I’ve been hot caustic bluing (hot salts bluing) for only about 25 years, so I’m no expert, but I’ll share a few things that I’ve learned. Hot salts bluing is one of my least favorite processes, it’s fairly easy once you get set up, but, it’s a real PITA overall. If I didn’t have to do it… I wouldn’t.
Most know that caustic bluing salts are basically lye (Sodium Hydroxide), pretty nasty stuff, which are superheated (280 deg+ well above the boiling point of water), in a super saturated solution meaning that you have to heat the water up for it to hold the amount of salts required.
Lye destroys most organic material: wood, leather, hair, skin, fats, oils, waxes etc.… which is why it’s such an effective drain cleaner.
Lye burns hurt, superheated lye burns hurt a bunch, and heal slowly, use the appropriate PPE and always keep a gallon jug of vinegar at the ready.
Keep aluminum away for the salts, the salts will quickly dissolve AL. Copper, Brass etc. will ruin the salts.
Bluing salts “creep”, they will creep out of your tank, and onto everything below it.
As for stainless steel, I’ve heard that it’s not recommended, but I have not had an issue. I bought my bluing set-up from a retiring gunsmith about 30 years ago, he had custom made SS tanks which I still use today. But keep in mind that there are over 20 different grades of SS, not all are created equal. I suspect that the majority of SS cookware: pots, pans, fryers etc. is now imported from overseas, no telling what the metallurgy is of that stuff. If I were making them today, I would look at 316.
Electric fryer, I wouldn’t recommend it: First, most heating elements are made up of Kanthal, Cupronickel, Nichrome… some contain copper and aluminum, most contain chromium. none you want in a super-heated lye bath. Secondly, the salts will creep into everything. Quickly destroying the electronics/ internals.
There’s a reason why the propane burner set-up is used in the majority of caustic bluing set-ups.
Hot salts bluing is temperature sensitive; you don’t control the temperature with heat (i.e. thermostat), you control the temperature by adjusting the solution.
We all remember 5th grade science class, pure water boils at 212 deg, no mater how much heat you apply you will never get pure H2O above 212 deg… it will be steam.
The super saturated lye solution needs to boil around 285 deg f, if it starts to boil too low, you ass more salts, too hot add more water… Which brings up another point, adding water to a super-heated solution, will cause the water to almost instantly turn to steam…. Lots of splatter…and not just water but super-heated lye. Wear protective clothing, and keep a gallon jug of vinegar handy.