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Damp or dry casting flasks

I am not going to do burnout right after the flasks have been
invested, and they will have to wait for about a week. I was taught
that when there is a several day delay between the time they are
invested and burned out, th ey should air dry about an hour, then
wrapped in a damp towel, and put in plastic bags to retain the
moisture in the flasks.

However, In the Tim McCreight book on casting, he specifically says
the flask should be dry before inserting in the kiln, and even
recommends hastening the drying time by setting the flask on a
radiator or in the su n to dry.

I will be putting the flasks in a cold kiln, and slowly raising the
temp to 300 degrees for the first segment of the burnout. Is there a
danger of d amp flasks exploding?


Hello Alma, I have been casting regularly for years, After I pour and
vacume my flasks I wait one hour and then put them in a cold oven.
I’ve never had one blow up. I’ve never even heard of one blowing up.
Have fun.

Tom Arnold


I have burned out dry and damp flasks. I prefer the damp flasks. I
just put them in a locking plastic bag until it is time to burnout.
If you stick to the burnout temps and times recommended by the
investment manufacturer you should be okay. I have only had one blow
out and that was because I made a mistake on the first temp and it
went to 1300 F right away and the steam build up so fast it blew the
investment out. No big explosion just a big mess. I seem to get a
better casting surface with damp flasks.

Good luck,

Ken Moore

Hi Alma

I usually put flasks in a plastic bag once I have removed the sprue
base, cleaned them up and numbered them. I then do the burnout the
next day. I start my burnout cycle by ramping up to 150C and holding
for 3 hours to dry the investment before ramping up to burnout. In my
case I use 90 mm perforated flasks for my vacuum caster.

Saying all this my previous experience when learning casting was
different. At that time I was doing the castings for a club. We
normally used a range of fairly small flasks 40-50mm with a
centrifugal casting machine. We normally invested on one day and did
the burnout and casting up to a week later. We simply left the
invested flasks ready for firing on a shelf until we got around to
casting. We also used a fast burnout cycle because we had to start
burnout before 9:00am and finish and clean up by 2:00pm because of
the opening hours of the clubhouse. The system worked quite well but
I expect I wouldn’t get away with it with larger flasks.

All the best

No, if you are starting with a cold furnace, the flasks will not
explode. Trials have shown me that the ramping process is largely
non-functional as well. I placed thermocouples at different points in
a test investment as well as at several points inside the burnout
furnace and started at high setting… I found that the temperature
inside the flask lagged behind the furnace to range around the
boiling point of water until all water was gone, then rose steadily
to the final temperature (1200F.). I hold the temperature at that
point for 2-3 hours but am not sure at this point that the holding is
truly functional.

I issue the caveat that this is with my investment and furnaces. I
have as a general rule used thermocouples inside the large flasks I
use for bronze and aluminum castings. These may weigh hundreds of
pounds (the investments, not the thermocouples). I heat these at
full heat and hold at 1100-1200F. for three days.

The trials with jewelry investments were burned out in an electric
muffle furnace and were the size for single pieces of jewelry such
as a ring, etc. For this kind of thing, incidentally, I use the cans
or tins that are used with tuna or similar products. They work very
well and can be used several times before being discarded.

I find that there are so many opinions as to how things "are done"
that I find it easier to run a few quick experiments or tests to see
how things work for me, myself, in my shop. I advise others to think
on this but to stay safe.

Gerald Vaughan

I find the idea of drying an investment before placing it into the
kiln to be unusual. I learned to begin a burnout immediately after
the investment was totally hardened. For large investments (to be
filled with bronze), if unable to begin burnout immediately, the
flask is dampened and securely wrapped in plastic sheeting to retard
drying. These large investments (30 to 200lb.) will begin to develop
cracks upon drying. My jewelry investments do not behave exactly the
same but will sometimes develop cracks if dried before burnout.
Perhaps this is because I use an investment composed of plaster and
SiO2 (very fine sand) with, on occasion, a few other minor additions.
Is this not basically what is in most commercial investment powder? I
know is MMuch less expensive.

There seem to be many ways of doing things which turn out to yield
similar results.

Gerald Vaughan