Le Fusil,
Wouldn't using Niter salts be Niter bluing rather than heat bluing? You are right about using a torch for larger items.
Mike
Nitre bluing is in fact heat coloring. Different temps give different results/colors. The salts do nothing more than maintain uniform heat/temp. A lot of folks temper springs and other steel parts in nitre salts, especially when it comes to “drawing” the steel back after heat treatment, which is especially important when making springs or using hardened mild steel when making a tool.
As was mentioned earlier, molten lead does the same thing.
Quenching in kerosene? That’s ballsy. I would try using something with a bit higher of a flash point (dealing with relatively high temps when coloring with heat, to include hot ignition sources, etc) like a dedicated quenching oil, olive oil, safflower, water, etc.
