Okay, here’s the thing. We are having an interesting problem at work
that we can not agree on, which has brought about an interesting
solution that we also can not agree on. I feel confident that some
wise Orchidians can explain the science behind what is happening.
The problem: We have a cast charm that is eventually attatched to a
chain bracelet. Sometimes the castings come through nearly perfect.
Sometimes they come out of casting with a terrible amount of
firescale (cloudy purplish haze in parts of the casting, or purplish
freckles everywhere). The firescale is definitely happening in
casting as the piece is never torch soldered. To make things worse,
this is never a surface firescale; it runs through the body of the
piece. Our customer requires the highest standards, and we must
reject all pieces with visible flaws, so this problem is a giant
pain in the neck.
The solution: Before the castings are polished and prepared, we send
them through our soldering / annealing oven. This oven has an
internal atmosphere that is oxygen free and runs approx 1505
farenheit. I think the heat of the oven is doing something chemically
to the cupric oxide (firescale) like removing oxygen(?). The
castings come out of the oven with a popcorn-like surface of fine
silver (?) where stuff has pushed out of the surface of the casting.
Once the pieces are polished, not all of them are perfect, but they
are much much much better, and a higher percentage of them are just
fine.
So what exactly is happening in the oven? What science is behind this
solution? I want to fully understand why this is working. None of us
here can agree.
Furthermore, how on earth does one produce a casting that is full of
firescale, when the casting is done in an oxygen-free chamber? One of
our team members has been experimenting with the quench times of the
flasks, but I dont think that can do anything to improve the
firescale. Im sure it can affect the hardness of the metal, but the
damage of the firescale is already done well before the
quench…right?
The soldering furnace uses disassociated ammonia or some other form
of nitrogen hydrogen mix for an atmosphere. The hydrogen molecule is
very small in relation to the size of the metal atoms and can
easily pass into the metals crystal matrix. During this diffusion
of hydrogen into the matrix if a hydrogen molecule (H2) comes in
contact with an oxygen atom you will end up with water (H2O) or
more precisely water vapor (steam). If there is a lot of oxygen
present the steam will expand rapidly and disturb the metal matrix,
this is what is creating the rough surface. If you leave the items
in the heated hydrogen atmosphere long enough then it will reduce
most of the surface and near surface oxides and you will be left
with a roughened surface that is free of fire satin.
As for why you are getting fire stain in the first place there are
at least two areas to look at. The first is the oxygen content of
the sterling casting grain. Silver loves oxygen and when molten can
hold up to 20 times its volume of oxygen in solution. When it cools
it releases the oxygen out of solution and does not form silver
oxides. However if there is copper present as in typical sterling
alloy the copper will bind the oxygen and you end up with cuprous
and cupric oxides and hence the dreaded fire stain. So if it is in
there in the first place you will not get rid of it in the melting
process. So you want to know how much oxygen is present in your
sterling shot. Larger manufacturers of sterling casting grain are
testing their shot for residual oxygen and can tell you how much is
there. I know of some casters that specify a maximum amount of
oxygen when they buy their shot. Second silver will still absorb
large amounts of oxygen even after it has started to solidify. If
you pull your flasks out of your “oxygen free chamber” before it
has cooled down substantially then it can still pull oxygen in
through the sprue button and even through the investment itself. I
am assuming you have a totally enclosed casting machine like the
NuTech. Try this, leave the flask in the machine for 5-10 minutes
after casting to allow the metal to cool down significantly. Then
break it out and look for fire stain. If you still have it you may
1 have a leak in your casting machine that is allowing air to get
in the chamber or 2 you are casting metal that is already
contaminated with oxides. If you reuse your metal as most of us do
and it has a large amount of oxygen in it you will continue to have
problems. It is possible to de-oxidize sterling with additions of
phosphorous but it requires a lab oxygen analysis of the batch to
be melted to know how much to use and if you use too much you will
end up with very brittle metal and if you don’t use enough you will
still have fire stain problems. Sterling is a tough metal to keep at
a high standard of fire stain free quality. This is why so many are
interested in the fire stain free alloys.