I have researched back through the archives on Tufa casting but did
not find anything on the subject of heating the tufa molds prior to
casting. I am doing some fairly detailed casts and have heard
through outside sources that heating the molds helps get a bit more
detail.
Really? OK, 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.
my experience with the Tufa has not been heating it, rather making it
smokey black - using the torch with a flame that makes the black
smoke. This is kinda like using a “release” material. The castings
have come out perfect, with no bubbles.
The main reason for heating the stone is to get rid of any ambient
moisture in the stone. The ambient moisture is the reason for the
volcanic reaction. What you are doing is creating steam. Try drying
out the stone in an oven for about four hours a 300F before you cast
your object.
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
be
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
metal
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.
A very sensible response, thanks. I have taken most of these factors
into consideration. As you say, outside ventilation is critical to
the outcome.
This boiling thing is really not backflow caused by internal air not
escaping the mold in my estimation. It seems to get worse the higher
the mold temp is. This seems a bit counter-intuitive I know but the
effect seems to be worse at 600F than at 400F.
Just to let you know… I am casting a concha that is about 3cm x
5cm requiring about 40g of SS including the gate plug.
Really? OK, 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.
Sounds like water vapor escaping from the stone when you pour the
silver. How are you heating the tufa? If you are using a torch or
other gas fired source (oven or kiln) you are probably getting steam
in the pores of the stone and this is coming out when the silver
hits it. Gas flames produce water vapor as a part of the combustion
process and that will tend to collect in the stone when you first
begin heating and will be slow to fully leave the stone. Also it will
have water vapor in it from just sitting around. In either case for a
porous object you should heat it long enough to drive off all the
water vapor which could take an hour or more for all the water to
leave at 300-400 F, Higher temperatures will speed this up but it
still will take time to dry it out.
James Binnion @James_Binnion
James Binnion Metal Arts