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Tiny holes in castings


#1

I keep ending up with tiny holes in the sterling silver castings I’m
doing. I can’t figure out why this keeps happening, nor how to
prevent it. I’m casting charms and small figures in sterling. Some
are carved from green or purple wax, some are blue injection wax.
Most of them come out OK after casting & cleaning. But some of them,
usually the ones that are most important, have tiny perfectly round
holes about the size of a 22/24 gauge wire. Some of the holes seem
to be very deep, others look like perfectly round hollow balls in the
wax. Are these being caused by bubbles in the wax? I checked this
last batch over very carefully under bright lights before sprueing
and investing them, and I didn’t see any bubbles. What could be
causing this, and how can I stop it from happening again?

–Kathy Johnson
Feathered Gems Jewelry
http://www.featheredgems.com


#2

Hi Kathy, Holes in castings can be caused by several things:

  1. Air bubbles in the wax. Air bubbles in carving wax is very
    rare. Air bubbles in injected wax is a little more common. One
    technique you might use, if you think you might have air bubbles, it
    to put the waxes under the vacuum table and pull a vacuum on them.
    If there are air bubbles near the surface the bubbles will burst.
    This does not solve the problems with air bubbles but saves you the
    time in casting flawed waxes.

  2. Incomplete burn out. There will be small particles of carbon
    (burned wax) left on the surfaces of the mold. This problem usually
    shows up immediately on the casting as the carbon leaves impressions
    in the silver. Maintaining the mold at the high temperature for a
    little longer should eliminate this problem.

  3. The combination of temperature of the mold and metal is too
    high. This problem caused the silver to absorb oxygen and usually
    does not show up, as bubbles in the silver, until the silver is
    polished. If the bubbles show up after polishing you might try
    lowering the flask temperature by 50 to 100 degrees. If you vacuum
    cast and use an electro melt furnace the temperature of the metal
    could be lowered slightly. If you torch melt be sure you do not
    overheat the metal. Do the bubbles appear on the items in one flask
    and not another? If this happens I think the heat combination is
    too high for the flask with the failed castings.

  4. The angles in charge of supervising your casting are asleep when
    you most need them. Sometimes, when problems exist, they cannot be
    explained.

If you make rubber molds of your originals the castings with bubbles
can sometimes be filled with epoxy. The surfaces is then taken back
to the original shape. This will allow you to save the work you did
in the wax carving.

Good Luck
Lee


#3

Hey Kathy, are you following investment mix instructions carefully?
The holes you are describing are very odd. Are you adding new metal
with each casting, not continually recasting old silver? The
uniformity you are describing is strange. I noted someone said air in
waxes, but unless they popped under vacuum that would not be. I have
not ever had that as a problem, and wonder at its possibility. Could
you be overheating the metal when casting? Are you vacuum or
centrifigal casting? Jay


#4

Kathy, one thing that might be causing your problem is an excessive
amount of borax flux. You might try cutting back on in. Too much
flux will leave balls of borax between the metal and the investment,
causing holes and many other problems.

Another is related to using “old” silver or gold. Never include
soldered metal in your castings. The solder will bubble and cause
pits. You can cut out the solder joints from your scrap and still
use the scrap for casting grain. Just make sure there is no solder
left on the metal you are going to cast.

To find the solder on your scrap, use an oxidizing flame and heat
the metal, using no flux or dip, until it gets to the blue color.
The solder joints should jump out at this point and be very easy to
see. Just make sure that you cut them out completely before using
the metal for casting.

Don at Campbell Gemstones


#5

Hi Kathy, It would be important to know if the tiny holes/pits are in
the thick area of the design or throughout the design. My best
recommendation is to take a bunch of sample pieces that are
problematic and lower your flask temp by at least 100 oF …possibly
more… What you have described is a classic temp related problem
either from the metal temp or the flask temp. Best Wishes,

Daniel Grandi Racecar Jewelry Co. Inc. We do casting finishing and a
whole lot more for designers, jewelers and people in the trade.


#6

It is improbable, but a hair in a wax will leave a hole. Hair burns
out and turns to carbon, leaves a hole that runs wherever the hair
was all the way though the whole piece.

Ricahrd in Denver