Does the buildup of this red material affect the quality of the
mixture of gold or silver? Am I possibly using too much boric acid
to flux the metal? Also, is it best to always use separate
crucibles for casting different kinds of metals?
No, No, and yes. The red is copper oxide that's been dissolved by the
boric acid flux, and remains in the boric acid glaze left on the
crucible, It's presence means it was not in the metal you melted,
where the boric dissolved it. You CAN use too much, when it ends up
flowing with the metal and causing defects in the ingot or casting.
Other than that, no. And if you use seperate crucibles for each
metal type, then the little tiny grains of metal that may remain in a
crucible won't alter the alloy in the next, different, color you
melt. If you're careful to see that none of those little grains
remain, then you can mix uses of the crucible, but it sure is safer
not to do so. at the least, reserve seperate crucibles for white
golds, vs the yellow golds or other silver/copper/gold mixes. Traces
of white gold are not good things to add to yellow golds...