Ok, enough from the experts:
Ive been offering hot (caustic) bluing to my customers for 25+ years. Ive also used most of the major brands out there: Du-Lite, Brownells, Williams and once or twice my own brew; so I might know a little something about it.
Depends on what you want to do: if its for an occasional bluing, screws, top levers, safe buttons, trigger plates and 4end irons etc. farm it out or Nitre Blue the parts.
First of all, its super-heated concentrated lye (295 deg), so its really not fun stuff to play with, lye burns hurt soprotective equipment.... Don't know where folks are getting 320/380 from.
Ventilation is a must.
Have a dedicated place to do it, lye destroys a lot of things brass, wood, aluminum, basically anything organic.
When not in use the salts creep, they will creep up and over the tank, and down the sides and onto anything below it, with the potential to mess it up Wood floors bad idea.
They are not cheap, just got another 5 gal bucket of salts in recently $325, the hazmat shipping adds a lot.
It does blue parts quickly, once up to temperature but it takes a while to get it up to temp 45 minutes.
The salts have a fairly narrow operating range, you either add water or add salts to keep it in that range; adding water to a solution that Is super-heated (295) can be an emotional event, think what happens when you put water on a hot skillet immediately turns to steam same thing, but with lots of splatter (think super-heated caustic lye).
I do lots of finishing work in the shop: Color Case Hardening, Nitre, Charcoal Bluing , Rust bluing Caustic Bluing, Caustic Bluing is my least favorite.
