From time to time I cast at a friend’s place where I melt my silver
using gas/oxygene from a torch. I have a hard time controlling the
melting temperature and as a result have a yellowish silica coating
on the pieces.
I would like to learn to better control the temperature with the
torch and maybe someone has a trick or two to share.
Also, is there anything, other than sandblasting or polishing, to
take off the silica coating? So far, my trials using acids and
thermal shocks proved useless.
If you’re getting a silica coating on your pieces, you’re using too
much flux. Generally, using a properly prepared crucible (i.e., the
crucible has been conditioned with borax flux so that the "raw"
ceramic is not coming into touch with the metal), I add a “pinch” of
borax first, then add my metal, then add another pinch of borax on
top of the cold metal. (I keep the borax in a metal sugar-shaker, so
a pinch is a shake or two from that container.) During the melt, I
don’t add more borax unless the metal is really showing up as gunky
on the surface. Once it’s reached casting temperature, I give one
more shake of borax onto the surface, heat for about another 3
seconds, then cast. The only place that there is EVER silica is
sometimes on the very bottom of the button - never enough to coat the
button and certainly not enough to impede reuse of the button metal
in a subsequent cast (the borax will melt first, so just place that
side of the button “down” in the crucible).
My pieces come out nice and clean, and while they always need
pickling and ultrasonic to remove the investment residue, I’ve not
had any problems with firescale or poor castings (yes, I realize I
probably just jinxed myself).
hope this helps!
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