Aluminum oxide crucibles for Pt/Pd


Has anyone used the aluminum oxide crucibles for casting platinum or
palladium? Are they stable enough or do you need to use zirconium
oxide, magnesium oxide, or the special chromium treated crucibles?
Graphite doesn’t seem to work too well. Also worry about
contamination. If the other choices are stable enough for platinum
can they withstand short periods when melting iridium to make alloys
from anyone’s experience?