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THE CORROSION PRODUCTS OF A N ANCIENT CHINESE BRONZE RUTHERFORD J. GETTENS Fogg Museum of Art, Haward University, Cambridge, Massachusetts
THE
corrosion of metals is not only a serious industrial problem and a household nuisance but it is also an interesting chemical phenomenon. It is often a complicated process which produces several distinct products in intimate juxtaposition. Some of the best examples of the complexity of this process are seen occasionally in the patina of bronze vessels and utensils excavated from the early graves of China. The diversity of copper and tin compounds disposed in the layered structures on their surfaces testify to the numerous factors that must enter into the formation of the mineralized envelope. To the collector these structures and products are of interest because their color and texture frequently enhance the value and reflect the age and authenticity of a piece; chemists and metallurgists might well study old bronze objects because the slowness of formation of the corrosion layers and the segregated distribution of the mineral, products undoubtedly could throw more light on the corrosion mechanism. Most of the observations reported here are not new. Beginning with Berthelot (2) several investigators, including Rosenberg, (129, Fink and Polushkin (5), Collins (4, Caley (5),Plenderleith ( I I), and the author (7,s)have described the layered structure on the surface of ancient corroded bronzes and have suggested chemical and electrochemical reactions and mechanisms to explain the formation of the several mineral products contained in them. Only a few Western investigators, however, have had opportunity to study the corroded high-tin cast bronzes from the Far East, principally, I think, because there has been so little study material available. Collectors have brought from China mainly whole and intact pieces of museum quality and have left the scraps and broken fragments behind. It was our good fortune sometime ago to be given a portion of an ancient bronze ceremonial vessel of the Chou period (112-255 B.c.) with freedom to do as we pleased with it. This vessel, with wide oval mouth and bulbous body, called a hu was purchased as a whole object in China during the war. Nothing was known of the exact origin of the bronze or the circumstances of its burial. It was shipped to this country but on arrival was found broken in several pieces (Figure 1). Examination showed it was, in part, a forgery. An incomplete lot of broken fragments of a bronze hu was found, probably in a burial; to complete the object and to make it saleable some Chinese artisan cut out and shaped pieces of heavy plate copper to fill in the voids. The old fragments and new replacements were joined with soft
Figure 1
be intact:
.
..
solder to make a complete shape, and finally the new metal surfaces and joins were cleverly concealed with an artificial patina made from green-colored stucco. The solder had adhered poorly to the old corroded metal, and the object fell apart in transit. In its broken condition there was no difficulty in telling the old from the new parts. While the trumped-up parts were amusing, the original earth-corroded pieces were of chief interest because of their value for analytical purposes. The bronze was cast, probably by the well-known cire perdu method. The surface was modeled in low relief with conventionalized animal forms set against delicate scroll and maze patterns so commonly used in that period. The surface in places is smooth graygreen in color but interrupted with irregular patches of red and bright and dull green. The patina is hard and adherent. A few small fragments were mounted in bakelite and polished for metallographic study. Microscopic examination revealed that there are three principal zones: (1) in places, in the center, small irregular areas of uncorroded metal, (2) an intermediate partially mineralized zone, and (3) an outer completely mineralized zone. The outer and intermediate zones have a layered structure within themselves (Figure 2).
JOURNAL OF CHEMICAL EDUCATION
68
zone of interphase penetration is generally 1 t o 3 mm. thick, but in places it goes entirely through the metal. Because it is so intimately mixed with uncorroded alpha it was quite impossible t o isolate the black material for microchemical tests. I t was possible, however, from a polished section, to isolate chips free from outer corrosion product and to subject them to X-ray diffraction analysis. The powder pattern showed mainly an expanded copper lattice, but in addition it showed faint lines characteristic of cassiterite and also one unidentified line. No lines of the copper chlorides were observed. Tin oxide could be expected, and, indeed, white tin oxide is plentifully seen in the outer corrosion layers. The faintness of the cassiterite lines indicate that the tin oxide is in a cryptocrystalline state. Why does it seem to be black here (Figure 4)? It is possible that it is stained black with cupric oxide, and it is well known that some forms of cupric oxide (like melachonite) are so nearly amorphous that they give indistinct diffraction patterns or none at all. If the black is cupric oxide it is doubtful if it is formed by direct oxidation of copper especially under the reducing conditions that seem t o prevail in the interior after corrosion attack is well along. There is a remote possibility that the black is formed from cupric hydroxide precipitated at cathodic area.s of local cells set up by electrolytic action in the salt environment. J. W. Mellor (10) cites (Vol. This unetohed dished oroas scotion of the old metal ahows the three princi~alzones: unrorroded methi core, (21 intermediate oartially minerrrliaed rone nod (3) outer completely, mineralized rone. The dark eras band ,"at ouiside the zone of dendritas 1s ooproas chloride. The bright reAeoting porous layer heyond that is red cuprite. . (Bright field, 5 0 X )
81
It was necessary to know the composition of the original metal. Although corroded completely through in places there are parts where the unchanged metal is still 1 to 2 mm. thick. Samples from these areas were cut out, carefully cleaned to remove oxidation products, and analyzed by standard methods. The results seen in the table show that it is a high-tin (above 20 per cent) bell metal, and it is just short, in respect to tin content, of being speculum metal, the alloy that was so extensively used in China at a little later period for casting mirrors. Occurrence of lead in this type of bronze is not unusual and this amount of 4 to 5 per cent is somewhat more lead than such a copper-tin alloy can take into solid solut.ion. Some unalloyed lead is dispersed in tiny globules throughout the alloy matrix. Spectrographic analysis showed the presence of several trace elements but among them arsenic was the only one reported in the order 0.1 t o 1.0 per cent. The corrosion zones and the several mineral layers were of chief interest. Most striking is the intermediate penetration zone where the eutectic (tin-rich phase) has been converted to a black product leaving dendrites of alpha bronze (copper-rich phase) nnattacked. It is a natural etch. 1nthe casting this bronze had appaxently cooled slowly, allowing alpha dendrites to form alld nearly perfectly (Figure 3), This
1 ,
rig"r.3 The original duplex stmcturr of the east copper-tin is revealed by the corrosion and dsrkening of the high-tin ~ h a s ewhich aurraunds the cryatals of copper-rich a b h s bronze. (Bright field, 100x1
FEBRUARY, 1951
69
111,p. 144) evidence that cupric hydroxide hydrogel in contact with water or salt solution can eventually dehydrate and turn black even below room temperatures although several months may be required to bring ahout the change. Bengough and May ( 1 ) state (p. 103) that in copper corrosion dark brown or black hydrated cupric oxides are important constituents of scales that form on the copper surface. This explanation of the tion bearing on it. On the outer edge of the intermediate penetration zone the alpha dendrites are attacked, and only disconnected remnants or islands of metal are left. From here outward only mineral corrosion products are found. The first of these is a grayish soft waxy layer (Figure 2) which was recognized as cuprous chloride (nantokite). This is the key mineral product and indicates that salt in soil water was perhaps the main corroding factor. It is not a t all unusual to h d cuprous chloride in ancient corroded bronzes from desert areas or other places where salt is present. I t is the material on old bronzes which gives rise in museums to the phenomenon called "bronze disease" which is nothing more than the oxidation and hydrolysis in humid weather of cuprous chloride to basic cupric chloride. When a polished cross section of the metal was exposed for a few hours in a humid Figure 5
Thisja
freshly polished crass section of the corroded metal efter it was laced m a humid chamber for some hours. On either side of the metal aors bright green blisters and eruptions of basic cuprio ahloride ~auaedby oxidation and hydratmn of the caprous chloride inner layer hare forrned. (Dsrk Add, 9x1
--
rigwe4 The inner advance front of corrosion as it penetrates among ol.mtsla of alpha bronse ia here alesrly seen. The initial corrosion product i. dark in aolor. (Bright field. 425x1
h
chamber a narrow line of bright green blistery eruptions formed just along the zone of gray cuprous chloride (Figure 5). When a sample of the pure translucent waxy material was dug out and treated with a drop of dilute hydrochloric acid and rapidly dried it was observed that the residue was a mixture of pale green crystals of cupric chloride and colorless t~trahedral crystals of cuprous chloride. Going outward, next comes a narrow zone of redeposited metallic copper. This is compact but crystalline as in an electroplate (Figure 6). Strangely it is not continuous but intermittent. Redeposited copper occurs also in tiny pockets throughout the initial p e n e tration zone among dendrites of residual alpha. Cathodically-formed copper is often observed also in corroded modem brass pipes and fittings. When formed, either in the dezincification of brass or in the destannification of bronze, redeposited copper indicates the electrochemical nature of this kind of corrosion. Next comes a thick band of cuprous oxide (cuprite) which is bright red in dark field illumination and bluegray in bright field. In places where there are small voids in the cuprite crust the comers of nearly perfect cubes of that mineral jut into the holloms (Figure 7). It is well known that in presence of water and air cuprous chloride is easily converted to cnprous oxide as
JOURNAL OF CHEMICAL EDUCATION
well as to basic cupric salt. The density and compactness of the cuprite layer is probably an important factor in preventing the complete oxidation of cuprous chloride. I t also explains the difficulty often encountered in attempts to rid the bronze of unstable cuprous chloride by electrolytic reduction or by other means. Tin oxide seems t o form no separate distinct interior layer. At high magnifications, however, white tin oxide is seen deposited mainly in little disconnected seams throughout the cuprite, like streaks of fat in a beefsteak. Ghosts of the original dendritic structure in places still persist in the white tin oxide. The tin corrosion product, unlike the copper, does not migrate because after formation it immediately becomes so insoluble. The initial products of copper corrosion, which are slightly soluble in the corrosion liquid, either deposit cuprite around the tin oxide or they are leached out leaving anearly solid tin oxide patina, as is often seen on ancient Chinese bronze mirrors (9). In certain areas on the surface copper is nearly all dissolved out leaving a smooth solid tin oxide. I t would seem that the copper in a bronze could be dissolved out completely
Figure 6
T h e narrow light band above and thc broad one below are of redeposited copper. One small pocket of redeposited oopper can also be seen among the residual dendrites in the partially mineralized zone. (Bright field, 5 0 X )
leaving a fragile skeleton of tin oxide as the only indication of the original shape. The vredominent exterior corrosion nroduct in other areas is green crystalline basic cupric carbonate (mal-
achite) mixed with some basic cupric chloride. This basic chloride, which an X-ray powder pattern showed to be paratacamite, occurs mainly in little pockets and patches of bright green powdery material. Frondel (6)has recently showed that paratacamite, dimorphous with atacamite, is found among the corrosion products of ancient copper and copper alloys. The considerable amount of malachite present supports the observation Analpi. of a Chinese Bronze Ceremonial Vessel Sample from rim (awerage of du licate ana~tsesl)
Sample from base (averageof duplicale
of Fink and Polushkin that the ultimate product of copper corrosion is the basic carbonate, stable in the presence of carbon dioxide. No corrosion products of lead were isolated or identified. Lead, however, may play a more important role in the corrosion process than we realize especially if it is dispersed as a separate metal phase in the duplex alloy structure. As these observations bear out, bronze corrosion is a complex process, and there is still much to be learned about it. I doubt if the corrosion process can be expressed in a neat series of chemical reactions, one proceeding from the other. I think Fink was correct when he suggested: "The fundamental corrosion reactions, while important, are probably only a small part of the story. Physical retention of semi-plastic corrosion products in an envelope about the metallic core may play a greater part in layer formation than the original corrosion reactions." The solid metal crystal stmcture appears to have been invaded by anion-bearingsolution, but how the ions migrated within this solid structure is hard to explain. I picture, however, the existence, even in the interior, of microscopic colloidal pipes like those seen during the precipitation of copper ferrocyanide and such as exists in a colloidal garden. Juxtsi posed anodic and cathodic areas and tiny concentration cells no doubt are involved. Blisters and locally corroded areas on the surface are evidence of this. The products we now see are probably, in part, solidified colloidal gels formed after the bronze was taken from the wet earth and dried. Existence of dried colloidal gels may be the explanation for the diffuse X-ray diffraction lines produced by the inner corrosion products. Although I may have seemed to disparage the employment of chemical step reactions to explain such a complex process, yet the layered structure does indicate they are to an extent valid and there is independent experimental evidence t o explain the mode of fn-mation of some of the end products. Space does not permit a
FEBRUARY, 1951
complete discussion here, but Caley's writings may be referred to for more extensive treatment of the theoretical considerations. To explain the observations made on this particular object, however, the following chemical reactions drawn from many sources might be used: In the inner penetration zone:
-
At anodic areas:
+
Cu++ Cu-Sn (bronze) 2NaCI HzO [0]
+
+
+ Sntt
-
+
CuClz 2NaOH (anodic) (cathodic) CuCI* 2NsOH CU(OH)~ 2NaCI (blue ~elatinous) . CU(OH)~ CuO H2O (black) Simultaneously: 2Sn 0~ 2H.O 2Sn++ +OHSn++ 4NrtCI 2H20 2[0] SnCI* 4NaOH 4NaOH Sn(OH), 4NaCI SnCI. (white gelatmnous) Cu
- + - + - + + - +
+
+
+ + + +
+
In the outer mineralized zone:
-
Copper from unattacked alpha, phase reduces CuCL Cn CuC12 2CuCI (nantokite) In water and air: 4CuCI [Ol Cu*O 2CuC12 (cuprite) In excess NaCI: 6CuCI 3 [ 0 ] 4HxO CuC12~3Cu(OH),.H,0 2CuCb (atscsmite) The red cuprite zone is banded %"d fissured. The ripht-anpled corners of nearly erfeot cubes of cuprite project into the w d e fissure seen near the In presence of carbon dioxide: top of t i e phatomiorograph. (Bright Rdd, 10OX) CO. C U C O ~ . ~ C ~ ( O H ) ~ . ~ H ~LITERATURE O CuCL. 3Cu(OH).. H.0 CITED (malachlte) 2HC1 (1) B E N G O ~ GG. H ,D., AND R. MAY, "Scventh Report to the Simultaneously: Corrosion Research Committee," J. lnst. Metals, 32, SnO? 2H,O Sn(OH), 81-142 (1924). (cassiterite) M., "Sur l'dteration lente dcq ohjets de ouivre (2) BERTHELOT, Cupric ions in contact with Cu-Sn eutectic au sein de la, terre et dans les mus6es," Comples Rendus, Cu++ Sn Sn++ +,Cu 118,768(1894). (redepos~tedcopper) (3) CALEY.E. R.. "The corroded bronze of Corinth." Proc. A r n . ~ h i lsic., . 84,689-761 (1941). ACKNOWLEDGMENTS (4) COLLINS, W. F., "The corrosion of early Chinese bronzes," J . Inst. Metals, 45,23-55 (1931). (5) FINK,C. G., AND E. P. POLUSHKIN, "Mi~ros~opic study of The fragments of the old bronze on which this work ancient bronze and copper," Trans. Am. Inst. Mining was done were loaned by Mr. Charles Fabens Kelly, Met. Engrs., 12490 (1936). Curator of Oriental Art of the Chicago Art Institute. (6) FRONDEL, CLIFFORD, "On psrstacamite and related copper The author is greatly indebted to Mr. Daniel Cushing, chlorides," Minerabg. Mag., 29, 34 (March, 1950). consulting metallurgist of Boston for the metallographs (7) GETPENS,R. J., "La corrosion recidivante des objets anciens en bronze et en cuivre," A4ouseion, 3 5 ~ 6 , 1 1 9(1937). used to illustrate the paper; Mr. Cushing has also taken (8) GETPENS,R. J., "Mineralieittion, electrolytic treatment, some very fine color transparencies of these subjects and radiographiphio exitmination of copper and bronze which unfortunately cannot be reproduced here. objects from Nuzi," Tech. Sludies Field Fine Arts, 1, Special acknowledgment is also made to Professor 118-142 (1933). (9) GETPENS,R. J., "Tin-oxide patina. of ancient high-tin Clifford Frondel and to Miss Mary Mrose of the Dana Bulletin of Lhe Fogg Museum of Art, 11, 16 (Jan., bronze," Laboratory, Department of Mineralogy, Harvard for ~. 1949). the X-ray diffractions analyses and to Professor H. C. (10) MELLOR,J. W., "A Comprehensive Treatise of Inorganic Harrison of Rhode Island State College for the spectroand Theoretical Chemistry," London, 1923. graphic analysis. Mr. Charles Fletcher of the Depart- (11) PLENDERLEITH,H. J., "Teohnical notes on Chinese bronzes with special reference to patina and incrustament of Geology mounted and polished the specimens tions," Tmns. Oriental Ceramic Society, 16, 33 (1938for metallographic study. Mr. Earl Robertson, now of 39). Eastman Kodak Laboratories, made the chemical (12) ROSENBERG, G. A,, "Antiquities en Fer et en Bronze," analyses. Glydenelske Boghandels Sortiment, Copenhagen, 1917.
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