| Proceedure |
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| Initial inspections were performed using x-ray
images of the entire sample. A seam was visible running up the side of
the cylinder. A portion of this seam and a cut portion of the soldered
area between the cap and cylinder were chosen for analysis. A jeweler's
saw was used to saw the individual pieces, taking great care for such
a brittle piece. The two slivers were mounted in epoxy resin and prepared
using sandpaper and diamond polish wheels (polished to 1/4 um). The specimens
were observed under LOM. A radiomicrograph and EDS spectrums were taken
of the jointed area between solder and base metal under the microprobe.
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Figure 2: Solder bounded by base metal in cylinder
seam sample. The metal plates are corroded but the original surface
can be seen.
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Figure 3: Break in base metal from excessive
corrosion. Original size of the plate can be seen under the corrosion
layers.
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| Results |
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| Under LOM, many phases were visible. The base
metal had almost completely corroded (figures 2, 4), while much of the
solder still remained (figures 2, 5). Lines were visible where the original
surface of the metal had existed. Three phases were visible in the solder.
EDS proved that the equi-axed grains were a silver-rich phase. Between
these grains existed two copper-rich phases, with significant amounts
of silver and arsenic (figures 6, 7). |
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Figure 4: Edge of corroded base metal. The
light boundary line represents the original edge of the metal before
corrosion. The internal banding is caused by preferential corrosion
of one metal over another in the alloy.
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Figure 5: Metal / Solder Interface. Note the
three phases visible in the solder in the top half. The metal plate
(lower area) is preferentially corroded.
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| Discussion |
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The initial sampling of the earring spool was
quite difficult. The corrosion present made it quite brittle, as it shattered
multiple times underneath the force of the jeweler's saw. The most durable
regions of the piece were the solders themselves, which made sampling
of the joined material fairly simple.
The earring spool was an excellent example of soldering performed by the
South American peoples. The shape of the sample indicated that they were
well-versed in metal sheet-working, as the cylinder is quite thin, yet
has been shaped into a near perfect circle. The seam of the cylinder and
the cap were joined with the same soldering material, a copper-silver-arsenic
alloy. Three different phases were found in the solder, each of varying
composition. This alloy was probably used because copper and silver form
a eutectic. Thus, the alloy would melt at a lower temperature than either
pure metal. |
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Figure 6: Compositional map (taken in the EPMA)
showing the enriched silver levels in the solder (bright areas).
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Figure 7: Compositional map (taken in the EPMA)
showing the enriched arsenic areas (brighter areas - intermixed with
copper).
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