|NIGEL MEEKS

|DEPARTMENT OF SCIENTIFIC RESEARCH

|BRITISH MUSEUM

Figure 1: Musicians pendant, Diquis, Costa Rica, height 72 mm. Photo: A Milton, British Museum.

| |Abstract: Technological examination of the Pre-Hispanic South American goldwork of the British Museum is currently in progress. A variety of ancient metalworking techniques and alloys used to produce a range of ornamental gold artefacts have been identified using scanning electron microscopy and microanalysis. Detailed investigation of some binary phase, copper |-rich castings and their depletion gilded surfaces are illustrated. Analysis of |fish hooks from the Cauca river gives a wide range of alloy compositions from alluvial argentiferous gold to alloys of gold with high copper content, along with both cast and worked wire types. Tool marks on |some gold objects are examined in detail.

|Resumen: Un examen tecnológico de orfebrería prehispánica suramericana de la colección del Museo Británico está llevandose a cabo actualmente. Usando microanálisis y |microscopia electrónica de barrido se ha identificado una variedad de aleaciones y |de técnicas de metalurgia antiguas usadas para producir un |amplio rango de artefactos de oro. En este art |ículo se ilustra la investigación detallada de algunas fases binarias y |de fundiciones ricas en cobre con superficies doradas. El análisis de anzuelos del río Cauca muestra el uso de un |rango amplio de composiciones de aleaciones, desde oro argentifero aluvial hasta aleaciones de oro con alto contenido de cobre, tanto para alambres martillados como para artefactos fundidos. Se examinan en detalle, igualmente, las marcas dejadas por las herramientas sobre los objetos de oro.

The South American gold collections in the Department of Ethnography of the British Museum were the subject of metallurgical examination to provide technological information for the exhibition "The Gilded Image". The exhibition displayed some of the finest gold objects in the collection which covers a wide geographical area -from Panama, Costa Rica, Colombia, Ecuador and Peru and includes many of the indigenous cultures from the period c500BC - 1500AD. Objects of cast gold and copper, false filigree, worked sheet, soldered objects and a range of gilt items that represent cultural variations in manufacturing techniques were chosen for examination. Some of the technological results were displayed in the exhibition adjacent to the objects to illustrate the metallurgical processes that are so characteristic of the South American cultures. Some of these results have also been published (Bowman, La Niece and Meeks 1997, McEwan, La Niece and Meeks 1996). Much technological work has been published on South American gold and it has been possible to categorise the British Museum collections within the existing framework of South American metallurgical technologies. This paper focuses on some further examples of objects with interesting metallurgical structures, compositions and toolmarks found during the detailed technological examination of the South American metalwork in the British Museum.

The first purpose of this paper concerns the characterisation of some depletion gilded, copper-rich, cast objects that were found to have unusual two phase metallurgical structures. These were not usual cast, cored structures but must be the result of part of the manufacturing process of the objects. The objects chosen for examination were a musicians pendant, Diquis culture Costa Rica, and an anthropomorphic figure from Panama. The second purpose of this paper is to examine a collection of wire fish hooks from the Cauca river, Colombia, from pre-Hispanic cultures, in order to study the range of compositions and wire technology used. The final purpose of the paper is to examine in detail the fine tool marks on a pair of Coclé earspools from Panama and a penannular nose ring from Ecuador.

Optical microscopy was first used to examine the antiquities (Leitz M8 Stereo-Zoom 6-50x magnification). This allowed assessment of the objects overall construction, surface texture and condition, and enabled potential sampling regions to be identified. Following optical examination, the majority of the metallurgical examination work on these objects and samples was carried out using scanning electran microscopy (SEM) and microanalyses with the attached energy dispersive X-ray analyser (EDX). The SEM is a JEOL JSM840 model with a modified backscattered electran detector to enhance composition differences in polished sections (Meeks 1988). The quantitative EDX analyser is an Oxford Instruments ISIS 200 system using a high resolution germanium (GEM) detector with a light element detecting window (SATW). Quantitative analyses are made with reference to gold alloy standards and the analytical precision is within ±0.5% for major elements and ±10% relative for minor elements. Accuracy with respects to analysed standards is of the same order.

Surfaces of objects were examined directly in the SEM and, where appropriate, tiny areas were scraped clean to expose core metal for accurate analysis.

(Comments concerning the composition of the surfaces of gold alloy objects is given in appendix 1). Tiny unobtrusive samples were taken from some objects and mounted in resin, ground and polished flat using standard metallurgical procedures so that internal structures could be examined. False colour is used in some SEM micrographs of polished sections to give greater visual enhancement to gilding, diffusion zones and phases, than is possible with the usual grey level images of the SEM.

The pendant shows musicians in mid-performance, the three small top figures blowing conch-shell trumpets while the two large and the three lower small figures play flutes (Fig.1). The main figures are also hollow bells with openwork backs. The border is of double rope-twist false filigree wire. The pendant is a lost wax casting in a depletion gilded copper-rich gold alloy (see table 1 below) and dendrites from the original casting are seen on some areas of the surface. Although the casting is predominantly copper coloured, it still retains some original gilding in the recesses away from the regions of wear. Viewing the surface of the pendant directly in the SEM at high magnification shows the characteristic porosity caused by removal of surface copper during depletion gilding. The porosity is about 0.3 - 1 micron in size and penetrates into the surface of the alloy. In cross section at high magnification the unburnished porous layer is seen to be between only 2-6 microns thick (Fig.4a) and is only just visible at lower magnification (x200) in Fig.4b which shows the relationship to the underlying body metal. The composition of the porous gilding varies between 70%-90% gold and is enriched from the copper-rich dendrites at the surface (which contain 39.5% Au) (Table 1 & Fig 4a).