[Jewelry making - Article 00163] - [Orchid] The Metallurgy of Keum-boo

[Jayne Redman]

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Hello all,

    I was privileged to have metallurgist Andrew Nyce in the keum-boo
    workshop I taught at Wolf Designs in Portland,
    Maine,<www.katewolfdesigns.com>. He very kindly wrote up this
    explanation of diffusion bonding for me today and gave me permission
    to pass it on to you. 

    Regards, Jayne Redman 

    THE METALLURGY OF KEUM-BOO 

    Keum-Boo in metallurgical terms is pressure-assisted diffusion
    bonding with the burnisher used to transmit the pressure. What is
    meant by diffusion? First we need to construct a three dimensional
    lattice of atoms (Au, Ag or alloys of Au or Ag). First draw a three
    dimensional cube and place circles (to represent atoms) on each
    corner and each face. You now have a face centered cubic lattice
    structure. The atoms of Gold and silver and their alloys arrange
    themselves in a face centered cubic array. To understand diffusion
    we must accept that as a metal is heated to temperature above .5 of
    its melting point, vacant lattice sites are created. As a result,
    the atoms are free to move randomly throughout the three dimensional
    lattice. If we were to move a silver lattice against a gold lattice
    and heat both above the temperature where vacancies are created, the
    gold and silver atoms would interdiffuse and bond the lattices
    together. In Keum-Boo of 18, 22 or 24 karat onto silver or
    conversely fine silver or sterling silver to gold or gold alloys,
    the atoms interdiffuse creating a metallurgical bond which is very
    strong. 

    If 24 karat sheet (.01to.05 mm) is simply laid flat against a silver
    sheet, only the high points would be diffusion bonded. By using a
    burnisher, the entire surface can be diffusion bonded if the
    burnishing action covers the entire surface. Trapped air or organic
    matter trapped during diffusion bonding will result in bubbles
    forming if the bonded surfaces are heated to a temperature higher
    than the diffusion bonding temperature. Oxygen can diffuse through
    silver but not readily through gold. Nitrogen will not diffuse
    through.  

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