I worry about my crucible, which has been turning red
/ deep brown inside. Till now, I have been casting Sterling. I do
not know the origin of the red substance. Could someone please
explain what it is and whether it is supposed to be there or not?
G’day. The red-brown colour is a copper/oxy/boron compound which
is derived from the copper in sterling and low carat gold alloys.
There are two oxides of copper; one is red-brown cuprous oxide and
the other is black cupric oxide, and that is the one which produces
the black, dreaded firestain on sterling. These oxides form when
sterling or just plain copper is strongly heated in the presence of
air, and the purpose of borax type fluxes is to dissolve these
oxides, thus keeping clean the surface of the metals be soldered or
otherwise worked when heated. The presence of this coloured borax
related compound is quite normal, but if the build up of flux in the
crucible becomes too great the excess may be removed by heating the
crucible very strongly indeed and allowing the flux to slowly run
out - it has the consistency of molasses on a cold day.
Would it (still) be safe to use the same crucible for gold?;
The answer is, “probably” , but I think that one should keep a
crucible for gold and another for silver to be sure; they aren’t all
Working with a Little Torch, I use a tip with six orifices
(for propane/oxygen) This is certainly a big improvement, but it
remains problematic to melt 45 - 50 gr. of Sterling. It takes a
long time before the metal begins to melt
It occurs to me that you may be using the form of crucible which is
the tall, hollow form. These are really better for use in a kiln, but
for torch heating small amounts up to about 100 grams of metal, the
type of crucible to use is a flat slab of ceramic which has a shallow
hollow to contain the metal. Thus the flame may be played directly
on to the metal, and a sprinkling of powdered borax on the pool of
liquid metal will instantly remove any scum of oxides which may have
formed, and allow it to run freely like mercury. This heating
method is far faster and will result in a lower formation of oxides
because you aren’t heating it for so long.
and, also, I have difficulties to obtain a reducing flame -
whatever I do, the flame doesn't seem to be a really reducing one.
If you could cut off a very small amount of air entering the torch,
using say, copper or aluminium foil, you can control the
oxidation/reduction properties of the flame, or in the case of an
oxy/fuel gas torch, simply cut the amount of oxygen fed in.
I have been working in the garage and it was really cold there.
Could that have something to do with it?
No; the ambient temperature of the working area has little to do
with melting temperature. – Cheers for now, and may 2003 be extra
to you John Burgess; @John_Burgess2 of Mapua, Nelson NZ