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Casting a dome ring


#1

I recently tried cast fine silver for the first time and got less
than perfect results. I cast a dome ring centrifugally using a "v"
sprue at the shank. I hollowed out the dome of the ring so it had an
even thickness throughout. What happened was some strange looking
porosity in the ring. It showed up on the surface of the inside of
the ring. I could see it right away even before I began to remove the
casting skin. The surface of the affected area was shiny and rough.
When I began to clean up the ring, I discovered that the porosity
went very deep. I was wondering if anyone could help me figure out
the source of this problem. I would really like to be able to cast
fine silver with out porosity. Thanks, Serina Omori


#2

Was the casting material virgin? The alloys in silver can be tricky.
Make sure your casting material is as clean as possible and make sure
you sprue it to the heaviest portion of the piece.


#3

Serina,

How did you melt your silver. With a flame and flux, in a melting
furnace, dile, digital, do you know the exact temperature of the
metal you poured? Was it a centrifugal cast, or vaccumm? How long
from
molten to injected into the mold? how hot was your flask?

It sounds like a pour temperature issue to me, but all of the above
variables are things that can affect the pour temperature.

Alex


#4

Pure metals are very unforgiving with regard to sprue and gate size.
Because the pure metal does not have a solidification range but an
exact solidification temperature it is very easy to freeze off a
sprue and starve the item for molten metal while it is cooling and
shrinking. It is critical to make sure the sprue or gate has a
thickness that is greater than the section of the item being cast
where it attaches and that the item is attached to the sprue(s) in
such a way as to have the heaviest sections being directly fed by the
sprue. If you ignore this you will end up with significant shrink
porosity when casting pure metals.

James Binnion
James Binnion Metal Arts


#5
Pure metals are very unforgiving with regard to sprue and gate
size. 

And with pure silver, you have an additional problem, since molten
pure silver has a very great ability to absorb oxygen into solution,
which it then cannot continue to hold as it solidifies. So as it
solidifies, the dissolved oxygen comes back out of solution, often
creating pits, defects, and really rough surfaces at that point. On
an ingot, it usually is confined to the poured end of the ingot, but
in a casting, it could happen almost anywhere if the metal was able
to absorb significant oxygen during the melting phase. You must make
sure your flame is reducing enough, and fully covers/protects the
melting metal when torch melting, so that the metal doesn’t dissolve
and hold oxygen. Be sure to use a proper melting flux too, to
provide additional protective cover to the melt. Be sure, too, that
your flask temperature is hot enough to allow the mold to fully fill
properly before the metal solidifies. You’ll need a hotter flask
than for sterling silver…

Peter Rowe