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Casting organic materials


#1

Hi

I’m looking for some help with casting-the burnout cycle
specifically. I have recently acquired a vacuum casting table and a
kiln. I bought Eurovest investment and I want to cast in silver.

I want to use organic items-sticks, flowers, insects, etc. I know
that the temperatures needed to burn out wood are much higher than
for wax and the cycle probably longer, but I’m a bit fuzzy on the
details and was hoping for some info. I invested a few flasks last
night (as the flowers were dying and would not last any longer) but I
am not sure exactly what to do with them now-a bit sad, I know.

Anyway, a few questions.

  1. Do the flasks need to stand on something, or can they go flat on
    the kiln floor?

  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)

  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?

  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
    true?

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.

I would so appreciate any help.
Thanks in advance.
Andrea


#2

Andrea, great questions here. First of all the materials you are
burning out all have different physical makeup so each will require
some inspection of the flasks before you cast.

To be certain on temps and what not most of these items will take a
longer and hotter burnout. However you never want to exceed the
manufacturers temps for the investment or you will have breakdown of
the investment if it is left too long.

This will be trial and error but I would use a burnout cycle that
would leave it longer at a higher temp. When the cycle is done turn
the flask over and see if there is any black residue left. If there
is then you will have to burn it out longer and hotter until all ash
is gone…no matter what the material is. Specifically, there might
be someone on here that does a lot of this type thing who can offer
other about each material.

I would always let the flasks be on something or make sure the holes
on the bottom are not covered for burnout.

Depending on the metal and the items being cast, thin leaves the
flask a little hotter and chunky things lower. Some trial and error
here as well. The kiln has a hole in the top or vent somewhere and
should allow for escaping gases. I have never heard of leaving the
door open.

To be exact in what you are doing…is the key. I know of an
instructor that used to cast spiders successfully…black widows to
be exact. The key was the spruing system for the legs and body, the
burnout and cast temp. 900 to 1000F on the spider, sprues on all the
legs adn the body, and a vent sprue for the total to have gases
escape to that area.

Russ Hyder
Former Casting instructor at GIA

One other thing…if you can’t get good results with regular
investment try using a higher temp investment for Platinum…


#3
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
flasks…

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
true? 

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.

Peter


#4

Hi Andrea,

I do quite a deal of work casting from organics mostly seed pods and
leaves. I haven’t tried insects yet largely because I have not worked
out what to do with them in an artistic sense. I do have a few
rhinoceros beetles which I might try in bronze sometime. They would
take well over 100 grams of silver.

Back to basic principles. For wax patterns I use following burnout
cycle for silver fro 85 mm perforated flasks

All temperatures are in degrees Celsius.

Ramp up to 200C hold for 3 hours

Ramp up to 450C hold for 1 hour

Ramp up to 650C hold for 3 hours

Ramp down to 450V and cast

When I’m burning out organics I increase the final soak temperature
to 700C and hold for an additional hour. This usually ensures a clean
burnout. However I still find the process somewhat experimental.

The additional experimental issue is how well different organic
objects cast. Seed pods usually work just fine though if they have a
porous surface it is generally a good idea to use some form of
sealant Depending on the object I sometimes apply wax with a wax pen
particularly if I need to build up thin areas to ensure a good
burnout and metal flow. Otherwise I use a spray sealant used by
artists for charcoal and pastel sketches. Hair spray works OK too.

When it comes to leaves unless they are particularly fleshy you need
to build up the surface to ensure a clean burnout and metal flow. If
the design is single sided (e.g., a pendant or a broach) I build up
the thickness from the back by welding on sheet wax again using a wax
pen.

I also ensure that organics based pieces are well sprued to ensure a
good clean flow of metal.

I have been told that it is easier to get clean castings in thin
sections when casting in gold (even 9 ct) because molten gold is
denser and more fluid than silver. I’m not sure casting with the
flask at a higher temperature. Higher temperature certainly shorten
the life of the gasket. I use a special high temperature gasket on
top of the silicone rubber one. Others may have an opinion on this. I
think it is more important to ensure that the metal is properly
molten (fluid as water) and to work expeditiously. Start the pump and
open the vacuum valve as soon as you have the crucible of metal ready
and pour promptly when there is sufficient vacuum.

I never fire the kiln with the door partially open. One reason of
course is the kiln wouldn’t work because of the safety cut out switch
on the door. However I do leave the top bung out until the fumes from
the burnout stop. It’s usually somewhere around 550-600C. This allows
the escape of steam from the initial dry out stage as well as the
burnout fumes. I then replace the bung to reduce heat loss. There
should be plenty of oxygen in the air in the kiln to achieve any
additional necessary oxidation.

You will find a variety of opinions of whether flasks should be
fired sprue down or sprue up If you fire them sprue down it is
necessary to space them up from the floor to allow a good flow of
oxygen into the mould. It is also a good idea to have some form of
drip tray to catch the molten wax at least in the early stages of
burnout. Otherwise it can damage the kiln liner. I actually fire with
the sprue up. I do so mainly because when I am casting I can pick up
the flask with the tongs above the flange and put it straight into
the vacuum chamber without having to turn the rather heavy flask
over. It simply makes the casting action simpler and less prone to
mishaps such as dropping a very hot flask.

You will still have failures when working with organics. The trick
is to understand why they failed either because your preparation was
unsuitable or because the material was unsuitable (e.g., too much
ash). The process no works reasonably well for me now by allowing a
few more pieces in the flask in case one or two fail. Sometimes none
fail and I keep the extras for future jobs… However this took a year
or so of experimentation before I sold my first gumnut bracelet. It
is a gorgeous piece.

All the best. I wish you success.
Jenny


#5

Peter Rowe and Russ Hyder pretty much put it down concerning casting
organic materials. One thing I might add is that when I cast insects
and leaves, I sprue BIG. I make those sprues fat, and I wind my spin
caster up another turn, so it does the quarter mile in 5 seconds. Then
I cast at 700C and that causes a “slam” effect. The molten metal hits
the cavity so hard it pushes any ash left to the topside of the
cavity. For an earlier comment to Orchid on the subject see

http://www.ganoksin.com/gnkurl/53

and also

http://www.ganoksin.com/gnkurl/54

And then there are various other links on the pages concerning
casting

Cheers, Hans


#6
and I wind my spin caster up another turn, so it does the quarter
mile in 5 seconds. Then I cast at 700C and that causes a "slam"
effect. The molten metal hits the cavity so hard it pushes any ash
left to the topside of the cavity.

Hmm. I’d bet, Hans, that it doesn’t manage to float all the way to
the outside, or even into the sprue. I’d bet that remaining ash
merely gets more mixed into the interior of the metal due to the
turbulence this approach will cause in the flowing molten metal. And,
a really high pressure cast like that, while it ensures total fill of
even the tiniest details, also puts a lot more stress on the
investment. Many natural objects have delicate details, many of
which can leave a slightly fragile and complex surface on the mold
cavity, which are prone to breaking off if you stress the mold too
much with too high a metal speed and pressure, giving you investment
inclusions. It can be a bit of a quandry, since for such highly
detailed models, one can be tempted to mix the investment very
slightly thinner, which makes it weaker, while mixing it with a tad
less water can make it stronger, but then somewhat less likely to
pick up every last detail in the finest most fragile models. For my
money, I’d leave the winding of the centrifuge the same, or even
less, for such delicate models, and let only the higher metal and
flask temp take care of getting a complete fill. For the ash, lightly
bump/shake the flask, sprue down, before putting it in the machine’s
cradle or on the casting surface if vacuum casting. You can often
shake most of the ash out. I’ve even sometimes done that, then
pointed a compressed air can into the sprue to blow out yet more. The
flask then goes back into the kiln to warm up again (the compressed
air blask cools the sprue too much otherwise) before actually
casting.

Peter Rowe


#7
I'd bet that remaining ash merely gets more mixed into the
interior of the metal due to the turbulence this approach will cause
in the flowing molten metal. 

No, It doesn’t. Often there would be a small mix on the top of the
insect that is easy to blend in with the normal texture.

Many natural objects have delicate details, many of which can leave
a slightly fragile and complex surface on the mold cavity, which
are prone to breaking off if you stress the mold too much with too
high a metal speed and pressure, giving you investment inclusions 

My early attempts did cause some investment problems. Later, when my
sprueing design improved I could cast even the fine hairs on a
spiders legs and body. (they came off during the finishing) But
casting the antenna of insects was simple, so my investment was
certainly strong enough.

First two or three layers of investment are painted on the insect or
leaf and certain sections of an insect would be thickened up with
wax as well.

My overall success rate was about 80%. I have never been successful
in casting a winged insect.

http://www.ganoksin.com/gnkurl/5j

This is a classic failure, even though the wings were thickened up
with wax.

Here is an example of an unfinished “Koring Kriek”.

Note the antennae have been folded back, so as not to break them
off.

http://www.ganoksin.com/gnkurl/5i

The mandibles, pedipalps and all the other mouth part come out in
exact detail. An unfinished centipede, cast at 700C.120mm in length.

http://www.ganoksin.com/gnkurl/5h

No ash, no problem.

Centipedes smell like baking cookies when the temperature is under
400C. You will not believe how delicious they smell. Over 500C, it
gets a bit different, though.

I have absolutely no experience in vacuum casting, even though I own
one. I just use the thing as a vacuum pump.

But my gut feeling says vacuum will not cast the equal detail that
spin casting does. Could be wrong, though.

And for what it’s worth, I mix my investment (Satin Cast 20) with
38% water at room temperature. One thing to remember is that casting
insects or organics is more art than science. A dung beetle will not
be spun up so hard, since it weighs +250 grams finished, whereas with
a hairy caterpillar, you going to spin that thing into a blur. And
when things do go wrong, the resultant spray of metal is somewhat
spectacular. For instance, when I cast this Pels Fishing Owl claw, it
was so large that I put water in my casting box, about 50mm deep,
like a mini swimming pool.

http://www.ganoksin.com/gnkurl/5g

Just in case, because I was casting nearly half a kilo of silver.
With this one, I painted ten or so layer of investment on it first.
It stands 200mm high and the bones, being calcium, are encased
inside the silver,because they can’t be burnt out in a reasonable
amount of time. One last thing.

All my organics are cast using a 95% silver and the 5% a deox alloy.
I makes for a more malleable cast, so that the legs and appendages
don’t break off easy when you adjust them.

http://www.meevis.com


#8

Thank you so much to everybody who has taken the time to reply to my
questions on casting. I am grateful for the

I have thus far cast on two occasions (casting 5 flasks in total).
The first run did not go so well, with only a couple usable pieces
from the three flasks, but the second attempt was quite successful-I
did a little dance all the way down the driveway and back up again.

The major difference in my second attempt is that I ended the cycle
on 710C instead of the 650C I tried first time round (I must just
point out that I only got to casting 10min or so after my kiln had
switched off the first time and that the oven had cooled to about
590C by then. Don’t know how quickly the flask cools; whether the
flask was then also 590C or weather they had retained their heat and
were still at 650C). Either way, the second lot were pretty great. I
need to do a whole lot more and see if they keep on coming out well,
but for the moment I am pleased and hopeful. (I was casting flowers
and twigs.) I cast a frangipani flower that almost came out perfect.
I can’t wait to do more organic casting.

Thanks again everyone!
J Andrea