[Jewelry making - Article 00022] - Re: [Orchid] Little Torch: switching over?

[Peter W . Rowe]

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http://www.ganoksin.com/orchid/archive/200409/msg01085.htm

>        The advantage is, according to my reading, that platinum is not
>     in danger of embrittlement. Acetylene is carbon-rich, and
>     apparently platinum soaks it up to its detriment. 

    so far as I know, platinum does not actually soak up carbon. The
    problem is that carbon, at platinum soldering or melting
    temepratures, is an exceptionally effective reducing agent.  And in
    our workshops, another material thats very commonly around are
    various forms of silicates. That includes the silicon dioxide (fused
    quartz) we might use for soldering blocks for platinum, or various
    silicates in fluxes (borosilicates), or any of a number of other
    possible sources, including even just common house dust. Carbon, at
    platinum soldering temps, is able to reduce silcates to silicon
    metal. And silicon metal DOES alloy with platinum. Even small amounts
    quickly cause brittleness and cracking. This is the reason why some
    people report being able to routinely do platinum work with
    acetyelene, withough having problems, despite the carbon rich flame.
    First, they're generally being careful to use a fully oxidizing
    flame, which gets rid of the free carbon. And, usually they're
    working in a manner that eliminates the contact of platinum
    soldering temps with both platinum and silicates, such as holding the
    work in tweezers, instead of on a block, and not using any flux.
    You'll find, if you try actually melting platinum in a fused silica
    crucible, with acetylene, a situation that really is tempting the
    fates, the incidence of contaminating the platinum will go WAY up.
    Even that, may be possible if the flame is quite oxidizing. 

    The same mechanism is part of the reason why it can be quite useful
    to get in the habit of cleaning your platinum, via steam, or
    ultrasonic, etc, before soldering or melting.  It removes things like
    fingerprints, oil residues, or other surface contaminants that might,
    upon heating, carbonize and give you a source of both silicates and
    carbon on the platinum surface. 

Peter

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