the propor- tion considerably. Sometimes the makers take out the lower benzene hydrocarbons and thus reduce the value of the coal-tar naphtha for the particular purpose ; in such case, the tempera- ture of distillation will be increased. The following table shows, in a comparative form, the proper- ties of turpentine, rosin spirit, shale and petroleum spirits, and coal-tar naphtha : — METHYLATED SPIRIT. 379 Turpentine. Eosin Spirit. Shale and Petroleum Spirits Coal-tar Smell, cold, Peculiar. T! , 1 • lurpentinous. None or slight. / Smell of \ coal-tar. ,, on warm- ) in g> • 1 (Ji rosm. olight. j Smell of \ coal-tar. laste, . Characteristic j Peculiar ( after-taste. Variable and slight. 0-730-0 -760 j- Strong. Specific gravity, 0-SG7 0-883 0-870-0-877 Fluorescence, . None. Slight. Slight. None. Rotary power, Strong. None. Slight; poly- ) merises on / Sulphuric acid j i > Slight. Slight. Nitric acid, . . heating. ) Strong. Strong. Boiling point, . 156° C. j High and ( variable. Low and variable. High and variable. Flash point, -J 1 36° C. 97° F. 37° C. 98° '5 F. Ordinary temperature. 40° C. 104° F. METHYLATED SPIRIT— Methylated spirit is a very useful article in the preparation of varnishes and enamel paints. It consists essentially of a mixture of two bodies, methyl alcohol and ethyl alcohol; but in the ordinary commercial qualities there are usually small traces of other bodies, some of an ethereal character, others of an acid character. The alcohols are a very large and important group of chemical compounds, many of them finding extensive application in the various chemical arts. The type of the group is ethyl alcohol, C 2 H 5 O H, the body usually understood by the term alcohol when used by itself. It is also known as spirit of wine, for to it is due the intoxicating effect of wines, spirits, beers, and all beverages which have undergone fermentation. Pure ethyl alcohol is a colourless, very limpid liquid, having a pleasant odour and a hot burning taste. It is very volatile when exposed to the air, passing off completely and leaving no residue behind. It boils at 78°-5 C. (173° F.) and distils over completely and unchanged at that temperature. It is only solidified when subjected to the very low temperature of - 130° C. The specific gravity of pure alcohol at 15°*5 C. (60° F.) is 0*7935; but it has such an affinity for water that the preparation of a sample absolutely free from water is exceedingly difficult, so that the gravity given above may not be quite correct, but the error, if there is any, is small. Alcohol mixes with water in all propor- tions ; if the two bodies are fairly pure the proportion of alcohol 380 PAINT VEHICLES. may be ascertained by simply determining the specific gravity (see table on p. 382). It mixes with ether, chloroform, tur- pentine, carbon bisulphide, and benzol, but not with petroleum products. It dissolves fatty acids and castor oil readily, but it has only a slight solvent action on the other fatty oils. It dissolves rosin and a few other resins, such as, shellac, sandarac, mastic, more or less completely ; but it will not dissolve the hard copals, animi and kauri. It is a powerful solvent for coal-tar dyes, and other bodies. When subjected to the action of oxidising agents it is first transformed into aldehyde, C H 3 C O H, and then, finally, into acetic acid, CH 3 COOH. It is obtained as a product of the fermentation of sugar ; this body, which is present in grapes, malt, and fruits of all kinds, if kept under conditions which cause it to enter into fermentation, loses carbonic acid and water, while alcohol is formed in fair proportions ; this passes into the water in which the process is conducted and from which it is separated by distillation, and redistillation with the aid of quicklime. The alcohol ordinarily met with in commerce is known as " rectified spirit of wine;" this has a specific gravity of 0*838, and contains 86 per cent, of real alcohol ; what is known as ••proof spirit " has a specific gravity of 0*926, and contains 49 per cent, of real alcohol. Alcohol alone is not used in the preparation of varnishes as the high rate of duty levied by the Excise Authorities prohibits its use for this purpose. Methyl alcohol is a homologue of ethyl alcohol ; and has the composition indicated by the formula C H 3 O H. When pure it is a colourless liquid, very mobile and volatile, which has a fragrant spirituous odour, and boils at 55° 0. Its specific gravity at 15°*5 C. (60° F.) is 0*8021, but authorities vary a little on this point. It is miscible in all proportions with water, from which it is not easily separated ; it also mixes freely with alcohol, ether, tur- pentine, &c, and possesses great solvent properties for resins, &c. When subjected to the action of oxidising agents it is first changed into formaldehyde, H 0 O H, then into formic acid, HCOOH. Methyl alcohol is obtained in large quantity in the dry distillation of wood. The wood is placed in iron stills or retorts in suitable furnaces, when there come over gaseous vapours, which condense, partly into an aqueous layer, and partly into a tarry mass. The aqueous layer, which has an exceedingly com- plex composition, contains acid, alcoholic, phenolic, ethereal and METHYLATED SPIRIT. 381 other compounds. It is separated from the tar, treated with slaked lime, and then subjected to heat; crude wood-spirit distils over, while impure acetate of lime is left behind in the still. The spirit is very impure, and is further treated by redistilling for quicklime, then treating with sulphuric acid (which removes ammonia and methylamine) and, finally, redistilled with lime. Crude wood- spirit, as obtained by the above process, is a liquid of complex composition, containing about 95 per cent, of methyl alcohol in the best qualities, although some samples do not contain more than 40 or 50 per cent. The following bodies are found in wood-spirit, or wood-naphtha as it is sometimes called: — Methyl alcohol, CH 3 OH; acetone (C H 3 ) 2 C O, sp. gr. 0*792, b. p. 56°*5 C.) allyl alcohol, 0 3 H 5 O H, sp. gr. 0-8604, b. p. 96°*5 C. ; furfurol, ketones, &c. The odour of wood-naphtha is characteristic and somewhat unpleasant. It is due entirely to the impurities which are present in the spirit. Its taste, for the same reason, is ex- tremely nauseous; hence the use of wood-naphtha in making- methylated spirit. Wood-naphtha is used for dissolving gums and resins in varnish making, and it is worth noting that many of the gums are more freely soluble in the crude wood-naphtha than they are in the pure methyl alcohol. The cause of this increased solvent power of the crude spirit must reside in the ethereal impurities it contains, many of which dissolve resins more freely than does methyl alcohol. The following reactions serve to distinguish wood-spirit from pure methyl alcohol: — caustic soda gives a brown colour, sul- phuric acid a red colour, which increases in depth on heating; mercurous nitrate gives a grey precipitate of mercury. Methylated spirit is a mixture of 90 parts of rectified spirit of wine with 10 parts of wood-spirit, and this mixture is permitted by the Excise authorities to be sold, under special regulations, for manufacturing purposes free of duty, the addition of the wood-spirit rendering the spirit undrinkable. Of late, however, owing to improvements in the manufacture of the wood-naphtha, much of the nauseous taste is removed, and the methylated spirit now made is not so undrinkable. On this account the Excise authorities have recently compelled the addition of \ per cent, of petroleum oil to the methylated spirit, with the object of rendering it still more undrinkable, but the use of the original spirit is still by special permit allowed. The methylated spirit is usually sold at a strength of " 64 over proof," and has a specific gravity of 0*821. It contains 90 per 382 PAINT VEHICLES. cent, of real alcohol. The meaning of the term "64 over proof" is that when 100 volumes of this spirit is mixed with 64 volumes of water, there is obtained "proof spirit," which is a spirit of such a strength that when mixed with gunpowder it will not set fire to the powder when a light is put to it. The term "proof spirit " is very vague, and should be done away with. It would be better to sell the spirit according to the actual quantity of alcohol it contains. The strength of methylated spirit may be fairly accurately estimated from its specific gravity. Tables have been constructed showing the quantity of alcohol contained in spirit of different gravities. Space cannot be spared in this book for the reproduc- tion of those tables, but the following table contains some infor- mation on this point which may be of use : — Sp. Gr. at 60° F. Per Cent, of Alcohol. Per Cent, of Proof Spirit. Sp. Gr. at 60° F. Per Cent, of Alcohol. Per Cent, of Proof Spirit. 0-79384 100 175-25 0*880 66-7 129-5 0-800 98 173 0-890 62-3 122-5 0-810 94*6 169-2 0-900 58-0 115-3 0-820 91 164-75 0-910 53-57 107 6 0-830 87*2 159-75 0-91984t 49-24 100 0-8382* 84 155 45 0 920 49-16 99-8 0-840 83-3 154-5 0-930 46-64 91-64 0-850 79-3 148-8 0-945 39-8 85-59 0-860 75-1 142*6 0-950 34-5 82 0-870 70-84 136 * Rectified spirit of wine. + Proof spirit. Methylated spirit generally has an acid reaction, due to the presence of small quantities of acetic acid and aldehyde ; besides these, it contains traces of higher alcohols (amyl alcohol, propyl alcohol), oily and resinous bodies, ethereal compounds, and water. Methylated spirit is used in making varnishes from shellac, sandarac, rosin, mastic, dammar, and other resins ; such var- nishes are very quick in drying owing to the volatility of the methylated spirit. It is also used in the preparation of enamel paints. The quality of methylated spirit may be ascertained by distill- ing 100 c.c, when nearly all should be distilled below 100° C, the great bulk passing over between 80° and 90° C. The specific gravity is also a good indication of the quality, as shown in the table given above. In making any determination of the specific FINISH. 383 gravity particular attention must be paid to the temperature at which it is determined, as small variations of temperature cause considerable alteration in the gravity; the standard temperature is 15° '5 C. (60° F.). The actual determination may be made by means of an hydrometer — either the glass one ; or the metal one, known as Sikes' hydrometer, which is used by the Excise authorities ; or the specific gravity bottle may be used. Finish or methylated finish is methylated spirit containing about 3 oz. of rosin to the gallon. For some purposes this may be used in the place of methylated spirit, as the Excise do not place so many restrictions on its sale. It may be distinguished from the pure spirit by its giving a very copious white precipitate when water is added to it. On the Continent distilled animal or " Dippels " oil is used for the denaturing (or rendering undrinkable) of alcohol ; the use of this material has not been adopted in this country. 384 CHAPTER XIII. DRIERS. Driers are a class of bodies added to oil for the purpose of causing it to dry quicker than it would otherwise do. The bodies generally used for this purpose are salts of iron, lead, manganese, and zinc. The following list comprises all the com- pounds used as driers in paints and varnishes : — Red lead, litharge, lead acetate, lead borate, manganese oxide, manganese sulphate, manganese borate, manganese oxalate, zinc oxide, zinc sulphate, and ferrous sulphate. Of these, the lead salts are most in use ; the manganese com- pounds are largely used ; the others but rarely. Red Lead is fully described on p. 93, et seq. Litharge is the monoxide of lead, and has the composition shown by the formula Pb 0. It is prepared by oxidising lead in a current of air at a temperature sufficiently high to melt the oxide as it forms. On cooling, the litharge separates out in the form of flakes of a red-brown colour, which, on being ground up, forms a buff-coloured powder. Litharge is sold in the two forms here noted. It is soluble in dilute nitric acid and in acetic acid, forming the corresponding nitrate or acetate of lead. Hydrochloric acid dissolves it on boiling, forming the chloride; while sulphuric acid does not dissolve it, but forms the insoluble sulphate of lead. Mixed with oils, a slow action sets in, resulting in the formation of lead soaps, which are insoluble in water and many solvents. This action occurs with linseed oil ; the lead linoleate so formed dissolves in the rest of the oil, forming a kind of varnish, which, on drying, leaves a lustrous coat. It is this feature of lead salts that makes them valuable in the production of paint. Litharge is a powerful drier, and should not be used too extravagantly in the boiling of oil ; about J lb. to the cwt. of oil is quite sufficient. Red lead is also a good drier, even better than litharge, from | lb. to 1 lb. being sufficient for 2 cwts. of oil. Its action on oil partakes more of the nature of an oxidising action than does MANGANESE DIOXIDE. 385 that of litharge, while it is dissolved in the oil in the nature of a lead soap, as is the case with litharge. Both litharge and red lead are largely used in the preparation of boiled oil. The oil so prepared has a dark red colour, but dries quickly, and leaves a coat which is elastic and yet firm to the touch, so that it is capable of resisting a great deal of rough wear and tear, as also exposure to considerable variations of temperature. Lead acetate, Pb 2 C 2 H 3 0 2 , is a white crystalline solid pre- pared by dissolving lead or litharge in acetic acid, and evaporat- ing the solution down to dryness, or until it crystallises. It is readily soluble in water, and to a small extent in alcohol. It is used as a drier, principally for mixing with paints, and then it gives good results. Paint to which lead acetate, or sugar of lead as it is called, is added dries better on greasy surfaces than paint to which nothing has been added. As a drier it is not equal to either red lead or litharge, but it has the advantage of not causing the oil or paint to become dark or coloured. It is used in making what are called patent driers. Lead borate is a white powder prepared by adding a solution of borax to one of lead acetate or nitrate. The precipitate is collected, washed, and dried. It is largely used as a drier both in boiling oil and in mixed paints. It does not lead to the dis- colouration of the oil so much as red lead, while its drying properties are nearly equal to those of litharge. From J lb. to 1 lb. is sufficient for 2 cwts. of oil or paint. Manganese dioxide, the black oxide of manganese, Mn 0 2 , is now very extensively used as a drier. It comes into the market from two sources, one natural, the other artificial. The natural manganese forms the mineral manganese or pyrolusite, and is found widely distributed in large quantities ; for use, it is simply ground to a powder with water and then dried. It forms a greyish-black powder insoluble in water. Artificially, manganese is obtained from the still liquors of the bleaching- powder manufacturer, who, to prepare chlorine, treats manganese with hydrochloric acid, when he obtains a solution of manganese chloride, Mn Cl 2 ; this is treated by a process invented by Wel- don, when all the manganese it contains is recovered in a usable form. While much of this recovered manganese is used over again in the preparation of chlorine, some of it is sold for other purposes. Manganese dioxide is soluble in hydrochloric acid with evolution of chlorine and the formation of manganese chloride, Mn Cl 9 ; in sulphuric acid it dissolves with evolution of oxygen and the formation of manganese sulphate, Mn S 0 4 . Essentially it is a peroxide, a class of bodies which may be 25 386 DRIERS. described as containing more oxygen than is exactly equivalent to the metal present in them ; this extra oxygen is often rather loosely combined, and ready to enter into combination with other bodies ; it is this feature in the composition of manganese which makes it useful in oil boiling, for the oxygen, during the process, combines with the oil and oxidises it, while the manganese dis- solves to some extent in the oil in the form of a manganese compound of the iinoleic acid of the oil. Manganese is in consequence a powerful drier; in fact, the most powerful known. The proportion usually added in the process of boiling is J lb. to 1 cwt. of oil, and it is not desirable to increase this propor- tion much, as this would give rise to too much drying action, and cause the oil to form a hard and rather friable coat, not a firm elastic coat as it should do. Unfortunately, manganese has a tendency to make the oil dark. It is not a good drier for mixed paint, chiefly because the tendency in using it would be to add too much. Manganese sulphate, Mn S 0 4 , is prepared by dissolving manganese in sulphuric acid, and evaporating the solution down to dryness. It is a crystalline salt of a faint pink colour and somewhat hygroscopic properties ; hence, it should always be dried before using as a drier. Its drying action is, perhaps, rather more powerful than that of the lead compounds, but less than that of the last-named compound. Rather less than ^ lb. should be added to each cwt. of oil or paint. It possesses one advantage over manganese in not adding to the colour of the oil. Owing, however, to its somewhat hygroscopic properties it is not largely used as a drier. Manganese borate is a powder of a faint pinkish hue prepared by adding a solution of borax to one of a manganese salt, such as the sulphate or acetate ; the powder is collected, washed, and dried, when it is ready for use. As a drier it is one of the best and most powerful, being superior to the lead compounds, but inferior to manganese, although it has the advantage of not leading to any discolouration of the oil. Between J and ^ lb. is required for each cwt. of oil. The use of manganese linoleate, prepared by adding a solution of linseed oil soap to one of manganese chloride, has been lately proposed and patented. By using this product in a special way " boiled " oils are obtained much heavier than ordinary boiled oil, yet paler in colour. Manganese oxalate, Mn 0 2 0 4 , has lately been proposed to be used as a drier for oil, and is said to have some advantages over other manganese compounds. It is prepared by precipitating ZUMATIC DRIER. 387 manganese salts with oxalate of potash or soda ; or by treating manganese hydroxide with oxalic acid. One advantage is said to be that during the process of oil-boiling it is decomposed, and that, owing to the manganese dissolving in the oil in combination with the linoleic acid and to the oxalic acid being evolved in the form of carbonic acid, the metal is able to exert its greatest drying power. From \ to J lb. may be used per cwt. of oil. Zinc oxide, Zn O, is used as a drier, but cannot act as such as it has no drying properties at all. It is often put in mixed driers (see below), where it acts as a diluent to decrease the drying power of the other ingredients. For a description of its properties see p. 58. Zinc sulphate, Zn S 0 4 , is also used as a drier, but its virtues in this respect are rather problematical. As the commercial product contains much water of crystallisation it is necessary to dry it before it is added to the oil. Ferrous sulphate, copperas, Fe S 0 4 , is frequently added as a drier, especially in the preparation of varnishes. It is in the form of pale bluish-green crystals, containing 5 molecules of water of crystallisation ; hence, before being used, it must be dried to dehydrate it. It is soluble in water. It is prone to decomposi- tion by oxidation, especially if the crystals be exposed to moist air ; it is this property of being changed by oxidation from ferrous sulphate to ferric oxide that makes copperas useful as a drier. It should be used with care, as its tendency is to harden the coat of paint or varnish, and thus impart a tendency to crack. It is not by any means such a good drier as either lead or man- ganese salts ; from 1 to 2 lbs. are required for 1 cwt. of oil. Besides the simple driers described above, a variety of com- pound driers, usually composed of mixtures of the single driers in various proportions or with some linseed oil or boiled oil, are made ; it is not intended to describe these in detail, but a few recipes for the production of those principally in use will be given. Patent Driers. — Take 15 lbs. of dried zinc sulphate, 4 lbs. of lead acetate, and 7 lbs. of litharge ; mix them with 4 lbs. of boiled oil, and grind well together. Mix 100 lbs. of Paris white and 50 lbs. of white lead with 30 lbs. of boiled oil, grind, and then mix with the first mixture, adding sufficient boiled oil to give the mass the consistency of soft dough. The composition of the commercial " patent driers " varies with different makers, but the above is a common form. Zumatic Drier. — 25 lbs. of zinc white and 1 lb. of borate of manganese are ground together. The object of the zinc white is 388 DRIERS. simply to dilute the manganese salt, and to form a powerful drier in a convenient form. The proportions generally used are 1 lb. of the drier to 25 lbs. of paint. Zinc Drier. — 6| lbs. of dry manganese sulphate, 6| lbs. of dry manganese acetate, 6 J lbs. of dry zinc sulphate, and 980 lbs. of zinc white are ground together. From 2 to 3 per cent, of this is usually added to the paint. This is called zinc drier, because it was brought out as a drier for zinc white. It is also known as Guynemer's drier. In both the above mixtures the manganese salts only act as driers ; the other materials are really diluents, and of themselves can exert no drying action. Oxidised Oil Driers. — Oxidised oil or well boiled linseed oil makes a good drier, very useful in many cases. 389 CHAPTER XY. VARNISHES. Varnishes form a very important group of the materials used by the painter in carrying on his art. They are liquid bodies, more or less coloured, although colour is not an essential feature. When applied to the surface of a body, they lose a portion of their constituents by evaporation, and there is left a coat of a highly lustrous and durable character, thereby increasing the lustre of the object, developing its beauty, and protecting it from the destructive action of the atmosphere. When varnishes were first introduced is very uncertain, but the kinds now in use are of modern origin, and mostly of English introduction, English varnishes being superior to those of any other country. Their use has, during the present century, in- creased very considerably, and has now attained very large proportions. The subject divides itself into two parts — 1st, Varnish materials. 2nd, Varnish making. 1st, VARNISH MATERIALS. The materials used in the manufacture of varnishes can be divided into six groups : — 1st, Drying oils. 2nd, Resins. 3rd, Gums. 4th, Solvents. 5th, Driers. 6th, Colouring matters. In the trade, the second and third groups are generally classed together under one head — " gums." 1st, DRYING OILS. — These have already been considered (seejp. 335, et seq.), and very little requires to be added here. 390 VARNISHES. Linseed oil only has as yet been used in the preparation of oil varnishes, although some of the other drying oils could be used; but it is very doubtful whether they will give such good quality of varnish as linseed oil, and they are more costly,, which is an important item in varnish making. The linseed oil used for making varnish should be of the very best quality — the best Baltic; other varieties of linseed oil only yield poor qualities of varnish. It should be kept at least twelve months before being used. It is best stored in old steam boilers, and it is essential that the air should be excluded. Nothing is definitely known as to the character of the action that goes on in the oil during the time it is thus stored; but there is a wonderful difference between unstored and stored oil in their varnish-making properties, the latter giving much the better results. 2nd, RESINS. — This is by far the most important group of varnish materials, for on these bodies the lustre and lasting properties of the varnishes depend. They are all of natural origin, being exudations from various species of trees. # They are very numerous. Some are used almost exclusively for varnish making, others are also used for other purposes, while some resins are not used for varnish making, but find use in other directions. As a class, they are distinguished by being more or less hard, friable or brittle, lustrous, generally clear and transparent, al- though some are slightly opaque, insoluble in water, and soluble in alcohol, ether, benzol, and other solvents of a similar character, to a greater or less extent. In composition, they are very com- plex, being mixtures of bodies having acid properties. A few only of these bodies have been isolated and their characters definitely ascertained. In their ultimate composition they are rich in carbon, poor in oxygen, and contain no nitrogen. They are more or less combustible, usually burning with a smoky flame. They are usually devoid of colour, which is a valuable feature for varnish making, although some are coloured. As a rule, they are free from odour, but some possess fragrant and characteristic odours. * All exudations from trees which form hard, more or less brittle masses, are termed in the produce markets and trade generally " gums " — e.g., gum arabic, gum tragacanth, gum copal, gum animi, gum sandarac, &c, no matter what their origin or properties. True gums are those which are soluble to a greater or less extent in water (see p. 414), and resins are bodies which are not soluble in water, but soluble only in solvents like alcohol, turpentine, &c. In the following pages the word " gum " will (except as a trade description) be used exclusively for gums proper, and " resin " for the true resins, of which copal and animi are examples. RESINS. 391 Classification of Resins. — The resins can be classed into various groups. Cooke classes them into three divisions : — 1st, Resins ; 2nd, Gum-resins ; 3rd, Oleo-resins. The resins possess the properties enumerated above, and will be again referred to below. The gum-resins contain a little gum as well as resin in their composition ; very few are used for varnish making. The oleo-resins consist essentially of a mixture of resin with a liquid oil which imparts to them a viscid character ; they are useful bodies, although few find their way into varnishes. The resins can be divided into two groups — hard or copalline, soft or elemi resins ; another method of grouping them is into oil- varnish resins, ethereal-varnish resins, and spirit-varnish resins. This latter classification will be adopted in this book as being of a practical nature. Characters of Resins. — Some of these have been pointed out above in a general manner, but it is advisable to deal with them in a more detailed manner. The characters of resins which are of the most importance are form, appearance, colour, hardness, specific gravity, solubility. Form. — Most resins occur in the form of knotty masses, some in the form of drops, and others in that of cylindrical pieces. The resin flows out of the tree in the form of drops ; if the resin solidifies quickly it keeps this form, as, for example, mastic ; if the process of solidification is slow, then the resin tends to form into tears or cylindrical pieces, as, for example, sandarac ; if, again, the process goes on slower and the resin collects on the tree or drops on to the ground, it forms into knotty masses of various sizes and shapes, as, for example, copal, animi, dam- mar, &c. Some resins come into the market artificially shaped, as, for instance, gamboge in cylinders, shellac in thin plates, dragon's blood in thin sticks or powder, benzoin and elemi in blocks. Appearance. — The appearance of many of the resins is charac- teristic. Animi is clear and transparent, and has a peculiar rough surface which, from its appearance, is known as the goose skin. Benzoin, elemi, and some others have more or less an agglomerate appearance, as if made up of two or three kinds of gum ; such a structure is called by mineralogists amygdaloidal. Animi and copal have a very lustrous appearance ; elemi, benzoin, K 460 INDEX. White lead, Manufacture of, Condy's process, 36. ,, ,, Cookson's pro- cess, 38. .,, Cory's process, 28. ,, Creed's ,, 18. ,, Dale & Milner's process, 35. ,, Delafield process, 35. .,, Dundonald pro- cess, 34. ,, ,, Dutch process, 11. .,, ,, Dutch process, Theory of, 15. ,, ,, Fourmentin pro- cess, 31. „ ,, Gardner's process, 21. ,, German process, 18. ,, ,, Hatfield's process, 20. ,, Kremnitz pro- cess, 24. ,, ,, Lewis process, 37. ,, Lowe process, 35. ,, Maclvor's process, 32. ,, Martin's process, 30, 37. ,, ,, Maxwell Lyte process, 38. ,, ,, Milner's process 29. ,, ,, Morris's process, 21. ,, Mullin's process, 25, 37. ,, Ozouf's process, 38.^ Pattinson's pro- cess, 34. ,, ,. Precipitation pro- cesses, 23. Richardson's pro- cess, 20. „ ,, Rowan's process, 35. Spence's process, 32. ,, Thenard pro- , »c et ccc. « . . " c cess, 25. White lead, Manufacture of, Theories of the, 15, 21, 23, 25, 33. ,, ,, Thompson's pro- cess, 20. ,, Torassa's process, 37. „ „ Watt & Tebbutts process, 35. ,, ,, Woolrich's pro- cess, 38. ,, Maxwell Lyte's, 52. ,, Non-poisonous, 50. Patent, 44. ,, Pattinson's, 87. ,, Properties of, 40. ,, Sublimed, 47. Wilkinson's, 87. White pigments, 9. Whiting, 75. assay and analysis, 78. ,, composition, 77. ,, properties, 77. Wilkinson's blue, 200. ,, white lead, 87. Wood spirit, 381. Yellow lake, 256, 271. lakes, 256, 276, 2S0. ochre, 99, 133. ,, pigments, 115. ultramarine, 199. Yellow, Antimony, 144. ,, Arsenic, 145. ,, Cadmium, 147. Cassel, 143. ,, Chrome, 115. Indian, 146. ,, Kassler, 143. ,, King's, 145. ,, Mars, 142. ,, Montpelier, 143. ,, Mineral, 143. Naples, 143. ,, Royal, 145. Solid, 144. Turner's, 143. ,, Zinc, 129. Young's theory of colour, 6. Zinc chrome, 129. ,, assay and analysis, 132. INDEX. 461 Zinc chrome, Clarke's process of making, 131. ,, ,, Murdoch's process of making, 131. ,, preparation, 1*29. ,, properties, 132. drier, 388. green, 175, 179. oxide, 53, 387. ,, as drier, 387. sulphate, 387. sulphide whites, 60. Zinc sulphide whites, assa} T and: analysis, 67. ultramarine, 199. white, 53, 54. assay and analysis, 59». Griffith's, 63. Knight's, 65. manufacture, 54, 57- Orr's, 61. properties, 53.. Zumatic drier, 388. BELL AND BAIN, LIMITED, 41 MITCHELL STREET, GLASGOW. ^ >£. 28 2852 GETTY CENTER LIBRARY CONS TP 935 H96 1892 BKS c - 1 Hurst, George H. Painters' colours, oils, and varnishes : 3 3125 00222 7789