Rob, crucibles are cheap! Treat yourself to some new ones. It’s just
not worth the time and effort to try to save one. You will be amazed
how much better your ingots and castings will come out when using a
fresh crucible after using that burnt up one you’ve been using for
so long. I replace mine regularly. As soon as I start seeing flakes
of zinc forming, it starts getting lumpy or the color starts
darkening, it’s done. Maybe twenty or twenty five melts?
I break in a new crucible by heating it to what seems like about
casting temperature and give it a dusting of casting flux (a 50 - 50
mixture of borax and boric acid in a salt shaker) and keep heating
it, applying another dusting or two so that it is evenly coated. You
want a thin but shiny coat, right up to the pouring trough (spout?),
but no more. If it pools at all, you’ve used way too much flux. Once
it’s fluxed, it’s ready to use immediately.
Another tip, use a separate crucible for each metal alloy. In other
words, have one exclusively for 14K yellow, another for 18K yellow,
and another for silver, etc. At a minimum, one for each color and for
each type of metal - one for yellow gold, one for white, one each for
rose and green and one for silver. Using one crucible for everything
can cause all sorts of grief. I keep an older one around too for
melting contaminated or garbage metals, like chains and sweeps, etc.
These tips apply to both dish style and centrifugal crucibles.