FT HE Importance 
 m= - of Chemistry 
 inthe Motion Picture 
 Industry. 
 
 ‘GLENN E, MATTHEWS 
 
 ss Research Laboratory 
 Eastman Kodak Company 
 eae, Rochester, Nj zy, 
 
The Importance of Chemistry in the 
 Motion Picture Industry 
 
 BY 
 
 GLENN E. MATTHEWS 
 
 Research Laboratory of the Eastman Kodak Company 
 
 in the development of photography is 
 aptly illustrated by a comparison of 
 the crude, cumbersome apparatus which was 
 necessary to make pictures years ago with 
 the simple, compact, cameras and the stable 
 and highly sensitive films available today. 
 When a wet plate photographer started out 
 for a day’s picture-making with his pack of 
 chemicals, plates, and dark tent on his back 
 he resembled a prospector more than a cam- 
 era man (Figure 1). Of necessity he was 
 his own manufacturing plant. He chemi- 
 cally sensitized his own plates just before 
 using, exposed them while wet, and de- 
 veloped them at once. Prints were made 
 later on paper which he sensitized himself. 
 With the introduction of the dry plate 
 and later the film, the crude methods of 
 wet plate photography disappeared, the prep- 
 aration of the materials became a commer- 
 cial operation, and photographers now pur- 
 chase almost all the materials that they use 
 from firms who manufacture them in large 
 quantities. This centralization has resulted 
 in a far greater improvement in quality than 
 would ever have been possible by individual 
 effort. 
 
 Motion picture film was first sold in Amer- 
 ica in 1889 when George Eastman supplied 
 narrow film “ribbon” to Thomas Fdison. As 
 now manufactured, it consists of a trans- 
 parent, flexible base or support on which 
 is coated a very thin layer of gelatin in 
 which are suspended microscopic particles 
 of a light sensitive silver salt. This upper 
 sensitive layer is called the emulsion. To 
 turn out millions of feet of film a year 
 maintaining an unvarying uniformity of 
 thickness, sensitiveness and quality requires 
 a highly skilled organization backed by 
 trained chemical research. In view of these 
 conditions it would be quite impossible for 
 an individual to prepare his own motion 
 picture film. 
 
 Experimentation must also be always in 
 progress to improve the film and to find new 
 methods of manufacture. In all this work, 
 chemistry plays an important part, not only 
 
 Ti PART that chemistry has played 
 
 Figure 1—A Photographic Pioneer 
 
 in the manufacture and treatment but later 
 in the processing, after treatment, tinting, 
 toning, and renovating of the film. On the 
 care with which these chemical operations 
 are conducted depends the wearing quality 
 or life of the film. 
 
 Chemistry in the Manufacture of 
 Motion Picture Film 
 
 In the manufacture of motion picture films 
 and other sensitized photographic materials, 
 absolute cleanliness is very necessary at 
 every stage of the process. All operations 
 must ‘be conducted in dust-free rooms and 
 
only clean, chemical substances: are 
 used. 
 
 Eastman motion picture film is manufac- 
 tured at Kodak Park, at Rochester, New 
 York. The plant consists of about 230 acres 
 situated in the northwest section of the 
 city. The output of this plant is roughly, 
 150,000 miles of film per year. To make this 
 quantity, over five million pounds of cotton 
 are used yearly, several millions of pounds of 
 gelatin, and over twelve tons of solid silver 
 
 per month. 
 
 pure, 
 
 The water necessary to take care of the 
 needs in manufacture is pumped through a 
 private pipe line into large reservoirs from 
 Lake Ontario, 4 miles away. The reser- 
 voirs have constantly on hand 
 water to supply a city of 150,000 people. The 
 temperature of the workrooms is rigidly con- 
 trolled at all times by refrigerating machin- 
 ry, having a cooling capacity equivalent to 
 the melting of 4,000 tons of ice every 24 
 hours. 
 
 In the preparation of film base or sup- 
 port, cotton is thoroughly washed in cir- 
 cular rotary vats with caustic soda solution 
 to remove vegetable gums and other im- 
 purities. After carefully drying in huge 
 dryers to eliminate all moisture (Figure 2), 
 it is treated with two acids, nitric and sul- 
 phuric, a process known as nitration. Ni- 
 ‘trating centrifugals, made of perforated 
 baskets rotating inside a vat, are used for 
 this process. The cleansed cotton is fed 
 into the basket and the acids run in until 
 the cotton is immersed (Figure 3). The 
 fibrous structure of the cotton is not de- 
 stroyed by nitrating but the treatment makes 
 it possible to dissolve the cotton later in 
 a solvent. When nitrating is completed the 
 acids are drawn off and the basket rotated 
 at high speed for draining. Nitrated cotton 
 is known as cellulose nitrate. The excess 
 acid is removed by placing the nitrated cot- 
 ton in centrifugal washers. After washing 
 in these machines, it is placed in large tanks 
 of water where it is drained and rinsed re- 
 peatedly: for several weeks. Centrifugal 
 wringers operated at high speed next re- 
 move all the water. All these elaborate pre- 
 cautions are necessary in order that the 
 cotton be freed from every trace of acid. 
 
 Washing and drying completed, the ni- 
 trated cotton is ready for dissolving in the 
 organic solvents. These are usually com- 
 pounds such as methyl alcohol to which cer- 
 tain other higher boiling liquids may be 
 
 sufficient. 
 
 =a 
 
 referred to as “dope.” It is thee piped : 
 large air tight tanks until ready for ce 
 ing (Figure 4). To remove any undissolved 
 specks and fibres, the dope is filtered und 
 great pressure. It is then Wehadiee. 
 
 solvents evaporate, the film dries, and is 
 peeled off. The thin sheets of trenipateng ; 
 base 2,000 feet long, 3% feet wide, and a 
 proximately 5-1,000ths of an inch in thick: 
 ness are wound up temporarily until ready 3 
 to be coated with the emulsion. : 4 
 
 For more pleasing presentation on he : 
 screen, motion pictures are often tinted by 
 bathing the flm in dye solutions which stain — 
 the gelatin. To save the finisher, the time 
 and trouble of this operation, Eastman posi-_ 
 tive film is supplied in several different col-— 
 ors of tinted bases. In this product the col- 
 or is impregnated in the film base. ery 
 
 For use in portable projectors which r. 
 quire a non-inflammable film, a special safer 
 base called cellulose acetate is manufacture a 
 It is made in much the same way as t 4 
 nitrate except that acetic anhydride is u 
 instead of nitric acid for treating the cottor 
 so as to render it soluble in the orga ic 
 
 solvents. 
 
 Preparing and Coating sig 
 
 We now turn to the making ats the of 
 sion or the light sensitive layer that | ‘ 
 the photographic image. It is. made_ in 
 grades, negative emulsion which is very 
 sitive to light and is used in the 
 and positive emulsion which is much less 
 light sensitive and is used for printing the - 
 pictures afterwards viewed on_ ‘the screen, 
 All emulsion making is conducted i in, 
 lighted with safelights which have bie e 
 cially preperie for ee PHOS ; 
 
 ee 
 
 in size wee in WP: ee he 
 are less than 1-10 as large. — 
 
 Silver, as used in pia '6 
 
 vices: (Figure 5). The bars are 
 in nitric acid and after recrys’ 
 porcelain dishes pure crystals of 
 trate are obtained. (Figure 6). 
 
‘auIyoOeU Surzeod (348A AMO) 
 2 eansy ‘syezsAad ozperAyIU ABATIS Surysem (19}U99 ABMOT) Q sAnNSYy ‘UOI[ING ASATIs JO steq (}J2] JAMO) G sANSY ‘sHUL. 
 aseioys adop,, (74511 doy) p eanSy ‘fuo0zjoo Sues (az9;uaes doz) ¢ sinsyg ‘faahap u0z}09 9y} sI (79, doz) Z oansiy 
 
 66 
 
gredients of the emulsion are potassium io- 
 dide,» potassium bromide and gelatin. If 
 these bromide and iodide salts are dissolved 
 in water and to the solution thus prepared 
 silver nitrate solution is added, an insoluble 
 yellow salt is precipitated which is very sen- 
 sitive to light, turning black after a few 
 minutes exposure. 
 
 If this solution is coated on the base, the 
 film would have very little sensitiveness and 
 for all practical purposes it would be worth- 
 less. For this and other reasons the pre- 
 cipitation must be conducted in the presence 
 of some material that will avoid these dif- 
 ficulties. 
 
 The material commonly employed is gela- 
 tin, a substance analagous to glue in com- 
 position, and lixe glue in that it is extracted 
 from the bones and hides of cattle. Photo- 
 graphic gelatin is usually prepared from calf 
 skin by soaking the skins in lime water, and 
 subsequently extracting with hot water. The 
 gelatin is dissolved in water and the bromide 
 and iodide solutions carefully mixed with 
 it. To this mixture heated to the correct 
 temperature, is added the silver nitrate solu- 
 tion. The precipitate of the sensitive silver 
 salt is held in suspension throughout the 
 solution by the gelatin and because of this 
 it receives the term, “emulsion.” 
 
 These actual operations are conducted in 
 silver lined steam jacketed vessels provided 
 with usitable agitators. Soluble salts formed 
 during the reaction must be washed out of 
 
 | ae 
 
 the emulsion. 
 
 Figure 8—Every photographic image is composed of coke-like masses of silver grains— i, = 
 shown here under a powerful microscope. a 
 
 This is accomplished by chil- — 
 ling it to a jelly, shredding it by pressing — 
 the mass through a chamber with a per-— 
 forated bottom and sides, and washing the 7 
 spaghetti-like strands many times with cold — 
 water. The shredded emulsion is then melted — 
 and coated. as 
 
 For coating the emulsion on the hase, spe- 
 cial and delicate machinery is necessary in — 
 order to carefully control the thickness. The © 
 film base is handled in such a way that ot 
 one side comes in contact with the heated ° 
 emulsion. (Figure 7). After the film F 
 coated, it is carried in large loops through ~ 
 chilling rooms to set and harden or become — 
 “conditioned.” When thoroughly dried, it — 
 is automatically cut into strips 13% inches 
 wide and wound into rolls varying from 100. 
 to 1,000 feet in length. 
 
 Perforating the film is carried on in a ~ 
 special department and the greatest care — 
 is required to have the work done accurately, 4 
 for unless the perforations are correct in — 
 spacing, the film will not run smoothly in~ 
 cameras, printers, or projectors and the pic- 4 
 ture will be unsteady on the screen. The 
 rolls of perforated film are then taken to — 
 the packing room to be wrapped in selected — 
 pure black paper and packed in tin cans 
 which are sealed to keep the contents air 
 and light tight. The cans are stamped with 
 
 the emulsion number, the footage, and are 
 then placed in strawboard containers ready — 
 for shipment. . a 
 
 To make film of the high average quality — 
 demanded, 
 
 inspection tests are necessary at — 
 
wegen pee 
 
 every step in the manufacture. These in- 
 clude the actual making of pictures which 
 are projected to show the photographic qual- 
 ity and to test the strength and wearing 
 properties of the base. Thousands of feet 
 of film are used up weekly in this way in 
 a critical inspection of the manufactured 
 product. No stock is permitted to reach 
 the consumer which does not come up to 
 the standard requirements. 
 
 Research on the Chemistry of 
 Emulsions 
 
 In the manufacture of photographic emul- 
 sions, the art has preceded the science. 
 Great refinements have been introduced in 
 manufacture on a large scale but the real 
 chemical causes and the factors controlling 
 the reactions have until recently remained as 
 much a mystery as in the early years when 
 all emulsions were coated by hand. As a 
 result of a large program of intensive re- 
 search that has been in progress now for 
 many years in the Eastman Research Lab- 
 oratory and other laboratories, some of the 
 uncertainty has been removed but much ad- 
 ditional work remains to be done. 
 
 To gain a better understanding of this 
 research work, something should be known 
 of the actual characteristics of the emulsion. 
 Ii a piece of exposed and developed motion 
 picture film is examined under a high power 
 microscope, the image will be found to be 
 composed of minute grains or clumps of 
 metallic silver, resembling tiny masses of 
 coke. (Figure 8). These grains are de- 
 rived from the original grains of the emul- 
 sion, which under the microscope are found 
 to be crystals varying in shape from spheres 
 to triangular or hexagonal plates in the 
 larger grains. (Figure 9). They are of all 
 sizes from very small grains to quite large 
 ones and the properties of the photographic 
 emulsion depend largely upon the various 
 sizes which are present. 
 
 One part of this comprehensive plan of 
 research has been the determination of the 
 systematic relations which exist between the 
 methods employed in the preparation of the 
 emulsion and the photographic properties of 
 the material obtained. That such relation- 
 ships exist is now definitely established and 
 before many years have passed a fairly com- 
 plete understanding of these will have been 
 arrived at. 
 
 One phase of this investigation has been 
 the direct microscopic study of the grains 
 in thousands of different samples of emul- 
 sions. This type of research is exceedingly 
 tedious and progresses very slowly but it 
 has proven one of the best lines of attack 
 
 Figure 9—Silver Halide Grains of a 
 Photographic Emulsion 
 
 on the problem. It will not be possible to 
 fully describe the method but some idea of 
 its complexity may be gained from the fol- 
 lowing statement. The emulsion sample is 
 coated as a layer only one =tiny grain in 
 thickness by a scheme requiring a high de- 
 gree of skill. A minute area of this layer 
 is then photographed so as to enlarge it 
 10,000 diameters. The grains are next meas- 
 ured, classified according to size and from 
 the results of hundreds of thousands of such 
 measurements, a tentative conclusion may 
 be drawn. This is essentially a_ statistical 
 method of attacking the problem. 
 
 The chemistry of gelatin has also come in 
 for a thorough study. That this is well worth 
 while was forcibly proven by the recent dis- 
 covery of a group of chemical substances 
 which must be present in samples of gela- 
 tin even .though in very small amounts in 
 order that the gelatin be useful for making 
 photographic emulsions. 
 
 These great problems of the chemistry of 
 the preparation of the sensitive materials 
 are only one part of the entire problem; 
 the other is the use of the photographic 
 materials. The faithfulness with which the 
 final print reproduces the different tone gra- 
 dations of the subject under various light 
 conditions is known as the problem of tone 
 reproduction. It may be reasonably said 
 that this problem is fully solved and a state- 
 ment of the accuracy of the reproduction 
 of the tone gradations. of any subject is now 
 possible on any photographic material under 
 any given condition of. illumination. 
 
Color Sensitivity of Motion Picture 
 7 Films 
 
 When a beam of white light (usually sun- 
 light) is passed through a prism it spreads 
 out into a multi-colored band called the 
 Visible spectrum. The normal eye can dis- 
 tinguish several prominent hues in this spec- 
 trum, violet at one end, then blue, green, 
 yellow, orange, and red.’ If this colored 
 spectrum is photographed upon ordinary 
 film, only the violet and blue would be com- 
 pletely recorded and the green very slightly 
 while the yellow and red would have scarcely 
 any effect at all. A red object therefore, 
 which appears relatively bright to the eye 
 photographs as black whereas blue and vio- 
 let objects photograph as white. The result 
 is a false reproduction of almost the entire 
 range of color tones. The chemist was re- 
 sponsible for making photographic emul- 
 sions sensitive to colors. It was found that 
 on adding certain dyes called sensitizing 
 dyes the sensitiveness of the emulsion to 
 green and yellow was increased. Such emul- 
 sions are called orthochromatic emulsions. 
 Negative motion picture film is of this type 
 but is relatively insensitive to red light and 
 may be handled safely in darkrooms lighted 
 with red safelights. It is manufactured in 
 two speeds, par-speed and super-speed film; 
 the latter being about twice as sensitive as 
 the former. Within the past twenty years 
 other sensitizing dyes have been discovered 
 which on incorporation in emulsions made 
 them sensitive to the entire spectrum. An 
 
 Figure 10—Spectrum photograph showing 
 sensitivity range of several emulsions. 
 
 emulsion of this type is known as a pan- 
 chromatic emulsion. (Figure 10). 
 color photography has been made possible 
 by the chemist’s discovery of these dye sub- 
 
 stances and their use in the manufacture ~ 
 
 of panchromatic film. Such pictures as 
 Douglas Fairbank's “Black Pirate’ could 
 never have been produced without panchro- 
 matic film. 
 
 Color Filters for Absorption.—Although 
 
 panchromatic motion picture film is strongly | 
 sensitive to red, yellow and green, it re-_ 
 
 mains more sensitive to blue and violet espe- 
 cially when photographing by daylight. To 
 correct for this extra sensitiveness to the 
 blue and violet, color filters are used be- 
 fore the lens. These filters consist of thin 
 
 Natural — 
 
 Se re ee 
 
 sheets of dyed gelatin cemented between two ‘ 
 
 pieces of optical glass. The dyes are care- 
 
 fully selected with reference to the por- — 
 
 tions of the spectrum which they trans- 
 mit and absorb. For example, 
 filter is most commonly used with panchro- 
 matic film since this filter absorbs a definite 
 
 a yellow 
 
 portion of the violet and blue light to which © 
 
 the emulsion is most sensitive thereby equal- 
 izing the exposure for all the colors. 
 
 tones of the subject. 
 
 When exposed to daylight or are lamps, — 
 
 Thea 
 result is a more accurate rendering of the 
 
 ——— 
 
 Eastman Panchromatic Negative Film is | 
 
 about equal in speed to Eastman Negative 
 Film, regular speed. 
 
 Negative Film. Because of 
 keeping qualities and its accurate rendering 
 
 of tone values, panchromatic film is now 
 
 being used extensively for both portraiture 
 and landscape work. 
 
 Panchromatic film can be supersensitized 
 by bathing for 1% minutes in 4 percent am- 
 monia at 50 degrees F., and drying as rapidly 
 as possible. When given this treatment, the 
 
 a a, ere 
 
 With tungsten lamps, A 
 it is considerably faster than standard speed — 
 its excellent — 
 
 —_— 
 x. 
 
 film is known as hypersensitized film and is — 
 
 about as fast as super speed negative film 
 for daylight work (see Figure 10). It should 
 be used as soon as possible after hypersen- 
 
 sitizing but if necessary to store for a week — 
 
 or so, it should be kept dry and at a tem- 
 
 perature not higher than 50 degrees F. The © 
 red and green sensitiveness of the film is — 
 
 increased three or four times by this hyper- 
 
 sensitizing treatment which is a great ad- 
 
 vantage if exposures through red filters are 
 to be made. 
 
 By the use of appropriate filters and © 
 treatment with certain sensitizing dye solu- — 
 tions, panchromatic film finds important ap- — 
 plications for making “night scenes” in the © 
 daytime (Figure 11), and for making distant 
 It may also be used 
 for making duplicate negatives from posi- ~ 
 
 shots through haze. 
 
Figure 11--“Night scenes” photographed in 
 daylight on specially sensitized film. 
 
 tives on tinted base when no other print 
 is available. Colored spots and stains can 
 be eliminated by duplicating in this way. 
 
 More complete information regarding pan- 
 chromatic film is given in the booklet “East- 
 man Panchromatic Negative Film for Motion 
 Pictures,” supplied on application to the Mo- 
 tion Picture Film Department, Eastman Ko- 
 dak Company, Rochester, N. Y. 
 
 ‘Chemistry in the Processing of Motion 
 Picture Film 
 
 After manufacture, motion picture film 
 has little contact with chemistry until it has 
 been exposed and is ready tc be processed. 
 The various treatments which it then re- 
 ceives such as development, rinsing, fixation, 
 washing, and drying, are all chemical and 
 determine in large measure the future per- 
 manency of the film. Besides the action of 
 the different solutions in processing the film, 
 there is considerable chemistry involved in 
 the actual mixing of the solutions and in the 
 action of the liquids on the vessels or tanks 
 used for containing them. 
 
 Too little thought is usually given to the 
 preparation of solutions used in photography. 
 
 “NI 
 
 We are apt to be satisfied to dump the 
 cheniicals into the water, stir the bath cas- 
 ually and proceed with the more important 
 business of processing the film. Conversely, 
 it is true, that it is unnecessary to take too 
 great precautions and waste too much time 
 in mixing the solutions, but more care should 
 be exercised than is usually given. 
 
 Although distilled water or rain water are 
 to be preferred for mixing solutions, experi- 
 ence has shown that it is only rarely that tap 
 water which usually contains dissolved salts 
 cannot be used. Providing the solution 1s 
 filtered through a canvas cloth or allowed to 
 settle before drawing off for use, very little 
 trouble need be anticipated. The important 
 thing is, however, to use only pure chemicals, 
 dissolve each separately before adding the 
 next, always mix them in the order recom- 
 mended, agitate the entire volume of solution 
 thoroughly as each constituent is poured in, 
 and finally make up the solution to a definite 
 volume with cold water. Hydrometer meas- 
 urements are best avoided in mixing solu- 
 tions (unless it is impossible to keep the 
 chemicals dry), because it takes considerable 
 time to adjust the strength of the solution. 
 Hydrometer readings also vary with the tem- 
 perature and no idea is conveyed as to the 
 percentage strength. 
 
 A good arrangement for mixing the solu- 
 tions is to place the chemical room directly 
 above the developing room. Wax impreg- 
 nated wooden tanks, enamelled vats or 
 smoothly glazed earthenware crocks are 
 recommended as containers connected with 
 chemical lead piping to convey the solutions 
 to the developing and fixing tanks in the 
 room below. 
 
 Further details may be found by consult- 
 ing the chapter on “Preparing Solutions” in 
 the booklet, “Elementary Photographic 
 Chemistry,’ published by Eastman Kodak 
 Company. See also “The Development of 
 Motion Picture Film’ by J. I. Crabtree, 
 Trans. Soc. M. P. Eng. No. 16, p. 163 (1922). 
 
 Developers and Development 
 
 The purpose of a developing solution is to 
 change the exposed silver salt in the emul- 
 sion to metallic silver without affecting the 
 unexposed silver salts. The constituent of 
 the developer which accomplishes this change 
 is called the reducing agent. The reducing 
 agents now generally employed are elon, 
 hydroquinone, pyro, and glycin. These sub- 
 stances are ineffective as developing agents 
 until the solutions are made alkaline, usually 
 with sodium carbonate, which activates the 
 reducing agent. In the presence of the oxy- 
 gen of the air, however, the reducing agent 
 is oxidized and the solution turns brown. A 
 
product somewhat like a dye is formed which 
 stains the film and slows up the developing 
 power of the solution. When the carbonate 
 is added this rate of oxidation is increased, 
 but if sodium bisulphite or sodium sulphite 
 is added, the oxidation tendency is reduced 
 and the solution turns brown very slowly. 
 The sulphite, therefore, generally should be 
 dissolved first as it acts as a preservative. 
 Besides the reducing agent, the activitator, 
 and the preservative, the developer contains 
 a restraining agent or potassium bromide 
 which assists in controlling the rate of de- 
 velopment and preventing developer fog. 
 
 The various reducing agents differ con- 
 siderably in their rate of development: elon, 
 for example, develops the image much 
 more rapidly than hydroquinone, but on pro- 
 longed development they produce similar 
 images (Figure 12). Both these developing 
 agents are usually added to a developer be- 
 cause hydroquinone, when used alone, de- 
 velops too slowly, especially at low tem- 
 peratures. For negative development, when 
 soft images are desired, the proportion of 
 elon should preponderate, while in the case 
 of a jeositive developer, when more contrast 
 
 cL 
 
 re 
 aan. 
 
 MIN. tein. 
 
 are 12—Graded strips eae comparative rates of develogaaan of (E) Elon 
 and (H) Hydroquinone. 
 
 is wanted, the hyarceiaean ‘seni be in ex % 
 
 cess of the elon. 
 The difference between the 
 blackness of the silver image of the lowest 
 
 exposure and the highest exposure is a meas- 
 
 ure of the “density contrast” of the nega- 
 
 density a 
 
 tive. This difference in density increases with 
 
 time of development, 
 usually occurring in the first 5 or 7 minutes 
 of development. Every picture is really a 
 
 series of varying tones and the particular 
 
 developer used, the time and temperature of 
 
 development of both the negative and the — 
 
 positive print all influence the range of the 
 density value of the tones. 
 
 If development is continued too long, a 
 
 chemical reduction of the unexposed grains 
 
 of the emulsion takes place which is com- 
 monly spoken of as “fog.” It is never ad- 
 visable to develop longer than one minute 
 less than the fogging point, and it is there- 
 fore important to know the time required 
 to produce visible fog with the type of film 
 
 being used. Occasionally substances get in 
 
 the developer which fog emulsions very 
 rapidiy. A serious trouble of this nature was 
 
 traced to the presence of certain bacteria — 
 
 we 
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 the greatest change © 
 
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 or fungi which acted on the sulphite in the 
 developer, changing it to sodium sulphide, 
 which fogs film very badly. Both the cause 
 and a method of eliminating the trouble were 
 worked out in the Eastman Research La- 
 boratory in connection with an extensive in- 
 vestigation dealing with the classification of 
 different types of chemical fog and the 
 chemistry of developer solutions. 
 
 Effect of Temperature.—The properties 
 of a developer solution are affected con- 
 siderably by temperature, especially if there 
 is much hydroquinone present. When the 
 temperature is raised, development is faster 
 and with a lowering of temperature, the 
 development rate is retarded. The fogging 
 point also changes with temperature. In 
 warm weather developers do not keep as 
 well because the higher temperature in- 
 creases the rate of aerial oxidation. It is 
 very important in view of these facts to 
 know the temperature of the solution and 
 to keep it fairly constant in order to dupli- 
 cate results. 
 
 Tropical Procedures.—For handling and 
 processing film under tropical conditions, a 
 different technique is required. Standard 
 methods have been worked out for insuring 
 that the sensitiveness of the emulsion shall 
 remain unimpaired and that the latent image 
 be retained after exposure and before devel- 
 opment. The secret of high temperature 
 development is to prevent excessive. swelling 
 of the gelatin. The most successful method 
 of doing this is to add an anti-swelling chem- 
 ical such as sodium sulphate, or sodium phos- 
 phate to the developer and immerse the film 
 in a hardening bath after development and 
 before fixation. Such a hardening solution 
 may be prepared with potassium chrome 
 alum in 3 per cent. concentration, which 
 works very well at temperatures from 75 to 
 85 degrees Fahrenheit. When higher tem- 
 perature up to 95 degrees Fahrenheit are 
 encountered, about 12 per cent. sodium sul- 
 phate should be added to this bath. Fixation 
 can be conducted in the usual way after three 
 minutes treatment of the film in this harden- 
 ing solution. More complete data are given 
 in a paper on “Handling Motion Picture Film 
 @eaiigh Temperatures.” by J. I. Crabtree, 
 feeties rans. Soc. M. P. Eng. No. 19, p. 39 
 (1924), | 
 
 Life of Developers.—The life or period 
 of usefulness of a developer depends on its 
 particular composition and whether it re- 
 ceives continuous or intermittent use. As a 
 developer is used, the solution accumulates 
 reaction products which tend to retard the 
 rate of development, and it is, therefore, 
 
 necessary to develop for a longer time te 
 secure a given contrast. Since reaction 
 products slow down development, if a rack 
 of film is allowed to remain stationary 
 in, a tank) there «is: an- accumulation’ ot 
 these by-products in the vicinity of the 
 film which induces further retarding effects 
 (Figure 13). Agitation of the rack and of 
 the solution will prevent this trouble and 
 give more uniform development. 
 
 With use a developer may become ex- 
 hausted in several ways: (1) By aerial oxida- 
 tion; (2) by accumulation of products re- 
 sulting from the decomposition of the de- 
 veloping agents; and (3) by formation of 
 sodium bromide and iodide from the reduc- 
 tion of these silver salts in the emulsion to 
 metallic silver. An old developer may there- 
 fore have to be discarded because it develops 
 too slowly or gives excessive stain or fog. 
 When film is developed on a reel the solution 
 is freely exposed to the air and if the 
 developer does not contain an excess of pre- 
 servative, chemical fog is produced. Ex- 
 perience has shown that the addition of about 
 5 per cent of old developer to a freshly mixed 
 new developer will lower the tendency for 
 chemical fog more than if the bromide con- 
 centration is increased above the normal 
 quantity added. The chemical explanation 
 of this effect is probably that the oxidized 
 developer acts as an anti-fogging agent. 
 Work is still in progress, however, to find the 
 best method of reviving used developers. 
 
 For an extensive discussion of develop- 
 ment, reference should be made to the paper, 
 “The Development of Motion Picture Film 
 by the. Reel and Tank’ Systems," by. J). 1. 
 Crabtree; Trans. Soca MiePs, BngmeNo sale, 
 De 1Oast 19227, 
 
 Developer Troub!es.—Occasionally during 
 processing, troubles arise caused by spots, 
 marks, and stains appearing on the films. 
 Methods of preventing, and removing many 
 of these difficulties have been found and pub- 
 lished, and data on others are being assembled 
 and investigated. 
 
 Stains may result from using old develop- 
 ers containing an excess of oxidation prod- 
 ucts, from particles of chemical matter in the 
 air of the room settling on the film, or from 
 undissolved solid chemicals in the developer. 
 They are sometimes produced by the forma- 
 tion of a scum on the surface of the develop- 
 er due to insoluble oxidation products rising 
 to the surface. Spots result from similar 
 causes and in addition may be produced by 
 bubbles of air clinging to the film on im- 
 mersing in the developer or by splashes of 
 oil which repel development. General stain 
 may be the result of chemical fog or a 
 
Figure 13—Streaks Caused by the Restraining Action of Products of Development. 
 
 coloring of the gelatin by an oxidized devel- 
 oping agent such as pyro. For full discus- 
 sion of stains, see paper on this subject by 
 J. I. Crabtree, Amer. Ann. of Phot. 1921, 
 p. 204 or Brit. J. Phot. 68, 294 (1921). 
 
 When film is developed on racks in a tank, 
 markings quite often occur at the points 
 where the film passes over the top and bot- 
 tom slats of the rack. These marks are 
 caused by convection currents set up by the 
 flowing away of chemical oxidation products 
 as the development progresses. (Figure 14). 
 
 TOP OF RACK 
 
 “BOTTOM OF RACK _ 
 
 Figure 14—Rack Marks produced where 
 
 film passes over slats of rack. 
 
 They may be very much diminished by agita- 
 tion of the rack, by moving the film on the 
 rack, or by the use of a special roller rack 
 which permits easier movement of the film 
 on the rack at intervals during the progress 
 of development. (See paper on “Rack Marks 
 and Air Bells,” by J. I. Crabtree and C. E. 
 Ives published in the Trans. Soc. M. P. Eng. 
 No. 24 p. 95 (1926). 
 
 Chemistry of Fixation 
 
 A fixing bath contains as the active chemi- 
 cal agent sodium thiosulphate or hypo 
 which dissolves the unexposed silver salts 
 without affecting the silver image. A double 
 salt of silver and sodium thiosulphate is 
 formed which is very soluble in water and 
 may be removed from the gelatin by wash- — 
 ing. Hypo is seldom used as a plain solu- 
 tion but usually in conjunction with a 
 weakly acid salt such as sodium bisulphite 
 or with an acid hardening solution. The 
 standard hardener contains a preservative, 
 sodium sulphite which prevents decomposi- _ 
 tion of the hypo; an acid, usually acetic 
 acid, to neutralize any alkali carried over in 
 the film from the developer, thereby arrest- 
 ing development since an acid developer will 
 not reduce silver salts; and a hardening 
 agent, either potassium alum or chromium 
 alum. = 
 
 It is important to mix a fixing solution 
 correctly. The hardener should be prepared 
 separately by dissolving the sulphite first and 
 when it is completely dissolved add the acetic 
 acid. After the sulphite-acid solution has 
 been thoroughly mixed, add the potassium — 
 alum. When the alum has dissolved make 
 up to final volume with cold water and add 
 the hardener solution slowly to the cold 
 hypo while stirring the latter rapidly. 
 
 There are certain criteria used in judging - 
 the efficiency of a fixing bath as follows: 
 
 ee 
 
 Rate of Fixation—When film is immersed 
 in a fixing bath, it is considered fixed when — 
 it has remained in the solution twice the — 
 time for the milkiness or opalescence of the 
 unreduced silver salts to disappear. e254 
 
 a ee 
 
 *, 
 , 
 
rate at which this takes place depends on 
 the strength of the hypo (30% to 40% hypo 
 fixes most rapidly), the emulsion used, that 
 is whether negative or positive, the tem- 
 perature of the solution (65° F. is recom- 
 mended), and the degree of exhaustion of 
 the solution. 
 
 Hardening Properties——A certain mini- 
 mum of alum is required to give the neces- 
 sary hardening while an excess of alum may 
 produce too much hardening and induce 
 brittleness. Normal fixing baths are care- 
 fully compounded to give a hardening of 130° 
 to 170° F., determined by immersing a strip 
 of the fixed and washed film in water and 
 heating the water slowly until the gelatin 
 flows away from the support. 
 
 * 
 ca 
 = 
 ‘ 
 . 
 é 
 
 i. Sludging Tendency.—A fixing bath may 
 -_-become cloudy or precipitate a sludge in two 
 
 different ways: (1) the hypo may break 
 down giving a pale yellow sludge of sulphur 
 which is the result of the temperature of 
 the bath rising too high or of adding too 
 much acid to the bath; and (2) the alum may 
 be decomposed and a white sludge of alumi- 
 num sulphite formed which is the result of 
 too low acidity, the presence of excess de- 
 veloper carried into the bath, or too high a 
 sulphite concentration. 
 
 Effect of Temperature.—Changes in tem- 
 perature of the fixing bath affect the rate of 
 fixation and the life of the solution. If a 
 film requires 95 seconds to clear at 65° F., 
 for example, it would take about 50 seconds 
 to clear at 85° F., but it is dangerous prac- 
 tice to allow the temperature of the bath to 
 rise above 65° F., as the solution is apt to 
 precipitate sulphur. Under tropicai condi- 
 tions where high temperatures and high 
 humidities prevail, it is obviously often im- 
 possible to keep the temperature within this 
 limit, and the fixing bath must usually be re- 
 placed oftener. A different technique must 
 be used for tropical processing as mentioned 
 previously under the subject of development. 
 
 Life of Fixing Solutions.—As a fixing 
 bath is being used, the hypo becomes ex- 
 hausted as a result of performing useful 
 work in fixing out the emulsion. When the 
 time for clearing negative film exceeds a 
 certain point, say ten minutes, the bath 
 should be discarded. The acidity of the 
 bath is being reduced by the developer car- 
 ried in although at first this tends to favor 
 a longer sulphurization life. With continued 
 use, however, the solution finally reaches a 
 
 point where a sludge: of aluminum sulphite 
 is precipitated rendering the bath useless. 
 On the other hand, the hardening properties 
 of a bath usually increase slightly during the 
 first stages of use after which they fall off 
 rapidly until the bath is revived. 
 
 Revival of Fixing Baths.—Since the acidity 
 of a bath and its hardening properties are 
 depleted before the hypo is used up, it is 
 general practice to revive the bath at in- 
 tervals by the addition of a definite quantity 
 of acetic acid. When the acidity is lowered 
 to two-thirds of the original value, enough 
 acid should be added to restore the initial 
 concentration. A fairly satisfactory method 
 is to add acid after a certain footage of film 
 has been passed through the solution. It the 
 bath has formed a sludge before it was re- 
 vived, the solution should be discarded. 
 
 Recovery of Silver—An exhausted fixing 
 bath contains dissolved silver salts and vari- 
 ous methods have been tried to profitably 
 recover the silver. The principal methods 
 used are: (a) the sulphide method wherein 
 sodium sulphide is added to the used bath 
 and the silver is thrown down as a sludge 
 of silver sulphide; (b) the hydrosulphite 
 method in which the addition of sodium 
 hvdrosulphite precipitates a mixture ot me- 
 tallic silver and silver sulphide depending on 
 the conditions of precipitation; (c) the zinc 
 method which uses zinc in various forms, 
 such as sheet, granulated, and dust; the sil- 
 ver being precipitated as metallic. silver; 
 (d) electrolytic methods which include the 
 use of metallic units and the actual applica- 
 tion of an electrical current. For recovery 
 of large quantities of fixing bath, the sul- 
 phide method is most efficient; for medium 
 quantities, the zinc dust method is satisfac- 
 tory, and for small quantities, the use of an 
 electrolytic recovery unit offers a simple 
 and economical procedure. 
 
 Fixing Bath Troubles—When the carbo- 
 nate in the developer is neutralized by the 
 acid in the fixing bath, carbon dioxide gas 
 is formed which produces blisters which ap- 
 pear on the film as tiny craters providing 
 the gelatin is too soft to withstand the dis- 
 ruptive action of the gas. If the bath has 
 good hardening properties and the film 1s 
 agitated on first immersion no trouble from 
 blisters need be anticipated. If the fixing 
 bath does not contain acid or if it is old and 
 exhausted and contains an excess of dis- 
 solved silver, a chemical fog called dichroic 
 fog is sometimes produced on the film. In 
 reflected light, film fogged in this way looks 
 yellowish green and by transmitted light it 
 appears reddish pink. Dichroic fog never 
 
occurs in a fresh acid fixing bath or if the 
 film is rinsed before fixing and the tempera- 
 ture of the bath is kept at 65° to 70° F. 
 When a partially exhausted fixing bath is 
 allowed to stand several days without use, 
 the hydrogen sulphide gas present in small 
 quantities in the. air reacts with the silver 
 thiosulphate in the bath and forms a me- 
 tallic-appearing scum on the surface of the 
 solution. The scum consists of silver sul- 
 phide and should be removed by drawing the 
 edge of a sheet of blotting paper across the 
 surface of the bath. Trouble from sludging 
 and precipitation has been discussed pre- 
 viously. Several different stains such as 
 white aluminum sulphite stain, sulphur stains 
 and yellow silver stains are occasionally pro- 
 duced. More complete discussion of fixing 
 troubles is given in a paper on “Stains on 
 Negatives and Prints,” by J. I, Crabtree, 
 Brit. J. Phot. 68, 294 (1921). 
 
 Chemistry of Washing 
 
 One might naturally think that there 
 is little chemistry associated with washing 
 film, and this is true so far as actual chemi- 
 cal changes are concerned, but it is a dis- 
 tinctly physico-chemical problem to deter- 
 mine the conditions that will ensure com- 
 plete removal of hypo and other fixing bath 
 components and oxidized products from the 
 film. The nature of the water supply is not 
 of vital importance, although if dirty water 
 or sea water have to be used, the film should 
 be subsequently given a thorough washing 
 in fresh water. 
 
 The problem of washing film involves two 
 principal operations: (1) separation of the 
 chemical substances from the film, and (2) 
 removal of these substances from the water 
 in the vicinity of the film. Obviously the 
 second operation must proceed equally as 
 rapidly as the first, or the film would still 
 retain some chemicals when taken from the 
 wash water and stains would appear later. 
 The first operation is really a problem of 
 diffusion since the chemicals are held in 
 the swollen gelatin layer and must find their 
 way out to the surface. It has been found 
 that as washing progresses under favorable 
 conditions, the hypo content of the film is 
 halved for each equal time interval. This 
 “half period” value can be determined for 
 each type of film. Most thorough washing re- 
 sults if the water is violently agitated at the 
 point of contact with the film. Practically 
 the ideal washing stream can only be real- 
 ized by using a spray or an excessively large 
 flow of water. Tanks should be as small as 
 is consistent with the film output because 
 the smaller the volume the more rapidly is 
 
 12 
 
 the stale water removed. To remove sur- 
 face hypo, a few seconds rinse in a separate 
 
 tank, or spraying with a coarse atomizer 
 
 previous to the main washing is recom- 
 
 If it can be arranged, the cascade 
 In this method, film 
 
 mended. 
 system is excellent. 
 
 is transferred successively through about 
 
 five baths in which the water is circulating 
 in a counter direction. In continuous tube 
 processing machines, the water should be 
 directed into the top of the last. tube, flow 
 from the bottom into the top of the next 
 tube, and so on. A rinsing loop previous to 
 the large washing tubes should be installed. 
 
 Compressed air admitted at the bottom of 4 
 
 tank or tube provides the most economical 
 
 and efficient method of agitation. 
 
 Washing Troubles.—If washing is incom- 
 plete as pointed out under the subject of 
 fixation, trouble from stains and spots is 
 often experienced. Sometimes these diffi- 
 culties do not appear until several months 
 or a year or two later, but the film is usu- 
 ally seriously if not permanently damaged. 
 Several theories have been advanced to ex- 
 plain this deterioration, but it is most prob- 
 able that the sulphur liberated from small 
 traces of retained hypo in conjunction with 
 bacterial action are the chief causes of the 
 fading that take place through the forma- 
 tion of silver sulphide. Most trouble is ex- 
 perienced with film that has been poorly 
 
 processed and stored in hot damp climates. 
 
 If the film is 
 troubles will arise from having to wash in 
 water whose temperature is 90° to 100° F. 
 When film is thoroughly fixed and washed 
 and subsequently stored at high tempera- 
 tures, it rapidly becomes brittle and in a 
 few years is completely destroyed by its own 
 decomposition products. 
 ant, therefore, to store film when practicable 
 at temperatures of 50° or 40° F., when the 
 rate of decomposition is negligible providing, 
 
 It is very import-— 
 
 properly hardened no © 
 
 of course, that the proper care has been 
 
 given the film during the processing opera- 
 tions. The motion picture industry is com- 
 paratively young, but there are samples of 
 
 film in very good condition which were 
 
 processed over thirty years ago. For more 
 detailed information on the subject of wash- 
 ing motion picture film, see the article by 
 K. C. D. Hickman, Trans. Soc. M. P. Eng. 
 No. 23, p. 62 (1926). 
 
 When moisture comes in contact with dry 
 film previous to or after exposure, or is de- 
 posited as the result of humid atmospheric 
 conditions, or is left on the film previous — 
 to or during drying, certain characteristic 
 
Figure 15—“Moisture Marks.” 
 
 markings are produced. The most common 
 example of these is a tiny white spot. 
 (Figure 15). Various other types have been 
 classified and several explanations advanced 
 as to the causes and means of preventing 
 such markings in a paper on the subject 
 published in the Trans. Soc. M. P. Eng. No. 
 17, p. 29 (1923). 
 
 Corrosion and Its Relation to Con- 
 struction Materials for Photo- 
 graphic Apparatus 
 
 In selecting a material for the construction 
 of photographic processing apparatus, it is 
 important to know the probable effect, if 
 any, of both the material on the solution 
 and the chemical action of the solution 
 On the material. A metal like tin, for 
 example, which is entirely satisfactory for 
 pipe lines carrying distilled water, is on the 
 
 _ other hand very unsuitable for use for con- 
 
 structing developer tanks as it reacts with 
 the solution giving very bad fog. An inves- 
 tigation of this- subject was made several 
 years ago and the results published in a 
 series of papers to which reference should 
 be made for complete information. (See 
 Amer. Phot. 18, i148, (1924) ). A few of the 
 conclusions may be of interest. Tin, copper, 
 _ or alloys containing these metals should not 
 come in contact with developers as serious 
 trouble from fog will be experienced. Sold- 
 ered joints in metal tanks are to be avoided. 
 
 If metals must be used for apparatus to 
 
 contain fixing baths, nickel, lead and monel 
 are the only ones recommended, and these 
 
 - should be electro-welded or soldered from 
 
 a the outside except in the case of lead which 
 _ should be burned. Aluminum, zinc, or gal- 
  vanized iron should not be used with either 
 
 ] 
 
 - 
 
 developers or fixing baths as these metals 
 react with the solutions forming precipitates 
 which deposit on the film and stain the 
 gelatin. 
 
 Single metals or alloys are to be preferred 
 to plated metals because when surface plat- 
 ing becomes worn or chipped they corrode 
 very rapidly. Porous earthenware, fibrous 
 materials, and rubber compositions should be 
 avoided since the solutions crystallize out in 
 the pores and subsequently disintegrate the 
 material. Lacquered trays and japanned 
 tanks are not suitable for containing strongly 
 alkaline developers or acid fixing baths. 
 Specific recommendations relative to the 
 most suitable materials for constructing 
 small apparatus, trays, tanks, tubes, troughs, 
 piping, pumps, faucets, etc., are given in a 
 paper on that subject which may be obtained 
 on application to the Service Dept. Eastman 
 Kodak Co., Rochester, N. Y. 
 
 Chemistry in the After Treatment of 
 Motion Picture Film 
 
 Reduction—Film is occasionally over- 
 exposed in the camera and although this can 
 be partly compensated for in the printing, it 
 is customary, to treat the film with solutions 
 which chemically remove some of the image. 
 This process is photographically termed “re- 
 duction” although it is not a chemical reduc- 
 tion but rather an oxidization since that por- 
 tion of the image which is removed has been 
 oxidized. 
 
 There are three general types of photo- 
 graphic reducers: 
 
 (1) Cutting reducers which remove the sil- 
 ver nearly equally from all parts of the 
 image; (2) Flattening reducers which attack 
 the heavy deposits more than the lighter 
 areas; and (3) True scale -reducers. that 
 attack both highlights and shadows propor- 
 tionately. Reducers of the first type are; 
 (a) Farmer’s reducer consisting of potas- 
 sium ferricvanide and hypo; and (b) the 
 permanganate reducer which is a slightly 
 acid solution of potassium permanganate. 
 With Farmer’s reducer, the silver is con- 
 verted to silver ferrocyanide which the hypo 
 dissolves. The permanganate reducer ox:d- 
 izes the silver to silver sulphate which is 
 sufficiently soluble in water to be dissolved. 
 The net effect with either reducer is that 
 the shadows of the negative are proportion- 
 ately attacked the most, since they have 
 less available silver to tose than the rest of 
 image. 
 
 When a negative image has excessive con- 
 trast caused by overlighting or over devel- 
 opment, a reducer of the second type 1s 
 
necessary. An acidified solution of ammon- 
 1um persulphate is the only solution known 
 to act on the heavy silver deposits more than 
 on the lighted ones. A fairly satisfactory 
 explanation of the chemistry of this reaction 
 has been reached and it is now known that 
 silver sulphate is formed which dissolves in 
 the solution. =. 
 
 When it is advantageous to reduce the 
 printing time on a good negative, a true 
 scale or proportionate reducer (type three) 
 is sometimes employed. This solution con- 
 sists of a compounded mixture of the per- 
 mangate and the persulphate reducers and 
 it weakens the image in direct proportion 
 to the amount of silver deposit present. 
 
 Intensification.—Although several methods 
 have been worked out for chemically reduc- 
 ing the photographic image, it is quite an- 
 other problem to add to or build up an 
 image. As the saying goes, “what isn’t there 
 is hard to put there.” A few solutions have 
 been found, however, with which an under- 
 exposed or an underdeveloped image may 
 be improved. 
 
 Usually intensification is performed by de- 
 positing a silver, mercury, or chromium com- 
 pound upon the image. The most common 
 mercury intensifier is Monchoven’s solu- 
 tion which uses a solution of mercury bich- 
 loride and potassium bromide for the bleach; 
 this reacts with the silver forming a mixture 
 of mercurous chloride and silver chloride. 
 The image may be re-developed in several 
 ways, with 10% sulphite solution, with an 
 elon-hydroquinone developer, with 10% 
 ammonia, and with a solution of potassium 
 cyanide and silver nitrate, each solution giv- 
 ing proportionately greater intensification. 
 
 A chromium intensifier consists of an acidi- 
 fied solution of potassium bichromate which 
 is used for the bleach. After a thorough 
 washing, the image is redeveloped in a regu- 
 lar developer such as elon-hydroquinone. 
 The chemistry of this intensifier is not very 
 well understood but its use has found in- 
 creasing favor owing to the ease and cer- 
 tainty of its operation and the permanency 
 of the intensified image. 
 
 Tinting and Toning.—In order to obtain 
 more pleasing effects on the screen, motion 
 picture film is often colored by treatment 
 with various chemical solutions. Tinting is 
 accomplished by evenly staining the gelatin 
 emulsion or the support by means of slightly 
 acidified dye solutions. It is rarely neces- 
 
 sary to tint the emulsion since positive film 
 on tinted support or base has been supplied 
 for several years in nine different colors by 
 the Eastman Kodak Company. This tinted 
 positive film is printed and processed in the 
 ordinary way. The use of tinted film thus 
 eliminates several extra operations which are 
 both expensive and troublesome. 
 
 Toning consists in changing the original 
 silver image to a colored inorgane salt of 
 silver or to a dye image. 
 commonly used methods of inorganic toning. 
 In the first method known as sulphide toning 
 the film is bleached in a ferricyanide-bromide 
 
 bleach and subsequently treated with a weak 
 
 solution of sodium sulphide which yields a 
 final brown image composed of silver sul- 
 phide. Very beautiful blue tones are ob- 
 tained by the use of a solution containing 
 potassium ferricyanide and ferric alum in the 
 presence of an alkaline salt of oxalic acid, 
 a mineral acid and certain other salts. The 
 silver image is thereby converted to a mix- 
 ture of silver ferrocyanide and iron (ferric) 
 ferrocyanide which forms the blue-toned 
 image. Tones ranging from chocolate to red- 
 dish brown may be produced in a somewhat 
 analogous way as the iron tones by the use 
 of a bath containing uranium ferricyanide, 
 the final image consisting of silver and uran- 
 ium ferrocyanide. 
 
 lf a silver image is converted more or 
 less to a silver ferrocyanide image and the 
 film immersed in a basic dye solution a mor- 
 danted dye image is produced. What hap- 
 pens is that the dye will attach or mordant 
 itself to the silver ferrocyanide whereas it 
 will not stick to the silver alone. This chemi- 
 cal reaction provides a method of obtaining 
 a wide range of tones which may be still 
 further extended by double toning. Basic 
 dyes are the most suitable for use since they 
 do not readily dye gelatin. Other effects 
 may be produced by combining tinting and 
 toning. For detailed information see the 
 
 booklet, “Tinting and Toning Eastman Posi- 
 
 tive Motion Picture Film,’ published by 
 
 Eastman Kodak Co. 
 
 Renovating Motion Picture Film.—Aiter 
 film has been projected and handled several 
 times, it accumulates a certain amount of 
 grease and dirt which detract from its pro- 
 jection value. If this is permitted to con- 
 
 - tinue, the film may be badly damaged by 
 
 scratching from grit. It is customary to 
 renovate the film by treating it with solu- 
 tions which will dissolve the grease and 
 loosen the dirt. Gasoline, benzine, toluene, 
 and xylene may be used for cleaning film 
 
 but because of the inflammability of these — 
 
 chemicals, commercially pure carbon tetra- 
 
 chloride is preferable. All cleaning chemi-_ 
 
 There are three 
 
 mh | 
 
 > a 
 
 SA of phe ie eid ‘) Me 2 eee 
 
 a Rie Behe eS 6 
 
 — Fa 
 
 oe ee Be Penne 
 
cals or solvents must be used with discre- 
 tion, however, and the liquid allowed to 
 completely evaporate before the film is 
 rewound, or the image may be subsequently 
 attacked. Traces of sulphur chloride pres- 
 ent in impure samples of tetrachloride prob- 
 ably cause fading due to deposition of sul- 
 phur which combines with the image form- 
 ing silver sulphide. If pure tetrachloride 
 is used and the film wound spirally on a 
 drum, and the solvent applied with a soft 
 cloth or velvet, the solvent will have suffi- 
 cient time to evaporate before rewinding the 
 film. Another non-inflammable solvent which 
 does not fade the film is tetrachlor-ethylene. 
 Similar precautions for cleaning on a large 
 drum should be used. There are machines 
 on the market in which the film passes over 
 several moist felt pads saturated with sol- 
 vents and then over a series of polishing 
 wheels made with small pieces of velvet 
 fastened around the periphery of the wheels. 
 The polishing wheels rotate very rapidly and 
 ensure thorough drying and polishing of the 
 film before rewinding. 
 
 It is now usual practice to apply a narrow 
 line of melted wax to new or first run prints 
 along the center of the perforation area 
 which provides against the liability of strain 
 in first projection. Similarly when film is 
 renovated it should always be rewaxed as the 
 cleaning chemicals remove all or nearly all 
 the wax. 
 
 Splicing and Varnishing Film.—Splicing of 
 film is essentially a chemical problem since 
 the film cement must possess certain prop- 
 erties, such as good adhesiveness, fairly 
 rapid evaporation on drying, and have no 
 corrosive action on the film support. When 
 film has been projected many times it 
 sometimes acquires scratches which fill up 
 with dirt and grease and show up plainly on 
 projection. Cleaning the film removes the 
 dirt from the scratches but as soon as the 
 film is put into use again the tiny groves 
 O\ap as much as before. To prevent this, 
 varnishes have been compounded for treat- 
 faethe film. Such varnishes have to be 
 made very carefully as they must possess the 
 saine refractive index as film base or in 
 other words the varnish layer must not 
 change the direction of the light rays when 
 ‘the film is projected. Furthermore these 
 varnishes must give a hard, non-abrasive 
 surface when coated very thinly on the film 
 and must not attack the support, the gelatin 
 or the image. 
 
 Chemistry and Color Motion Pictures 
 
 A field which is demanding more attention 
 yearly 1s that of natural color motion pic- 
 tures. This problem is both an optical as 
 well as chemical one; optically it demands 
 unusual refinements in the design of lens 
 systems and chemically it imposes a dif- 
 ficult problem in processing and in final dye- 
 ing of the films. There are two general 
 classes of natural color motion pictures: 
 those produced by additive and those by 
 subtractive methods. These are further sub- 
 divided according as they use three color or 
 two color ranges in color reproduction. In 
 the additive process, several distinct color 
 records are taken and projected separately 
 and are either superimposed or shown in rapid 
 succession, the colors being added to or 
 built 
 
 up on the screen. These processes 
 usually require complicated and expensive 
 apparatus. Whereas, in the _ subtractive 
 
 method which has found most public favor 
 the color records are taken separately but 
 are finally incorporated on a single film and 
 projected in the same way as standard pic- 
 tures. 
 
 In the foregoing description of the value 
 of chemistry in the motion picture industry, 
 it has not been possible in view of the nature 
 of this article and the diversity of the sub- 
 ject matter to discuss in much detail the 
 actual chemistry involved. It is hoped that 
 some idea may have been gained, however, 
 of the importance of chemistry in every 
 phase of the industry from the assembling of 
 the raw material for manufacture to the final 
 projection of the film. 
 
 Additional References 
 
 “The Fundamentals of Photography,” by 
 C. E. K. Mees, published by Eastman Kodak 
 Co. 
 
 “Significant Progress in Research on 
 Photography,” by C. E. K. Mees, Annals of 
 Amer. Acad. Polit. Sci., 69, 10 (1925). 
 
 “Application of Microscopy to the Photo- 
 tographic Industry,” by A. P. H. Trivelli and 
 Re P, Loveland, J. Rov; -Micro, Soc., 1925, -p: 
 293. 
 
 “The Home of Film,” published by East- 
 man Kodak Co. 
 
 “Moving Pictures,’ F. A. Talbot, Lippin- 
 Cote bub. Co, Phila... Paz 1923. 
 
 “Handbook of Projection,” F. H. Richard- 
 son, Chalmers Pub, Co., New York, N. Y. 
 “Chemistry in Industry,’ Chapt. 18. 
 
 Chemical Foundation, Inc., 1924. 
 
 The