[Jewelry making - Article 00261] - Re: [Orchid] Argentium Sterling and solder

[James Binnion]

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>>    Trevor, why is indirect heating better than more heat? 

>     Um, because it works? Sorry, not trying to be a smart-mouth, I just
>     don't have a better answer. I think it's probably got something to
>     do with the low melting stuff in the solder burning off easily (or
>     perhaps oxidizing quickly) and thus leaving a higher melting
>     residue but I don't know that for certain. What I do know is that
>     heating the Argentium Hard in such a way that the flame doesn't
>     touch it until the last second(s) of it's melt seems to allow it to
>     flow much better. Needless to say this could be due to my torch, the
>     fuel gas I use, my particular flux, the color of my eyes... ok maybe
>     not that but you get the idea: your mileage may vary. But FWIW this
>     is not an unheard of phenomenon, it's just that regular sterling
>     solder isn't all that prone to this so we don't notice it that
>     much. 

    What you are experiencing is called liquation. The leftover non
    melted bits of the solder are called a "skull". This occurs with
    solders (any solder not just Argentium solders) that have a wide
    melting range when they are slowly heated through their melting
    range. To quote from the AWS Brazing Handbook: 

          "Liquation is more often encountered when pre-placed filler
          metal (solder) is used. For example, when a filler metal with a
          wide melting range is pre-placed at the joint and gradually
          heated within this melting range, there may be time to allow
          the low melting fluid portion to flow into the joint capillary
          or to plate onto the outside of the components being joind.
          This leaves behind a "skull" The tendency for a filler metal to
          liquate depends on the relative amounts of liquid and solid
          phases present as the filler metal is heated through its
          melting range." 

    Liquation also often causes incomplete joints as the low melting
    phase flows partially into the joint and bonds with the base metals
    in the joint and ceases to be liquid as it diffuses into the base
    metal and effectively seals the joint so that no further solder can
    flow into the joint. 

    The solution to the problem is to either heat the work more rapidly
    through the melting range or to use the pick method of placing the
    solder on the joint as it reaches the proper soldering temperature.
    Either one will greatly reduce or eliminate the skulls left behind
    and a addtional benifit is that the solder will flow much better and
    further into the joint if it is applied to a properly preheated
    joint. 

James Binnion
jbin AT mokume-gane.com
James Binnion Metal Arts
http://www.mokume-gane.com
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