Hi Glenn, The discolouration you describe is completely normal. Some
of the alloys degrade, oxidize, and leave deposits behind in a
mixture of flux (your boric acid) and burned dust. Because it pours
out so slowly, this goop remains in the crucible after you pour the
metal. If you get too much accumulation, it will be difficult to
pour past it, and you should break some of it out. This can be
acomplished fairly easily while it is cold, using a broken file and
a small grinding wheel to smooth the surface. You will have to clean
the dust away and re-flux after. You might also try super-heating
it until it becomes molten enough to drip out, if the accumulation
is not severe.
As for the question regarding diferent crucibles for different
metals, yes, I would reccomend this method. Sometimes, small amounts
of metal will remain in the crucible, trapped by the goop, and cause
contamination of a different metal which may not become apparent
until you are well into a project. Mark each crucible by drawing on
it with rouge, using it like a crazyon.
David Keeling
www.davidkeelingjewellery.com