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Solving large flask blowout problem

Up until now, I’ve been casting mostly in 2 1/2" X 4" flasks. I got a
few used 3 1/2" x 7" flasks on ebay recently. Yesterday I loaded one
of them up with 42 waxes of an antique button I wanted to reproduce
for a very detailed Ren Faire costume. I was very, very careful to
space all the waxes a good distance apart on the tree and far from
the sides of the flask. There was nearly a full inch of solid
investment between the top wax and the top edge of the flask. I
weighed the investment and the water carefully, mixed and vacuumed
it well, and did a long, slow burnout (I had 10 other smaller flasks
in the kiln with the behemoth flask). Got Big Boy out of the kiln at
1000F, put it on the vacuum table, and as soon as I turned the
vacuum on, the flask blew out (fortunately this happened before I
poured 320 grams of hot bronze into it…).

I’m SO bummed. I was as careful as I could be with the whole
process. What did I do wrong, and what can I do to prevent this from
happening again? I’ve got both of the other huge flask bases set up
with waxes, but now I’m afraid to invest them for fear that I’ll lose
all that work. Are there special techniques for working with really
large flasks that I’m not aware of? Any suggestions/advice would be
greatly appreciated. I’ve got a bunch of casting to do this week for
an upcoming event, and I need to decide asap whether to remove all
those waxes from the other big flask bases and move them to my usual
2 1/2" ones.

Kathy Johnson
Feathered Gems Jewelry

Kathy, I’m not a caster (my brother, Hans The Jeweler could probably
give you some guidance, as he casts all sorts of stuff in all sizes)
but it would seem to me that if you have a close deadline you should
re-sprue to the smaller flasks for now and experiment later with the
large flask.

OK, I spoke with Hans The Jeweler who asked me to relay the

He assumes that by ‘blew out’ you mean the blind end of the flask
caved in. if so…

1" is not thick enough for a large solid flask, should be more like
inch and a half for vacuum casting. One inch would be OK for
centrifugal though. Mixing is critical.

You didn’t mention if this was a perforated flask. If so, one inch
would usually be OK, given that the vacuum draws thru the entire
investment, spreading out stress.

If the flask gets inadvertently knocked along the way, that can
cause failure too.

He hopes that helps and I hope I relayed all that correctly.

Hi Kathy,

Welcome to the mystic world of casting. Some problems have an
immediate recognizable reason for happening. Others just happen
without any logical explanation.

I vacuum cast flasks from 2 1/2 inch up to 6 inches in diameter. In
33 year I have had two blow out failures.

One was a failure in a 4 inch flask and the other was in a 2 1/2
inch flask.

The wax in this flask was a 3 1/2 diameter piece of my pottery with
the bottom of the wax near the non sprue end of the flask. When I
pulled the vacuum the sound was strange indicating there was a
problem. When I lifted the flask to check things out there was a 4
inch diameter investment plug laying on the table. The plug has the
design of the wax on the surface. After that I invest my 3 1/2 inch
diameter waxes in a 5 inch flask.

The other blow out was a hole in the center of the flask. Not sure
why this happened as the conditions of the flask from investment to
burn out was the same as 15 other flasks that I cast that day.

There are a lot of maybes that can cause blow out failures. Maybe
the investment was old. Maybe the mixing was not complete. Maybe the
burn out cycle was not correct. Maybe there was not enough investment
above the wax. Maybe the layout of the wax near the top end of the
flask caused the investment to crack. Maybe the manufactures
investment is not as strong as other manufactures. The maybes go on
and on.

I try to follow some rules when I invest my waxes. I must admit I
violate these rules all the time.

I try to have at least of investment for every inch of flask
diameter above the waxes.

I try to always use a flask that provides at least a of an inch
between the wax and the flask. I have cast some trees that have less
that 1/8 of an inch between the wax and the flask.

I use Ransom and Randolf investment.

I would suggest you provide more investment above you wax. If you do
not have a taller flask you can very gently use a hot tip tool and
melt the tree off the sprue base that melt a portion of the tree off
the whole an re-wax the remaining tree to the sprue base.

You sound like you covered all the maybes and should not have had
blow out failure.

There is still one more maybe that I didn’t mention that could cause
the failure. Maybe the gremlin who hates lost wax castings caused the

Lee Epperson

Hi Kathy,

I know what it’s like to spend days making waxes, cleaning them up
all nice, spruing very carefully, etc, only to have the investment
blow out on the vacuum table. I have found a few things that help
though. One inch of investment (or even 3/4 for smaller flasks) over
the top wax has always been enough for me if everything else is done
right, but if you want to be safe go a bit further.

Make sure your investment is relatively fresh and hasn’t absorbed
water from the air. I live in a very wet place in Alaska and do my
casting in an unheated shed that regularly has water all over the
floor, but I have found that I can keep my investment dry and
working well for up to a year by keeping it in a tightly sealed
plastic bag with a plastic container of “Damp-Rid” (a chemical that
absorbs water from the air) on top of the investment. If the
investment is old and has absorbed water it will take longer to set
and will be softer and more likely to blow out. If the investment is
fresh and is mixed at 78 degrees farenheit it should gloss off in
about 10-12 minutes, more than that means it’s either too cold,
wasn’t mixed well, or has absorbed water. If that’s the case then
you risk blowout problems due to soft investment.

Old investment has been the main cause of flask blowouts for me. If
you want to make the investment harder you can increase the water
temp, as well as mixing it at a faster speed.

For myself, I have pretty much eliminated all blowout problems by
keeping my investment dry with Damp-Rid, as well as using hot water
to mix the investment, since my shop temp averages about 50 degrees
farenheit and the investment is the same temp, I have to use 100-105
degree water to get it to gloss off around the 11 minute mark. Also
a well controlled burnout is essential, I don’t know how I lived
without a digital kiln controller but it is one of the best tools I
ever bought.

Hope this helps you, Good luck.

Hi Kathy,

I had a similar problem a few years back when I was casting large
silver belt buckles. It only happened when using large (4X6) flasks,
and seemed to happen regardless of how much top space I allowed. I
finally got new investment and mixed it with a little less water than
usual, about 62 - 38 ratio, and paid extra attention to the water and
investment temperature on the advice of my investment manufacturer.
That solved the problem. I don’t know if it was old investment or
just that I wasn’t using the proper ratio as I changed both at the
same time, but I think it was the ratio. The old investment was still
glossing off correctly at 11 minutes and worked just fine in small

If you try changing the ratio, be prepared for it to set up a little
bit more quickly than normal. Try to be finished at about 8 minutes.
Might be a good idea to mix up a small test batch and time it.

If you have access to a centrifugal machine, that might prove to be
a good solution as well. I’ve never had a blow out using centrifugal,
although I suspect it could still be a possibility if you are casting
a substantial amount of metal with less than a inch of top space.

Good luck and I feel your pain!

Hi Kathy,

I did some work with a local sculptural foundry making bigger and
bigger casting flasks, 18"x24" anyone?

The solution to flasks breaking out of the bottom was to drill a
1/4" hole across the diameter of the flask just above where the flask
base is and put in a length of stainless steel studding, held with
stainless nuts.

If you leave 1" sticking out either side you can easily hook them
out of a top loading kiln.

4" flasks are mostly OK 6" ones this extra support is vital

regards Tim Blades.
(Who now has a shiny new hard drive and is going to back up everything
THIS time…honest)

Thank you to everybody who replied with advice on the large flask
issue. I’m going to try again tonight. Here’s what I did to try to
address the problem:

First, I removed the “valuable” waxes from the other two tall sprue
trees that were awaiting investment, and put them on shorter trees so
I won’t lose them if the big flask blows out again. I put expendable
waxes (which I can certainly still use if the casting works) on the
taller sprue tree as a test run. This is superstitious behavior, but
I figure that if I don’t care what’s in that flask it’ll come out
just fine. If it’s something I desperately need, it’ll blow out.

Second, my husband took the used tall flasks to a friend’s house,
where he ground them down flat and flush on both ends with a large
disk sander. The ends of the flasks were rusty, and perhaps there was
an air gap that caused a problem.

Third I opened a new box of investment, and invested the flask with
over an inch of investment above the end of the flask. I used the
40/100 mixture again because I didn’t get some of the later replies
until after I’d already invested the flasks.

Fourth, I got a brand new rubber pad for the top of the casting
table, which I will use first for the big flask.

Fifth, on the advice of a private reply, I tested the tall flask on
the vacuum table before I de-waxed it. I was told that if it held up
to the vacuum when solid, odds were better that it would hold up
after burnout. It didn’t blow out.

The 3 1/2 x 7" flask, along with eleven 2 1/2 x 4" flasks, is in the
kiln right now, and I’ll be casting them all this evening. I’ll
report back with results later.

Kathy Johnson
Feathered Gems Jewelry

Great suggestions so far! But as I remember the original post (which
unfortunately I’ve deleted) the investment popped out before there
was any attempt to cast, so the type of casting machine probably
isn’t an issue. Based on my own experience I’d check for:

  1. too much water in the mix (or old waterlogged investment) or
  2. not enough set-up time.

I’m more inclined to suspect the first, recalling a similar problem
of my own.

Rick Martin

Well…I set up the large flask as I described in my earlier post.
I steam de-waxed it, and burned it out in a kiln with 12 smaller
flasks. I saved it for last to be cast so the electromelt would be
good and hot for the larger amount of bronze.

Once again, as soon as I turned on the vacuum table, the flask blew
out–a divot right out of the middle that covered approx. 1/2 to 2/3
of the top surface.

I won’t be using the big flasks again until after I get the casting
done for this event and return home from the show. Then I’ll
experiment with thicker investment mixes and much shorter sprue trees
to see if that makes a difference. Those are the only two things I
haven’t tried yet. If those don’t work, I’ll have some big flasks for
sale, cheap…

Kathy Johnson
Feathered Gems Pet Jewelry

Possibly the cold flask got put into a hot oven, Not good!

But as I remember the original post (which unfortunately I've
deleted) the investment popped out before there was any attempt to
cast, so the type of casting machine probably isn't an issue. 

My investment failures seemed to be similar to Kathy’s in that the
investment popped as soon as the vacuum hit it, meaning that the
vacuum pulling on the bottom caused the weakened investment to fail,
not the weight of the metal. With centrifugal, there is no pulling
force applied to the bottom, so it can’t blow out that way. Still, it
shouldn’t blow out, even with vacuum on all but the largest flasks
(18 x 24? Now that’s a big flask!), so I would agree that vacuum
casting would present more of an extenuating circumstance by pushing
a weakened investment mixture beyond it’s limit rather than being a
primary cause for a blow out. But by using a centrifugal machine and
not exposing a weak investment to the surprisingly strong pulling
action of a vacuum table, it might be possible to get it to work
without having to trash what’s left of a questionable bag of
investment, as long as it’s working OK otherwise.

If you want to see how strong that pull really is, put an empty
flask on the table, turn on the vacuum and put the palm of your hand
on it. Make sure you can release the vacuum with your other hand
before trying this though, or you might be stuck for a while! You’ll
wonder how even well mixed and strong investment ever holds up to
that kind of force.

Kathy, once you get the problem figured out, check your casting
table plumbing to make sure it’s not full of investment. I forgot to
mention that mine filled up with broken pieces that eventually found
their way into the pump and killed it. Investment in the oil makes a
very abrasive mixture that resembles lapping compound used for
lapping engine valves. Good for a valve job, very bad for the pump.

I’ve always suspected unexplained casting malfunctions might
actually be the work of mischievous space aliens, but I’ve never
actually caught one in the act. Watch out for Lee’s lost-wax-hating
gremlin. He might be the real culprit.


Possibly the cold flask got put into a hot oven, Not good! 

Nope. It went straight from the steam de-waxer into a room temp oven
that warmed up to 300 degrees over the next 2 hours and progressed
from there. It never got a chance to cool off at all between the
de-waxer and the kiln.

Kathy Johnson
Feathered Gems Jewelry

My investment failures seemed to be similar to Kathy's in that the
investment popped as soon as the vacuum hit it, meaning that the
vacuum pulling on the bottom caused the weakened investment to
fail, not the weight of the metal. 

Reading this brought back memories.

A big 5" X 8" centrifugal induction machine with a 22" vacuum
assist. After drilling silver out of the vac lines 3 or 4 times I
quit using the vac. (youth induced slow learning syndrome :slight_smile: Soon to
follow were the rather expensive to maintain argon and immersion
pyrometer. Without all the fancy features it did function as a rather
nice manual machine.

Demand Designs

Possibly the cold flask got put into a hot oven, Not good! 

Good point, especially since we are talking about steam-dewaxed
flask: They should go directly from steam into the pre-heated oven.
Don’t let them cool down.

There must be something going on with the flask blow out described
in the posts.

I have invested and cast flasks from 2 1/2 inches to 6 inches for
thirty years. I have had only two blow outs that I can remember.
Something in the steps I take prevents blowouts or the casting
gremlins are busy elsewhere when I cast.

The frustrating thing about casting is that what works for one may
not work for others. Knowing this I risk telling what I do in
investing and casting my flasks. Some of the techniques will probably
leave casters thinking “NO WAY THAT WON’T WORK.” However I have had
success following these steps.

Maybe these steps will lead to clues as to why blow outs happen.

I try to follow the rule of allowing inches of investment above the
wax for every 1 inch of flask diameter. Note I said I try to follow
the rule. Some times on smaller flasks I violate this rule.

If the upper part of the wax forms a solid plane like the bottoms of
my large pottery I provide a much larger amount of investment above
the wax.

I do not worry about the distance between the wax and the flask on
most of my waxes as long as the wax does not touch the flask. On the
larger pieces of pottery I try to provide at least of an inch between
wax and flask. The only investment holding the top of the investment
to the bottom of the investment is the ring between the flask and the

I do not use tap water but use only water bought from local water
treatment stores.

Both the investment and water are at room temperature.

I use Ransom and Randolph investment. I store the investment in my
house. I have had investment sit for many months between usages.

Weight out 5 pounds of investment and measure the recommended amount
of water. I pour the recommended amount of water into the mixing
bowl. Pour the investment into the water. Mix the investment with an
electric mixer on high speed for 1 min 15 seconds.

Place the mixing bowl under a bell jar and pull a vacuum. The vacuum
is maintained for 1 minute after the investment pancakes.

Place the flasks to be invested on the rubber pad and pour the
investment through a kitchen strainer into the flasks. I find the
strainer will sometimes strain our improperly mixed investment.

Five 2 1/2 inch flasks can be invested at a time.

Maintain a vacuum for one minute after the first bubble appears in
the investment.

If all goes well I will have enough time to invest a second set of
flasks with the remains of the first 5 pounds of investment. I try
to complete all the investing within a seven minute period.

Let the flasks sit for three hours.

  • I scrape the top of the investment with a curved trowel. This
    gives a slight concave shape to the investment. I believe the vacuum
    will be more uniformly distributed across the entire face of the
    flask. The slight curve in the investment might, and I am just
    guessing, give the investment a little more strength.

  • My oven holds 16-2 1/2 inch flasks on the first level. I will add
    additional flasks if they are short. The oven will handle flasks up
    to seven inches tall. The taller flasks should be placed in the
    center and front of the oven. There is a danger of overheating the
    heating coils if tall flasks are placed against the back and side
    walls. If I cast very large flasks I place them in the center of the

I normally do not cast unless I have a full oven.

Place flasks in the burnout oven and increase the temperature from
room temperature to 250 degrees and hold for 2 hours. Increase the
temperature by 300 degrees per hour until the temperature of 1350 is
reached. If the oven is really packed the oven may not be able to
increase 300 degrees per hour.

I maintain the burnout temp of 1330 for 8 hours. I have found it
takes 8 hours to completely remove all the wax in the full oven. Then
cool the flask down to 860 degrees if silver is to be poured. The
flasks are allowed to sit for at least one hour before pouring the
first flask.

The silver is melted in an electro melt furnace at around 1370

The flask is placed on the pad when the system is set to pull a
vacuum. It might be safer to place the flask on the pad then increase
the vacuum to the flask.

Pour the silver.

Place the hot flask immediately on to a wax covered pad. Throw wax
on the sprue and cover the flask with a larger flask then place a
solder pad on top of the cover flask. This if my way of preventing
fire scale on the silver casting. The process is better described in
Orchids “Tips from the jeweler’s bench.”

Allow the melt furnace to stand for around 5 minutes before a second
pour is made. This allows the upper ring of the graphite crucible to
heat up to pouring temperature. If the ring is not heated up again
before pouring the silver will chill as it flows over the ring.

I quench the smaller flasks in about 7 minutes after pouring and
around 15 to 20 minutes for the very large flasks.

Note: The gripping tongs normally used to hold the flasks do not
work well on the 4, 5 and 6 inch flasks. To solve this problem I
scrape some investment away form the top and bottom edge of the flask
so that I can grip the flask with vice clamp pliers. The flask is
removed from the oven by clamping the pliers on the non sprue end.
The flask is placed on a flat surface and the pliers are moved to the
sprue end. The pliers are left clamped to the flask as the silver is
poured. The flask and head of the pliers go under the surface of the
quench water when quenching.

Now that I have said all that the casting gremlins will probably
cause some blow outs when I cast four 4 inch flasks this coming

I will attempt to add some photos and drawings to illustrate the
above to my blog next week.


Here’s couple of more thoughts.

How old is the investment? If it sits for more than six months it
will start to separate. Like redi-mix cement sitting in a bag. After
a while you have gravel on the top and the heavier ingredients on the
bottom. Use new investment or mix the unused investment once in a
while (put a mask on). You can feel lumps on investment sometimes.

Do you measure you powder and water? It comes with three water
powder ratios to use. Try a thicker mix. Do you have hard water (lots
of minerals)?

Make sure you have at least one inch of plaster above your waxes. It
does not have to be to the top of the inside of your flask. Within
1/8th inch is fine.

Do not burn out above 1350 f. If you leave your flasks burn out
overnight you may be overheating the flasks. This will weaken the
investment and cause blowouts. Probably more on larger vacuum flasks
than spin casting. Make sure the color of your flask is all white.
Grey would indicate an incomplete cycle.

How fast do you ramp your temperature to 1350? This should be