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<p>[QUOTE=&quot;Suarez, post: 4897637, member: 99239&quot;]There&#039;s two types of toning. Accretions which (potentially attractively) obscure the coin&#039;s surface and that which forms from a chemical reaction with the coin&#039;s metal. Most ancients that have any toning will have a combination.</p><p><br /></p><p>That rainbow toning is a subset and can happen in both cases. You could apply a thin coating of a translucent material or, for a bigger challenge, you can induce oxidation. The color shifting is determined by the thickness of the layer. Depending on what the layer is made of it reflects or absorbs all light except for a very narrow range of the visual spectrum. However, it&#039;s not just the thickness. You also need an underlying layer that is reflective. This is why it&#039;s much more commonly seen in silver than in copper since ancient copper will have non-reflective patinas in pretty much every case unless stripped. It also explains why you don&#039;t see it naturally in gold since this metal does not oxidize.</p><p><br /></p><p>Rasiel[/QUOTE]</p><p><br /></p>

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