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Overheating silver


One of our older students is complaining that her silver is "ruined"
and not reusable because, during casting, flask was too hot and also
that the torch was set too high for the melt. Her casting apparently
did have a lot of fractures in it, so that may have indeed been
true. However, it is my understanding that silver is not “ruined” in
this case, but can be reused as long as you mix it half and half with
new casting grain. Can some of our metallurgists clear this up?
Thanks so much. Emie Stewart


The overheating by itself is not an issue as you cannot really ruin
sterling by overheating if you got really really carried away you
could possibly vaporize some of the silver but you would need to get
to 3924F which is probably not really likely, that is quite a bit
hotter than the melting point of platinum and actually quite a bit
hotter than the melting point of most crucible materials. The
cracking of the silver was caused by a phenomenon called hot
cracking. It is caused by the metal staying hot for too long and as
it shrinks in cooling it tears because it remains soft over an
extended period of time and strains against the mold. The big issue
in this case is the investment breaking down from overheating and
producing sulfur dioxide gas which is then absorbed by the sterling.
This will end up causing issues with gas porosity that cannot be
eliminated by anything short of full refining. This can be somewhat
mitigated by diluting the amount of sulfur dioxide by adding some
fresh sterling. So I would say to reuse it with the addition of 50%
new metal was you proposed.

James Binnion
James Binnion Metal Arts


Yes you can reuse it. Sounds like it was quenched in cold water when
they mold was too hot. Had this happen once, castings were cracked.
Quench only from black heat and then only by dipping it in and out of
the water. Hope that helps. Regards Hamish