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Originally Posted by Joepy
A friend of mine that gunsmiths uses a recipe of 50/50 (IIRC) sodium hydroxide (Lye) and ammonium nitrate in a boiling solution. Yields a really durable black oxide finish. I think the solution boils at 250F minimum. Steel parts are dunked for 5 - 10 minutes.
Depending on where you're located, obviously it may be edgey trying to get the ammonium nitrate. But, you ought to be able to get a small quantity without a lot of headache, even these days.
joe
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Sorry about the bad memory. I was way off. Here's a more accurate recipe and process that I found on Google:
Here is the information I promised, taken from "Gunsmithing" by Roy
F. Dunlap. (Excellent book, btw.)
5 pounds sodium hydroxide.
2.5 pounds ammonium nitrate.
1 gallon water.
The following is a condensed version of his instructions:
Working temperature is between 285 and 295 degrees F. When you mix
the solution, do it outdoors, as a considerable amount of ammonia gas
is given off. From 15 to 40 minutes in the bath are required. To use
this solution, the metal parts are degreased either by a commercial
detergent like Oakite, a solution of lye, solution or washing with
solvent, dried, and placed in the tank. After bluing, they are rinsed
in water, either cold or warm, and dried and oiled.
If you have hard water, it is a very good idea to use rainwater or
distilled water.
According to Dunlap, this finish is more durable than any other he has
seen. I have used it for a couple of shotgun receivers and various
small parts, and have been very pleased with the results. It seems to
wear at least as well as the original finish from Miroku.
Ole-Hj. Kristensen
Joe