[Jewelry making - Article 00079] - Re: [Orchid] Intimidated by casting

[Fred Sias]

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Vera:

    My response to your post should be in two parts: economics and
    fears. I will address the fears below but you must evaluate the
    economics yourself. 

    Economics: 

>     I have been sending my models to a casting company for a few years,
>     but after evaluating how much this is costing me per piece, 

    The cost per piece depends on whether you cast one piece at a time
    or many. This is true whether you do the casting yourself or send the
    work out. The whole casting process has a long elapsed time because
    of the investing and burnout steps. Not all of the burnout requires
    your constant attention but some time is involved and the amount
    depends on whether you have a simple manual burnout oven or one with
    a computer controller (more expense). At any rate, if your work
    schedule calls for casting each piece separately, you will find that
    you will have a significant investment of your time: eg. 15 minutes
    or so for investing and perhaps 3 or more hours for burnout (that
    does not require your constant attention). On the other hand, if you
    make molds and cast 50 or more duplicate pieces at a time, the labor
    cost per piece is very low. 

    The same logic applies to your casting company: Are they casting one
    piece at a time or many for you at one time? 

    Your fears: 

>     I get an order for an expensive gold piece. After measuring and
>     ordering the exacting grams I need I somehow make a mistake and the
>     cavity in the mold isn't completely filled. 

    This should never be a problem as you should always use enough metal
    to fill the mold, the sprue system (passages thru which the metal
    enters the mold), and a "button" of excess metal that remains in the
    pouring cup. The button is important because it supplies metal to
    model that shrinks while cooling. This prevents "shrinkage porosity." 

>     So I re-melt the metal and try again won't this eventually lead to
>     porosity? 

    One always saves the button and sprues from a casting. An equal
    amount of new metal is generally added to the used button to retain
    the good properties of new casting grain. 

>     How difficult is it to learn vacuum casting? 

    There are a lot steps and details to remember when vacuum casting,
    or centrifugal casting, for that matter. But none of the concepts are
    difficult to understand. Most of the details are before you get
    around to doing the actual casting which requires only a few minutes
    to melt the metal and pour it into the mold. 

>     I think the part that causes me anxiety is the proper measuring
>     and melting of the metal. Would one of the cheap pocket gram scales
>     easily purchased off of Ebay be sufficient/accurate enough for me
>     to use? 

    Any scale should be adequate for calculating the amount of metal to
    use. Simply weigh the wax model and the sprue system. Multiply by the
    specific gravity of the alloy being used to get the weight of metal
    needed to fill the model and sprues. Then add an extra amount of
    metal sufficient to produce a reasonably sized button. 

>     And how do I gauge the temperature of my metal as I'm melting it? 

    This depends on whether you are melting your metal with a torch or
    with a melting pot with a pyrometer on it. When using a pyrometer,
    heat the casting metal to around 150 degrees above the published
    melting point of the alloy and then pour. 

    Torch casting requires experience and judgement to determine when
    the metal is sufficiently fluid. Often it is suggested that the
    surface of the melted metal in the crucible will be shiny and will
    swirl slightly when ready to cast. Expensive industrial casting
    equipment solve these problems for you by constantly monitoring the
    metal temperature with a pyrometer. 

>     Work space I've decided that I am just going to use my garage to
>     cast in because of the fumes, silica and other dangerous air born
>     particles involved in the process. 

    Just be sure to have an exhaust hood over your burnout oven and wear
    a good respirator mask when working with investment in the powder
    form. Steam dewaxing or other procedures will help eliminate wax
    fumes. 

>     A significant dent in my work time. But if it isn't too difficult
>     to learn, I think I should get past this curve pretty quickly. Any
>     opinions? 

    Read a good book that will reveal the details and/or take a several
    day course at a workshop. Workshops at professional jewelry schools
    are expensive but one can often learn the basics at relatively
    inexpensive workshops sponsored by gem and mineral societies. (See
    www.amfed.org/sfms/lapidary-workshops.html) I think the hands-on
    experience at a workshop is the best way to decide whether you want
    to invest in a casting shop. 

Hope this helps,
Fred
Fred Sias -- Woodsmere Press
Books for the crafts and jewelry trade
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