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
Redirect to the new SFMS website) 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
