Just very quickly and to oversimplify, the polished part is heated to around 400 degrees F, which causes it to darken to a golden yellow or "straw" color. It takes a fair bit of skill to keep from over-heating and getting the part too dark, or getting an uneven color because of the varying thicknesses of metal in the part. Now do a search and see what the experts had to say.
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If it's made after 1918...it's a reproduction
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