Huh. I get quoted in the first reply, and this is the first time I've looked at this thread! Looks like most of the queries have been answered. I'd just add that while I'm not a metallurgist, the effect of silicon causing the "plum" color was originally explained to me by an old gunsmith who does some of the most beautiful hot bluing I've ever seen, and was confirmed by folks at Essex Arms and Caspian, both of whom manufacture cast 1911 frames. I originally asked the question after my old gunsmith friend blued an Essex frame for me, and it came out of the tank plum colored. Other parts of other guns, including the forged slide I'd fit to the Essex frame, coming out of the same tank at the same time, came out a beautiful, deep blue. Obviously, it's not the bluing salts. It had no WD-40 applied, so it's obviously not that either. And no, WD-40 has no silicone in it, nor silicon, for that matter.