It doesn't look like there is any cover on that crucible at all, just the color of burning charcoal. There was a guy on the Marlin forum that tried to reproduce this stunt and had his receivers come out scaled and grey (exactly what you would expect).
Ive spoken with Jim on the Marlin forum a number of times. What could be a possibility from the film is that there is indeed a cover sitting inside of the crucible, and that it is than covered with charcoal so we dont see it (why, I dont know, but it could be). I dont think that Jim plunged his crucible into the quench tank like the Marlin film photos show, and from seeing Jims pictures it looks like he loses the majority of his charcoal when he turns his crucible over (as his is rotating on a parallel plane with the quench tank), which is why his first parts showed zero color. Had he used an internal cap, much like the one he developed later to hold three actions at a time, then covered it with charcoal, there most likely would have been colors. An internal lid would prevent the charcoal from washing away as quickly, and especially if they practically dipped the crucible in the water.
3 to 4 hours is also believable, but obviously not very specific because Marlin doesnt want to give away all of their secrets. For example, Jim on the Marlin forum runs his actions for at least 2 hours at a specific temp, then drops to a lower temp for an additional hour. The total time the action sits in the furnace could be at least 3 hours simply becuase it takes time for the furnace to reduce in heat and then stabilize for an additional hour.
As for grabbing the actions out of the water as soon as they are quenched, that you can do because they cool instantly when quenched. As for the difference in color in crucibles, could it be possible to determine whether or not the frames were cut or edited through modern technology? Keep it coming boys, the board needs more topics like this!
