Gilded penannular ornaments from Ecuador

The collection includes a number of small penannular ornaments, either nose- or ear-rings which were acquired in 1938 with the provenance "from ancient graves discovered when working for gold on the shorelines of the Máte and Ostiones rivers, Ecuador" inscribed in the Museum register. The Máte (or Mátes) and Ostiones are two minor rivers running into the North Pacific between 'the towns of Esmeraldas and Valdez in the Department of Esmeraldas. This places them within the cultural area of Tumaco-Tolita, but the date of these pieces is unknown and they mar belong to a later culture. The ornaments are a heterogeneous collection of forms and material s, varying in diameter from 1 to 4 cm; some are round in section, some flat and some are hollow C-sectioned gold sheet, some are gold, some gilded and the rest are copper, usually with trace levels (less than 1 %) of arsenic and gold detectable by X-ray fluorescence analysis. One is certainly modem, being made of machined brass robe.

Photograph 14: High magnification detail of image in photo 13. The gilding layer is very sharpy defined and shows some coring. The dark pink/orange junction between the gilding and copper body indicates that diffusion has taken place. [photo S. La Niece, British Museum].

Two corroded but gilded ornaments were selected from this group for closer examination and identification of the gilding technique. As has been pointed out by David Scott (1986a: 318) it is not easy to determine from examination at low magnification whether a layer of gilding over copper is foil gilding (covering with a layer of gold foil) or fusion gilding (coating with molten gold), or, as is illustrated below, depletion gilding (enriching the surface of a base gold alloy by removing copper). Metallographic examination is the only way to conclusively resolve the problem. A small sample of metal was taken from each of the ornaments and mounted in epoxy resin in cross-section, to allow study of microstructure of the gilding layer (cast or worked) and examination of the junction between the gilding and the body metal of the ornaments. The sections were polished and then examined using a lEaL 840 scanning electron microscope with analytical facilities.

Penannular ornament 1938-10-25,31 (photograph 11) has a bright, gilded surface, bubbling up in places because of the corrosion beneath. At the damaged tip, the core metal appears.-Bark and corroded. The polished section reveals a distorted cast structure with corros ion zones deep inside the core metal. The composition of a relatively uncorroded area of the core metal is 44% gold, 52% copper and 4% silver (EDX analysis, ±1% accuracy level), rising to about 78% gold at the surface, with corresponding decrease in copper content. The gold enriched surface is very uneven in thickness, at most only a few microns deep. It has been burnished. The conclusion from examination of this section is that the ring is depletion gilded, that is, it is cast from a low gold alloy from which copper has been deliberately oxidised and dissolved out of the surface. The resulting gold-enriched surface has been burnished to give a shiny golden appearance. The distortion in the cast structure suggests that some work was done at the tip of the ornament after casting, perhaps to bend it round.

The second ornament (BM 1938-12-25,28) under low magnification looks very similar (compare photographs 11 and 12). It has a bright gilding layer, which is peeling away in place s to reveal the dark, corroded body metal. Photograph 13 is a false colour SEM image of the polished section through this object seen at high magnification, the width of the field of view is 2 mm. In this photograph the grey-scale of the black and white SEM image is replaced with a spectrum of colours which provide more contrast than a range of greys. In this image the brighter end of the scale, ie. the gold-rich layer on the surface appears pink. It varies in thickness between about 60 microns and 25 microns. The darker end of the scale, that is the copper, appears green.

Analysis of this section shows that the ornament has a heavily corroded copper core with only about 5% gold and less than 1% silver in the alloy. The dark blue areas are the most deeply corroded, and interestingly, these areas are where the few percent of gold in the core metal are concentrated (see paper by Meeks for further examples of the preferential corrosion of gold-rich zones in tumbaga alloys). The yellow areas on the image are islands of relatively sound metal.

The gilding is a discrete layer of gold-copper-silver alloy, with a thin layer of copper corrosion over the top (green in the image). The gilding has a cored structure (the darker pink areas are richer in copper) suggestive of casting and shows no sign of working, certainly not the extensive working required to hammer out gold sheet or foil. Although the gilding layer is sharply distinct from the copper core it has flowed into every pit and hollow in the surface. The dark pink and orange band in the gilding at the junction with the copper is indicative of diffusion between the two zones due to heat. Area analysis of the gilding layer gave a composition of 40% gold, 56% copper and 4% silver. Photograph 14 shows the gilding layer at higher magnification. There is a very thin, discontinuous, gold-rich layer (white) at the surface of the gilding. The layer of copper corrosion seen in photograph 13 over the gilding suggests this is the result of corrosion during burial, rather than deliberate enhancement during manufacture.