Molten silver can adsorb about 20 times its volume of oxygen. As
the metal cools this oxygen can no longer be held in solution and
it comes out as bubbles. On surfaces exposed to the air this can
show up as mini volcanos with metal spitting out as the metal
cools. In a mold the surface isn’t exposed but the Oxygen has to
come out as the molten metal cools therefore buibbles and
porosity. This problem is controled by:
1.Using a reducing atmosphere.
2.Maintaining a flux cover during melting and pouring
3.Using an inert Atmosphere such as Argon or Nitrogen during
melting and casting such as done in production machines.
4.Using a deoxidizer metal as a component of the alloy such as
Silicon. Someone mentioned recently that trace aluminum worked in
a Electromelt crucible that had been used to melt aluninum
peviously. There is some evidence that even .05% aluminum might
be enough deoxidizer. The proprietary Silver alloys uswe silicon
or whaterver fot this function.
5.Combinations of all the above.
With Sterling the copper will grab the oxygen as well but not
adsorb as much, but the effects show up as surface discoration or
firescale.
The electromelt furnace may work better for control because of
the Graphite crucible providing by combining with oxygen
peferential to the melt making a reducing atmosphere. The
mecanical furnace cover over the melt as well as a flux cover
also helps. My two cents today> Jesse