[Jewelry making - Article 00102] - Re: [Orchid] Verifying Gold Alloys

[Kevin]

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    Dietrich, I'm sure you are proceeding in the right direction,
    especially with being concerned that touchstone methods may cause
    damage (although only important I would think in the case of
    absolutely mint examples) and, more importantly, that they only test
    the surface. 

    Your proposed method gives you the overall density of the piece,
    subject to experimental factors that I'm sure you're aware of
    (accuracy and precision of the balance, temperature, method of
    suspension, no adhering bubbles .....).  The question then has to be
    .... _how happy are you to take density as an indication of metal
    fineness?_ 

    For gold items there shouldn't be too much problem, as it's not easy
    to find a base material sufficiently dense to serve as a fake.  The
    only one that springs to mind is tungsten.  This wouldn't be alloyed,
    but I suppose a disk of tungsten, which is becoming increasingly
    available today (OK, in the form of a tungsten composite together
    with some other metal, sometimes cobalt) could be given a coating of
    gold so as to have the right density for a fairly high carat item. 
    Unlikely maybe (maybe not).....  But bear in mind that for a given
    high density, although it indicates that the item is (probably)
    composed of noble metals, there will be a huge range of possible
    alloys that give that density, comprising gold, platinum, palladium,
    silver, copper, so you wouldn't be able to assign a gold carat value
    on the basis of density alone. Touchstone testing would help
    considerably. 

    Silver is much more problematic, on at least two counts.  First,
    lead has a density of about 11.34, and has a low melting point,
    making it pretty well ideal for fakes.  Secondly, a wide range of
    silver alloys have been used for medals (indeed for all silver
    items).  So, a perfectly genuine silver medal may be only say 70
    percent silver, and so have a lower density that Sterling. 
    Presumably reference books on medals would (might?) list the actual
    fineness used.  Remember also that some genuine medals are actually
    made of base metal with a silver or gold plating.  Again, you'd need
    to know this from reference works. 

    X-ray fluorescence wouldn't help either, unfortunately.  The metal
    plating industry (especially printed circuit manufacturers) has
    bench top instruments that will give the gold content of a small spot
    on an item, but unfortunately it's only looking at the top most layer
    of the surface, just a micron or two.  The only certain answer, as
    you know, would be to melt the item down and analyse a representative
    sample..... 

    I imagine that density determination would be just a part of the
    procedure needed to determine whether a medal is genuine or not.  A
    necessary part, but not sufficient.  I can imagine that it wuld also
    require a careful optical examination by a medals expert, who would
    consider things such as exact dimensions, crispness of detail and
    similarity (or not, hopefully) with known fakes.  For example, for a
    medal of sufficient value it could be worthwhile for a forger to use
    18ct gold or whatever the original had. 

    Thanks for bringing such an interesting and problematic topic to the
    forum, and I wish you good luck in your work. 

Kevin  (NW England, UK)

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