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Bright rainbow toning on silver

2005

Hello,
I am a coin collector. Oxidation and physical preservation of original surface conditions are a big factor in this hobby.
Recently, I have seen a variety of silver coins for sale that have very brightly colored "rainbow toning". This looks quite unnatural, and I am very suspicious that this is being created artificially.
Is it possible that this is being forcibly created by someone?
What sort of reaction would cause this type of coloration?

         

David St.Thomas
Hobbyist - Boulder, Colorado, USA

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First of two simultaneous responses --

Sulfides and oxides are the most commonly observed naturally formed films on the surface of silver substrates and they are generally dark.

The electrochemically formed oxide films on Ti and Zr alloys give different shades of colour depending on the thickness of the oxide film. However, the colouration of films formed by such techniques is uniform on the surface. Chemical methods employed to colour a variety of metals and alloys also give uniform colouration.

In the photos you have provided, the colouration is not uniform. So I do not think it is forcibly created.

T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note Nov. 2017: The good doctor has a fascinating blog at https://advancementinscience.wordpress.com)
2005

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Second of two simultaneous responses -- 2005

Rainbow colours are probably caused by very thin corrosion layer .This layer can be artificially made by patination.
In your case this is probably result of long storing of coins in improper conditions (wooden boxes, paper or cardboard, textile).

Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia