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Casting porosity


#1

I need some advice on casting. We’re used to casting brass, but last
night we were doing a silver casting, and the results are horrible.
None of us have done much with it before. There’s obviously
something basically wrong with what we’re doing, incomplete pieces,
monstrous porosity. There are pieces which look normal, and then
right next to them, shrivelled blackened pieces. The normal looking
pieces are quite brittle, porous and red inside (cuprous or cupric
oxide I suppose). We thought the metal had been heated properly -
completely melted and just a little beyond. We’re using a graphite
crucible with induction heating. Adding a little - a small pinch -
boric acid. The metal, at least compared to brass, is very sticky,
sticks to the stir rod, sticks to the crucible. There’s a slag on
top which doesn’t seem normal from what I’ve read. Using clean pure
silver and copper shot, with a little less than 50% old sprues (cast
by someone else, who was trustworthy). Any clues?

Yours,
Hunter Robertson

Colin Robertson Jewelry
www.minoanjewelry.com


#2

Hunter, Sounds like you have a bag of worms.

It might be helpful if you described you casting procedure in more
detail. Vacuum or centrifugal casting? What was the metal and mold
temperature? How long did you burn out and what type of wax were
the models? Carving wax requires a longer burn out. What was your
burn out temp? In general you description might indicate incomplete
burn out which could be the cause of surface porosity and incomplete
castings. The mold should be pure white when it comes out of the
burn out oven.

Incomplete castings could indicate that the metal/mold temperature
was too cold. For vacuum casting I use a metal temp of 1850 and a
mold temp of 860 degrees. That combination seems to work for all my
big and small castings. The sticky silver might indicate the metal
was not hot enough and was chilling on the surface of a cold stir
rod. There was a time in my early use of an electro melt furnace
where the silver formed a thin layer of silver on the top of the
crucible as I poured. I found that the top part of the crucible was
too cold and was chilling the silver as it poured. I don’t pour
until about 5 minutes after the metal reaches 1850 degrees. That
allows the ring of the crucible to heat up. I would be very
suspicious of the scrap silver you used. That might account for the
excessive slag. Was the copper shot pure or an alloy? You should
try to remove the slag before you pour the metal. Slag in the mold
could cause poor castings. The ratio of new vs scrap silver should be
at least 50-50. I use 60 new and 40 old. That’s my 2 cents Lee
Epperson


#3

Lee, thanks for the input. A bag of worms indeed!

So, we’re using a centrifugal casting machine, motor driven, with an
induction melting coil, and a graphite lined crucible. We’re casting
about 700 grams of metal per tree. Metal temperature I couldn’t say,
doing it by eye, molten and a little beyond. We did try heating it a
bit more on one cylinder, but had the same results. The flasks we
tried at 1120, which is the standard temperature we use for brass,
1040, and 800. No discernible difference. Burn out was 3 hours at
1380, and using Kerr Flexplast injection wax. This is our standard
set-up for brass, and has worked well for us with it, for the same
reason I don’t think sprueing is the problem.

We tried (This we is my sister and I. My father passed away
recently, and as he had done a good of the brass casting, and all of
the silver casting, we’re at somewhat of a loss!) one cylinder using
only the new metal, and got the same result. As far as I know the
copper is pure, but I tried another using electrical wire, and again
the same. We have scraped the metal right before releasing it, but
the slag seems to form again right away. It has a slight greenish
tinge upon cooling.

A guy my sister was talking to who has a foundry in Britain said
that alloying the metal with the induction set-up might be the
problem, but that was what my father was doing, though of course
with the benefit of a lot more experience. Its mainly the outer most
parts, near the cylinder wall’, that are affected, but then there
are occasional pieces right in the middle of normal looking ones
which look like torched raisins.

Thanks again.
Yours,
Hunter


#4
So, we're using a centrifugal casting machine, motor driven, with
an induction melting coil, and a graphite lined crucible. 

Are you using any melting flux? Even with a graphite crucible, some
means to prevent hot gasses from dissolving in the liquid metal may
be needed. Even though the graphite creates a reducing atmosphere to
help control actual oxidation, you may be getting enough simple
absorbtion of gasses like the nitrogen in the air, etc, (oxygen does
this very much, in fine silver), that when the metal solidifies, the
excess dissolved gas has to then come out of soluition somehow (like
bubbles in your pepsi), and it can make a terrible mess of otherwise
good metal. Try using, or using a bit more if you already use it,
some mixed borax and boric acid (half and half) powders added to the
metal. They form a liquid barrier on the surface of the molten metal
that greatly slows down or even prevents the absorbtion of gasses in
the metal.

it’s just a thought, but worth trying.

Peter