If you are looking to duplicate charcoal bluing colors then you will need to heat your salts more than 625F. If you are trying to get some form of temper color like fire blue then 625F will work just fine. To get charcoal blue color (blue-black) I have found that 730-750F works for most steels. On some modern steel parts I have had to increase the temp. It takes about 15 minutes to get the correct color. The parts need to be recently polished, degreased,and dried. When the parts first go in I gently agitate them until all of the bubbles are off the parts. When the desired color is reached I take the parts out, let them cool, rinse in water, and soak in oil. On some steels (like 2nd generation SAA cylinders) the parts need to be submerged several times.
I first learned to nitre blue from Oscar. He heated his salts to over 900F. I used to do this but found that the same charcoal blue color developes on most steels at a lower temperature. I have mixed my own salts from salt peter and used Brownells. Both salts give the same results. Oscar was concerned that Brownells salts could not be heated to the correct temperature without breaking down. I have heated Brownells salts to 900F with no problems.
Some steels are more difficult to blue. With a little practice you can duplicate the charcoal blue color found on pre-war II guns. I have blue about 15 Colt, 6 Winchester actions, 3 Marlin actions, and over 25 shotgun trigger guards using this method. I have compared the blue color obtained this way with original Colts in high condition. The colors are identical. I hope this helps.
