I am getting ready to cast my wax models using my vacuum caster. In
reviewing Tim McCreight's Practical Casting, Page 41, he emphasizes
the necessity of having the flasks bone dry before putting them into
the kiln. However other sources stress that it is necessary to have
some moisture in theflask and not to let them get absolutely dry.
In the past, I have always put my flask in a cold oven, when the
investment has glossed over and has solidified, but is not bone
dry---just very slightly damp.
However, I have been concerned that as the kiln heats up there is
some steam arising from the flasks during the initial time of the
burnout, and wonder if this is harming my kiln.
I keep the vent open until all the wax has dripped out, which does
keep any moisture from being trapped in the kiln. but am puzzled as
to whether the flasks should be bone dry, or slightly damp.
Thanks for your suggestions. Alma
Hello Alma Ive been casting since 1996 and the way I was taught and
the way we almost always burnout is as follows:
after vacuum/investment is done, we settle for two hours and then
take away base and sleeves (if perforated)
then we steam for an hour or more depending on the size of the flask
and directly from there to a kiln at approximately 250 F
the Kiln is at 250 F for 2 hours raising all the way up to 1350 F
(the times and duration depends on flask size and material to be
cast) then the temperature is brought down to casting temperature
(this also being subject to size of flask and pieces and alloy)
The heavier the pieces the lower the temperature has to be (given
that the spaces where metal shall flow will be bigger thous allowing
metal to flow free)
If pieces are small more temperature will be needed so metal can
flow without cooling in the process.
Alloy content will influence casting temp as well as the method used
for casting itself (in my case centrifugal or vacuum assisted)
Cynthia the artist in this family, I am the "technical guy", uses a
plasticized wax (carvex is the old name for it) and to cast it
she/we feel it necessary to put a wet flask in a HOT kiln to get the
wax to melt before fracturing the investment, as it heats up. It,
Carvex, is a high temp. melting wax. For lower temp., "softer" waxes
one can burnout the sry flask but we normally do even these wet.
Before you cast your masters, do a few test castings and see what
works for you.
Also, we now mostly make a silicon (very low shrink) RTV mold of the
master, keep the master, inject a wax into the mold and cast that
wax. Then, if a disaster occurs, we have a mold to make another
castable wax if/as needed. Just OUR WAY and there are many other
ways that work fine.
I used to let my flasks dry out, but since someone on this forum
recommended burning them out after a half hour of set time. In the
class I help teach at our local senior center we half to wait a week
before we can burn out. So, we seal the flasks up in zip lock style
bags to keep the moisture in the investment. The reason is the
moisture that is left in the investmant is like a internal steam
cleaner. It forces the residue wax out of the pores of the
investment and gives less porosity of the finish. Our castings have
drasticaly improved since using the damp investment procedure.
Good luck, Ken
You want a little moisture. I always let mine sit overnight before I
load them. If they're going to sit much longer than that, I put them
in a plastic bag with a wet paper towel. Never had a problem in 12
years of doing it that way. Usually let the student flasks sit for a
week between investing and burnout. (in a bag, with wet towel.)
If you're going straight for burnout, the last time I read the
directions, they said to let them sit for 2 hours after gloss off
before you move them.
I *have* had issues with flashing once or twice when I was forced to
move them early. Patience is a virtue when it comes to casting.
Meanwhile, why are you worried about steam hurting your kiln? If
it's gas fired, it's an overgrown BBQ grill. It's basically a brick
box, with a torch under it. What's it going to do?
If it's electric, it's a brick box with coils of hot wire. Water is
going to flash to steam and get *out* of there fast. The real killer
for electric kilns is the carbon soot from the wax. Turns out carbon
soot is conductive. Stuff enough flasks in there that the soot gets
dense enough, and you can cause arcing between the coil loops of
your elements, and burn them out. (Ask me how I know this....)(Hint:
it has something to do with why I know how to re-wind kiln
elements...) This is one reason I *really* like the old Neycraft
fiber furnaces: the elements are buried in the walls, and shielded
from the burnout. MUCH tougher design than the open coil kilns, if
more expensive. If you've got one of those, you're bulletproof.
Steam isn't much of a risk. It's looking for a way out pretty
aggressively. The water comes out of the flasks slowly enough that
the density never gets high enough to do much.
My experience is that it doesn't make much difference between having
your flasks quite dry or a little moist as long as your burnout cycle
is fairly gentle up to 400C. When I first learned casting it was
through a club where we invested and cast for other members using a
centrifugal caster. We used to invest as members handed in their
patterns and kept the flasks with their sprue bases attached until we
had accumulated 10 flasks or so at which time we removed the bases
and fired and cast in a fairly fast 3 hour burnout cycle up to 650C
and then down to 450C. We rarely had any problems and none associated
with burnout. The flasks were fairly small, 40 to 50 mm diameter.
My own setup consists of a vacuum casting machine mostly using 95mm
diameter perforated flasks of which I have 3. Unless I am in a hurry
to complete a job I wait until I have 3 trees of patterns set up on
their sprue bases.
Usually then I invest on one day, typically in the late afternoon,
and after I have removed the bases and sleeves, leave the damp
flasks in the kiln overnight unless I don't intend to fire them the
nest day in which case I put them in a plastic bag. I normally start
firing first thing in the morning by first gently taking the kiln,
with the vent open, up to 200C and holding it there for 2 hours with
the vent open to dry the investment and melt the wax. I then take the
kiln up to 400C and hold it there until most of the wax has burnt out
but there is still some soot around the sprue base but no flame. I
then close the vent and take the kiln up to 650C and hold usually for
2 hours --- an hour longer if my patterns contain other organics. I
then drop the temperature back to 450C for casting.
The only time I have ever had problems with this cycle is when the
investment had gone stale by absorbing moisture in storage. Keeping
it in a plastic bag within a tightly lidded plastic drum solved that
problem. I am also quite meticulous in measuring both the investment
powder and the water on digital scales accurate to 1 gram when
mixing investment. For the flasks I normally use I add 750 grams of
investment powder to 300 grams of water from a rainwater tank.
All the best
I was told in college when I first learned casting was that you want
some moisture in the flask. Not to let them dry out even overnight. I
let mine cure for about an hour after gloss. I've pushed that to 1\2
at times still works. I think the manufactures recommendations are
that as well. Check the investment. The way vacuum works is by having
tiny voids in the flask that air can be drawn through. Otherwise it
Thank you everyone for your helpful replies. I shall continue to use
the slightly damp flasks as I have done in the past. Was puzzled
whenTim McCreight recommended dry flasks, but I guess it must work
Thanks again to you all. Alma
About 5 years ago an old friend who had taught jewelry making at one
time was cleaning out his belongings getting ready to move to a
smaller home. He ran across a flask he had invested back in 1974.
He brought it to us. When we asked him what was in there he said,
"Are you kidding me? I invested that 40 years ago". We did not
rehydrate it. We just burned it out on our usual cycle and cast it.
It came out just fine.
Have fun and make lots of jewelry.
There is an old saying: "Ask six jewelers, get seven answers.
Everybody's got two or three mutually exclusive ways to get the job
It was once explained to me that one would have problems with a
partially dried flask. When teaching I typically put flasks in a hot
(1350F) kiln shortly after the investment has set. I keep flasks
hydrated if they have been sitting around for an extended period and
might be partially dried.
Have only had one flask blow up and scatter investment all over the
That flask had been invested by a student with investment that had
started to set and probably left cavities that collected steam during
the burnout process.
I repeat: Flasks always go into a hot oven within an hour of
For what its worth,
This is true for many others than just jewelers. As I have always
said, There are a zillion ways to skin the cat!!!!!!
I've observed that nearly everyone does nearly everything a little
differently, usually with good results for all. We end up doing it
how we do it and over time come to think our way is best. Funny
thing is when you try to teachsomeone how to do it your way instead
of their way, their results are oftennot as good as yours. Even
though they seem to be replicating your technique. That's convinced
me of the importance of the things we do without thinking, the
subtle nuances that we overlook when teaching another person. It's a
Anyway, I usually let flasks sit overnight. But sometimes only a
couple of hours. Good results either way. Mark