Fragmented Soldered Bead

Period: ??

Peru

ca. ??

Report by :

Kevin Elliott

May 15, 2003

The purpose of this study was to examine a pre-Columbian Peruvian ornamental ball sample and to determine the authenticity of the sample (Sample 02-01-25A). A picture of the sample is given as Figure 1. We also attempted to determine the metal composition and forming techniques of the ornamental ball.

Tribes in Pre-Columbian Peru, specifically the Moche and later the Chimu who were located in northern coastal Peru from between 250 BC to 600 AD and 800 AD to 1500 AD, respectively, hammered copper alloys to make ornamental objects. Pre-Columbian metalsmiths specialized in a copper alloy called tumbaga, which is primarily gold and copper, perhaps with some silver because silver typically occurs with gold when found in Andean stream beds [1].

Tumbaga can achieve a gold color, despite being as low as twelve percent gold through a process called depletion gilding. Depletion gilding requires pickling a sheet of tumbaga in an acidic solution1. Acidic pickling solutions could be easily obtained by the Moche and Chimu by soaking certain plant roots2. A schematic portrayal of Depletion Gilding is shown as Figure 3.

The pre-Columbian Peruvian's primary interest in metalworking stems from a strong sense of spirituality. They believed that a divine power gave them the ability to "magically" make colors and designs appear in materials from where they had not previously existed, such as bringing a gold color out of a copper colored alloy. One can clearly see how the pre-Columbians would see depletion gilding a copper colored alloy and making it turn gold as a very spiritual activity since they did not understand the metallurgical principles behind depletion gilding [2].

When viewed under a color microscope, the grains are visible, as well as thick layers of corrosion on the surface. The most striking feature is the gold color visible on both sides of the sample next to the outer corrosion layer. The gold color on the outside of the sample and copper color on the inside of the sample shows that this is definitely a depletion gilded sample. The microstructure of this sample is given as Figure 3. Note that the more gold colors seen on the edges show as gray and copper shows up as white on black and white photographs. While the exact composition of this alloy could not be determined without a Scanning Electron Microscope (SEM) which was not used due to time and cost constraints, the composition is believed to be around 80% copper, 18% gold, and 2% silver based upon research [3].

The thin shape of the sample and noticeable dents on the surfaces are indicative that the sample was flattened into a spherical shape by hammering. The sample was most likely formed into thin, curved halves and then entirely depletion gilded, which would account for both sides being depletion gilded even though the inner side (shown on top of Figure 3) was never meant to be seen. The depletion gilded halves were then most likely joined together with solder. Joining with a solder is a very common method in pre-Columbian metal pieces to obtain a smooth spherical surface for hollow shapes.

This sample is most likely an authentic artifact due to the process used to achieve a gold outer color, which looks different from more modern plating techniques such as electrochemical plating [1].

References

1. Lechtman, Heather. "Pre-Columbian Surface Metallurgy."
2. Bernstein, David. Lecture. New York Museum of Natural History. 14 April 2002.
3. Scott, David A. Metallography and Microstructures of Ancient and Historical Metals. Los Angeles: The J. Paul Getty Trust, 1991.