Metal has to be absolutely clean. Cleaner than you would accept for hot blueing purposes.
Don't let it sit around after final polishing. Even an unseen to the eye oxidation starting to form will cause a poor blue finish.
Insulate the sides of the pot you're melting the salt in if you can. We used to use the all purpose asbestose paper. But since that stuff is more hands off than a hypo on the sidewalk, there must be something else available now.
A steel heat shield around the sides with a layer of insulation helps dramaticly in heat up, stability of temp and being able to reach the higher temps with a common heat source.
Have some absolutely dry and oil free fine steel wool handy for the small air bubbles that will sometimes form on parts in the salts.
Sometimes just lifting the parts and carefully dunking them back and forth into the salts will break up the bubbles. If needed, swipe gently over them with just a wisp of the steel wool snagged around the end of a new clean bronze cleaning brush on a pistol cleaning rod for easy handling. It can get hot quick too, but you're wearing your gloves right!
If the part doesn't color to your expectations, it'll need to be polished up again to be redone. Trying to touch up a spot or two never seems to work out well.
Plain sodium nitrate works fine. Ammonium nitrate works well too. Most any chemicle salt that'll melt around 350 and not boil till after 900F can be used.
The Brownells stuff works good. I've used it & plain sodium nitrate both for many years for straw & blue all the way to a charcoal blue look at 825/850F.
Works nicely as a spring temper medium in the lower ranges.
Ammonium Nitrate was used in a pinch a couple of times and it worked just fine too.
Some metals will just not blue well with the process due to their alloy content or heat treatment.
We used both nitre and at the time the start of a charcoal blue process at DTR.
Charcoal blue was done with bone char (same stuff from the CCH process) and a Mobil synthetic oil (can't recall the #) in the place of carbonia oil of the original process.
The charcoal blue process was done in a self contained canister type device we fabricated. Air tight with the exception of a tiny hole in the removeable end to allow the smoke out and (hopefully) no atmosphere in.
It was placed inside one of the existing CCH ovens.
It was more a mimic of the later carbonia oil process of the American Gas Furnace Co., than the earlier real charcoal bluing process.
Two different processes giving similar results. One replacing the other as more labor & time efficient in industry in the early 20th Century.
The canister had to be manually rotated a 1/2 turn every 20min IIRC. The batch ran for 2hrs to complete. Results were at times spectacular, other times not acceptable for a number of reasons. Usually air leaking into the canister around the cover gasket somewhere, moisture in the char/oil mix, improper mix, parts not clean,temp not correct,,lots of variables..
But we were on the right track. Then it went into secret mode for refinement like most things there.
That was in the early 90's. I don't know what process(es) they use over there now.
Like CCH, it's not a terribly complicated process. Just one that needs constant repetition once a system that yields good results is found.
