Hot caustic bluing is somewhat dangerous. The chemical salts are caustic, like lye, and will cause chemical burns. They also operate at about 285F, so they will burn with physical heat. The fumes are not healthy.
The chemicals will destroy aluminum. I saw a guy do it in gunsmithing school… he lowered the part into the tanks on wires and pulled out absolutely nothing but the wires!! Anyway, steel parts only!
Nickel Steel will often turn red instead of black. Temperature is critical. Pre-finishing the parts is critical. Degreasing the parts is critical…any spot of grease or oil will show up in the finished bluing. Rinsing the parts is critical. Post bluing penetrating lubricant is critical. It is a PAIN IN THE BUTT… but that is only my opinion! 🙂 🙂
There are some guys that are good at it and they are worth whatever they charge!
There are other finishes both blue/black and brown, such as hot water bath, steam cabinet rust finishing, parkerizing, etc. The chemicals are a lot less dangerous but they are not easy either. Cold bluing is simple but it is only for touch-ups. There are other finishes, like Cerakote or Diamond Black, but they require special equipment.
I’m sure the guys that do bluing will chime in at some point and give you more info.
