From the Conservator's Bench: Cleaning Gold Artifacts
In preparation for the 2011 exhibit To Capture the Sun: The Gold of Ancient Panama, many of the magnificent objects underwent conservation treatment and were cleaned to remove debris and oxidation. But why was cleaning necessary? Why did this gold look tarnished? Isn't gold inert?
Although not technically inert, pure gold doesn't oxidize or corrode and is less reactive than most other metals. Tarnish and corrosion on gold is a sign that it is actually composed of an alloy, a chemical mixture of metals. The artifacts are also not identical in color: some are bright yellow, while others have a pinkish or greenish tinge.
Pure gold, in elemental metallic form, has the bright yellow color and shiny luster we know and love, and doesn't corrode or discolor like most metals. It is also very "ductile," meaning that it can be hammered very thin and easily bent without breaking; a characteristic that makes gold ideal for jewelry and many industrial uses. Pure gold is so soft, in fact, that it is usually alloyed with other metals in order for it to be able to withstand everyday use. The metals used in the alloy will influence the color and alter mechanical properties. Gold alloys of lower purity are less resistant to corrosion and have some of the characteristics of the other metals in the alloy.
Many of the gold pieces in To Capture the Sun are made of an alloy called tumbaga by the Spaniards. Tumbaga describes the gold and copper alloy and technique that was in use in pre-Columbian Mesoamerica and South America. Tumbaga does not refer to a precise percentage of each metal, so there is a wide variety of color, but the technique is characteristic. Called depletion gilding by metallurgists, the last step of the technique involves an acidic chemical bath. The acid dissolves away the copper in the outer surface, but does not dissolve the gold. The resulting object has an outer shell of pure gold that can be rubbed to a bright sheen, while the core of the object retains the hardness of the gold/copper alloy. Over time and with use, the thin, pure gold layer can wear through, and the underlying alloy can be seen.
If you have questions about artistic techniques or preservation, you can ask Victoria Book Lupia, our chief conservator. Send questions to: GMConservation@utulsa.edu.