[Jewelry making - Article 01051] - Re: [Orchid] Black Blobs in my Sterling

[Peter W . Rowe]

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>        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

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