THE METROPOLITAN MUSEUM OF ART THE RESTORATION OF | ANCIENT BRONZES AND OTHER ALLOYS ve - en s Tie see q ; < way, ~ 7S THE METROPOLITAN MUSEUM OF ART The Restoration of Ancient Bronzes and Other Alloys Biv Corin G? Fink, PH: D. Professor of Electro-Chemistry at Columbia University AND (HARTES fi, ELDRIDGE, 6.9. ® First Report NEW YORK : MCMXXV A a a 1 COPYRIGHT BY THE METROPOLITAN MUSEUM OF ART MAY, 1925 INTRODUCTION Ir 1s a general impression among visitors to museums that once a work of art has been properly installed, once it has been set on a pedestal or hung upon a wall, and more especially if it has been protected by glass, its life is assured for an indefinite future. So far is this from being the fact, however, that exactly the con- trary might almost be said to be true. Many objects bring with them the germs of deterioration if not of actual destruction, others though sound when they are received are highly sensitive to relative degrees of heat and cold, moisture and dryness, and to the atmospheric impurities of our modern cities, never so serious a menace as they are today. These dangers are not con- fined to any one material or class of materials, indeed there are very few—if any—which can be regarded as immune from one or another of them; and when we add to this the knowledge that one “diseased” specimen in a show-case may and frequently does infect others, it will be realized that eternal vigilance is the price at which a curator keeps his collections in condition, though he is often at his wits’ end to know how to arrest or prevent the disaster with which he finds himself threatened. Happily a few chemists have been coming to our assistance during recent years, by making a serious study of our problems in the light of the advances which have been made during this period in their own science, especially electro-chemistry, with the promise of results which we should formerly have believed impossible. This has been notably the case in the laboratory established in the British Museum by the Government Depart- ment of Scientific and Industrial Research under the able direc- tion of Dr. Alexander Scott, F.R.S. Following that example the Trustees of the Metropolitan Museum of Art determined a little more than two years ago to broaden this field of endeavor by undertaking independent investigations in one type of ma- terial which presents particularly vexatious problems; namely, ancient bronzes. The evils with which these are most generally afflicted and which are the most difficult to overcome may be roughly divided into two classes. One is the corrosive green crust with which objects in bronze or other copper alloys are apt to become thickly coated through long burial in the earth, and which not only completely hides the details of the modeling and the engraved designs, but frequently eats deeply into the surface of the metal itself, thereby rendering the object value- less, and sometimes destroying it altogether. ‘The other is what is now familiarly known among collectors as the “bronze dis- ease.’ Both of these highly disfiguring and destructive agents are described and discussed in the following pages, as well as the difficulties in dealing with them. In entering upon our task we were fortunate in having the interest and assistance of the chemical authorities of the Na- tional Research Council, under whose guidance we secured the services of Professor Colin G. Fink of Columbia University, whose experience in the study of metals made him singularly fitted to undertake investigations of the kind required. The Museum is also under great obligation to Columbia University for its friendly codperation in allowing us the free use of its chemical apparatus, and in setting aside a special laboratory exclusively for our investigations. Without these combined privileges it would have been difficult indeed for us to pursue them. With the assistance of Mr. Charles H. Eldridge, Dr. Fink began his studies and experiments in January, 1923, and al- though a final solution of all the problems involved has not yet been reached, what has been achieved thus far is so extraordi- nary as to warrant the publication of a preliminary report giv- ing the results accomplished up to date, inasmuch as the Trus- tees of the Museum desire to share the benefits they are deriving from these researches with the whole scientific world, a desire in which Dr. Fink joins in the most liberal spirit. In doing this we are again following the example of the British Museum. As to the results, even a layman can see from the illustrations in this report, showing objects before and after treatment, that so far as the removal of the crust is concerned these may be fairly described as sensational. We are now well on the way to salvaging brilliantly thousands of bronzes which might other- wise have been ruined by crude attempts at ‘“‘cleaning”’ or dis- carded altogether as worthless. The artificial patination of such objects as may require it after treatment, and the cure or pre- vention of the bronze disease, still call for further study, but considerable progress has been made in both, and we are justi- fied in believing that satisfactory solutions are not far distant. In conclusion, I should like to offer a word of caution. The museum official or collector who hopes to find in this report a ready-made formula for treating bronzes which anybody can apply will be disappointed. While Dr. Fink and Mr. Eldridge have shown that ancient bronzes can be cleaned and preserved they have demonstrated with equal force that this can be done only by a highly trained expert who like a physician watches and deals instantly with every symptom that develops during his treatment, as otherwise his patient may be lost instead of cured. The conclusion is obvious. The only way of putting the knowledge gained by these investigations to practical and gen- eral use is by the establishment of a central bureau or labora- tory, available to the country at large, to which museums, pri- vate collectors, and dealers may send their specimens for treat- ment with the assurance that this will be of the highest scientific character. With the financial support of one of our great Foun- dations the National Research Council might be well equipped to effect the organization of such an establishment, and if its investigations were to be extended to other materials than bronze it would be of incalculable value in preserving our works of art for posterity. Is it too much to hope that such a consum- mation may be brought about ? EDWARD RosBINson, Director. March 2, 1925. RESTORATION OF ANCIENT BRONZES AND OTHER ALLOYS REPORT OF AN EXPERIMENTAL RESEARCH CARRIED OUT AT COLUMBIA UNIVERSITY FOR THE METROPOLITAN MUSEUM OF ART HE restoration of objects of antiquity has been practised for a good many years but the methods applied have been almost invariably of a haphazard “‘hit or miss” type. Further- more, it has usually been the practice in the past to keep secret the methods applied in restoring or trying to restore a valuable find. Accordingly, it is not surprising that there are very few references in the literature that give any details whatsoever on “Methods of Cleaning or Restoring.” It has been unfortunate that men who undertook to clean, repair, and restore usually had no artistic or scientific knowledge or appreciation but were pri- marily interested in the sum to be realized upon completion of the “cleaning.” That methods employed by the various “anti- quarians”’ are not at all alike and are conducive of very diver- gent results is everywhere evident: we need merely compare a collection of bronzes found in the very same locality but treated by different agents and observe the widely different results. There has almost always been a desire on the part of the “restorer” to bring out details in design and yet not make the object appear as though it were of recent origin. In looking over collections of ancient bronzes we furthermore observe that methods of cleaning applied were in many cases of a most rad- ical kind. For example, a number of ancient bronzes have been studied by us where there is little doubt that strong chemical reagents, such as mineral acids, were used to remove the crust. In other specimens, chisel marks were observed, indicating § RESTORATION OF ANCIENT plainly that the method of removing the crust or hard outer layers—so common with bronzes that have lain buried in the soil for ages—was of a very crude mechanical sort. Ruined or partially ruined bronzes such as these encouraged the present writers to search for a more reliable method of cleaning and restoring. At the time the present research was undertaken there were two rather distinct studies presented for investigation: 1. An improved and more general method of restoration. 2. The best means of combating the bronze disease. The lack of scientific publications on the restoration of an- cient bronzes is probably to be attributed to the difficulty in developing a truly scientific process, due primarily to the very wide diversity in the composition of the bronzes and other an- cient alloys. A method carefully worked out and applicable to a certain specimen of bronze is not necessarily applicable to any of the other bronze objects in a particular group or acquisition. Even when the composition of the bronze proper is identical— or very nearly so—in two specimens, it does not follow that the method which gave satisfactory results in the first specimen will give equally satisfactory results in the second. As a concrete example, the crust of the first object may be very thin and readily removed with a weak chemical reagent whereas in the second, the crust may be hard and much thicker and the weak reagent have little or no effect on the crust. Furthermore, dif- ferent sections of the same bronze object are often differently corroded, and one of the older methods, such as a dip in nitric acid, would not affect the two sections alike: it might “clean” one and badly pit the other. Another inevitable obstacle in beginning the pursuit of a purely scientific research on the restoration of bronzes and other metal objects of antiquity, 1s the absolute lack of information as to whether, upon the removal of the outer crust or mineral- ized layers of the bronze, we shall find any artistic design at all. BRONZES AND OTHER ALLOys 9 Accordingly, a really serviceable and generally applicable method of restoration must not be dependent upon differences in composition of the alloy or of the incrustation or patina, but must work well with all; and the resultant product must not leave any doubt in the mind of the restorer that perhaps some other method might have been more satisfactory. The Process of Corrosion Before briefly outlining the method we finally developed and adopted, and found very satisfactory indeed for a large variety of bronze compositions in various stages of decomposition, it is well to consider what takes place in the process of corrosion which gives rise to the green, blue, brown, and red layers of the outer shell or crust covering the bronze. Upon examination we find that these outer layers consist primarily of various copper minerals such as the carbonate (green malachite or blue azur- ite) the chloride and oxide or oxychloride and occasionally the sulfate and sulfide. Then there are tin and lead minerals present in some incrustations. Bronzes containing silver and other silver alloys often contain silver oxide or silver sulfide, in the crust; but in the presence of the less noble metals, such as copper, tin, zinc, and lead, silver in the crust is usually present as metal. Gold is almost invariably unattacked. The copper as carbonate or other compound in the mineral crusts is derived almost al- ways from the underlying bronze. There are specimens that indicate that the metal in the crust has been derived from out- side sources, but these cases are comparatively rare. The nature and causes of the corrosion of ancient bronzes have been investigated in the past by Rathgen,* Frisch,” Mar- gival,’ and others. The chief corroding agents are usually chlo- I. The Preservation of Antiquities, by Dr. Friedrich Rathgen, Director of the Berlin Museum, 1905, English translation published by the University Press, Cambridge. 2. Natural Patina and Artificial Patina, by Ch. Frisch, Revue de Chimie Industrielle, Vol. 15 (1904), pages 169-174. 3. Bronze Patinas, Francois Margival, Revue de Chimie Industrielle, Vol. 22 (1Q1T), pages 304-310, 331-332. 10 RESTORATION OF ANCIENT rides and nitrates present in moist soil. The constituents of the bronze are slowly converted into an outer hard green crust con- sisting of oxychlorides and oxycarbonates of copper mixed with a certain quantity of tin compounds, chiefly tin oxide; and underlying this green crust is a layer of copper oxide, and very often below this again, a core of metal. In extreme cases the corrosion has gone all the way through the specimen, and noth- ing of the original bronze metal remains. In such a case we have no longer a bronze object, but a body, consisting of a shell or crust of copper compounds, usually green in color, covering a soft brittle core of copper oxide, red or brown in color. Apart from the corrosive action of the chlorides and other salts in the soil, the prime important soil constituent necessary to bring about the most frequently met with type of corrosion of ancient bronzes is water in which the chlorides or other salts are dissolved. Water present merely as moisture in the soil in which the bronze lies buried is sufficient to bring about mineralization of the surface of the bronze and eventually of its entire body. We have had under investigation a number of bronzes that had been completely mineralized in comparatively dry locations. It is generally conceded today that the corrosion of metals and alloys is due to an electrolytic process, and it occurred to us that if this electrolytic process of corrosion could be reversed a method might be devised whereby the metal compounds in the crust would be reduced back to metal. This line of attack of the problem seemed most hopeful, since other methods, such as dip- ping the bronze into strong mineral acids, are far too radical and seldom produce satisfactory results. It will be appreciated at once that any acid method is very difficult to apply, since it is almost impossible to regulate the action of the acid in such a way as to avoid the pitting of the underlying metal. Furthermore, the metal contained 1n the com- pounds of the incrustation is removed by the acid whereas in the ideal case it should be reduced and put back into place. BRONZES AND OTHER ALLOYS 11 L ype of Bronzes Investigated The word “bronze” conveys to the minds of most people the idea of an everlasting and never-corroding metal, but such a bronze does not exist. Whenever specimens of very ancient bronze are met with that are in an excellent state of preserva- tion this is due to favorable conditions of location, such as an absolutely dry grave or tomb. Many bronze articles that throughout the ages have lain embedded in the ground in con- tact with corrosive reagents are badly encrusted and outwardly deformed. It is with corroded bronzes of this latter type that we are primarily concerned in this report. Some of these bronzes are covered with a crust so massive that recognition or identification is almost impossible. In other cases, the metal constituents have been completely mineralized. With others again the corrosion crust is so thick that thin objects, such as mirrors or plates, are warped, owing to unequal expansion. Along with such extreme crust formation, parts of the original bronze object will often be entirely corroded away, the metal having disappeared even beyond the outer crust. Object of the Investigation The object of our investigation was to start with specimens of badly corroded bronze as described above and to so treat them that the ugly crust should be removed and at the same time to restore or to preserve intact any detail of shape, design, engraving, tool mark, or other signs of workmanship remaining on the specimen as found. Now this may appear an almost impossible task; and it would be, except for one redeeming fea- ture. Almost always the details of design or workmanship of the original article have been found to be preserved and copied exactly by the layer of copper oxides which underlies the ugly and deforming green crust. Having established this, our first step was to find a method to remove or reduce the green crust 12 RESTORATION OF ANCIENT and to expose intact the oxide layer, partially or wholly reduced, beneath. We have developed a method which has given very sat- isfactory results, and which will be described more fully below. Our method of restoration has been successfully carried out with specimens in which the oxide layer still has a true metal core, and likewise with specimens in which the metal of the in- terior is completely corroded, and where only a core of weak porous oxide remains. In either case, our method of treatment is practically the same, except that the completely mineralized articles call for extreme care against mechanical breakage. The Methods of Other Investigators The idea of making ancient bronzes more presentable by re- moving the crust is not a new one. It is not uncommon among collectors and dealers to treat such a specimen with an acid so as to dissolve away the crust. The usual result of such an “acid test”? is to dissolve the corroded parts along with some of the metal core, leaving a rough, unlovely metallic surface. Fine details will be obliterated, and we shall have a “stripped” surface, often pitted. Other investigators or so-called “dealers” have in the past applied purely mechanical methods for the removal of the crust. Such methods are, however, most difficult to carry out and seldom produce satisfactory results. Again others have used the water bath, that is, placing the bronze object into a deep dish of water so as to completely immerse the object. By this method a soft crust will loosen and gradually drop off after days of submersion; if the underlying surface is hard or me- tallic and resistant to the action of water very pleasing results will be obtained, but if the underlying surface is a porous brown or black oxide there is always the danger of destroying the details. At all events we have not found this water-bath method to be a safe and reliable one and would recommend it only in special cases, cases where the bronze or silver object is in a very BRONZES AND OTHER ALLOoys ie} good state of preservation and merely requires simple removal of a thin layer of soil or earthy matter loosely adherent. Finkener was one of the first to use an electrolytic method. This method is described in detail in Rathgen’s book. Finkener used a 2 per cent. solution of potassium cyanide. Under the title, Natural Patina and Artificial Patina, Ch. Frisch (loc. cit. ), outlines electrolytic methods for the removal of the heavy green patina. Later, in 1911, Francois Margival also discusses elec- trolytic methods of attack. Francesco Rocchi, an Italian, presented a paper entitled Contribution of the Experimental Sciences in Art and History, in the Rassegna d’Arte, Vol. 7, 1920, pages 258-264. He illus- trates the results of an electrolytic method for cleaning and re- storing bronzes. Several photographs show objects before and after treatment with gratifying results. However, no details of procedure are given, nor even the composition of the electro- lytic bath. British Museum I nvestigations In 1919, the Department of Scientific and Industrial Re- search of the British Government organized a laboratory at the British Museum, and there under the direction of Dr. Alex- ander Scott, F.R.S., attention is given to the work of cleaning, restoring, and preserving museum objects. The work covers prints, enamels, silver, lead, iron, copper, and copper alloys. A very interesting booklet was published in 1921, entitled The Cleaning and Restoration of Museum Exhibits, bulletin No. 5, Department of Scientific and Industrial Research, which cov- ered the work up to that date. In 1923, a second report with the same title was published, describing Dr. Scott’s most recent work. In our work at Columbia we have freely used Dr. Scott’s re- ports and have received much valuable aid from them. Dr. Scott, however, uses ordinary chemical methods for removing crust formation and makes no reference to electrolytic methods. 14 RESTORATION OF ANCIENT Results of Investigations at Columbia As indicated above, the original purpose of our investigations at Columbia University was to remove the unsightly crust by any means possible, so as to restore the object to a state more nearly as it appeared ages ago. We felt, however, that if we could eliminate or reduce this crust and at the same time un- cover and preserve the detail underneath, that many museum objects now nothing but an ugly green mass could be made pre- sentable and attractive. This we believe we have succeeded in doing. We have given a large variety of methods a most careful trial and observed the results. We very soon decided in favor of an electrolytic method of attack. The fundamental idea was to replace cathodically the metal that had passed into the crust rather than to completely eliminate the crust with its metal content by corrosive acids or by mechanical means. Our electro- lytic method as finally developed 1s of very general application and can be used with absolute safety in practically all cases of restoration. [his is the one strong argument in favor of the electrolytic method of restoration—it is safe. In a large number and variety of cases, ancient bronze speci- mens were received at our laboratory that were very badly cor- roded and extremely fragile. Usually investigation showed that the entire metal mass had been completely mineralized. Upon submitting the specimen to our electrolytic treatment, we suc- ceeded in recovering most of the detail, and in revealing inscrip- tions, markings, etc., put on by the early artist. In cases such as these mechanical removal of the crust 1s out of the question on account of the extreme fragility of the specimen. And an acid bath would have destroyed the details. In developing our electrolytic process we tried out various electrolytes used by earlier workers, as well as other electrolytes suggested by our own laboratory observations. We finally de- BRONZES AND OTHER ALLOoys By cided upon a 2 per cent. solution of caustic soda (Na OH), as the safest electrolyte to use. Finkener’s electrolyte of 2 per cent. potassium cyanide (KCN), was not found to be satisfactory. We do not recommend its use. Potassium cyanide is extremely poisonous and its use even by skilled workers is dangerous. Fur- thermore, the cyanide solution has a very marked solvent action on bronzes, in particular those covered with a delicate copper oxide layer, even when the bronze object is made cathode. The Method in Detail Without any preliminary cleaning, the bronze object to be treated is hung as cathode into the 2 per cent. caustic soda solu- tion and a low amperage direct current is applied. The object is suspended with soft copper wires and is completely immersed into the solution. In case the object is very soft and fragile or is completely mineralized, fine annealed copper wire is wrapped around the object, one to two turns per inch, and electrical con- nections are made with several turns of this wire. Where there is danger that the object might not hold together upon the re- moval of the hard supporting shell, we have found it advisable to pack the whole object in clean white sand, after making proper electrical connections, and then filling the containers with the caustic soda solution. The anodes are hung on either side of the object. Iron, duriron, and platinum anodes have been used with success. A rectangular glass battery jar of one litre capacity serves well as a container tor the treatment of small bronzes. For large objects we have used stoneware tanks and there is no objection to the use of large tanks made of heavy sheet iron welded at the joints. For a small object of about two to ten square inches of ex- posed surface, the cell is connected in series with a rheostat and the 110 volts direct current circuit, so as to send from 0.1 to 0.5 amperes through the circuit. A slight gassing at the anode will occur. Sometimes the crust resistance at the point of contact of 16 RESTORATION OF ANCIENT the copper wire is so high that an appreciable current will not pass through the cell at first. Rather than file a clean contact, which is hardly ever to be recommended, it is best to start the cell at night, allow the solution to slowly penetrate the crust and usually by the next morning, the 110 volts potential will be sufficient to have broken through the submerged crust with- out injury to the surface. Often wetting the copper wire contact with electrolyte will break down the dry resistance of the crust at that point. The object being treated is always made cathode. The action of the electrolysis is to evolve hydrogen at the cathode and to so reduce the crust to finely divided or spongy copper. This is effectively accomplished in the caustic soda solution. At times reguline copper metal will be deposited in place. Very low current densities are preferred. The reduction of a thin crust 4%. to % inch thick usually re- quires three or four days. In the case of clayey crusts it is a good plan to change the electrolyte once every twenty-four hours. The use of too strong an electrolyte, or too high a current density will cause excessive gassing at the cathode (the object being treated) and may give rise to warping and falling apart of the object unless it is underlaid by a strong metal core. Complete reduction is indicated by a free evolution of gas at the cathode with comparatively low current values. The object is then removed from the solution and carefully washed by soaking or steeping in several changes of warm water. This will remove all but traces of caustic soda. We now arrive at a stage in the process where we must use judgment enough to modify subsequent treatment to fit the general appearance and physical strength of the specimen. If the object treated originally had a hard metal surface under a thin sandy layer of crust, we may remove the reduced copper film or layer by a gentle brushing with a stiff bristle brush or soft brass-wire brush. If, however, examination preliminary to electrolytic treat- BRONZES AND OTHER ALLoys 17 ment indicates that no true metallic core is present, we do not dare subject the treated surfaces to pressure or friction of any kind. In this case a weak acid dip may be necessary and is used as follows: After thorough washing to remove all but traces of caustic soda, carefully dip the reduced object into dilute nitric acid (one part acid to four parts of water). There will result a rapid action and evolution of gases, as the reduced outer layer of cop- per dissolves. By alternately dipping into warm water and into dilute acid, we can control the extent of the treatment. As the reduced copper dissolves away, we bring into view the hard, brownish-colored copper oxide surface which preserves the de- tail of design. This oxide layer does not dissolve readily in the weak acid used, but takes on a greyish tint (due probably to intermixed tin compounds ). Inspection with a magnifying glass will show when the treatment has proceeded far enough. It 1s an exciting moment when we see the copper dissolve, showing a smooth surface, or design, beneath. The oxide surfaces now exposed are almost as smooth to the touch as metallic surfaces and they will take on a greenish tint or patina when dipped into dilute ammonium acetate and dried in an oven at 40° to 60° Centigrade (104° to 140° Fahrenheit). Sometimes, especially if the original surface shows cracks in the patina or conspicuously high lumps or “‘boils” of crust, we would find the underlying oxide layer pitted and roughened in spots 1f we had used the weak acid dip. Such regions have been highly corroded by localized action and the treatment in our electrolytic bath causes brown copper to fill the pits. Therefore in these cases the weak acid dip is omitted. Treatment of Individual Objects The best way to show the possibilities of the electrolytic treatment is to describe results obtained on various individual objects. As the objects were received for treatment each was 18 RESTORATION OF ANCIENT assigned a serial number. Such serial or laboratory number will be used to identify the objects in this report. The first step in each case was to submit the object to a very careful examination. It was measured and weighed; the crust or patina was studied and analyzed if necessary, in order to get an estimate of the probable composition of the underlying base. Detailed records were kept of each object investigated. BRONZE VASE (Serial No. 25) This is a pear-shaped vase about 3% inches high. Fig. 1a shows the vase as received for treatment. As received the object was covered with a hard green deforming crust. Here and there over the surface were to be found warts or knobs of hard crust. Preliminary examination indicated that no metallic core re- mained, and that the vase was corroded or mineralized all the way through. This case was selected to show permanently on one specimen the difference in the appearance of the surface before and after treatment, and therefore only the lower half was treated. This was subjected to cathodic reduction in dilute caustic soda for one week. After such treatment the reduced surface was treated with weak nitric acid. After removal of the red spongy copper, the oxide core was revealed beneath; it was smooth, detailed, of a brick-red color, and with the aspect of unglazed earthenware. This oxide core was very weak mechanically and required the most careful handling to avoid breakage. In fact, it was about as hard and strong as a thin cake of chocolate. After the electrolytic reduction treatment, in the 2 per cent. caustic solution, the whole object was dried and then covered with a high-grade mat lacquer, commonly used for silverware, such as the lacquer known under the trade name Zapon Aqua- nite. As the specimen now stands (Fig. 1b) there is a striking contrast between the rough, green, untreated upper half and the brick-red reduced lower half, showing the details. The mat Fig. 1b BRONZE VASE (Serial No. 25) 26 RESTORATION OF ANCIENT lacquer, besides protecting, strengthens such an object and 1s itself practically invisible. BRONZE SSL KLE (Serzal No. 69) The results obtained in this case were almost spectacular, and illustrate the possibilities of treatment of a piece of bronze so completely mineralized that no metal core is left. The object is a bronze strip and when received from Rome was approxi- mately 4 inches by 6 inches. Figs. 2a and 2b show front and back as received, Figs. 2c and 2d show the finished object. At one time this object was part of a bronze plaque which served some decorative purpose, probably architectural. It was originally surfaced on one side with gold leaf. After ages of corrosion all we have left is a completely mineralized copper oxide strip, exceedingly weak mechanically. The strip proper was about /% inch thick, and before treatment was covered on the back with an ugly green-grey crust also about % inch thick. The crust on the front was much thinner, and after treatment revealed a flimsy gold leaf and a design underneath. At once this question came up: why is the crust on the face so much thinner than that on the back? Does this mean that the gold leaf protected the face? This object was reduced as cathode in dilute caustic soda solution for several days. Next dilute nitric acid was used to dissolve away the sponge copper covering the gold. This treat- ment also brought out the natural bright lustre of the gold leaf. The dilute acid treatment has no bad effect on the oxide core other than to give it a greyish tint (due very likely to the forma- tion of a meta-stannic acid ). The grey color of the oxide helped to distinguish it from the undissolved red spongy copper, and enabled one to decide when the acid treatment was completed. Immersing in dilute ammonium acetate solution and drying at 4o° Centigrade in an oven added a desirable greenish tint to the oxide layer. This bronze strip or plate was finally lacquered with a good mat lacquer. 40, 2a BRONZE STRIP—FRONT (Serial No. 69) (69 ON 1049S) MOVA—dINLS AZNOUA pe Sly BRONZES AND OTHER ALLOYS 20 BRONZE FRAGMENT (Seréal No. 84) Figs. 3a and 3b show this specimen before and after electrol- ysis in dilute caustic solution. The results are about the same as for the bronze plaque above. In this case a bronze core was found underneath the copper oxide layer. The specimen was finally given a patina (of a green tint) and lacquered. BRONZE BRACELET (Serial No. 17) As received (see Fig. 4a), we have an oval bracelet, bent on about a 34-inch radius. It was covered with a hard green crust, and at one point we found a large lump, like hard enamel, badly cracked. The photograph shows this lump at about 1% inch from the end. After electrolytic reduction in caustic soda solution and careful rubbing with a stiff bristle brush, we bring to light a pretty little bronze bracelet as shown in Fig. 4b. In this case we have a rather hard core with most of the design intact. A snake head is plainly seen at each end. As is usually the case a deep pit was found underneath the enamel-like lump. In general a conspicuous growth of crust indicates severe cor- rosion beneath. BRONZE LAMP (Serial No. 32) Figs. 5a and 5b show this lamp as received. We see front and back covered with a hard, lumpy, green patina. The bowl is 2% inches in diameter. There were no suggestions of detail in design. After electrolytic reduction the surface was covered with brown spongy copper. This was carefully brushed off with a stiff brush. Figs. 5c and sd show the lamp after treatment. After brushing, the surface was of a dull copper color. BRONZE COINS (Serial No. 48) Individual bronze coins so badly encrusted as to be illegible have been successfully treated by reducing electrolytically in 24 RESTORATION OF ANCIENT caustic solution followed by a gentle rubbing. A good example of such treatment is shown by Figs. 6a and 6b, which show a bronze coin (Lab. No. 48) before and after treatment. The strangest thing about coins reduced in this way is that a freshly reduced coin shows the design best when wet. A clean dry treated coin is hard to decipher. The above coin was artificially patinated to bring out the design. Old bronze coins sometimes seem not to respond to treatment. We must remember, how- ever, that coins were often badly worn while in use; these had a smooth surface before the actual corrosion in the soil started. Usually nothing is gained by examining such worn coins under the microscope. The appearance to the unaided eye is nearly always the best test. Photographing a treated coin while wet will show the design even better than visual examination. BRONZE FIBULA (Serial No. 67) This small decorated fibula is about 3 inches long, and when received it showed a pretty pale blue original patina under a thin sandy crust. In one or two spots the patina appears clean, bright, and enamel-like. On boiling in concentrated solutions of ammonium acetate for a few minutes, we removed all traces of the sandy crust and exposed the original design, a series of grooves or rings. Figs. 7a and 7b show this fibula or handle before and after treatment. In this case we decided not to use electrolysis and tried out a very weak chemical solvent (am- monium acetate), which gave great promise on other small bronze fragments. We suggest that this solution be tried along with the chemical solvents recommended by Dr. Scott for mild cases of corrosion such as this fibula. Gold and Silver Ornaments Much to our surprise and pleasure, several heavily encrusted ancient specimens, classed as copper-tin bronzes, turned out to BRONZE FRAGMENT (Serial No. 84) Fig. 4a Fig. ab BRONZE BRACELET (Serial No. 17) Fig. 5c Front Fig. 5d Back BRONZE LAMP (Serial No. 33) BRONZES AND OTHER ALLOYS 925 | be silver or gold (or the respective coin alloys) on treatment. These will be described in detail below. We generally assume that gold and silver are corrosion-proof. Such is not the case, however, if these metals are hardened or debased by alloying with copper. Dr. Scott in his 1921 report, page 7, most ably describes the mechanism of silver alloy cor- rosion, and gives some excellent chemical methods for treating it. Rathgen, pages 49 and 148, also goes into the theory of cor- rosion of silver alloys. In brief, a silver alloy containing copper, after centuries of burial in the earth, subject to the various agents of corrosion, will deteriorate. Depending upon the fineness of the silver, we shall get anything from a thick, hard green crust to a soft white or lilac film or patina. The same holds true for gold alloys, only not to the same degree. For pure gold or pure silver, our remarks do not hold, but objects of the chemically pure metals are very seldom found. It is our opinion that gold and silver objects, hardened or alloyed with copper, and subjected to underground corroding conditions, will undergo a copper migration or out- ward copper diffusion. Let us call this ““decuperification” of the alloy. This action seems analogous to the dezincification of brass condenser tubes. Perhaps aided by the membrane action of the first formed tarnish layer, or patina, we get a gradual outward migration of the copper originally alloyed with the more noble metal. After centuries of corroding influences, we have left, not a silver or a gold alloy, but an object consisting of three layers, (1) an inner core of porous noble metal, (2) an intermediate layer consisting mostly of copper, and (3) an outer crust of green oxy-compounds of copper. The inner core of “decuperized”’ gold or silver will be more or less porous and weak mechanically, depending upon the extent of the corrosion. It is this inner core, however, which carries and preserves the details of design. The intermediate layer, of red secondary copper has migrated from within, and must be re- 28 RESTORATION OF ANCIENT moved. The outer green crust is usually very unlovely and is best removed by electrolytic reduction in caustic soda. The simplest way to outline such a treatment is to describe in detail below the restoration of several silver bracelets, silver coins, and gold specimens. SILVER BRACELET (Serial No. 1) After cleaning, this bracelet weighed 57 grams and was 14 inch (6 mm.) thick. The radius of curvature was about 114 inches (32 mm.). When received, it was covered with a hard green crust, from 4%. to % inch (1.6 to 3.2 mm.) thick, and was catalogued as “bronze.” The crust was unattractive and to all external appearances, it seemed to encase a bronze object. In this instance the treatment was chiefly electrolytic in 2 per cent. caustic soda solution. This resulted in a reduction of the green crust to spongy copper which was readily brushed off. However, we found beneath the spongy copper a sheath or envelope of red or secondary or reprecipitated copper that had presumably dif- fused from within. This 1s a good example of “‘decuperification”’ of a silver alloy. When this layer of secondary copper was re- moved partly as anode in very dilute sulfuric acid, and partly . mechanically, a smooth white silver surface came to view. Fig. 8 shows the restored bracelet. Note the four grooves cut near each end. These were evidently made with a small chisel, as each individual cut is apparent. Possibly a setting formerly fitted between the ends. Through an oversight no photograph was made of the bracelet before treatment. SILVER BRACELET (Seng This was made of round silver alloy wire, 34 inch (5 mm.) thick, bent on a 1-inch (25 mm.) radius. Fig. ga shows its con- dition when received, classified as ‘‘bronze”’ ; it was covered with a hard green crust. After cathodic reduction in dilute caustic soda solution, the loose spongy copper was brushed off. Next, Fig. 6a Fig. 6b BRONZE COIN (Serial No. 48) Fig.7a Fag. 7b BRONZE FIBULA (Serial No. 67) Fig.8 SILVER BRACELET (Serial No.1) Fig.oa Fig.9gb SILVER BRACELET (Serial No. 18) BRONZES AND OTHER ALLOoys 31 the secondary copper was dissolved in weak ammonium chloride solution. The silver core in this case was very weak mechan- ically, and broke at several points owing to some slight strain in handling. Again we have a decuperized core of spongy silver, soft and mechanically weak. Fig. gb shows the restored bracelet. SILVER COINS § (Serials Nos.74.and77) Two groups of silver coins were subjected to treatment. These groups consisted of thirteen coins very tightly cemented together by the products of corrosion, a mixture of silver and copper compounds. It is possible that originally they had all been united in one lump which was partially broken during or after * excavation. The mass was completely covered with a hard green crust, and its appearance suggested nothing other than bronze coins, which the authorities of the Metropolitan Museum assumed them to be. Figs. 10a and 10b show them as received. They were given various treatments, mainly chemical, as out- lined by Dr. Scott. One group was first reduced as cathode in 2 per cent. sodium hydrate solution. The results of the treatment are shown in Figs. 1oc and 1od. The large coin at the bottom is of bronze. The coins were found in all stages of decomposition. The best preserved are shown in the photographs. Some of the coins are covered superficially with a sheathing of soft, spongy silver which resisted any attempt to remove it by chemical or mechanical means without harming the coin design beneath. It is very difficult to give any hard and fast rule for treatment of silver coins in such an advanced state of corrosion. Boiling with formic acid, followed with boiling in acidulated ammo- nium and stannous chlorides as outlined by Dr. Scott, works well in many cases. In the 1922 report of the British Scientific and Industrial Research, pages 63-67, a brief report is made by Dr. Scott, as follows: “The mixture of ammonium and stannous chlorides in acid BD. RESTORATION OF ANCIENT solution has proved particularly effective in the treatment of coins that have been encrusted and stuck together by corrosion. The reagent enables the coins to be completely separated with- out mechanical force and, moreover, renders their inscriptions easily legible.”’ We are of the opinion that the presence of the large bronze coin was the cause for most of the severe corrosion. In other words, had these silver coins been buried out of contact with the bronze coin, the corrosion would not have been so marked. This seems borne out by the fact that the coins nearest the bronze coin are the most corroded. HANDLE OF A GOLD STRIGIL “Senay We have received but one gold antique for treatment. Figs. 11a and 11b show this specimen before and after electrolytic re- duction in caustic solution and treatment with dilute nitric acid to remove spongy copper. Note the clearness with which the small Greek inscription has been brought out. Here, as in the silver ornaments, we were troubled in spots with a tight-sticking layer of secondary copper which is difficult to remove. We found that an electrolytic reduction in caustic soda, followed by boiling in ammonium stannous chloride solution, is most effec- tive for objects of this type. GREEK BRONZE GOAT (Seréal No. 100) As received this little bronze goat was covered with a hard grey-green crust. The fore feet were encrusted or cemented to- gether. [here were scratches on parts of the body indicating that attempts had been made to remove the offensive crust by me- chanical means. The goat was submitted for three days to the electrolytic reduction process in 2 per cent. Na OH solution. The treatment worked wonders, as is shown in the accompany- ing photographs (Figs. 12a and 12b). Fig. 1ob SILVER COINS (Serial Nos. 74 and 77) Fig. iod Fig. toc SILVER COINS (Serial Nos. 74 and 77) BRONZES AND OTHER ALLOYS 35 DROINZE CAT WITH KITTENS: “(SertalNo.io2) This is an Egyptian bronze. The surface was badly corroded and covered with an ugly grey-green crust. The bronze had also suffered mechanically, being warped and cracked. The heads of two of the kittens had been broken off years ago and the frac- tured parts were badly corroded. The object was given the electrolytic caustic soda treatment and the results obtained are indicated in the photographs (Figs. ieaand 13b). COPPER PLATE (Serial No. 26) An excellent example of restoration is shown by the photo- graphs of a copper tray or platter about 6 inches (15 cm.) in diameter. This tray, as received, was very badly corroded and covered with an ugly, hard green crust hiding all details. After electrolytic reduction and restoration the almost complete de- sign engraved in the copper was revealed. The photographs (Figs. 14a and 14b) give a view of the front, before and after treatment. SievhUb DLE sOR SSIS a Serzal No237) This is another Egyptian bronze, lent to us for experimenta- tion by Mr.V. Everit Macy (Figs. 15a, 1 5b before and 1 §c, 15d after treatment ). It is about 11 inches high and weighs approx1- mately 3 pounds. The white spots in the “before”? photographs are “‘diseased”’ regions consisting of copper carbonate plus chlo- ride. The left hand of Isis was very badly corroded. The crust over the remaining portions of the body was hard and dark green in color. The statuette was suspended in an earthenware tank containing 14 litres of 2 per cent. Na OH solution. Electrical connections were made to various parts of the figure, without, however, cutting through the crust. In all of our work great pains were taken never to disturb the crust or patina before treat- ment. The current was 114 amperes and the first treatment 30 RESTORATION OF ANCIENT lasted seven days. The second electrolytic treatment lasted fourteen days. The statuette was then washed in warm water and the loosely adhering particles of reduced metal and foreign material were brushed off. The details were revealed in a most striking fashion as is shown in the photographs. Details of de- sign such as the necklace, the hair and headdress, toe and finger- nails, etc., which had been completely hidden by the crust, were now distinctly revealed. The diseased portions were repaired by the same process except the one on the left thigh, where the crust had disappeared before receipt of the statuette at the laboratory. The statuette after treatment was patinated by submitting it in an enclosed chamber to fumes of ammonia and acetic acid. This patinating method is discussed more fully below. Patinating Restored Bronze Surfaces Most of the bronzes that have been investigated at our lab- oratory were covered with greyish-green or brownish-green granular crusts of an unattractive, inartistic appearance. Those bronzes that have an enamel-like, hard green, blue, or red patina require, as a rule, very little if any special cleaning. The hard glassy patina has preserved the details in design and upon re- moval of the outer clayey or sandy crust, often by merely washing with warm water or with weak salt solutions, the beau- tiful patina underneath is revealed. At times gentle brushing with a soft bristle brush will be necessary to remove the last traces of clay or sand. Bronzes that do not have this hard protective patina are almost always covered with a loosely compacted unsightly crust. Of course it will be at once appreciated that it is impos- sible to bring forth from under this unsightly crust a hard, at- tractive, natural patina where it no longer exists. The inev- itably unlovely appearance of such bronzes after they have been through the treatment for the removal of the ugly crust is a Fig. 11b HANDLE OF A GOLD STRIGIL (Serial No. 83) semen Fig. 12b GREEK BRONZE GOAT (Serial No. 100) Fig. 13a Fig.13b BRONZE CAT WITH KITTENS (Serial No. 102) BRONZES AND OTHER ALLOYS 39 matter of common experience no matter what the method for the removal of the crust has been. Dealers as well as museum authorities, recognizing the fact, have sought to overcome it by supplying an artificial patina, often deceptively successful. There is no doubt that a large percentage of the ancient bronzes in public and private collections have been artificially pati- nated. In view of the circumstances, the procedure seems to be amply justified, provided that the bronze itself suffers no injury from it and that the patina can be readily removed if desired. Upon that understanding, Mr. Edward Robinson, the Director of the Metropolitan Museum, wished us to carry on experiments to the point of determining the most desirable form of artificial patination to be applied to bronzes requiring it. Upon treating many of the bronzes by our electrolytic reduc- tion methods a smooth metallic or metallic-like surface is ex- posed. This surface can be much improved in appearance by developing over the surface a patina very similar to that met with naturally. We felt that if we reproduced the natural pati- nating conditions in the laboratory and intensified these we ought to obtain effects very similar to those obtained after years of exposure. Accordingly, we subjected bronzes to the attack of various gases such as ammonia, carbon dioxide, sulfuretted hy- drogen and acetic acid. This investigation is still in progress, but the best results up to date have been obtained by exposing the bronze object to a simultaneous attack of ammonia and acetic acid gas. Beautiful bluish-green patinas are obtained which lose the bluish tint upon heating the bronze in an oven at about 110° Centigrade, bringing about a gradual change. We do not favor the use of any chloride as we consider chlo- rine one of the chief destructive agents of bronzes and the cause of much of the so-called bronze disease (see below). Patinating by the gas method is far more satisfactory than the older methods of dipping the object in ammonium chloride solution or painting the object with copper carbonate, or—as 40 RESTORATION OF ANCIENT has been done in some cases—painting the object with a green or blue aniline dye. The gas methods bring out artistic effects which are practicalky impossible by any dipping or brush methods. The procedure we recommend is to place the object in an air- tight cell or box at the bottom of which are two small vessels or jars, one filled with concentrated ammonia solution and the other with strong acetic acid. It 1s not necessary to heat the chamber. The time required for a satisfactory patina is from ten to thirty hours, although even longer treatment will be found necessary in individual cases. To render this artificial patina proof against weather conditions such as exist in New York, the surface must be protected with a mat laquer or with a solution of beeswax. It is important to note that a long exposure of the bronze surface to a weak corroding atmosphere will bring about more artistic and lasting effects than a short exposure to strong reagents. We therefore do not approve of heating the reagents or the cabinet itself during the gas treatment. Horns in his book on Metal Coloring and Bronzing recom- mends “‘a judicious and thorough scratch brushing” in order to secure uniformity of surface. We have tried this but found that the results were far from satisfactory. The beautiful artistic green, blue, brown effects brought about by the gas treatment are more or less destroyed. In some cases we have found that a preliminary gas treatment in sulfuretted hydrogen and a subsequent treatment in ammonia and acetic acid gas produced a dark green to black patina. On the other hand, we have found the action of carbonic acid gas alone too slow for practical purposes ; a mixture of carbonic acid gas and acetic acid gas, with the former in large excess, is more active. Chlorine gas or volatile chlorine compounds such as ‘muriatic acid and salammoniac give very beautiful effects, but we do not recommend any of these on account of the treacher- ous nature of chlorides in the patina. Fig. 14b COPPER PLATE (Serial No. 26) 42 RESTORATION OF ANCIENT The Bronze Disease We have referred to this bronze disease repeatedly above. Very often it is due to a phenomenon similar to the well-known “tin pest,’ when tin is gradually transformed from one allo- tropic modification to another. We have found that the usual immediate cause of the bronze disease is the presence of a trace of chloride, and the action is primarily electrolytic. We have closely studied a number of bronzes that were in- fected with the bronze disease. The first intimation that the object is infected is usually the appearance of a few fine grains of light green dust on the base or platform upon which the bronze is mounted. These grains are so fine and so light that unless the bronzes are encased in glass they will be easily blown away. Upon carefully examining the bronze object and tracing the origin of the grains of light green dust we find an area, often not larger than that of a pin-head, which is rough in appearance and free of all lustre as compared with the rest of the surface of the bronze. Taking a fine needle we note that the infected area is comparatively soft. If neglected the area will grow at a more or less rapid rate, depending upon conditions such as the pres- ence of moisture in the surrounding air. How the bronze disease actually starts at one or the other spot on the surface of the bronze it is very difficult to determine. We have observed that when a mere speck of lampblack or coal soot is deposited on a sheet of bright clean copper and the sheet exposed to our laboratory atmosphere that the black speck soon becomes the centre of a corroding area. Whether it is a trace of sulfurous or sulfuric acid originally present in the carbon speck that accelerates the corrosion of the copper in the immediate vicinity of the speck, or whether it is a trace of one or the other constituents of the atmosphere such as ammonia, sodium or ammonium chloride, or a sulfide which has been adsorbed and condensed by the carbon speck and brought about an acceler- Fig.15a Fig. 15¢ STATUETTE OF ISIS—FRONT (Serial No. 87) Fig. 15b Fig. 15d STATUETTE OF ISlo— BACK (Serial No. 87) BRONZES AND OTHER ALLoys 45 ated corrosion, is very difficult to determine, on account of the very minute quantities of material dealt with. Copper sheets given a thin film of ammonium chloride and kept in a fairly moist atmosphere soon become pitted, due to a slow but continuous action of the chloride. Only a very minute amount of chloride is needed to convert a large mass of copper into malachite or azurite in the presence of moisture and car- bonic acid as found in the air. For combating the bronze disease various methods have been suggested in the past. A common method is to coat the affected part with pitch or bitumen but this is very unsatisfactory as it leaves an ugly black spot. We recommend the following pro- cedure: clean the affected parts thoroughly by the electrolytic method described above, using the 2 per cent. sodium hydrate solution. Then wash and dry carefully and apply a mat lacquer (cellulose acetate). This lacquer should be renewed at least once in four years. : Conclusion Ancient bronzes that had been badly corroded have hitherto been subjected to various treatments for the purpose of restor- ing their surfaces. A new, comparatively simple method has been developed in the investigations described above, which has been successfully applied to a wide variety of bronze objects. The method is primarily an electrolytic one and its chief advan- tage lies in its being absolutely safe as compared to older acid dip methods or the chisel and hammer method. The new method we have described in detail and the accom- panying photographs show the results that have actually been obtained. Methods for patinating and for combating the bronze disease are only briefly described because this part of the investigation is still in progress, and what we should regard as absolutely satis- factory methods have not yet been achieved. We hope, however, to make these the subject of a subsequent report. 46 RESTORATION OF ANCIENT APPENDIX EXAMINATION OF BRONZES TO ESTABLISH, IF POSSIBLE, THEIR GENUINENESS In the course of our investigations on ancient bronzes, we have had occasion to examine a number of objects concerning which there was some uncertainty as to whether or not they were genuine antiques. It need hardly be mentioned that there are a large number of spurious antiques sold to people who are not familiar with even the simplest means of determining the composition and age of the object and deciding whether or not the claims made by the dealer are fact or fiction. Just as there are means and methods of detecting a counter- feit “Rembrandt” so likewise are there methods of detecting an imitation or spurious “antique” bronze. Although it is true that absolute identification of the fraud is not always possible, in the large majority of cases we can by a systematic method of phy- sical, chemical, and metallographic analyses determine whether or not the bronze or other alloy is of comparatively recent origin. We have divided our method of investigation and procedure into three distinct steps: 1. Microscopical examination of the patina or crust. 2. Metallographic examination of the metal core. 3. Chemical analysis of the metal. In the examination of the patina we may find distinct crystal- line growths of malachite or azurite, the tiny crystals partially imbedded in the underlying oxide film. Such crystalline growths are most difficult to imitate and patinas of recent origin are almost always amorphous or non-crystalline. Nevertheless, the BRONZES AND OTHER ALLOys A7 patina may be recently formed or applied and yet the bronze may be genuine—in fact, very many of the genuine bronzes exhibited nowadays have patinas of recent or artificial for- mation. Accordingly, if the microstructure of the malachite or azurite particles in the crust or in the patina 1s distinctly crystalline and interlaced with the copper oxide layer, the bronze is very likely genuine. But an amorphous malachite deposit does not neces- sarily imply that the underlying bronze is not genuine. Under the outermost coating of malachite or azurite or clayey material there usually is found, in the case of genuine bronzes, a layer of copper oxide, reddish-brown to reddish-black in color. This layer has been repeatedly referred to in the body of this report. It is usually of considerable thickness and is not readily applied by any rapid artificial process. In the metallographic examination of the bronze we find ad- ditional proof as to whether or not the bronze is genuine. Taking a very fine slice of metal from some inconspicuous part of the bronze, and preparing this by polishing and etching for micro- scopical examination, we note in the case of a genuine bronze that there is a gradual change in structure as we pass to the outer exposed edge of the bronze; furthermore we will some- times note changes in the size of certain crystal components of the bronze, changes that are brought about by a very slow proc- ess of “‘annealing.”’ We have referred to the process of decuperi- fication in the body of this report and will cite another definite case below. Chemical analysis must be carried out with great care. We can support the findings of our metallographic investigation by determining chemically that the main body or interior of the bronze has a decidedly different composition from the layers under the outer oxide or patina surface. How the above method of investigation and detection may be applied can best be illustrated by the two examples cited below. 48 RESTORATION OF ANCIENT SILVER BRACELET (No. 22.50.3) (a) The Pendant Pan This is a small silver pendant attached to a silver link brace- let (Figs. 16a and 16b). Upon examination under the micro- scope we found on the back and on the arms of the image of Pan irregular deposits of tarnished metal. Further study indicated that these deposits were of secondary formation, very similar to those reported in the case of the two silver bracelets (see page 23). These irregular deposits on the back and on the arms are due to a process of corrosion which we have termed “‘decu- perification.”’ The process of decuperification is very similar to the process of dezincification, commonly met with in the case of brass, where the surface of the alloy decomposes and as a result of the operation of corrosion, pure copper is found on the surface of the brass, the zinc having disappeared into the crust or patina or beyond. In the case of the image of Pan and the links to which it is attached, the alloy consists of silver with small percentages of copper. During the process of corrosion, there has been a ten- dency for the copper to pass out of the surface and leave pure silver behind. As the result of this process, silver is redeposited in irregular patches and these pure silver patches are tarnished by the sulfur gases of the atmosphere. Besides copper, there may be other metals in the alloy of which the image is composed, which tend to make the metal more resistant to corrosion. At all events the secondary deposits of silver on the back and on the arms of Pan are darker, in some places jet black, as compared with the rest of the body of this little image, the pure silver tar- nishing more readily than the original alloy. We surmise that the image had been cleaned before we received it and that it was at one time covered with a dark green crust. Patches of the green patina are still present; for example, there is a small patch of green patina underneath the beard, also examination shows green patina on the right arm. These green patches indicate the Fig. 16b The Pendant Pan SILVER BRACELE (GVo.22.560:43) MIRROR (No. 22.50.1) BRONZES AND OTHER ALLOYS Sal presence of copper in the alloy. We hesitated to make a chemical analysis of the image since the bronze was too small and we were afraid we would deface it. (b) The Chain The links and the hook are rather odd and not usually met with in modern times. In the case of a number of the links, it is difficult to see where the metal has been forged together. The workmanship has been very careful and accurate. Upon closer examination of the links and of the ring connecting the two halves of the chain to which the pendant is attached, we ob- served the same type of secondary deposit of silver referred to above which has turned black on exposure to sulfur gases. The second link to the left of the ring shows this black silver most strikingly. These secondary deposits of silver take many years to form and it would require a very clever imitator to bring about results which would at all resemble the metal deposits on the chain and on Pan. Our conclusions, therefore, were that the chain and pendant are composed of a silver-copper alloy. This alloy has been decuperized on the surface and deposits of pure silver have formed. Since this action is extremely slow, since the patches are very natural in appearance, and since, further- more, the chain and pendant are very similar in characteristics to the two silver bracelets previously examined, we therefore concluded that the chain and pendant were very old. MIRROR (No. 22.50.1) The mirror (Fig. 17) had been originally turned over to a “repairer,” by whom it was cleaned. Small tufts of the cotton used in cleaning still adhered to the surface of the mirror, in particular on the reverse side. The mirror was at one time fastened to a wooden backing or support and plaster of paris had recently been applied to hold the metal to the wood. Samples of the patina, the wood, and the metal were taken. 52 RESTORATION OF ANCIENT In sampling the patina it was very difficult to find portions that had not been disturbed or altered during the cleaning process or the recent application of the plaster of paris. ‘wo small slivers of metal from the back of the mirror were taken. These were almost black in appearance. The patina was carefully examined under the microscope. We found malachite in abundance, some of which was distinctly crystalline in structure, showing the fine needlelike prisms. This crystalline structure was a very strong indication that the patina was not of recent origin. It is practically impossible to produce patina of crystalline malachite artificially. The crystal- lization process is very slow and the malachite or copper car- bonate produced by ordinary chemical manufacturing processes is amorphous in structure. Ihe element of time is a very im- portant factor in producing malachite crystals. Besides malachite we observed very fine crystals of calcite and, although these too were crystalline, the proof as to age is not so strong as it is in the case of the crystalline malachite. The front face of the mirror frame was protected by a thin layer of gold and only where the gold had separated from the underlying bronze was excessive corrosion apparent. The two bronze slivers were covered with a very dark oxide of copper and over this in spots were small growths of malachite. The metallographic analysis of a sample of the bronze indicated that the alloy was very old, although the evidence is not so strong as in the case of the patina. Owing to the contamination of the patina samples with cot- ton fibre used by the dealers in cleaning the mirror, the chem- ical analysis was not reliable and no definite conclusions could be drawn. The wood sample was examined under the microscope. The fibrous structure was highly characteristic of very old wood. The cells were distinctly visible and small white crystals of mineral matter were scattered throughout. There seems very little doubt, BRONZES AND OTHER ALLOYS 53 to our minds, that the wood is of ancient origin. Of course, this does not necessarily prove that the mirror is of ancient origin. However, on the basis of our microscopical examination of the metal and of the patina we concluded that the mirror was of ancient origin. OF THIS BOOK ONE THOUSAND COPIES WERE PRINTED MAY, 1925 Ino ie