Pre-Columbian Peruvian MetalsSample 02 |
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Soldered Bead Early Chimu North Coast, Peru ca. 900-1100 AD |
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Report by : Wendi Croft Feb 10, 2003 |
| Introduction |
Figure 1: Macroscopic view of the artifact
as received. Photograph by W. Croft
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| The origin of this ball is suspected to be on the coast of Peru where the Chimu were once established. The Chimu Empire began in Chan Chan and expanded covering up to the Ecuador border and down to the present day city of Lima. An image of the ancient Peruvian coast and the land covered by the Chimu can be seen in Figure 1. The Chimu were very skillful metallurgists who manipulated silver, gold and cooper in techniques similar to the ones found in this large silver ball. The time period of this piece is estimated to be between 1150-1476 A.D [1]. The purpose of this large Peruvian ball could have been multi-fold, such as tomb decoration or jewelry. | ||
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| Figure 2: Image showing
the over etched region of the sample. |
Figures 3 + 4: Image showing the non-etched region of the sample (left). Image showing a Knoop microhardness test site as well as the region of change between the etched region and the non-etched region (right) |
| Sample Preparation | |
| The artifact was previously fractured and a broken piece was mounted in Epoxy between plastic clips. Following mounting, it was ground to 800 grit and polished to .25 m with diamond spray. It was then etched with 100mL of H20, 100mL H2O2, and 32g of FeCl3. The object was also observed under the LOM before and after etching. During the observation process digital images were taken using a Nikon digital camera in coordination with a LOM. Knoop microhardness tests were conducted on the sample. Results can be found in Table 1. Finally, the sample was carbon coated for observation in the SEM using EDS X-ray analysis. Results can be found in Table 2 and Figure 8-10. | |
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Figure 5: Center of the sample after etching
showing microstructure. Grains are clearly visible as is the gold enriched
surface.
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Figure 6: Image of the center of the sample
before etching showing minimal inclusions nd some grain shapes.
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| Interpretation and Discussion | |
| Initially, this object was suspected to be
a pure silver ball. The macroscopic images of the artifact can found in
Figure 2. However, after X-ray spectra investigation it was shown to be
a silver-gold-copper alloy, Figures 8-10. As noted in the EDS results
in Table 2, gold was the most prevalent element
throughout the majority of sample. There was also a large percentage of
silver present. Copper showed to be a much smaller portion of the composition.
From the EDS graphs an estimate of the ternary phase diagram was created.
Using the peak heights as reference, the rough composition of the sample
was found and plotted. From this information, a comparison was made to
the ternary diagrams of the Chimu. Additionally, provided background information
on the Chimu suggests that this piece was most likely created by one of
their talented craftsmen [2]. After more observation of the EDS results it was noted that the outer layer (right side in Figure 5) was depleted of silver and had more gold then the inner regions. Moreover, in Figure 6 it can be seen that there is a small fraction of the outer surface that has faint microstructure shown. These findings suggest that depletion gilding could have occurred. Depletion gilding is a process that involves hot working (cold working and annealing combined) in conjunction with pickling. This combination will rid the sample of cuprous oxides and leave it enriched with silver and gold on the surface [1]. A schematic of the process can be seen in Figure 13. These expected results directly correlate to the findings on the EDS for the ball sample. |
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Figure 7: X-Ray image of the beads showing
that they were joined in the center of the sample.
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Figure 8: EDS results taken at the inner edge
of the sample. This area corresponds to the left hand side of Figure
5
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Figure 9: EDS results taken at the center of
the sample.
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Figure 10: EDS results taken at the outer edge
of the sample. This area corresponds to the right hand side of Figure
5.
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| Discussion (con't) | |
| In addition to EDS results, Knoop microhardness
results were also evaluated. The results can be seen in Table
1. After comparing data with a fellow investigator's information
on a similar Au-Cu piece large differences were discovered. The silver-gold-copper
alloy was surprisingly hard in comparison to the tumbaga (Cu-Au alloy)
sample. This was a very strange finding due to the fact that copper is
generally regarded as the hardest of the three materials and has the lowest
percentage in the Ag-Au-Cu alloy. A reasonable explanation for this finding
could not be achieved [3]. Also interesting to note, half of the sample stained after etching, and the remaining half showed faint grain structure formations on the other outer edges. These images can be seen in Figures 5, 4, 3 and 2. Unfortunately, an explanation for this behavior could also not be reached. The manufacturing process of the piece was very vague. However, it is suspected that the piece was first a sheet material that was depletion gilded, as reasoned by the EDS results. The sheet was then formed through the use of a hammer-like took into a ball with overlapping sheet that were joined with heat [4]. This joint can be seen in Figure 7. This image shows an x-ray of the artifact and its joint can be clearly noticed as the brighter line around the artifacts middle section. The authenticity of this sample was assumed to be good. The accuracy and similarity in the ternary diagram comparison to the Chimu was very convincing. Moreover, the piece was somewhat corroded, as can be noted in Figure 4. All of this would lend to the integrity of the piece. |
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Referneces:
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