Home Machinist's Journal

Firstly, I'm not the smartest guy on the planet for metal surface prep. This is my Hobby Shop experience. I have done hot and cold surface bluing/blackening. Used lye and potassium nitrate and heated it up. Works good and will not affect temper. But is dangerous and the waste chemicals are hard to dispose of. Cold bluing with store bought bluing is so so for me, easily comes off. Then I tried heating the metal up to a little over the purple color, like a silver gray color and dunking it in dirty synthetic engine oil. WOW, it's fast and looks great! I have no idea on its rust prevention and surface wear properties. But for me, I'm sold. This process will affect temper, but I was doing it to 12L14 so no problem. Do it outside, stand up wind and have a fire extinguisher handy.
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I've had mixed results with gun gluing.
I made a bunch of qctp holders some time ago and blued them. Same batch. Some went blacker than black and others barely did anything.
I will say tho, 6 years later they're still not rusty. Granted they'll be covered in machine oil, but I also haven't used that lathe in over a year sitting in the old shop that's not longer climate controlled.

Now we send everything out for black oxide (when necessary). Comes back beautiful and it's inexpensive.

I knew an old gunsmith 50 odd years ago that blued gun barrels using a hot water bath and black walnut shells.
Worked good and looked nice.

I've done "blacking" per say in the past with varying results. I've also worked at places where we had inhouse phosphating setup for the parts we manufactured. Now days we send the stuff out to be phosphated. I haven't checked pricing lately but the last time I had it done it was around 50 cents a pound.  Yeah, they charge by the weight of your part not by how much detail your part has in it.
So far the best phosphate blackening kit I've found was one I bought from Brownells. Been a quite a while since I've used it. Works best if the parts are cleaned with dishwashing soap then run hot water over the parts just cleaned, dired with clean hand towel or paper towel.  Then immediately while the part is still hot, 95° - 120°F, apply the phosphate solution with a cotton ball until the ideal darkening is achieved.  Let set, then clean with water, dry quickly.  Then apply an oil like Starrett M-1 or a ISO-46 oil. And it's done.