Blueing or 'blacking' is controlled oxidation. The pattern ('browning' or black & white) seen is damascus barrels is related to the different rate of oxidation of the iron (rapidly rusting) vs. the steel when exposed to a mild acidic solution. The 'plum' appearance over time is from uncontrolled oxidation, from which the metal underneath the forend is somewhat protected.
I'll let the fellas that actually do this stuff comment further smile
http://docs.google.com/Doc?id=dfg2hmx7_192fcpp4khn

Last edited by Drew Hause; 02/21/11 11:13 AM.
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