[Jewelry making - Article 00764] - Re: [Orchid] Black Blobs in my Sterling

[Peter W . Rowe]

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>        Then I saw these blobs grow out of the metal. Crazy! Black
>     secretions from inside the metal. 

Very strange indeed, Brian.

    I'd guess that it's flux (some casting fluxes are black), or a
    combination of flux and graphite from crucible breakdown, or some
    such goop, and the casting process was such that the stuff got mixed
    with and carried along with the metal during casting, forming flux
    inclusions within the metal.  I'd also guess that, as with many
    fluxes, it's at least somewhat water soluable, so that with the first
    pickling or two, moisture was able to penetrate the metal's porosity
    (all castings, after all, have some degree of porosity, and your's
    with those inclusions, might have more) and be absorbed by the
    included flux, so then the next heating created enough steam pressure
    within the metal to force heat softened flux out of the castings? 
    Just a guess.  Perhaps it's not directly too much flux added during
    casting, but perhaps use of a melting crucible that was deteriorating
    or already had too much flux in it, or some such. 

    I'm recalling occasional castings I've seen that were porous but
    with only small surface pores but apparently significant internal
    cavities/shrinkage voids, etc, that after cleaning off polishing
    compound in an ultrasonic, or other such soaks in liquid, subsequent
    heating would cause liquid to bubble up out of seemingly solid metal.
     Very freaky surprise indeed.  In the case I'm thinking of, the
    situation only became apparent after totally finishing the casting
    and polishing it, , as the initial cast surface seemed solid, and the
    castings looked and acted just fine.  Finishing off the casting took
    off a surface skin, exposing slightly increased porosity, but even
    then it didn't look bad.  I later cut cross sections of it, and it
    turned out most of the center areas of the heaviest parts of that
    ring shank were pretty much filled with shrinkage cracks and voids. 
    Enough to have absorbed a significant amount of liquid... 

Peter

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