Figure 3: Gold-copper phase diagram showing the lowest melting point at 80% gold composition and the ordered phases that form in the solid state (a´= Cu Au, a´´ = Cu3Au, and a´´´= CuAu3). (Phase diagram from Metals Handbook 8th edition, Vol 8, Lyman 1973)

The gold/copper phase diagram (Fig.3) indicates that for this alloy containing 26% copper an ordered phase can form (Scott 1983a, 1991b, Reed-Hill 1973, Rapson and Groenewald 1978, Wise 1964) but does not explain the extreme non-equilibrium composition of 97.7% gold in the gold-rich phase that we find in the Diquis sample. This is well outside the normal composition ranges for the ordered phases (a'- Cu3Au up to 62% gold, al"- CuAu up to 85% gold, and a´´´- CuAu3 up to 92% gold). Also, the melting point of the gold-rich phase is about 1020 °C which is well above the melting point of the bulk alloy of 915°C. Clearly a non-equilibrium structure has occurred in this interdendritic region, ie. where the lowest melting phase should form. Thus the gold-rich phase could not solidify out at the time of casting from this bulk composition alloy, but must be a product of later solid state transformation. Although much is written on the theory of ordering in gold-copper alloys (Rapson and Groenewald 1978, Wise 1964, Prince et. al. 1990) there is a paucity of published micrographs on the subject for comparison with our ancient material. It is possible that our macroscopic two-phase microstructures are associated with long-term ordering and thus only archaeological alloys might show this structure.

Figure 4a and 4b : SEM detail of the cross-section through the musicians pendant showing a: the thin porous depletion gilded layer (mag- x1700), and b: the region where the gold-rich ordered phase is corroding, while the copper-rich matrix is not (mag. x200). [photo: N.D. Meeks, British Museum].

The formation of this structure in archaeological gold seems to involve several steps. Rapson and Groenewald (1978) note that fast cooling can push the equilibrium structure towards the gold-rich side of the phase diagram which could explain the non-equilibrium structure that we find. However, fast cooling, such as quenching as part of the depletion gilding process (see below) initially has the opposite effect by suppressing formation of the ordered structures which results in softening the metal. Scott (1991b) describes how indigenous South American Indians quench copper-rich alloys to soften them for further shaping. However, the ordered structure should form by solid state diffusion over time and this ageing process can be accelerated by heating (Rapson and Groenewald 1978), but in this case we mar be seeing the effect of natural ageing and ordering over archaeological time.

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