[Jewelry making - Article 00159] - Re: [Orchid] Drag lines on silver

[Peter W . Rowe]

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>        With this system of creating sheet, they seem to get what I
>     call, though it is probably not accurate "twisted crystals".  They
>     are not so much pits as knotted areas that polish out at a
>     different rate than the area around them. 

    Metals we use form crystals in the cubic system.  Fully symmetrical
    in all directions, and capable of distortion in any direction with
    working/rolling, and during annealing, such distorted crystals relax
    and reform smaller, grains with similar universal symmetry.  Twisting
    metal, or it's crystals, does nothing different from any other form
    of working it. 

    Continuous casting methods of sheet production produce a continuous
    strip shaped ingot, unlike traditional methods of casting brick sized
    ingots.  The traditional method usually employs a step where they
    mill off the exposed surfaces of the brick, to remove surface
    irregularities.  The result is then rolled into sheet, and given the
    larger size of the "brick", it goes through a lot more reduction.  so
    traditionally rolled sheet may end up with slightly more uniform, and
    smaller, grain size.  Continuous cast sheet, on the other hand,
    doesn't  need the surface milled off, since the machines give
    uniform smooth ingot surfaces as it casts them, and the process
    reduces the tendency for components to segregate out from the center
    to the surface, as can happen in the large brick sized ingots.  So in
    theory, continuous casting should also give a very uniform even raw
    metal to feed to the rolls, and uniformity of the alloy composition
    will be superior.   But, if in the process, inclusions form, such as
    oxides mixed into the melt, or carbides (some metals  can combine
    with carbon, such as from graphite melting crucibles, to form
    carbides, which show up as hard knots in the metal), then these can
    end up on the surface of the sheet metal, behaving just as you
    describe.  The hard bits you found are probably not just distorted
    crystals of good metal, but rather inclusions of a different
    material, either oxides, bits of foreign material entirely, or
    carbide inclusions, or something of that sort.  The highly automated
    nature of sheet production from continuous casting means, among other
    things, that there is not so much human observation of the sheet as
    it's rolled to it's final form, so defects, though rare, are more
    easily not noticed by the processors. 

Peter


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