INDIAN AND INDONESIAN Copper, Brass and Bronze.—The founding of copper, brass and bronze by the tire perdue process is universal in India, and of high antiquity; but it may be remarked that the use of bronze is comparatively rare in the case of images, most of the so-called Indian bronzes being made of pure copper. After finishing, the casting is usually gilded by the mercury process. In the case of Nepalese copper figures, and occasionally in Jaina brass figures, a decoration with inlaid gems may be added. Small images are cast solid; much larger images were cast by the same process, but on an earthy core composed of clay, sand, charcoal and rice-husks, burnt to a cinder-like consistency. The colossal copper Buddha from Sultanganj, of Gupta date, now in the Birmingham museum and art gallery, was cast in this manner, but in several sections; the same process is widely employed in Farther India, and is especially characteristic of the later Siamese "bronzes." In the case of utensils such as water-vessels and ceremonial vases, lamps, etc., the same process is employed, but the casting is finished by turning on a lathe and polishing. Vessels of copper, brass or bell-metal were formerly made exclusively for domestic and religious use, but more highly ornamented types are now made largely for sale to tourists, in Benares, Madras and Ceylon. Amongst the standard forms may be mentioned water-vessels for hand use (iota), those with long necks (surahi) used for carrying Ganges water to great distances for ritual purposes, and the vessels for specifically religious use in worship, such as the for consecrated water. Spouted vessels are used for drinking water, not from the spout by direct contact, but as a continuous stream poured directly into the throat (see DRINKING VESSELS). A special form of vessel formerly in use by Brah manical and Buddhist ascetics was the kamandaiu or kwadikd, usually of earthenware, but sometimes of bronze, in which the long neck formed the true spout, and a lateral opening served for filling ; this form passed with Buddhism to China and Korea.
The tire perdue process is also employed in the making of peasant jewellery in base metal. A very interesting variety of this technique is practised at Bundi in Rajputana, where the flexible anklets called stint are cast in one piece. A composition of wax, resin and oil is prepared in a long string, which is then twisted spirally round a stick of the diameter of the intended links; one cut along the stick separates the links, which are then interlaced every one into two others, and joined up by the application of a hot knife. When 6o or 7o rings have been thus united, the ends of the chain are joined, and the whole gently manipulated until it forms a perfectly flexible model of the future anklet. This is then dipped into a paste of clay and cowdung, and finally enclosed in an outer layer of clay. When dry the mould thus prepared is scraped until a small piece of each link is just visible, then a wax leading line is attached all round, and the whole again covered with clay. Two such moulds being placed side by side, the wax leading lines are led into a hollow at the top of the mould; this hollow cup is filled with metal and borax, and covered with clay, leaving only a blow-hole. The whole is then placed in the furnace, the wax melts, and the metal takes its place. When the mould is opened it is only necessary to remove the leading lines and file down irregularities, to have a pair of flexible anklets ready for use. Small images and toys are often made in a similar fashion from prepared wax, all the ornament being applied in the shape of the wax string; brassware similarly decorated always retains an applique effect in the finished product.
Some of the finest examples of Indian metal-work are afforded by temple bells, and standing and hanging lamps and lamp-chains. The bells range from those used in the hand to the great bell cast for King Bawdawpaya in Burma in 1790, the second largest in the world, and weighing over 8o tons. Lamps for burning oil are cupped for one or more wicks; those made in candelabra form, like a tree of which the branches bear innumerable lights, are especially beautiful.
Brass or copper vessels may be decorated by chased or engraved ornament or by inlay or encrustation of some other metal, accord ing to the method already described in the case of iron. Brass inlaid with silver and copper is often very effective. Silver is applied to copper with specially good effect in the case of the huqqa covers of Purnea, Bengal. More rarely a copper vessel may be decorated with inset gems, like goldsmith's work. The char acteristic brass ware of Moradabad, still extensively produced, is engraved with minute designs, relieved against a background of coloured lac, which is filled into the excavated ground by means of heat; in some of the best examples the ground colour is all black, producing a sort of imitation bidri, in others red and green are also used. True enamel seems to be applied only to gold and silver, niello (q.v.) only to silver. The excellent tinned copper ware of Kashmir and the Punjab is mainly used by Mohamme dans for domestic purposes, and is of Persian origin.
Bidri.—One of the most distinctive of Indian metal wares is that known as bidri, from the chief and, perhaps, original place of manufacture at Bidar in Hyderabad. Another main source of production, both in the 18th century and at the present day, is Lucknow. The objects made are for domestic use (Pl. xvii, figs. I, 2, 3). The alloy is composed mainly of tin or zinc, with smaller proportions of lead and copper. After casting, and shaping on the lathe, the surface is then engraved for the application of silver in lay; finally the surface is polished and darkened by the application of a sal ammoniac and saltpetre paste mixed with oil. The designs are geometrical and floral and in the best taste (except in some modern examples, where the effect is too thin), standing out brightly in silver on the dead black ground colour of the alloy. (See also ARMS AND ARMOUR; COPPER; LIRE PERDUE; SILVER SMITH'S AND GOLDSMITH'S WORK; PEWTER; MEDALS AND COINS.) BIBLIOGRAPHY.-B. H. Baden-Powell, Handbook of the Economic Bibliography.-B. H. Baden-Powell, Handbook of the Economic Products, Art and Manufactures of the Punjab (1868-72) ; W. Egerton, Illustrated Handbook of Indian Arms (188o) ; Sir G. Birdwood, Industrial Arts of India (188o) ; C. E. de Ujfalvy, Les Cuivres Anciens au Cachemire et an Petit-Thibet (1883) ; L'Art des Cuivres Anciens dans l'Himalaya occidental (1884) ; T. N. Mukharji, Art Manufactures of India (Calcutta, i888), Brass and Copper Manufactures of Bengal (Calcutta, 1894) ; A. K. Coomaraswamy, Mediaeval Sinhalese Art (19o8), History of Indian and Indonesian Art (Leipzig, 1927) ; S. Hadaway, Illustrations of Metal Work in Brass and Copper, mostly South Indian (Madras, 1913) ; G. Mukhopadhyaya, Surgical Instru ments of the Hindus (Calcutta, 1913, 1914) ; O. C. Gangoly (Ardhendra Kumara Gangopadhyaya), Nepalese Incense Burners (Rupam, 192I). See also articles by E. A. Gait, J. Griffiths, E. B. Havell, J. L. Kipling and Mrs. Rivett-Carnac in Journal of Indian Art, vols. i., iii., vii., ix. (1886, etc.) ; B. H. Baden-Powell, "Indian Arms and Armour," Journ. Ind. Art., vol. v(i. (1896) ; O. C. Gangoly, "South Indian Lamps," Bur lington Magazine (1916) ; G. Groslier, "Objects Cultuels en bronze," Arts et Archeologie Khmers, vol. i. (1923). For the manufacture and history of iron and steel in India see I. E. Lester, Indian Iron (Stour bridge, 1912) ; P. Neogi, Iron in Ancient India (Calcutta, 1914) ; W. Belck, "Die Erfinder der Eisentechnik," Zeitschrift fur Ethnologie (191o) ; trans. in Ann. Rep., Smithsonian Inst. (191I) ; Sir R. Hadfield, "Sinhalese Iron and Steel of Ancient Origin," Journ. Iron and Steel Inst. (1912) , and Proc. Royal Soc., A., vol. lxxxvi. (1912) . (A. K. C.) The art of fabricating metal objects such as spearheads, daggers, urns, vases, statues, bracelets, etc., dates back many thousands of years. There are on exhibit to-day in all of the important museums of the world many precious specimens, often of rare beauty of design, that bear witness to the highly developed skill of the ancient metal workers and artists of Egypt, Greece, Etruria and other countries. These metal objects when unearthed to-day are at times in a very well-preserved condition. This applies, for example, to most of the bronze, silver and other metal articles found in the tomb of Tutankhamen. This tomb may be looked upon as an "ideal location" in view of the fact that corroding influences such as moisture, salts and acids were practically absent throughout the countless days of seclusion. On the other hand, there have been many old graves uncovered in which the metal implements and jewellery were found to be completely disin tegrated, no vestige of the original metal or alloy remaining. The corroding agents, ever present in most soils, had converted the copper, tin, iron and lead back into mineral compounds such as oxides, chlorides and carbonates.
Between these two limits—complete preservation and complete disintegration—there are many intermediate stages. Very often a statue or a coin reveals upon examination a distinct metal core, the outer incrustation being composed of minerals formed from the original metal or metals. The process of restoration, briefly outlined below, and discovered and developed a few years ago (C. G. Fink and C. H. Eldridge, Report, Metropolitan Museum of Art, New York, 1925), applies in particular to art objects of bronze and of other alloys which have suffered partial or complete disintegration while being buried or being otherwise exposed to corroding influences.
The Process of Corr5sion.—It is now conceded by most authorities that the process of corrosion of a metal or an alloy is due to electrochemical action. One or many "cells" are formed between the more noble or "positive" areas or components, and the less noble or "negative" areas of the metal. Very often there is a distinct plating out of pure silver due to disintegration of wsilver-copper alloys. In the case of the bronzes the products of corrosion are the oxides of tin and copper besides the pretty green (malachite) and blue (azurite) basic carbonates of copper. At times chlorides and sulphur compounds of copper are likewise present. Silver, when not alloyed, is found either as metal or as ebony-black sulphide. Gold almost invariably occurs as metal, either pure or alloyed with copper or silver.
Certain salts such as chlorides, sulphates and nitrates present in rain and in moist soil greatly hasten the process of corrosion. There are, however, other active agents, for example, acids and salts of organic origin, notably carbonic acid, which serve as electrolytes and tend to hasten the decomposition of the metal or alloy.
Of prime importance in this process of corrosion is the presence of water or moisture. In completely dry locations corrosion does not occur. 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.
The author's examination of the cross-section of a number of metal objects which were in the last stages of complete mineral ization revealed some interesting facts : The cross-section of a Greek bronze bracelet showed a small residual metallic core, around this a in. thick layer of copper oxide (cuprite) and stannite and outside of this a layer of green basic carbonate of copper (malachite). The cross-section of another bracelet showed a metallic core composed primarily of copper and silver, then but a short space from the surface of this core, a ring of pure silver and then beyond this a layer of malachite.
It might be well to recall at this point that of the various copper minerals occurring in the earth's crust, malachite is one of the most stable, in particular, under those conditions of the atmosphere and the soil of the localities in which the human race has lived and prospered. Malachite is a decomposition product of other less stable copper minerals such as the sulphide, chalcocite, or the chloride, atacamite, or the sulphate, brochantite. During the process of corrosion of a bronze there are, as a rule, undoubtedly unstable compounds of copper first formed which are eventually transformed into the basic carbonate or malachite.
The Older Processes of Restoration.—In the past, metal objects of art had been found in dry locations, in a good state of preservation, and these needed very little preparation or cleaning before being submitted to museums or collections. On the other hand, when the metal article was covered with an earthy crust of appreciable thickness—and usually unsightly in appearance—various "radical" measures were resorted to by dealers and others. Old bronzes showed fresh chisel marks, in dicating plainly that the method of removing the crust or hard outer layers was of a very crude mechanical sort. Other "restored" bronzes showed all the pit marks of strong acid cleaning. Those who have resorted to these radical chisel or acid methods of "restoration" in the past overlooked one very important fact—a fact which was only brought to light a few years ago in the course of our own researches. The fact is this : During the process of corrosion of an art bronze the detail of design is "held" or "pre served" in the crust but not on the surface of the metal core. The core of a badly corroded bronze is almost invariably deeply pitted and ugly in appearance.
The New Process of Restoration.—Conceding that the corro sion of metals and alloys was an electrolytic one, it was evident 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. After a series of experi ments, expectations were fully realized, and this record, with the illustrations herewith, is the restoration process as finally worked out by the writer.
Without any preliminary cleaning, the corroded bronze object to be restored is hung as a cathode in a 2% caustic soda solution (room temperature) and low amperage direct current applied. The object is suspended with soft copper wires and is completely immersed in the solution. When there is danger that the object might not hold together during the process of reduction or restoration, it is 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. Two or more anodes are hung up near the edge of the container. Iron, duriron and platinum anodes have been used with success. A rectangular glass battery jar of one litre capacity serves well as a container for the treatment of small bronzes. For large objects, stoneware tanks may be employed and there is no objection to the use of large sheet-iron tanks welded at the joints. A very low current density is essential. In general, the more completely mineralized the object, the lower should be the current density. From one to five amperes per square foot of exposed cathode crust surface is usually suitable, but even lower current densities are to be resorted to when the gassing at the cathode is too vigorous. It must be understood that violent gassing will cause serious injury to a soft crust, especially during the first stages of reduction. Upon first closing the electrical circuit it often happens that no appreciable current passes through the cell, due to the poor con tact between the copper wire and the crust. However, this is only a temporary phenomenon and after a short time the entire crust becomes conducting. An important requisite for success is the allowance of ample time. The heavier the crust the longer the time. Bronzes with crusts of to 4 in. in thickness usually require three to six months continuous electrolytic treatment. The metal compounds in the crust are slowly reduced back to metal and after further electrolysis the finely divided metal becomes more and more compact. Very often it is not essential to proceed to this very last stage. By careful handling and drying the finely divided metal can be conveniently compacted by means of a good grade of shellac. This latter procedure is particularly useful where detail of design is of first importance and where the object treated will be kept in a glass case. Silver alloy objects which have been badly decomposed while buried in the soil usually contain a layer of pure silver within the crust, as mentioned above. Now it is a very fortunate circumstance that the details of design of the original silver-alloy bracelet or other article are faithfully reproduced in this layer of pure silver, the product of the de cuprification of the alloy. Accordingly, when restoring articles of this alloy by the electrolytic method, we reduce the malachite and other copper compounds in the crust outside of the silver. A layer composed largely of metallic copper is produced on top of the silver. In order to expose the silver underneath the article in question, at the completion of the electrolytic treatment, it is carefully dipped alternately in dilute nitric acid (ro%) and in warm water until all of the copper is removed. For the removal of the last traces of copper deposited on the silver it is safer to use formic acid instead of nitric and thus avoid any possible etching of the silver surface.
Among the Greeks and Romans, and likewise among the ancient Chinese, it was often customary to apply hammered gold leaf to the surface of bronze objects. Take, for example, the author's investigation of a decorated Roman bronze plaque which had been used for architectural purposes. The original plaque was of solid bronze, and to the surface was applied gold-leaf about l in. thick (the human hair is about ,.o o in. in diameter). During the process of corrosion the copper and tin salts passed through the pores of the gold-leaf and formed a heavy green crust. To remove the crust by one of the older methods of restoration, such as the chisel method or the acid method, would have resulted in complete failure, since the bronze proper was entirely mineral ized and the gold-leaf had no mechanical strength, although it was tightly embedded or anchored in the red-green crust. By ap plying the electrolytic method of restoration, a "restored" plaque, composed of an upper and lower layer of metallic copper and in between the layer of gold-leaf was obtained. By very careful manipulation, the upper layer of the copper was dissolved and the gold underneath exposed.
Preserving the Restored Bronzes and Other Alloys.—In the case of the ordinary copper-tin and copper-tin-lead bronzes, the surface obtained by the electrolytic restoration method was coppery in colour, very much like the surface of freshly cast high copper alloys. This appearance of "newness" is usually an objec tion from an artistic point of view. Nearly all ancient and many modern bronzes on exhibit to-day have coats of patina either naturally formed or artificially applied. One of the commonest reagents used for patinating bronzes is salammoniac . However, we can not too strongly warn against the use of this salt as it will often give rise to that treacherous "bronze disease" more fully described below. Examining a large number of natural patinas, in particular those formed in Egypt, Greece and ancient Rome, a number of different shades were found : There is the beautiful red of cuprous oxide, the green of malachite and the blue of azurite. In this investigation it was evident that the closer the approach to the natural conditions under which the patinas are formed, the more beautiful and artistic would be the results. Accordingly, such methods as applying solutions with a brush or cloth were eliminated at the start. The final procedure as now used consists in exposing the bronze to carbon dioxide gas after a preliminary exposure to fumes of ammonia or acetic acid or both. Beautiful shades of blues and greens are produced in irregular patches. In our American and European climates the greens are the most stable, the blue shades turning to green after some months. To produce the red shades of patina is decidedly more difficult. Artistic effects can be produced by submerging the bronze in a suspension of precipitated chalk to which has been added a little iodine, as tincture of iodine. This treatment had best precede the carbonic acid treatment described above. After the bronze object has been patinated, it is then carefully dried in an oven (232° F) and upon partially cooling it is sprayed with a dilute solution of bees-wax in benzol. There is left behind an unbroken film of bees-wax more lasting and protective than most of the lacquer preparations commonly used to-day.
Silver objects of art are usually kept in the bright polished con dition, although a few collectors prefer the black sulphide patina. Accordingly, in the case of the silver articles restored by our process these are, after restoration, carefully dried in an electric oven (gas ovens tarnish the silver) kept at a temperature slightly above the boiling point of water. Thereupon, the bees-wax coat is applied as described for the bronzes.
Gold or gold alloy objects seldom require a protective coating. However, if the gold is present merely as a thin leaf or film, it is better to apply the wax coating as for bronzes, in order to pro tect the base metal underneath the gold-leaf. But even this wax film is not an absolute protection against the destructive action of the contaminated atmosphere of a modern city. All metal objects of art should be kept in glass cases. The atmosphere within the case can be "corrected" by placing within the show case small open containers filled with sticks of caustic soda or caustic potash. As soon as these sticks turn to a thick liquid paste the containers should be cleaned out, dried and filled with fresh caustic. The less frequently a case is opened and the better it is sealed, the longer will the caustic remain effective.
Bronzes infected with the disease should be washed in repeated changes of boiling distilled water. In case the disease is deeply rooted the only safe procedure is to make the bronze object a cathode in a 2% sodium hydrate solution and electrolyze as in the case of the restoration process described above. During electrolysis the chloride, sulphate and other acid radicals pass to the iron anode there reacting to form basic iron salts which precipitate out of solution.