[Jewelry making - Article 00291] - Re: [Orchid] Sterling Silver and Nickel

[James Binnion]

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>     My curiosity has been engaged by Mr Binnion's comment on the
>     solubility of nickel in a sterling alloy. To provide more "spring"
>     in a clasp or catch, it is not uncommon to alloy the sterling by
>     using coin metal. New Zealand "silver" coins were an alloy of 75%
>     copper/ 25% nickel. The resulting sterling alloy therefore runs out
>     as 925/1000 fine silver, 56.25/1000 copper and 18.75/1000 nickel.
>     Not being a metallurgist I am now curious to know why the melt
>     appears to absorb the nickel when it is presented in an already
>     partially alloyed form via the "silver" coin as distinct from the
>     apparent difficulties when adding pure nickel. 

    The 1.8% nickel alloy you are talking about is possible because of
    the copper content but even so the resulting metal will be composed
    mostly of silver/copper phases and some copper/nickel phase but the
    silver and nickel will not form a solution or alloy with each other
    of more than a fraction of a percent. You could look at it with a
    microscope and if etched properly see separate distinct areas of
    these phases. The question is how well the copper nickel is
    distributed in the majority silver/copper material. If the nickel
    content is too high then it will fall out of solution and end up as
    separate nickel rich areas. This material will be not very ductile
    and may tend to crack when even moderately worked due to its less
    than homogenous nature. It can also lead to hard spots in the alloy
    that cause defects in the polishing. It will be harder which is why
    it makes a good spring. It is not so much that you can't add nickel
    in this way to a silver casting but it will be like mixing sand into
    water rather than sugar into water in that the nickel will not
    dissolve in the silver. 

    It is not too unusual to add insoluble material to some alloys to
    make them harder or make the grains smaller ( like iridium in some
    gold alloys) but you have to be careful how much as it can lead to
    cracking problems if there is too much in the mix. Generally it is
    in the less than one percent range. 

    So for your spring alloy it can be a useful addition but as a
    standard casting or wrought alloy it will have too many undesirable
    features to be used. 

Jim

James Binnion
jbin AT mokume-gane.com
James Binnion Metal Arts
http://www.mokume-gane.com
360-756-6550


 
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