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Silver Artifacts
There are really only 3 major chemical changes that you will face dealing with ancient silver. Silver combines with sulfur, and chlorine and, less readily, with nitrate ions and oxygen, mostly resulting in gray to black patinas.
The first two important chemical reactions are of the silver itself with it's environment, and the third, brittleness, is the result of the copper which is almost always present in silver alloys.
- Silver Sulfide (gray to black): This is the commonly known silver tarnish. It occurs rapidly and consistently to nearly all silver artifacts.
- Silver Chloride (horn silver): Horn silver is present on many ancient silver coins. It is relatively soft, and can be difficult to remove due to its tendency to smear and obscure features of the coin. Horn Silver is the combination of part of the silver in the coin or artifact with chlorine to form silver chloride. It has a somewhat purple-ish to silvery-yellow in appearance, and projects slightly from the surface of the artifact or coin, affecting it's appearance.
- Embrittlement: Embrittlement is the condition commonly know to collectors as crystallization and is primarily the result of inter-crystalline corrosion at the microscopic level due to the selective precipitation of copper from the silver alloy at the crystal boundaries. Silver can only contain about 1% copper at room temperature. This amount increases as the silver is heated, thus the use of heat to combine silver and copper in alloy, and the degradation of this alloy at room temperature. Most silver alloys are what is called super saturated, meaning they contain more copper than is stable at room temperature. When the silver is cooled rapidly from high temperature the copper is trapped in solution and then precipitates out over time.
Discontinuous precipitation of copper may soon provide a new method for the dating and authentication of silver artifacts. Discontinuous precipitation occurs primarily in silver alloys with a 1.5% - 10% copper content, and shows up as a crystalline or jig-saw grain pattern on the surface of the artifact. Higher copper content silver alloys are resistant to this phenomenon. (Internet reference 12)
See also patina &:
Bronze, Definition, Formation, Index, Silver, Verdigris, Winds
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