With using a thin sprue the metal streams into the cavity and fills
the far side of the piece before returning to the eddies. Where the
hotter incoming metal meets the backwash it seems the backwash must
already be too cool to recombine resulting in the "cracks". The
cracks, under close observation, may have rounding to the backwash
side of their edges. The roughness where the sprue comes in could
be an indication that that part of the casting is solidifying before
the main body of the piece. The same process can also occur in wax
injection. Wax temp,, mold temp., and too much talc are the factor
to look at. Use an inspection light to check the waxes.
It is difficult to establish directional solidification in flat
Like you said, changing the sprue design is one easy thing you can
try. If you take the weight of one of your other pieces and compare
the cross section area where the sprue joins the piece, with similar
measurements on the flat pieces, you might find that the two samples
have different sprue cross section to weight ratios. Because
rectangular shapes cool faster than cylindrical ones, the ratio
should be more area for the rectangular, (or oval) sprue entering the
flat piece. Making sure the sprue solidifies after the piece will
prevent directional solidification coming from 2 directions and
pulling molten metal out of the piece.
One way that directional solidification is routinely established in
the foundry industry is through the use of cast iron "chills".
Basically the mold maker sticks a big piece of cast iron into the
sand touching the area of the casting where it is desired that
solidification should begin. As the molten metal enters the sand it
starts to cool. Because of the much greater conductivity of the
cast iron over the sand, the metal touching the cast iron will start
cooling faster than a similar section touching only sand. In this
way mold makers can control where the shrinkage occurs, either by
hiding the shrinkage porosity in the interior of the piece, or
forcing the shrinkage to occur outside the piece in the risers
The dental catalogs sometimes sell reservoir sprues. They look like
a piece of sprue wax stuck through a ball of wax. The ball, which
is located several MM from the piece, is designed to keep the sprue
area next to the piece molten longer. while still allowing for
reasonable sprue removal.
Surface texture increases surface area, so a piece with texture will
have faster cooling than one without. Texture can be good for
pushing porosity problems into the inside of a piece, and can be
used a a directional shrinkage facilitator.
Sometimes we add small pointed nibs to the piece. These are later
ground off. Like the tiny strings of rubber on new tires. The nibs
cool first and localize the start crystallization, and aid in
filling both the casting and the wax mold. (No doubt this is wrong
in some circles)
In your post you didn't mention the method of casting. Bronze
sculpture is usually cast from the bottom up, like filling a bath
tub from the drain rather than splashing down from the faucet. This
is why some casters put their sprues on at lower angles.
A recent AJM article claims that square sprues cast better because
"90% of the air goes back out the main sprue" and not through the
Bad sprueing works for some, just look at those adds for stone in
place products, where they show 10 inch trees of stone in place
rings all cast through the thinest cross section at the back of the
ring. (this is wrong)
I am still learning so correct away.