[Jewelry making - Article 00053] - Re: [Orchid] Quenching to Harden Ear Posts

[James Binnion]

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    Sterling silver is at its softest when it is annealed at 650C (1202F)
    then immediately quenched, at this point it's hardness is 56 Vickers
    and its tensile strength is 300 Nmm -2. If it is air cooled it will
    be only slightly harder at about 60 Vickers due to the difference in
    crystal structure. Quenched sterling is closer to a uniform solid
    solution and air cooled will have a less homogenous structure and
    will as a consequence be slightly weaker. However if you look at the
    data for work hardened sterling its hardness can be as high as
    140-180 Vickers and its tensile strength is up to 550 N mm -2 so the
    effort in cold working the ear post will pay off with a greatly
    improved hardness and tensile strength (as long as you don't over do
    it and twist the darn thing off). 

    To quote from Handy and Harman's : 
    The Handy Book of Precious Metals 
    http://www.handyharmancanada.com/hbpm/hbpm.htm 

          The alloy [Sterling] is entirely liquid at 1640F (890C.) -and
          entirely solid at  1435F (780C.) However, the degree of copper
          solubility in the solid  alloy depends on the heat treatment
          used, and the overall physical  properties of the sterling can
          be materially affected, not only by heating  the silver to
          different temperatures, but also by employing different
          cooling rates." 

    Also

          Specific annealing procedures for sterling alloys should be 
          available from the supplier, and these recommendations should
          be followed scrupulously, since even small differences in
          temperatures and/or cooling  rates could significantly affect
          the physical characteristics of the alloy.

          Some people confuse air cooling with the age hardening process
          where sterling silver can be greatly hardened by heat treating.
          to do this you first heat the sterling to 750C (1382F) and hold
          it at this temp for 30 minutes then rapidly quench  the item is
          then heated to 300C (572F) for 1 hour. this will result in a
          sterling piece that has a hardness of 110-120 Vickers and a
          tensile strength of 350 Nmm -2. The heat treatment can only be
          done on items that are not soldered as 750C will melt all
          silver solders if the item has been soldered prior to treatment
          and soldering after treatment will anneal the item and remove
          all the hardness gained. 

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
http://www.mokume-gane.com
jbin AT mokume-gane.com
Member of the Better Business Bureau

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