Pre-Columbian Peruvian MetalsSample 01 |
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Soldered Bead Early Chimu North Coast, Peru ca. 900-1100 AD |
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Report by : Timothy Anderson May 14, 2003 |
| Introduction |
Figure 1: Ornamental Bead
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| Sample 02-01-01a was recovered on the North Coast of Peru. It has been dated in the time period from 900-1100 AD. The most significant people from the region during that time were the Chimu. The sample is a spherical ornament, most likely a bead from a longer chain of similar spheres. The piece was recovered in a damaged state, with a large puncture hole on one side (Figure 1). | ||
| Proceedure | |
| A small portion of the sample was first sectioned off by tearing off a loose tab from the broken edge. The tab was prepared by modern metallographic techniques. Epoxy resin was used as the mounting medium. The sample was ground and polished using diamond polishing wheels to 1/4 um. The sample was etched with an ammonia-hydrogen peroxide solution and observed under a light optical microscope (LOM). This etchant only highlighted a discolored outer layer. It was speculated then that there was a presence of copper, so a copper etchant, ferric chloride, was then used. Microhardness tests were done on the sample, using the Knoop test. The final test performed was to examine the sample under a microprobe and take an electron dispersive spectroscopy (EDS) spectrum to determine composition. | |
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Figure 2: Tip of section before etching, some
variation in surface morphology is visible on the lower side (inner
surface of the bead).
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Figure 3: Other side of the section after etching
in ferric chloride. Large grains are clearly seen with enriched surface
(gold) due to depletion guilding.
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| Results | |
| The initial etchant revealed an outer layer much lighter in color. Based on comparisons in the literature, this was believed to be gold. The second etchant outlined very large grains, some spanning the width of the piece. The Knoop test gave hardness results that were lower than anticipated. The results are tabulated in Table I. The EDS spectrum specified gold, silver, and copper as the elements present in the sample. The EDS also proved the existence of a very thin secondary layer on the side opposite to the visible layer. It exhibited a composition similar to the visible outer layer. The two outer layers displayed lower concentrations of copper than the base metal layer. | |
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Figure 4: Close up of the large grains and
inner and outer (lower and higher) surfaces. Knoops hardness testing
indentation is shown.
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Figure 5: Inner gold enriched area (unburnished)
after etching with ammonium-peroxide etch.
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| Discussion | |
| The data gathered seems to correlate: the bead is a depletion-gilded
tumbaga. The outer edges of the sample were shown via EDS to contain lower
concentrations of copper on the outside layers. The inside of the curved
piece displays a thick copper-depleted layer. It is thicker than the opposite
layer because the inside of the bead was not burnished smooth. The opposite
edge's depleted layer was not visible under LOM. However, an EDS spectrum
proved the existence of a thin layer similar in composition to the thick
layer on the other side. Depletion gilding was valuable to them, since
precious metals were symbols of worth [3]. Microhardness tests of the base metal layer were much lower than hardness tests of a similar sample. One possible explanation for the difference in hardness may lie in the quantities of the elements present. EDS only revealed the presence, and not the concentration, of individual elements. Higher concentrations of copper in this sample would make it harder. A better explanation may be that this bead was annealed more than the harder bead. This would cause recovery of dislocation strains in the lattice. The large grains found are certainly an indication that the bead was heat-treated. |
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| Description of Production Technique | ||
| Depletion gilding has been associated with alchemy, the ancient study of making gold from more common metals. It has the power to make a copper-rich alloy look very similar to gold or silver, yet still maintain strong mechanical properties. This was helpful in forming, as the copper-based crafts would hold their shape during forming. The metal used in the process was called tumbaga; it is a copper-rich alloy containing gold and sometimes silver [2]. A cast or sheet-worked piece of tumbaga would be quenched following a normalizing heat treatment. Several steps of annealing and quenching may have occurred to create the right hardness. The sample was then heated to the point where an oxide coating would form. Aqueous solutions would be used to leach copper and silver oxides out of the surface. The exact composition of the solution can only be surmised, but comparable results have been duplicated using oxalic acid [3]. Burnishing the depleted surface gave the tumbaga the appearance of gold. Different solutions were also used to create a silver finish [1]. The process is accomplished because the copper-rich phases oxidize faster than the Au-rich phases. Many Spanish conquerors were fooled by depletion gilded tumbaga, believing it to be gold. | ||
| References
1. Jacobson, Dr. David M. Jacobson. "Transmutation of Base Metals into Gold." Interdisciplinary Science Reviews. Vol. 17, no. 4, 1992. pgs. 326-31. Retern 2. Scott, David A. Metallography & Microstructure of Ancient and Historic Metals. Getty Conservation Institute. pgs. 22, 90. Return 3. Bray, Warwick. "Techniques of Gilding and Surface-enrichment
in Pre-Hispanic American Metallurgy." Metal Plating and Patination:
Cultural, Technical, and Historical Developments. Oxford: Butterworth
& Heineman, 2993. Return |
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