If you're talking about the white ceramic ones, and not the black
ones used in electric melt pots, the excess flux should be removed.
Just heat it with your torch until it becomes a glassy liquid and
pour it off into some very dry sand. Too much borax causes rough
castings. If you use a graphite or quartz rod and draw through the
molten metal before pouring it into your mold, not only does it mix
the metal consistently, say as in an alloy, but it also removes the
excess borax and the entrapped junk. You will have to reheat your
metal again to the liquidus state, as the rod absorbs heat.
Occasionally you have to get rid of the junk on your rod by gently
tapping it, ever so gently, with a hammer against a steel block.
I read somewhere that one could coat the cruicible by filling it
1/3 full of borax, and putting it in the kiln to heat, rotating it
around so that the molten borax completely covers the sides of the
crucible. If this is the proper proceedure, what temperature should
I heat my kiln.?.
OK, I'm a low techie kind of person. I mix boric acid, which is
available at a pharmacy, and a little finer grade of borax, with
alcohol. You want the boric acid to just be at the supersaturated
stage in the alcohol, where it won't pick up any more boric and some
of it is still in the bottom of the mixing vessel. Then paint it onto
the walls of your crucible, as well as top and bottom, and any pour
gates and fire holes. Then fire it with your torch. Do it several
times until you've built up a nice glaze.
Just the way I do it, which may, or may not be "correct".