if you are using 20 mule team borax, often the fillers in ti will
turn a fused clay crucible black. Not a problem though if you ensure
it is glazed thoroughly and that it is stored in a moisture free
environment and that it’s dedicated to one metal - not using it for
many metals (i even have mine dedicated by karat).
Another consideration is that sometimes depending on the heat source
and O2 consumption the oxides are “oxidizing” s you are melting…
still not a problem as long as it’s a well glazed interior and the
pour hole (if any) is also glazed (just to make it easier to pour
quickly through the smallish hole on burno style crucibles even when
held in tongs (as opposed to in the machine they are inteded for).
hope this helps.