[Jewelry making - Article 00696] - Re: [Orchid] Using MAPP gas for silver work?

[David J McIntyre]

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Good Morning All,

    The extensive discussions over soldering, melting, alloying,
    casting, heat-treating, etc. these past several years makes it clear
    that a good many of our correspondents are artists. Our good and
    gentle colleagues who don't consider themselves engineers or even
    technicians and really don't want to become such. I feel driven to
    make a point that I hope will aid us all in understanding and
    communicating the issues of gasses, torches and fire. There are
    untold variations and details that some of us could haggle over until
    the goats come home to Topanga, and I'm going to make some rather
    loose generalizations. However, I'm not trying to whack the hornet
    nest so let's not get involved in details, just let me make a point
    that I think will help. 

    A torch is a device for the controlled mixing of a fuel gas with an
    oxygen source. The mixture ratio is controlled by the valves and
    openings within the torch and the pressures of the gasses applied to
    them. The volume of mixture or rate of flow is largely determined by
    the opening in the nozzle and the pressure of the aggregate mixture.
    We light the mixture where it comes from the nozzle to make a flame
    for our use. Let's look at the flame. 

    The flame is a jet of mixed fuel and oxygen which is busy combining
    chemically, breaking down the fuel molecules to form a residuum of
    CO2 and water and minor "impurities". We are interested in the heat
    that this chemical union produces and we are interested in two
    aspects--the temperature of the heat and the quantity of the heat. I
    realize that this concept has been abused in the last few decades as
    marketing forces have pushed various new technologies. For example
    when the neodynium-YAG laser was introduced to eye surgery it was
    termed a "cold laser", but, good heavens, in the few nanoseconds of
    its pulse discharge at its microscopic focal point the temperature
    reaches about 9000 degrees Kelvin.! That's more or less the
    temperature of the surface of the sun, but the quantity of heat is
    infinitesimal. This is my point, think separately of THE TEMPERATURE
    OF THE HEAT and THE QUANTITY OF THE HEAT. 

    The TEMPERATURE is determined primarily by the type of fuel and
    ratio of oxygen in the mixture. The QUANTITY of heat is determined
    primarily by the volume or amount of mixture being burned. Jim
    Binnion has given you the TEMPERATURE that can be achieved by each
    fuel gas when mixed with the optimum ratio of pure oxygen and when
    mixed with the optimum ratio of room air. Clearly all of them are way
    above the temperature you want your work piece to reach for
    soldering, So how do we understand what we want to acheive and how do
    we get there? 

    In the case of a solder joint we want to raise the temperature of
    the joint and the solder to it's flow point without overheating the
    work-piece and slowly enough that we have time to think and react.
    This brings up some more technical factors. The rate of heat transfer
    is mostly determined by the temperature differential, as from the hot
    flame to the cool metal. The distribution of heat that is transfered
    to the metal is a function of thermal conductivity. Silver and copper
    have very high thermal conductivity while gold (and especially
    platinum) have rather low rates of conduction. Now lets look at what
    actually happens when soldering with our trusty torch. 

    At one extreme is platinum. We play our torch on the joint, the heat
    stays near the joint but we need to get it really hot, so we use a
    very hot but rather small flame. At the other extreme is fine silver.
    The heat from our torch is rapidly conducted away to heat up the
    entire work piece. The whole piece gets hot but our solder flow point
    is much lower than the platinum example. For the silver piece we need
    a fairly large QUANTITY of heat but not a really high TEMPERATURE;
    with these requirements a tiny, super-hot flame can get you in
    trouble much faster the a larger, cooler flame. 

    Think separately of TEMPERATURE and QUANTITY, it may help you (I
    hope so). 

Dr. Mac

 
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