David,
I have successfully nitre blue hardened parts one of two ways. The first is to nitre blue the part with a first pass. Usually the first blue will have a light blue grey color with patches of blue-black. Next, polish off the blue with 400-600 and reblue. Often times the second blue takes. I believe that this works because the heat from the first blue tempers the steel. Sometimes this doesn't work very well on all steels. The second method I have used is to put the parts in the salt bath several times without polishing between blues. Each time more blue builds up. This is the only way I have been able to blue 2nd generation SAA cylinders. I have to experiment with temperatures but hardened steels will usually turn blue-black somewhere between 750-850F. Good luck.
I have a gunsmith acquaintance who claims to have spoken with a former employee of Turnbull. According to him Turnbull uses 825F. He said the employee claimed that Turnbull used what he described as a "yellowish liquid salt bath" to charcoal" blue and not charcoal. The former employee would not tell him what the salts were. The "yellowish liquid salt bath" sure sounds like nitre salts to me. I have no way of knowing if this is true or not. It was just what I was told second hand.
