Re: What is that rainbowy finish used on antique guns?
Yes, as Dave said, color case hardening. It is a difficult and tedious thing, not a finish really. It is made by heating the metal in a special atmosphere with carbon in contact with the surface and then by dunking it into a bath of bubbling water typically. The reason for it is to apply a hard outer layer for wear protection, the beautiful colors are a secondary thing.
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