I recently experimented a bit with Brownells Nitre Blue salts. I added manganese dioxide per Oscar's formula. Starting at about 600°, I immersed well polished steel samples until color change was complete, usually 10-15 min. I ran samples until I got to about 850°. I found that 750° was hot enough to do a good job with full color development in about 10 min. with my particular samples(shotgun barrel sections) I could see the double color change as the oxide film grew thicker. Final color was a rich blue approaching black on mirror polished
samples. All my steel samples were the same composition, but I think different alloys will yield slightly different colors.
A few years ago I tried to use the salts without added MgO and a burner set up that was limited to about 600°. This really didn't produce satisfactory results on a low carbon steel receiver.
I can't prove it, but the MgO seems to aid oxide growth. The higher temp. really is key to good color and I am satisfied that the Brownells salts are capable of doing a good job.
OB
