[Jewelry making - Article 00229] - Re: [Orchid] Brittle gold-about alloys

[Watchman]

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    One thing which seems to have been overlooked in this discussion is
    the crystallography of the metals. In any metal which has been cast
    at some time there is a crystalline structure which, depending upon
    the amount it has been worked since casting and the various component
    parts of it, will be more or less permeable to gases and liquids.
    Unless the alloy is at the 'eutectic', the point at which the amounts
    of the constituent parts exactly dissolve in one another, there will
    be a surplus of one or more metals which will crystallise out
    separately and simply form a 'mixture' in the 'alloy'. These crystals
    will be available to be attacked by anything which can get to them. 

    In the horological field we have a problem which is known as 'brass
    embrittlement' which is very similar to the problem you have
    described. This is caused by cleaning the brass parts of clocks and
    watches in Ammonia-based cleaning solutions and, as in this case,
    makes the grains of the metal simply fall apart so that the metal
    turns to dust. In this case, it is thought that the mecanism involved
    is that ammonia vapour which is released as the cleaner does its work
    seeps into the gaps between the grains of the crystal structure and
    becomes trapped there. When the part is then exposed to damp air over
    a period of years, the ammonia combines with hydroxyl radicals from
    the moisture to form Ammonium Hydroxide which crystallises and
    increases in bulk and, in so doing, forces the crystals apart
    physically until the whole matrix crumbles. 

    The amount that the brass has been 'worked' seems to have some
    effect on this process and, surprisingly, old brass which has been
    hammered after casting to compact and harden the metal, is often
    affected more than unworked castings. Presumably the hammering
    distorts the crystals so that the bonds between them at an atomic
    level are weakened so allowing microscopic cavities to develop. 

    So, thinking along these lines, could it be possible that exactly
    the same process is causing the embrittlement but with Chlorine ions
    diffusing into the crystal lattice and reacting with the Zinc to form
    Zinc Chloride (perhaps even Copper Chloride also)? 

Best wishes,
Ian
--

Ian W. Wright
Sheffield, UK



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