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What caused this colorful rainbow?
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<p>[QUOTE="messydesk, post: 8092190, member: 1765"]Short answer: The "shape" of the toning. Lacquer pools up in crevices of the coin differently from how toning does.</p><p><br /></p><p>Longer attempt at an answer: The thin film interference that causes toning is from layers of light-absorbing material building up on the coin in the 200-1000 nm thickness range. Once it's too thick, the coin is black. </p><p><br /></p><p>For lacquer, the root cause of the thin film interference is not likely the thickness of the lacquer, as the variation of the thickness across the surface of the lacquer on the order of micrometers, not nanometers. What is causing the rainbows in the lacquer is a change in the refractive characteristics of a layer of the lacquer nanometers deep. This is influenced both by the thickness of the lacquer and the exposure to air, which is causing the change. Put another way, the lacquer itself tones as it dries and cures over time. I imagine that with another 50 years' exposure to warm, dry air, the rainbows on the OP coin would change, and eventually disappear altogether as the lacquer layer became completely homogeneous.[/QUOTE]</p><p><br /></p>
[QUOTE="messydesk, post: 8092190, member: 1765"]Short answer: The "shape" of the toning. Lacquer pools up in crevices of the coin differently from how toning does. Longer attempt at an answer: The thin film interference that causes toning is from layers of light-absorbing material building up on the coin in the 200-1000 nm thickness range. Once it's too thick, the coin is black. For lacquer, the root cause of the thin film interference is not likely the thickness of the lacquer, as the variation of the thickness across the surface of the lacquer on the order of micrometers, not nanometers. What is causing the rainbows in the lacquer is a change in the refractive characteristics of a layer of the lacquer nanometers deep. This is influenced both by the thickness of the lacquer and the exposure to air, which is causing the change. Put another way, the lacquer itself tones as it dries and cures over time. I imagine that with another 50 years' exposure to warm, dry air, the rainbows on the OP coin would change, and eventually disappear altogether as the lacquer layer became completely homogeneous.[/QUOTE]
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