•WOR/6615 Grimwade Tech web 18/8/99 3:57 pm Page 16 The Surface Enrichment of Carat Gold Alloys – Depletion Gilding* MARK GRIMWADE Consultant, UK Introduction because the treatments described recipes for refining, testing, alloying The techniques of surface enrichment have a relevance to some of the more and soldering of gold, for gilding of jewellery and other artefacts made modern processes such as wet and silver and other metals, and for the from alloys containing gold have dry colouring. preparation of base metal alloys. An been practised for well over 2000 The fact that base metals and silver English translation of the text was years. The techniques are known as can be removed by chemical means published in 1926 (4) and discussed Depletion Gilding or Mise en Couleur from gold alloys is well known. For more recently by Hunt (5). Of and, in essence, the process involves example, when gold is assayed by particular interest in the context of the removal of base metals and silver cupellation (fire assay), “parting “ with depletion gilding is Recipe No. 25: from the surface layers to leave them nitric acid is used to remove silver “For treating gold or for thoroughly considerably enriched in gold. This is leaving behind a residue of fine gold. purifying it and rendering it obviously radically different from Similarly, refining by “inquartation” is brilliant: misy 4 parts; alum 4 parts; other gilding processes such as done by alloying gold scrap with salt 4 parts. Pulverize with water electroplating, electroless plating and copper such that the gold content is (to make a paste) and having the older techniques of water gilding not greater than about 25%. It can then coated the gold with it place in an and fire gilding where gold is applied be treated with nitric acid to remove earthenware vessel put in a from an external source onto a silver and the base metals to leave furnace and luted with clay (i.e. substrate, usually of base metal or silver. gold. Many early cultures in different sealed with clay) and heat until Similar changes in surface parts of the world used chemical above named substances have composition to enhance the colour and means for the surface enrichment of become molten; then withdraw it finish of carat gold jewellery are gold-containing alloys and even for (the gold) and scour carefully.” achieved nowadays using processes gold refining before nitric acid was According to Pliny, misy is either such as Wet and Dry Colouring, bright available. In Europe, for example, iron or copper pyrites or their acid dipping and pickling and chemical nitric acid has only been in use since oxidation products, i.e. the sulphates. ‘bombing’ with cyanide/peroxide the twelfth century mixtures although these do not Pre-Columbian Goldsmiths necessarily all involve surface Mediterranean Cultures The pre-Columbian cultures of Central enrichment of gold. The last named Analysis of fifth and fourth century BC and South America were highly skilled process is particularly hazardous Greek “white gold” and electrum coins at metal working, and archaeologists because of dangers of spillage and has shown the existence of a gold- have drawn attention to the methods toxicity and there are other enriched surface layer with that were used to produce gold- disadvantages as will be seen later. concentrations of over 70% gold being enriched surfaces on items made from There appears to be very little found in places, as compared to about alloys containing a low proportion of recently published or readily 35% for the bulk of the metal (2,3). A gold. Gonzalo Fernandez de Oviedo available practical information for cementation process was described by (1535-1548) wrote (6): jewellery manufacturers on these Pliny in the first century AD for “... they know very well how to processes although Rapson has refining gold. In this process, impure gild the objects and items they reviewed some of the scientific gold is placed in a crucible in intimate make from copper and low grade reports on selective dissolution and contact with a mixture of salt and brick gold. And they have such ability corrosion of gold alloys in acid media dust and then heated. Base metals and and excellence in this, and give (1). This paper is an attempt to silver are gradually removed from the such a high lustre to what they disseminate some of the information gold and absorbed into the brick dust gild, that it looks like good gold of so that manufacturers are encouraged leaving behind a residue of relatively 23 carats or more... They do this to investigate the use of alternative pure gold which can be separated with a certain herb, and it is such a processes to chemical bombing. from the brick dust by washing. secret that any goldsmith in Another source of information on Europe or in any other part of Historical background ancient techniques is the Leyden It is pertinent to briefly review the Papyrus X, a metallurgical text *This is based on a presentation given at the 13th Santa Fe Symposium on jewelry manufacturing historical and archaeological written in the fourth century AD by a technology, held in Albuquerque, N.M., USA in May evidence for depletion gilding goldsmith, and which contains 1999 (see report, this issue). 16 •WOR/6615 Grimwade Tech web 18/8/99 3:57 pm Page 17 Christendom, would soon become chlorides were then absorbed by the a rich man from this manner of brick dust. Additional ingredients gilding.” may have been ammonium chloride, potassium nitrate, copper and iron The depletion gilding techniques sulphates, vinegar and urine. After that were most likely to have been cooling, and subsequent washing to used by the pre-Columbian Indians remove residues, the brilliance of the have been investigated by Lechtman gold-enriched surface was improved (7,8). The alloys used were generally by burnishing. of two types. One type was known as An alternative method was to tumbaga, reddish, bronze-coloured immerse an object to be gilded in an Figure 2 - Cross-section of Vicus disc copper-gold alloys produced with aqueous paste or solution of alum, showing depletion gilded layer. x200 differing gold contents, and probably iron sulphate and salt at room containing silver as an impurity. The temperature. After about ten days, it other type were pale greenish-white was taken out and washed in a strong ternary silver-gold-copper alloys salt solution to remove surface scale. containing a high proportion of The object was then heated to silver. These were not dissimilar to convert the spongy, gold-enriched the electrum of the Mediterranean surface to a smooth compact richly world and appear to have been coloured surface. Both the widely used in Peru. cementation and the aqueous paste The methods used by the pre- methods work equally well on alloys Columbian goldsmiths also fall into with a high silver content and with two groups. The first is Mise en tumbaga. Figure 3 - Vicus disc showing gold flecks Couleur, a name given in Figure 1 shows discs which were in corrosion products. x200 archaeological circles to processes made in Peru probably during the for depletion gilding objects made early Vicus period. The classical Vicus from tumbaga, in which the alloy was period was between 400 BC and 100 The Vicus culture was followed by rubbed with the juice of a plant and AD, although there was a later period the Mochicas who excelled in a then heated so that it assumed a gold which extended to about 700 AD. It is variety of metalworking techniques, coloration due to the removal of thought that these particular discs are such as raising, forging, chasing and copper from the surface layer. This from the earlier period because of the lost wax casting using mainly the process could be repeated many lack of surface decoration. The discs high silver-containing alloys, similar times to increase the thickness of the had lain buried for many centuries to electrum. They in turn were gold-enriched layer and improve its and traces of black corrosion followed by the Chimu (1000-1470 colour. It is believed that the plant products may be seen in the AD) who were highly skilled in was a species of oxalis and that the micrographs of cross sections of the depletion gilding objects made from juice contained oxalic acid. It is discs, Figure 2. A thin, gold-rich layer tumbaga and electrum type sheet. possible that ammonium carbonate is clearly visible, which is surrounded Jacobson and McKenzie (9) and soaked in urine was also used. by corrosion products in places. Tiny Scott (10) put forward the argument Lechtman has pointed out that flecks of gold can be seen within that low gold alloys containing although depletion gilding of corrosion products, Figure 3. These copper were first heated in air to tumbaga works well with these particular discs were made from produce a black copper oxide scale processes, they do not remove any tumbaga having high copper and low and then treated to selectively silver that might be present in the gold contents. remove the copper and silver. alloy. Gold alloys of the electrum type containing high silver were probably gilded using a cementation process or by using special aqueous pastes. In the cementation process, the object to be gilded was placed in a crucible and surrounded with a powdered mixture which invariably contained alum (potassium aluminium sulphate), common salt (sodium chloride) and brick dust. The crucible and its contents were heated and the mixture reacted with the surface of the alloy to form chlorides of silver, copper and other Figure 1 - Depletion gilded tumbaga impurity metals.