but how much heat? I've tried 400-500F which seems to
help a bit but the sterling seems to boil in the mold when poured.
A bit like a volcano actually, spitting up through the sprue even
with plenty of side channels.
You only need to heat tufa on fairly large pattern molds. I do it
with a torch, just before I melt my metal in the crucible. I heat it
up until there’s a nice red glow on both sides. You have to remember
that the tufa method relies on gravity flow, so the problem is to
keep the metal in a fluid state as long as possible, so it can reach
to the bottom of the mold.
Now your metal splashing out of the mold is a different problem.
This sounds like backflow or too small of a gate. The gate needs to
sizeable, much more so than with a centrifugal or vacuum cast mold. I
am presuming your mold is dry, so you’re not getting into the
possibility of a steam explosion. The gate should be about thumb
circumference at the top, about an inch long, and a minimum of a
pencil top eraser size at the bottom of the gate. Large casting
require a larger opening at the bottom of the gate leading into the
mold. Keep in mind that the top of the gate is just guiding your
down into the mold quickly, so that’s why it’s so large. Gravity pour
means you have to get a lot of hot metal in the mold as quickly as
possible before the metal loses it’s liquidus state.
Backflow is where you have recurves. Any place that will cause
turbulence when the metal is trying to get to the extremities of the
mold, even if the ‘extremity’ is in the middle of the mold, must have
air vents that lead to the outside edge of the mold. It has to be
able to push out the gas, to the outside–into the air, as the metal
flows. If it doesn’t have access to the outside of the mold, the air
gets compressed and pushes back the metal. With tufa you’ll probably
use twice as many air vents than you would sprues for conventional
casting. Careful attention needs to be paid to the recurves.
I hope this helps to solve your problem.