If any air comes into contact with the metal you will get an ugly grey scale. PeteM is right (thank you much for posting the photos by the way). Many older films and probably many of todays modern documentaries do not reveal the entire process simply because it wouldnt benefit them. I havent seen the film, but from the looks of the pictures he has that crucible right over top of the (flowing?) water. Then it looks like he practically plunges it in the water while simultaneously turning over? This would surely prevent any air from contacting the steel during the quench, as there appears to be no drop distance between the crucible and the water like many of us are doing, but Im not certain it would produce optimum colors. There is also the possibility that he placed a flat lid on top of everything and then re-covered it with charcoal, but who knows.
Theres just something about that sh*t eatin grin on his face and how his bare hand is so close to the freshly remove 1000F+ crucible that still makes me wonder?
