This doesn’t really address the original post but I thought I’d post
it in any case.
When I first learned how to cast (centrifugally) with an oxy/propane
torch the goal was to get a complete casting, fully filled. Wouldn’t
that be great?
With that hurdle met and as I began to cast more the goal shifted to
the quality of the casting. You all know what that means:
-no inclusion pits, such as flux or investment
-no hot tears or cracks
-no shrinkage (shrink spot) porosity.
-ductile, workable metal.
Casting is one of those things that is a sum of its variables.
Trouble can begin in the wax, the spruing, the investing, the
burnout, the casting, the cooling and/or the finishing.
If the waxing, the spruing, the investing and burnout are all good
there’s still something big: temperature. Not only the metal melt. A
really big piece of the casting puzzle is the temp. of the flask when
it is pulled from the oven or kiln. In my experience this is keyed to
what is inside, how much volume of metal there is. Whether it’s a
fine, netlike wax or a heavy sterling signet.
My results greatly improved when I learned that a lot of trouble can
happen as the metal mass cools. The longer the cooling takes, the
more coarse the casting and the more brittle it will be. It’s a bit
counterintuitive (it was to me) in that a larger mold cavity is
easier to fill: it takes longer and the air can evacuate more
quickly. A finer casting fills more quickly and so can be harder to
fill due to the back pressure of the trapped air which must evacuate
more rapidly. Even though there is less air it is harder for it to
The metal is cooling as soon as the torch is pulled away so I pull a
flask containing a finer model from the kiln at a higher temp since
it will be more difficult to fill. Also, the mass of metal is much
less, so cooling will happen more quickly and the casting will be
finer “grained” --the crystals will be smaller. (Jim Binnion, please
correct me if my understanding or terminology is off).
Heavier mold chambers are comparatively easier to fill. The
compromise here is that less advantage is lost due to a cooler flask
(it will still fill) but much is gained by having the larger metal
mass cool quickly.
Proper spruing and things like chill vents can control shrinkage
porosity but mold temperature has always been super important in my
studio. If I were making multiples, I had the luxery to play with
flask temps, lowering them until there were incomplete castings and
then bumping back up again.
Obtaining a fine grained casting was even more important when I used
to cast crowns and bridges in a dental office. It’s been my casting
goal since then.