Back to Ganoksin | FAQ | Contact

Adding boric acid to molten metal


#1

While melting gold or silver in a crucible, I usually take a small
pinch of boric acid and add it to the metal right before casting.
This is to reduce the amount of oxidation that occurs while melting
the metal. I am noticing that the crucible now has a reddish color
to the bottom, and it seems to be building up. Does the buildup of
this red material affect the quality of the mixture of gold or
silver? Am I possibly using too much boric acid to flux the metal?
Also, is it best to always use separate crucibles for casting
different kinds of metals? Thanks.

Glenn Block
Shardan Jewelry
Allentown, PA 18104
610-433-0833


#2

Hello Glenn: It is advisable to use seperate crucibles not only for
silver and gold but also for different colors of gold and some would
say for different karats. The problem is if small amounts of the
metal you cast before melt into what you are casting now you could
have a purity problem or a slight color change. Add some silver to a
yellow gold melt and it gets greener. I suggest when starting with a
new crucible too heat it and add pinches of boric acid followed by
the torch again until the entire inside and top edge are covered with
the melted glase of boric. The boric will discolor over time and use.

When adding Boric acid to your melt you should heat the end of your
stir stick and dip it into the boric powder. The boric will stick to
the stick and then you can add it to the metal without removing the
torch. This will reduce the amount of oxygen that the metal absorbs
since your flame is always covering your melt.

I have a fairly thick layer of boric in my crucibles and every now
and then I hold them upside down and heat it with my torch until some
flows out.

Michael R. Mathews Sr. Victoria,Texas


#3
Does the buildup of this red material affect the quality of the
mixture of gold or silver?  Am I possibly using too much boric acid
to flux the metal? Also, is it best to always use separate
crucibles for casting different kinds of metals? 

No, No, and yes. The red is copper oxide that’s been dissolved by the
boric acid flux, and remains in the boric acid glaze left on the
crucible, It’s presence means it was not in the metal you melted,
where the boric dissolved it. You CAN use too much, when it ends up
flowing with the metal and causing defects in the ingot or casting.
Other than that, no. And if you use seperate crucibles for each
metal type, then the little tiny grains of metal that may remain in a
crucible won’t alter the alloy in the next, different, color you
melt. If you’re careful to see that none of those little grains
remain, then you can mix uses of the crucible, but it sure is safer
not to do so. at the least, reserve seperate crucibles for white
golds, vs the yellow golds or other silver/copper/gold mixes. Traces
of white gold are not good things to add to yellow golds…

Peter