I have found the royal yellow and 18k nickel white alloys to be most
susceptible to this problem. Some things that may be contributing,
or that you may try: A different brand or manufacturer of crucible,
if available for your furnace. Store the crucible in a dry
environment. Minimize the time the crucible spends hot, especially
above 1800 F, it seems breakdown occurs more rapidly above this
temperature. I used to change my crucible about every six casts but
I've found that I can extend their life by fluxing and boraxing only
the portion of the crucible that extends above the top of my
Electromelt. Avoid application to the contacting surface between the
lip of the crucible and the top of the furnace. (So it doesn't "glue"
itself down.) For the same reason, one must use care not to over
apply as the borax glass may run down the side of the crucible and
get on the bottom surface. I've avoided fluxing any of the inside
surfaces, so the graphite can do it's job of eliminating the oxygen
from the melt environment.
Hope this helps!
david lee jeweler