It's solved ... it was my fault. I talked to the jewellery
casting company (a major precious metals supplier and casting
house) and a man there analysed the blob material. I might have
mis-heard but I thought he said it was a high proportion of copper
(21.5%) in the form of cupric oxide (CuO). Possibly the result of
uncontrolled heating during annealing.
Brian, I’m not sure I believe this, if you were using a standard
sterling silver, ie 7.5 percent copper and the balance silver. I’ve
many times heated silver to a nice orangy glow, sometimes for
extended times, playing with, among other things, the grain growth one
gets with overheating during annealing, as well as various variations
on depletion gilding, and in a few cases, intentionally trying to
damage the metal in various ways to just see what happens. If simply
overheating sterling silver, even for extended times, could create
islands of cupric oxide that would ooze out, we’d all have seen it.
While it’s likely that your unintended extended heating brought out
the problem which you’d not seen in other batches not treated that
way, I suggest that the extended heating was not the actual cause,
merely the condition that made the problem evident.
While extended heating might oxidize the copper, certainly, the
normal mechanism is for the copper to migrate towards the surface as
it oxidizes, forming our familiar nemesis, fire stain and it’s black
surface cousin, fire scale. Neither of these have ever in my memory
even remotely resembled the odd blobs you experienced. While your
refiner may well have found that the gunk was high in cupric oxide,
something that might make sense given the basic copper content of the
metal, the form it was in suggests something much more fundamentally
wrong than just overheating or overannealing, since that does not
produce islands of oozing goo in small spots. it produces a fairly
uniform series of surface and subsurface oxide layers, none of which
seem prone to ooze out.
I remain convinced that the problem is related to the casting
process, such as porosity problems, inclusions from the crucible,
inclusions of flux, or some other similarly related problem, and not
to what you did after the pieces were cast. You did not heat them
enough to melt them, after all. A pinkish cast after extended
annealing isn’t a surprise, just a confirmation of my statement to
expect fire stain and fire scale, which could easily do exactly
that. The ooze you saw is something different.
Just because the guy works for a major supplier, and knows metals,
does not automatically mean he’s got his guess as to the cause
correct. If what he’s done is determine that it’s high in copper
oxide, well that’s good info, but not yet an explanation. And keep in
mind that there is a clear conflict of interest. He doesn’t really
wish to admit that it’s the fault of his company, after all, so
deciding this was your fault is clearly in his favor. Try consulting
an independent matalurgist, and don’t be so quick to assume the
caster is right and you’re at fault. He may well not know what caused
this, in spite of his facilities and knowledge. I’ve known several
large respectable companies who’s knowledge of their own product’s
engineering and performace was surprisingly lacking, such as the long
time manufacturer of a platinum investment who’d done many
engineering tests of the material’s set up time response to burnout,
etc, all nice and scientific, but who’d never actually cast platinum
in it, and were thus unaware of a metal to mold reaction that
produced rougher than necessary surfaces.
Peter Rowe