The Deterioration of Gold Alloys and Some Aspects of Their Conservation Author(s): David A. Scott Source: Studies in Conservation , Nov., 1983, Vol. 28, No. 4 (Nov., 1983), pp. 194-203 Published by: Taylor & Francis, Ltd. on behalf of the International Institute for Conservation of Historic and Artistic Works Stable URL: http://www.jstor.com/stable/1505967 REFERENCES Linked references are available on JSTOR for this article: http://www.jstor.com/stable/1505967?seq=1&cid=pdf- reference#references_tab_contents You may need to log in to JSTOR to access the linked references. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms Taylor & Francis, Ltd. and International Institute for Conservation of Historic and Artistic Works are collaborating with JSTOR to digitize, preserve and extend access to Studies in Conservation This content downloaded from 128.194.18.45 on Thu, 18 Jun 2020 14:47:09 UTC All use subject to https://about.jstor.org/terms THE DETERIORATION OF GOLD ALLOYS AND SOME ASPECTS OF THEIR CONSERVATION David A. Scott Abstract-The different ways in which gold alloys wide can occurrence, both for the production of coinage deteriorate are described. The principal alloys discussed and for a variety of decorative metalwork [7]. The are gold-copper, gold-silver and gold-copper-silver. South American gold-copper or gold-copper-silver Examples of South American gold alloys from Colombia alloys were frequently finished by a depletion gilding and Panama are used to illustrate the different types of process [6,8] which removed the alloying con- corrosive attack. Some problems related to the conserva- stituents from the outer surfaces leaving a thin, por- tion of gold alloy antiquities are mentioned, together with the difficulty of the preservation of technological evidence. ous layer of gold which could then be polished to create a gilded surface. The presence of these deple- 1 Introduction tion gilded layers over debased gold alloys often creates severe corrosion which will be discussed in Gold in the native state, whether obtained from lode detail below. or placer deposits, usually contains appreciable amounts of silver (typically from 5% to 45% by 2 Types of deterioration weight), and often a little copper (about 0-1%-5%), as the major accompanying impurities. High percen- It is often assumed that pure gold will not undergo tages of silver, from about 40% upwards, constitute any corrosion during burial, but this may not be so; natural electrums which can be pale green or silvery it is known, for example, that biological processes in colour. This alloy could also be fabricated, mayof result in small concentrations of gold being course, by the addition of extracted silver to golds accumulated by certain plants, whilst water-borne containing lower impurities of silver. Copper can decomposing vegetable matter has been shown to also be added to make a range of alloys varying from deposit gold particles from solution if flowing over moderately debased gold, containing perhaps 5%- auriferous rocks [9]. The gold, therefore, must be 15% of copper, to copper containing small additions taken into solution, and it is thought that both cyanogenic glucosides in plants and a variety of of gold (about 0-5%-5%). Small additions of gold are thought to have been included in some copper amino acids in micro-organisms are responsible for alloys to influence the surface colouring characteris- the dissolution of gold. A saturated solution of tics of the alloy after various forms of surface treat- sodium chloride can also attack gold, as can bromides which may be present near marine envi- ment [1-3]. Within this broad spectrum of gold alloys lie anronments. One would not expect serious decay of important range of compositions corresponding togold antiquities necessarily to arise from exposure to the gold-copper and gold-silver-copper alloys used these types of corrosive agents, although some superficial attack of the immediate surface is cer- especially in South America. The gold-copper alloys are often referred to as tumbaga alloys, the composi- tainly possible. Returning to the consideration of tional data for which varies from copper contents goldof alloys, the different forms of possible deteriora- tion can be classified as follows: about 90% down to about 10%. European Bronze Age and Egyptian gold artifacts are also occasion- i) Tarnishing. ally found to be made from fabricated gold-copper ii) Dissolution of anodic constituents. alloys [4,5], although the Old World utilization is oniii) Stress corrosion cracking and embrittlement. a limited scale compared with the many thousands iv) Changes due to order-disorder transformations. of tumbaga alloy objects which are known from South America. Silver was more plentiful in the 2.1 Tarnishing Peruvian area, and many gold alloys had deliberate Alloys of gold with copper or silver, or with both additions of both silver and copper [6]. Since the metals, are liable to exhibit thin films of surface cor- rosion which can either be derived from the deposi- gold used to make the tumbaga alloys usually con- tained some silver as an accompanying impurity, tion of metallic ions from solution (such as iron these alloys are also in fact ternary gold-copper- staining) or from reactions with sulphur-containing contaminants, particularly hydrogen sulphide, H2S. silver alloys. Alloys of gold and silver are of very It is well known that unprotected silver objects are Received 17 December 1982 liable to react with gaseous pollutants with the for- 194 Studies in Conservation 28 (1983) 194-203 This content downloaded from 128.194.18.45 on Thu, 18 Jun 2020 14:47:09 UTC All use subject to https://about.jstor.org/terms The deterioration of gold alloys and some aspects of their conservation mation of grey-brown films of silver sulphide, Ag2S. observed by the author during the exposure of Vernon [10] found that H2S can produce discolora- polished sections of some gold-copper-silver alloys tion even after careful drying of the air. Very low in a high concentration of hydrogen sulphide (2% relative humidities in themselves are therefore not v/v) at 80% relative humidity for 70 hours. Some of enough protection if the atmosphere is contami- the sections were etched by the exposure to reveal nated. Higher relative humidities promote more grain boundaries and casting segregation as well as severe tarnishing, although the presence of con- two-phase areas of precipitation in those composi- densed water on the surface may retard the reaction. tions lying in the two-phase field of the phase dia- The same reactions which lead to Ag2S formation gram on (see McDonald and Sistare [17], for further pure silver occur with copper-silver and gold-silver discussion of the ternary equilibrium diagram). The alloys [11]. A case of whisker-like crystal growth results on of this test are briefly listed in Table 1, in gold jewellery in the Waiters Art Gallery, Balti- which it can be seen that only an alloy with 80% more, has been mentioned by Weisser [12]. goldThe content was unaffected. Figure 1 shows an crystals were of silver sulphide, illustrating the reac- example of an ancient Colombian tumbaga alloy tivity of silver within the gold-silver system under with a heavily tarnished surface. Although the alloy poor storage conditions, although the composition contains about 20% gold, 70% copper and 3% of the gold objects attacked was not reported. Thesilver, it has been finished by depletion gilding and rate at which tarnishing occurs in gold-silver alloys has surfaces on both sides enriched in gold. decreases as the gold content increases. Some experimental work by Tamman [13] was carried out using solutions of ammonium sulphide and sodium sulphide to determine the reaction limits in solution, that is, the limiting percentage of gold below which corrosion can be expected. For (NH )2S2 and gold- copper alloys a value of 51.5wt% gold was obtained, whilst for gold-silver alloys the value was 46.5%. Sodium sulphide solutions gave similar results. German [14] examined the sulphide tarnishing sus- ceptibility of a number of low carat gold-silver- copper alloys in the cast condition. Evidence of tarn- ishing with the formation of Ag2S in a dendritic pat- tern was found over a wide range of gold contents, from 41% to 81%. Indeed, Wise [15] notes that a gold content as high as 95% may be necessary Figure to 1 Fragment of a tumbaga alloy from the Naritio prevent tarnishing problems occurring in contami- area, Colombia, from c. J0th-12th century AD, showing nated environments. This was confirmed in dental sulphide tarnishing. studies by Tuccillo and Nielsen [16] who found that 92% gold content was not enough to prevent tarn-2.2 Dissolution of anodic constituents ishing. Electron microprobe examination showed Gold-copper, gold-silver and gold-copper-silver either silver sulphide or copper sulphide to be pres-alloys as well as gilded surfaces are all liable to cor- ent in ternary gold-silver-copper alloys; in many rosion during burial. The process is usually aggra- cases the onset of tarnishing was associated with vated by the presence of depletion gilded layers specific microstructural features. This was which also are gold-rich compared with the underlying Table 1 Results of exposure of some gold alloys to H S at 80% RH for 70 hours.