[Jewelry making - Article 00738] - Re: [Orchid] Lost lead casting

[jkbrennan]

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    A couple things that would probably counter indicate the use of lead
    for  lost  pattern mold making: 

    Lead has a very high thermal expansion  which would probably crack
    molds  during melt out. Wax can and does crack improperly designed
    and dewaxed modern shell molds. 

    Lead easily forms a lot of  oxide dross on heating that can
    contaminate the mold surface and cause detail  loss and pitting. 

    Incomplete  dewaxing of modern molds have surface degradation
    problems  from carbon residue. 

    Burn outs of natural objects  leaving carbon and ash residues also
    cause surface degradation. 

    Lead is the basis for fire assaying in which gold and silver in ores
    are dissolved in metalic lead reduced from lead oxide by heating. The
     lead is then boiled off by continued heating in air. It looks to me
    like pouring gold or silver into a dross contaminated mold would
    possibly  result in a reverse reaction contaminating the object
    surface metal. The surface would probably then degrade overtime and
    be fairly obvious. 

    Then there is Occam's razor: The simplest way is probably the way it
    was done. 

    Bees wax and rosin are ubiquitous  on most of the inhabited earth
    surface. They can be  compounded into modeling -carving waxes that
    produce very fine work today. The Japanese Mitsuro wax modeling is an
    example of work with these totally natural substances from some time
    in the past to the present. 

    It appears that lost wax -clay mold casting developed independently
    at many locations in the world and it worked any place there was wax,
    clay and the beginnings of metallurgical knowledge. 

    Investment casting as an industrial process developed  during and
    after WW2. By 1950 The low melting CERRO bismuth based low melting
    alloys and various solders  had been investigated  for  lost pattern
    molding and abandoned. At the same time ( late 1940's) lost frozen
    mercury was used  for a while as a real process when the current
    investment  casting  methods were being developed. It was  a patented
    process in actual use in  the early 50's at least. I don't know when
    it was discontinued. I remember concern over mercury vapor toxicity 
    as an  industrial hygiene issue  in 1962 and not before then. But? 

    Are you familiar with "Pirotechnia" by  Biringuccio, Vannoccio.
    (1540) ? This is one of the earliest books printed. The is first
    technical book and is a good compilation of the metal arts  until
    that time-- and not a bad reference even today. Cellini was
    rediscovering lost wax casting at the same time. 

    DE RE METALLICA. Agricola, Georgius. the second metals book was
    printed in 1565  and has a little different coverage 

    I think these two cover the state of the art then  pretty well  and
    lead as a modeling media isn't mentioned. Cellini was rediscovering
    lost wax casting at the same time.  It now seems that the Dark ages
    were not as dark as we were taught. 

    While these books in print are not old as knowledge may go. They
    probably are a fair practical representation of realistic, successful
     metallurgical practices until then. There may have been something
    different in China . But?? 

    People seem to have gotten around a lot more than we were taught and
    there was knowledge transfer. 

    I believe any unmolded lead patterns would have not  survived ambient
    exposures. There is no way to tell  if a process was tried and
    abandoned or just lost. To test a premise you don't have to make
    special  models just start with some commercial wax patterns. 

    Today non destructive SEM based surface analysis techniques can
    analyze for this type contamination even below the surface. 

    In spite of the ease of doing analysis with the only practical
    constraint being cost, I doubt if there would  be any one willing to
    expose their objects to analysis 

    If you have the credentials --Maybe. 

jesse

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