Darryl, The reason that you keep the gas flowing is to prevent
the carbon crucible from oxidizing and prematurely wearing out.
What I do is turn the gas down to 2 liters/min and leave it on
for 20-30 min. Most of the glow is gone from the crucible at that
point so I think most (not all) of the oxidation that would have
taken place has been prevented. Seems to work, as my crucible
life seems to be pretty good.
Freedom Design and Contracting