[Jewelry making - Article 00238] - Re: [Orchid] The Metallurgy of Keum-boo

[James Binnion]

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    The key to what he said is "metal is heated to temperature above .5
    of its melting point" this should be written for the rest of us as
    50% of the melting point. In Kuem-boo the metal is not quite that
    hot I think typically you only reach about 500  which is about 25% of
    fine golds melting point and about 28% of Fine silvers  but you have
    several things that contribute to the process that allow the bond to
    occur. The first is the lack of oxides both fine gold and very high
    carat gold are basically oxide free surfaces the oxides on the
    surface of other metals and alloys are what keep the surfaces from
    bonding. If you were to try Keum-boo with say copper instead of gold
    you would find you cant do it this is because of the oxide layer that
    is in/on the surface of the metal . Fine silver at low temperatures
    (in the range of temperatures that Keum-boo is done at) does not form
    stable oxides so there is no oxide present on the fine silver surface
    that is formed by the depletion of copper from the surface of the
    sterling by repeated heating and pickling. So you have two metal
    surfaces that are brought in contact with each other rather than two
    oxide layers. This allows the surfaces to even attempt to bond to
    each other. The second thing that aids the bonding process is
    pressure from burnishing.  This pressure adds energy (just like heat
    adds energy to the metals, by adding enough energy to  metal it can
    no longer remain in a solid form and melts!)  to the  the interface
    between the gold and silver and it provides more intimate contact
    between the layers which increases the number of points that can 
    bond  together.   So by combining the elevated temperature and the
    oxide free surfaces and the pressure and intimate contact you create
    a environment where the rest of his description of the crystal
    matrices occurs. 

    You can create diffusion bonds in metals at room temperature if the
    surfaces are flat enough and clean enough and they are left in
    contact for a long enough time. Gold plated onto copper will diffuse
    into the copper over time unless there is a barrier layer of nickel
    in between them. Diffusion is just speeded up with the application of
    heat and pressure. Jim

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