Thanks Claude,
Thought I'd post this I found about the original question:
"To Color Iron or Steel Blue (Niter Bath). Mix equal parts of sodium nitrate and potassium nitrate in a heavy iron steel pot. When the nitrates are thoroughly melted add a bit of manganese dioxide, about 1/3 ounce to each pound of nitrate.
Parts to be colored in this bath must be buffed to a mirror finish, cleaned scrupulously, covered with a film of light oil (dipping in an oil bath is the most satisfactory way of accomplishing this) and then lowered into the bath.
Immersion time for a dark blue color varies with the type of steel and its mass.
Since the working temperature of this type bath runs around 800-900F. soldered parts (lead or silver) or springs should not be put in it. Actually it should only be used for blueing pins, screws and similar small, nonoperating parts.
After the desired color has been achieved, the blued articles should be dunked in room temperature water, then in boiling water (to rinse away any adhering salt) and finally in oil."
From "PROFESSIONAL GUNSMITHING" By Walter J. Howe, Samworth Books, 1946
Trust some of you will try it out and show the results.
JC
