Gunplummer,
Interesting about the air bubble effect. Must be doing a sort of selective application of oxygen to the hot surface. New one on me. Thanks for the info.
Sc928porsche,
Keep in mind he is color case hardening, so he needs the water for the color to come out right. Also, he will be pouring red hot charcoal embers into the quench which could set even a flame resistant quench oil on fire. You can watch how the spilled carbon flames up on contact with air in first twenty seconds of
this video.