Fire bluing, or 'Nitre blue'?

[TennBill2]

It appears from my 'research' that Borchardts, and perhaps early Lugers were 'blued' by way of the above process using potassium nitrate, or 'saltpeter'. If I understand the process, it seems that it is merely a controlled immersion heating process rather than a chemical reaction. For example, I've 'case hardened' BP gun frames just using a propane torch. Supposedly, Spectricide stump rotting chemical is a source of potassium nitrate. I saw a Youtube example of a guy coloring small parts in a pot, the potassium nitrate was hard like solid rosin before heating. I guess the blue occurs around the 700 degree F range and is only a way creating a large, constant heat for an entire gun, as opposed to small parts. I tried some of the Laurel Mountain Browning on a sample and it is too black for my Borchardt project. I've never seen, first hand, any 'early' Lugers, or other examples of the nitre blue process. I think I read early Winchesters were done by this process too. Anyone tried this? Bill