David A. Scott
Abstract
Before the Spanish Conquest, the many different cultural periods and regions of the Continent gave rise to unique developments in terms of precious metal metallurgy and gilding techniques. Mercury amalgam gilding was never used in the New World: consequently other approaches to the covering of materials with gold were developed. The most obvious technique, used extensively in both the Old and New Worlds was to make a thin gold foil or leaf to cover the substrate, which could be of stone, wood or other metal. The application of this gold foil could be achieved by mechanical of metallurgical bonding to the substrate, and both techniques were used. One of the major difficulties in tracing the technological evolution or cultural choice underlying gilding methods is the paucity of datable material which is available for study. As a consequence of this problem there is still much work to be done to understand the use of even such a simple technique as leaf or foil gilding. The Conquistadors noted in some of their accounts of the Conquest of Peru, that entire walls of buildings were covered in gold, besides the gold pectorals, diadems, bracelets and headdresses of important chiefs. The majority of metal usage in ancient South American cultures was for decorative and religious purposes rather than utilitarian, and the metals which predominate in their manufacture are copper, silver and gold. The earliest evidence for goldworking occurs at around 3,000 BC in Peru, but we do not know when the use of gold foil or leaf gilding really began to be used. Examples of this technique will be discussed in the paper drawing on the study of an Ecuadorian nose-ornament from La Compania, Provincia de Los Rios, nose-rings from Colombia and an elaborate nose-piece for the Frias cultural area, Peru.
By the time of the Moche domination of the North Coast of Peru, during the first few centuries AD, a variety of gold alloys were in use as well as different forms of gilding. Examples will be discussed relating to some of these developments. The techniques which appear to be known at this time include: fusion gilding, depletion gilding and electrochemical replacement gilding. In the fusion gilding process, a shaped and finished object is covered with a gold-copper alloy by dipping or coating with the gold alloy. The resulting surface gold layer is often rather thick, of the order of several hundred microns, and there is an extensive diffusion zone at the junction between the copper substrate and the gold alloy coating. Recent studies have shown that fusion gilding has been used on some Sipan style Moche gold items such as a jaguar head for the gilding of the small fangs inset in the mouth. The use of fusion gilding also occurs in both Ecuador and Colombia, but only appears to spread into the Northern Colombia area of the Narino Highlands around 1,000 AD, while related techniques were used at an earlier period in the Esmeraldas-La Tolita region along the Pacific littoral, during the period from the early centuries BC – 600 AD.
The remarkable development of electrochemical replacement plating of gold alloys over copper, without the use of a battery, began to be used extensively along the North Coast of Peru, during the Moche period. Laboratory studies by Heather Lechtman have demonstrated the feasibility of the technique, which depends on the ability of the ancient metalsmiths to obtain gold in solution without the use of strong mineral acids, which would have been unavailable at the time. The thoroughly cleaned copper object must then have been dipped into the gold solution, following pH adjustment The reason for the pH adjustment is that the initial gold solution, employing minerals such as ferric sulphate and alum has a very low pH, perhaps 2-4, and would be too corrosive to copper without first raising the pH to a more alkaline level, such as pH 8-9. The method produces extremely thin gold coverings, in the order of 1 – 2 microns which includes the diffusion zone formed by gently heating the object to allow proper adhesion of the gold to the copper substrate. Examples of dangles from the Vicus period, Peru, Moche, and North Coast Peru and Ecuador will be discussed and illustrated, as examples of the electrochemical replacement plating technique. It is perfectly reasonable that this astonishing discovery should be limited to the region of the North Coast of Peru, for the necessary mineral deposits would not have been available in less arid regions, such as Colombia, Panama or Costa Rica.
In Ecuador and Colombia, the most important alloys which were developed were those of gold and copper. Since native gold always contains some silver, they are essentially ternary alloys of gold-copper-silver, very similar in many respects to modem jewelry 10 – 22 carat alloys used today. The deliberate alloying of gold and copper gave rise to methods of making the surface appear to be of much higher gold content than the bulk material: these techniques were often called mise-en-couleur in more traditional texts, more often referred to as surface enrichment or depletion gilding in our terminology today. Depletion gilding can be used to remove either copper, or both copper and silver from the surfaces of the gold alloy objects. Lambayeque-style gold alloy masks from Peru were often treated to remove both silver and copper from the surface of the object, partially because many of these objects were made from deliberate ternary alloys of gold-silver and copper, probably with deliberate additions of silver a well as copper. Corrosive mineral recipes are quite capable of ensuring that both copper and silver can be depleted from the surface over a few microns depth. In Colombia, Panama and Costa Rica, the prevalent technology relied on gold-copper alloys with removal of copper from the outer surface, followed by burnishing, creating a smooth depletion gilded gold layer. In the Narino area of Colombia potential differences in surface color was skillfully exploited by the Indians to create multi-colored flat circular gold alloy discs with matte gold, burnished gold and copper colored designs. Examples of these will also be discussed, together with an account of some similar items, found in Ecuador across the border for the Narino area. Other examples of depletion gilded metalwork will be discussed from different cultural areas of Colombia, and examples from Costa Rica.
Technical examination of objects from ancient South America becomes complex when decisions have to be made concerning the gilding technology employed on a particular piece, especially when it is not possible to remove a small sample for metallographic examination. This is, unfortunately, often the only way to be sure that visual observations can be corroborated with technical studies, especially in regions where fusion gilding, foil gilding, electrochemical replacement gilding and depletion gilding were in use. Corrosion of the gold alloy objects during burial increases the difficulties and in some cases, gold enriched surfaces may peel away rather like gold foil from the surface, so that the visual assessment is entirely incorrect. Examples of a few corroded gold alloys will be discussed in relation to this problem and attention will be drawn to gold-copper alloys where the interior metal has become completely oxidized.