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Casting Silver into Ingot Problem

Dear All

'ELP… Some common sense advise needed 'cause I seem to be making
a really silly mistake…

Anyway, Lately I’ve been having problems with my smelting. The
resulting sterling silver bar has craters all along the top. These
craters seem to form after the bar starts to cool in the ingot.

I know that the possible problems could be invected metal granules
or/and I’m smelting the metal to hot. What I would also like to know
is for how long, once the bar is in the ingot should 1 leave it,
before quenching it? Would that also be causing problems? Any other
ideas welcome too ;O)

taa eps Cape Technikon, Cape Town, RSA

Molten silver has the ability to dissolve or absorb a tremendous
amount of oxygen. The best sources say this about 20 to 22 times the
volume of the silver. I have seen a source that says very much more
but it is probably incorrect. When the silver solidifies they
oxygen this to come out. This will cause pitting porosity in the
metal most especially in the last metal to solidify. All the pits
will not be big enough to see unaided. The “cure” is to melt
under a good flux cover and in a reducing atmosphere. You must
maintain the flux cover during pouring until the metal is solid.
Melting in a carbon crucible in an electromelt type furnace will
help. Ideally the metal would be melted in an inert gas, high vacuum
and or in a controlled reducing atmosphere. This is not easy to do
on a small scale. Jesse