As promised, here's part 2 of 3 of my steel bluing guide.
This one covers caustic bluing, a process that allows getting a finished bluing in a matter of minutes, making steel parts as black as they can be. As usual I'm only using readily available chemicals.
Differently than rust bluing, this process is dangerous because of the large abounts of caustic soda required. Do not attempt without proper personal protective equipment (PPE).

A special thanks goes to Michelangelo Neri Orliani for helping in the making of this project

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