The following is taken from Wikipedia: "Tintype is a photograph made by creating a direct positive on a sheet of iron metal that is blackened by painting, laquering or enamelling and is used as a support for a collodion photographic emulsion. Photographers usually worked outside at fairs, carnivals etc. and as the support of the tintype (there is no actual tin used) is resilient and does not need drying, instant photographs can be produced only a few minutes after taking the photograph."
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Making Tintypes (YouTube)