1) Do the flasks need to stand on something, or can they go flat
on the kiln floor?
It’s good to have them supported up a little bit, so there is air
circulation around the sprue hole, allowing cumbustion/vaporization
gasses to more easily escape. Many Kilns have a rough enough floor
surface that this is enough, or you can get ceramic perforated bricks
intended for barbaques, that will work too. Or use your imagination.
Be sure what ever you use can withstand the temps without dumping the
2) How long should the cycle be and what temperature sequence do I
set it to? (I have a programmable kiln, it can take a
sequence-once I have read the manual)
You use the same burnout cycle you’d use for wax for the first
parts, since the initial part of the cycle is aimed at the needs of
the investment, not the burnout of the model. Your actual burnout
temp is limited to around 1300 degrees by the limits of standard
investments, and this is within the range of what waxes are often
burned out at. It will work too, for the organic materials. The
waxes burn out quicker because most of the wax melts and flows out,
and doesn’t need to actually burn out within the mold. So you’ll need
a longer time at that maximum temperature to get a full burnout. How
long, though, depends on what you’re burning out. At a guess, I’d
think for most small things, at least three hours for a small flask,
up to six for a large one. Some organics won’t completely burn out,
but will leave a non-combustable ash remaining within the flask
cavity. Sometimes you can shake this out before pouring the metal,
while other times you just have to put up with it. Your end casting
temperatures would be the same as you’d use for wax models of similar
weight and thickness. If highly detailed, you could raise the temp a
little more, if you like, and if experimenting shows you it’s needed.
3) What temperature must the kiln/flasks be at when I take the
flasks out in order to pour in my metal? Would this vary depending
on whether I am casting thin leaves or chunky bits of bark or pods?
Yes. See above. The main thing is extending the time at peak
temperature where the model is first reduced to carbon, then oxidized
from there leaving a hopefully clean mold. After that, though, the
cycle is the same as for similar shape/weight/detail in wax.
4) I read somewhere that the kiln needs to have an oxidizing
environment and that I should leave the door open a bit. Is this
You’re burnout kiln does need to have an oxidizing atomosphere for a
good burnout. This is true for wax burnout too. If your kiln is made
and sold as suitable for casting, then it should be fine already.
Most such kilns have a vent hole in the top to allow gasses to
escape, and this also allows enough air circulation to pull outside
air and oxygen in. Some enamelling kilns, though, may not have
adequate ventillation to give a good oxidizing atomosphere while
burning out (unlike enamelling, while burning out, the oxygen is
being consumed by the process, so a sealed kiln may start with enough
oxygen in there, but won’t be able to maintain it.) In some cases,
these can be modified by adding a vent hole, usually at the top, but
you’d have to check with the kiln manufacturer for how and where to
do that. If, though, you just leave the door open, you’ll find the
kiln will have trouble reaching the desired temperatures. An open
door will be too much ventillation with most kilns, which will give
you poorer results and a higher electricity bill.
I did a bit of casting a couple of years ago when I was studying,
but we were not allowed to do very much. The teaching we received
wasn't very amazing and my organics were never successful.
They can be difficult to get good results. For one thing is that
issue with remaining ash in the mold. Not much cure for that unless
you can shake or tap the mold before casting to get the stuff out.
Not everything you burn out will do that, but some do. You can also
improve some things by coating them with wax or a sealer like spray
lacquer or the like. Some of your natural objects are porous enough
that water from the investment will be sopped up, and some investment
right at the surface may end up not as strong as desired. Coupled
with the more difficult burnout, this can give you pretty poor
quality surfaces on the castings. Sealing the surfaces helps some,
but then you may loose some fine detail too, so this takes some
finesse to get right. The sealers also may help some things like
wilting flowers to retain the right shape through the investing
process. In some cases, you’ll find natural objects may be too thin
and delicate to cast well. In these cases, you can sometimes build
up the back surfaces with a little wax or something to increase the
thickness of the model, letting it cast properly. Leaves, some
flowers, insect wings, etc, often fall into this catagory.
And keep in mind too, your other option. Room temperature
vulcanizing silicone rubbers or similar pourable liquid mold
materials can be used to make a mold of these delicate items at room
temperature. Then wax is injected into the mold to give you a
standard type wax model which will be easier to cast. Plus, if the
first one doesn’t work, you still have the mold, which isn’t the
case if you cast the original.