I guess you are using an induction furnace. Graphite burns itself with
the oxygen when temperature is more than 400 centigrade degrees. It
lasts 50 - 100 hours if you protect the crucible with an inert gas
such as CO2, or argon. When more exposed to the air, it lasts less.
There are some crucibles that are externally protected with a ceramic
coat. You have to fill the crucible until the top, so the inner part
is full covered with the melted metal. Also you need a ceramic lid to
prevent the air can go into the crucible. Another trick is to put a
little piece of charcoal inside the crucible. It consumes the oxygen
while it burns. When a crucible is dissolved and it is full of holes,
you have to replace it with a new one. There is no way to fix it. I hope
this can help you a little. Write me if you have more questions.
Best regards from Daniel Mischelejis
Buenos Aires, Argentina
web page: http://www.mischelejis.com