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vickyswaggo t1_ixb4ier wrote

https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/celc.201800435

This is an archaeology paper from 2018 discussing one method of dating archaeological gold. I'm unsure if it's open access because I'm accessing it from my college campus, so I shall summarize it:

The authors use a corrosion clock and electrochemistry (specifically something called VIMP which stands for voltammetry of immobilized particles). Gold forms oxides, which can be studied with electrochemistry. The authors designed a "calibration curve" using known compositions and then fitted historical samples onto this curve.

You mentioned the possibility of melting and recasting. When a metal is melted, the metal oxides decompose into the metal and oxygen. Specifically for gold, Gold (iii) oxide can form. It decomposes at 298°C. Gold melts at 1064 °C. As you can see, gold (iii) oxide would decompose long before the gold itself could be melted.

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Macrophage87 t1_ixcp2qr wrote

Couldn't that corrosion be faked by some chemical process though?

Also, wouldn't storage conditions change corrosion rates substantially.

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vickyswaggo t1_ixd8sec wrote

Corrosion could be induced by putting the gold in a more oxygen rich environment and perhaps making it a bit moist, but it's unlikely that an authentic corrosion profile could be forged. The paper I was using as a source mentions:

"Despite its chemical stability, gold possesses a rich oxidative electrochemistry resulting in the formation of gold oxide coatings and, in the presence of complexing agents, oxidative dissolution processes. Such electrochemical processes are sensitive to changes in the textural properties of the metal surface, reflecting most minute alterations of
the gold surfaces upon aging."

This indicates that it's not just the corrosion that is important, but rather the corrosion and the fine mini reactions.

For your second question, yes storage conditions change corrosion rates substantially. However, this can be accounted for chemically (acidic soil vs basic soil, wet vs dry, etc). This plays a part in the "presence of complexing agents" quoted earlier.

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As well, the authors used known specimens as calibration standards. Those specimens were in various storage conditions, so can be compared to new specimens found in similar conditions.

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