Depletion gilding, workshop practice and microstructure

The depletion gilding process of copper-rich alloys is well documented and various methods of selective removal of copper from the surface of the alloy (and silver from ternary alloys) are recorded (Lechtman 1973, 1984, Bray 1978a, 1993, Scott 1983b). In its simplest form depletion gilding can be achieved by heating the copper-rich objects in an oxidising hearth to allow surface oxidation of the copper in the alloy which is followed by pickling in mildly acidic mixtures to remove the oxide scale. Cementation processes, involving heat and salt mixtures, would be used to remove both silver and copper from the alloys (Lechtman 1973).

Goldsmiths traditionally pickle their handywork to remove the discolouration and oxidation that occurs on the surface during any high temperature heating operation (Untracht 1982, Maryon 1954) and depletion gilding is simply an extension of this routine process. One of the most important features of the pickling process recently seen in a traditional goldsmith's workshop in Lima was that the gold item was heated to red heat immediately before being dropped into the hot pickle solution (quenched). The surface of the item was instantly brightened and gold enriched compared to the original alloy colour - a modern version of depletion gilding. It would seem logical that ancient South American goldsmiths could have used a similar process, and Bray (1993) and Scott (1983b) quote hom a sixteenth century document describing a process which involved polishing, heating and quenching in a solution several times so that the object 'attained the colour and finish it should have' (Tamalameque 1555). The advantage of quenching directly from the oxidising hearth would be to speed up the gilding process which could be repeated until the desired golden surface was obtained. The gilded surface would then be burnished to consolidate the porosity and impart a reflective golden shine.

Although it mar not be fully clear what is causing the binary phase structures in archaeological gold, it is suggested that the non-equilibrium composition and ordering that we find in the Diquis pendant mar be evidence of the workshop practice of depletion gilding by quenching from high temperature into a pickle bath, possibly combined with boiling in the solution which could initiate phase ordering at that temperature (d'Heurle and Gordon 1961), the binary phase structure fully forming over time. This structure does seem to be associated with (at least these) cast, depletion gilt objects, while the structure does not appear in objects of similar composition (so far examined) which were not deliberately depletion gilt and therefore not been through the high temperature oxidation and pickling process.

Figure 5: Anthrapomorphic figure fram Panama, height 57 mm. [photo: A. Milton, British Museum].This figure has recently been generausly donated to the British Museum by Prafessor Warw,ick Bray of the Institute of Archaeology.

Anthropomorphic figure, Panama, no.115

The figure is an unusual piece combing stylistic forms of different regional cultures but is probably in the Veraguas/Chiriquí style, post 700AD (Warwick Bray personal communication, Lothrop 1950, Cooke and Bray 1985).

The figure has a humanoid upper body and the legs turn into animal heads, Fig. 5. The figure has lost part of the headdress and the proper left leg/ animal head is fragmentary due to corrosion of the core metal (Scott 1983a). The broken edges are black, primarily due to oxidation of copper in the alloy. The gilded surface is, by comparison, in good condition and appears to be quite thick and is probably holding the object together. In arder to further examine the metallography of the figure a small sample was taken from the damaged animal' s mouth where the animal' s lip is made from false filigree wire. The sample was mounted and polished for SEM metallography and analysis.

Viewing the unburnished surface of the figure directly in the SEM at high magnification (x3500) shows the thick depletion gilded surface is a very porous 3-dimensional sponge (Fig.6). The porous surface has formed as the result of heating, oxidation and pickling which caused vacancy diffusion and coalescence as copper was lost from the copper-rich solid solution (Reed-Hill1973) and at the same time gold-rich islands formed. The whole structure developed into an interconnecting maze-like structure of islands and channels (Forty 1979, Scott 1995). This surface structure is common to unburnished, depletion gilded copper-rich gold alloys. On more exposed areas the figure has been bumished and the porosity is compacted to a solid and shiny surface. The thick gilding has not been worn through on any part of the figure.

Figure 6: SEM detail of the surface of the anthropomorphic figure showing the porous structure of the unburnished depletion gilded surface. (mag. c x3500). [photo: N.D. Meeks, British Museum]