The evidence of both the analysis and the microstructure eliminates most of the common gilding techniques. It is not depletion gilding as there is not enough gold in the core metal to produce that thickness of gilding and the gilding layer is too sharply distinct from the body metal. It is not foil gilding because it lacks the evidence of working and is very uneven in thickness. Mercury gilding can be discounted (no mercury detected), and electrochemical gilding (too thick and with a micro-structure not seen in electro-deposited metals). The microstructure and the close junction with the irregular surface of the copper indicate that the gilding was applied molten, so it is concluded that this is an example of what has been termed in the literature as fusion gilding or wash gilding (Bergsoe, 1938 i Scott, 1986a and b). By reference to the ternary Au-Ag-Cu diagram it can be calculated that the gilding alloy of this ornament (44%Au, 56%Cu, 4%Ag) would have been molten at about 960°C, while the copper alloy core (estimated from analysis of the corroded metal as approximately 95%Cu, 5%Au) would not begin to melt until the temperature reached over 1060°C. This margin in melting temperatures of 100°C, while not great, is sufficient to have allowed the gilding layer to be molten without damaging the ornament.

Photograph 15: Gilded cooper owl mace head.  Moche (Peru) (height 6.5 cm). BM 1949 Am 22.217 [photo A. Milton, British Museum].

The practical application of fusion gilding is still not fully understood. It was originally identified by Bergsoe (1938) on objects from the La Tolita area on the Pacific coast of Ecuador. More recently, gilded sheet copper fragments from the Department of Nariño, Colombia, have been studied by David Scott (1986, a and b). A cast Moche mace head in the form of an owl's head from the British Museum's collections (Photograph 15) has also been found to have this type of gilding (Bowman et al 1997). The two methods which have been suggested for applying this gilding are dipping the copper object into a bath of molten gold alloy or applying the molten alloy to the heated copper ornament and allowing it to run over the surface. Neither of these methods would fully account for features seen on all of this class of gilded object. There are practical problems with dipping a copper-gold object into a molten gold-copper-silver alloy, not least the difficulty of controlling the temperature so that the item to be gilded does not melt in the crucible of molten gold. There is also the problem of holding it securely while coating the whole surface evenly. The Moche owl has strong relief decoration, which would have been flooded by either dipping in molten metal or running it over the surface, but the gilding on this piece is fairly evenly distributed over the flat areas and in the channels. How this was done is not known but perhaps fine grains of gold alloy were applied evenly over the surface in a paste with a flux to prevent oxidation and heated until the grains melted.

It is quite possible that different methods were used for different types of object. The fusion gilding of sheet metal, which requires further forming to make a finished object might well be applied by a different method from the fusion gilding on an object cast to its final shape. Scott (1986a: 322) has reported sheet copper which has been gilded on only one side, so could not possibly have been dipped in molten metal, but on other objects he found evidence of dipping, particularly a silvered copper nose-ring. Whatever the method used to apply the plating, it is apparently not a common technique. It seems to be largely confined to the coastal areas of Ecuador, reaching at its northern extent up into the Nariño area of Colombia and clown as far as the Moche and Vicus regions of northern Peru. No fusion gilding has been identified anywhere else in the world though molten silver-copper alloy was a method used by the Celts and the Romans for plating copper (La Niece 1993:206). How far this reflects the real limits of the use of fusion plating, rather than the difficulty in identifying it, remains to be seen.

The results presented here form a part of a scientific study of the pre-Hispanic gold objects in the British Museum, which like most such collections lack any real archaeological context. The work is still in progress and will be published in the catalogue of the collection. The aim is to make the technological characteristics of the pieces available for comparison with stylistically similar pieces elsewhere. A broad database of analyses and manufacturing techniques will assist in the understanding of where these objects belong and will do something to ameliorate the loss of archaeological information.