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.