| Proceedure |
|
| A macro photo of the sample can be seen in Figure 1. A small
piece was cut off the tip on the right side with a jeweler’s saw.
This sample was then set in a plastic clip and mounted in epoxy. The sample
was ground with 12 um Silicon Carbide paper on the automatic polisher
at 150 rpm. Sample 22 was then polished with 6 um diamond paste on a manual
wheel, followed by 1 um and 0.3 um alumina polishes on a stationary wheel.
Between each polishing step the sample was cleaned ultrasonically in ethanol
for one minute. The sample was then final polished using 0.05 um non-agglomerating
alumina polish. Photos were taken using a Nikon camera mounted to an Olympus
Light Optical Microscope (LOM). The sample was then etched using an Aqua
Regia solution of 20 ml nitric acid and 30 ml hydrochloric acid for 90
seconds. Photos were again taken using the camera and microscope. The
sample was then taken for Energy Dispersive Spectrometry (EDS) analysis
on the Scanning Electron Microscope (SEM). Microhardness testing was also
performed with a 200g load and a 15 second dwell time. |
| |
|
| Figure
2: Unetched LOM photo. Picture width is 1.5 mm. Cloudiness indicates
that this sample may not be pure gold. |
Figure
3: Etched LOM photo. Picture width is 0.75 mm. Evidence of cold
work lines can be seen. Also evidence of potential precipitation |
| Results |
|
| LOM analysis before etching revealed that there
was some cloudiness to the sample, indicating that it was not pure gold.
This can be seen in Figure 2. Upon etching cold work lines were strongly
visible as seen in Figure 3. The average microhardness value of the sample
was 156.7 Vickers, supporting that the sample was worked and not pure
gold, since the value was so high. Precipitates can be seen in Figures
4 and 5. EDS analysis determined that this sample was a gold matrix with
silver precipitates; the scans can be seen in Figures 6 and 7. Figure
6 is a matrix scan and Figure 7 is a precipitate scan. |
|
|
 |
| Figure 4: Etched photo showing
fine precipitates and cold work lines. Picture width is 112 um.
|
Figure 5: SEM photo showing silver precipitates. |
| Discussion |
|
Contrary to what the collector originally thought, this
sample was not pure gold, rather silver and gold. In addition, the presence
of cold work lines and the absence of porosity indicates that it could
not have been a casting as originally thought. Cracking on the sides of
the piece, as shown in Figure 4, also indicate working and could be an
indication of folding from extreme working.
This sample proved exceptionally difficult to polish and etch. Final etching
still did not resolve grains, but only cold work lines. Also there is
burning of the second phase which can be seen in Figures 3 and 4. This
discoloration indicated a second phase even without the higher resolution
of the SEM.
Little evidence could be found to support that this piece was actually
authentic. No examples of similar pieces were found and there is not indication
that the Mayans used silver- gold alloys.
The precipitates in this piece are quite strange since the gold - silver
system is completely miscible and no precipitates should form. Precipitates
may be due to a chemical reaction during etching bringing silver to the
surface, similar to the process of depletion gilding. |
|
|
|
Figure 6: EDS scan of the matrix,
showing mostly gold. |
Figure 7: EDS scan of the precipitate
showing that it is silver rich. |
