I know this is an old post, but some may find it interesting, so I'll comment. For small parts like screws, temperature control isn't as critical in a niter bath because they heat fast and more evenly, so you can remove them from the bath when you get the color you like. For larger or odd shaped parts, it gets more critical because they aren't going to heat up as evenly, so the temperature needs to be controlled according to the color you want. (Brownell's has all the stuff; thermometer, salts, etc.) But you might get satisfactory results out of a tin can with home-made salts. Burn the finish off the can first, and if possible get a can that doesn't have a separate bottom. The salts can be made using saltpeter and lye. The old guy who told me about this (he'd be about a hundred by now) didn't know proportions, so I started out using saltpeter and adding a tablespoonful of lye at a time. It's been too many years and I don't remember proportions either, but it seems like it was close to 50-50 by the time I got satisfied. Colors show up better if the steel is polished real good. But the color range goes just like steel being heated in a forge. Straw is one of the first colors, and grey comes last, right after blue. Then if it gets too hot, needs to be polished again and done over. This mix isn't as good on whole guns as the Brownell product, but I did use it to blue an old double barrell belgian wire twist shotgun for a wall-hanger. The twist pattern came out very noticeable with different colors ranging from straw to light blue; something which ordinarily doesn't happen in typical hot tank blue jobs, but the Brownells' salts melt more evenly and don't separate. But for screws and the like, it may not be worth buying salts in bulk and paying a haz-mat charge, so it has it's place. It's interesting to see how creative you can get with the colors, though it takes a lot of time, but the highest temperature color range can be reached, then cooled and different areas "erased" with 800-1000 grit sandpaper and heated again, using a lower temperature, so it's possible to have blues, golds, purples, browns, etc. on the same piece if you plan ahead. Its interesting to fool around with, but I don't think a guy would make any money at it. But for himself, it might be worth it. I don't have to tell anyone about safety, but with temperatures in the 600 range, even the wires get real hot. Also, this isn't the type salts to get any water near either, or a guy will have a volcano on his hands, literally. And don't forget your eyeballs.
