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Drying investment completely for burnout


#1

I am a bit puzzled as I have seen different instructions for
preparing invested flasks prior to burnout. In some of my books the
instructions are that the investment should be totally dry before the
flasks are put in the kiln, and that there should be no steam coming
from them.

Other books say the investment should be slightly moist and not dry.
In fact they caution against letting the investment dry throughly
before putting the flasks in the kiln

So which is it?. Should the flasks be totally dry, or still slightly
damp?.

I am using Kerr Satin Cast
Alma


#2

Hi Alma,

I really don’t know what the “correct” answer is, but I have had the
best results from making 24 hours my shortest time to wait. In our
class we invest one week then burnout and cast the next.

I think I had a flask that was still too wet and when I burned it
out it exploded and made a mess all over the inside of the kiln. It
actually had dried for overnight. But the work place was cold and I
suspect it didn’t dry as much as it needed.

Just my two clams worth,

Ken Moore
www.kenworx.com


#3
So which is it?. Should the flasks be totally dry, or still
slightly damp?. 

The latter is to me correct. Get the flasks in to the kiln while
"wet".

John Dach


#4

Slightly damp, this aids the wax elimination as the water helps to
keep the wax from infiltrating the investment as much as it would if
the investment was completely dry.

James Binnion
James Binnion Metal Arts


#5
So which is it?. Should the flasks be totally dry, or still
slightly damp?. 

Totally dry is more likely to crack, and will get more carbon
residue in the mold that needs to be removed.

You normally want investments like Satin Cast, or similar, to set
for two to three hours or so after investing, before going into the
oven. Can be as little as a half hour for small flasks (though one
hour is a lot safer). If the flasks will sit for some time, drying
out before burning out, you can either keep them in a plastic bag to
keep them moist, or just dunk them in water for a few minutes prior
to burning out to resoak the investment.

When you burn out the flasks, the water turns to steam, which not
only helps conduct heat into the flask for those critical first
couple hundred degrees, but importantly, the wet plaster also doesn’t
absorb as much melting wax. The steam actually helps expel the
melting wax. So there’s less wax residue carbonizing in the mold.
That helps

Totally dry investment still works, especially with smaller flasks,
but you need to heat up the flasks for the first few hundred degrees
more slowly to avoid cracking. Once the flask temps are past about
350, there’s no longer any difference between flasks that started
damp, or those that started dry.

Peter


#6
So which is it?. Should the flasks be totally dry, or still
slightly damp?. 

I let them ‘set’ for at least an hour and throw them in the kiln.
Either a cold or 300 F kiln. 1/2 dry ( sitting round for a day+ etc)
and they get a soak until they stop bubbling.

My thoughts are that the steam pressure from the investment keeps
some of the wax from soaking into the investment. Probably a better
theory for injection waxes but it seems to work for carving wax with
a fast enough ramp to 300 F

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#7

Alma,

If you let the Investment completely dry out it can cause problems
with the casting.

Investment is porous and the carbon from the wax during burnout will
not completely burn out if it is too dry. The carbon will work it’s
way into the Investment.

The steam helps remove all traces of the wax and carbon. I generally
Invest my cylinders the afternoon before I will cast giving the
Investment no more than 24 hours before I start my burnout.

If you let a cylinder dry out completely, simply dunk it into a
bucket of water for a few minutes and then let it dry for a few
hours before burnout.

Happy Holidays
Greg DeMark
www.natureinspiredjewelry.com


#8

Having just done a LW casting over this pas weekend…

I have always commenced burnout with a slightly moist investment. In
my recent case I was using a 2.5"x3" flask; pretty small really. I
time this thing so that I insert the flask in a 400* preheated kiln
exactly one hour from the time I first began mixing the investment.
Works perfectly every time.

The theory here is that steam pressure from the risidual moisture
will help remove the wax and open the pores in the investment walls.

Cheers,
RC2


#9

We put ours in the kiln about an hour after investing while it’s
still slightly cool and damp. We burn out very slowly at first to
get the wax to drain out before taking to a higher temp. If they have
to set too long, say overnight, it’s a good idea to rehydrate before
burn out.

Jo Haemer
www.timothywgreen.com


#10
If they have to set too long, say overnight, it's a good idea to
rehydrate before burn out. 

I have not had any problems steam dewaxing and putting flasks in the
oven on a time delay for 7 hours before the kiln came on.

I have had kilns died during burnout, unloaded flasks, rebuilt kiln,
reloaded kiln and started burnout all over again, sometimes not
restarting the kiln for a day and there was no problem.

I was burning out 9 3"x7" flasks at one time, 250-350 grams per
flask.

Richard Hart G.G.
Denver, Co.


#11

I use Kerr Satin Cast and very small (2.5 X 2.5) flasks. I put them
in a 400F oven after 30 min. to an hour. I have not had investment
related defects. I follow the Kerr mixing ratio and time exactly.
This method has served me well for 20 years.

Happy Holiday,
Kevin


#12

Thanks everyone who answered my question about drying investment.
Moist or slightly damp it will be. And thanks especially for telling
me why. Once one understands the reason for things it makes for
better workmanship.

Alma


#13
I really don't know what the "correct" answer is, but I have had
the best results from making 24 hours my shortest time to wait. In
our class we invest one week then burnout and cast the next. 

Ken, at 24 hours, it’s actually drier than most casters would like,
though still OK. Your class is a special situation, since the
instructor has to arrange the timing of when to invest and when to
burnout and cast, to fit the class schedule. In many such school
programs, flasks are allowed to sit sometimes for days, as is done
with your class. In the last one I taught, while in grad school, the
routine was also invest one class session, burnout and cast the next
week, but flasks were dunked in a sink of water and allowed to soak
up enough water they’d stop “bubbling” before going into the oven.

I think I had a flask that was still too wet and when I burned it
out it exploded and made a mess all over the inside of the kiln.
It actually had dried for overnight. But the work place was cold
and I suspect it didn't dry as much as it needed. 

As other posts in the thread have also pointed out, moisture in the
flask helps drive the wax out, rather than letting it soak into the
investment, so burnout is more easily complete. As well, in the
initial stages, water is a better heat conductor than is dry
investment, so initially, it helps even out the heat transfer into
the flask during the initial period when wax is being melted. Flasks
can “blow” for a number of reasons. Often, the initial heating stage
is too rapid when people burn out in kilns not equipped with a
programmable controller. The quartz (silica) in the investment goes
through a structural change while passing through the temperature
range in the 300 to 350 degree area and there is a slight dimensional
change to the crystals that goes with this. Going through that temp
range too quickly puts stress on the investment. Also, when
investing, it’s important to pay decent attention to the water to
powder ratio. Too much water gives a weaker investment, more prone to
cracking. And carving waxes are also harder on the investment than
the softer and lower melting injection waxes.

For what it’s worth, virtually all the manufacturers of casting
investments used for silver or gold recommend a wait time, after
investing, of just a couple hours or so before beginning burnout, and
don’t suggest needing the stuff to be fully dry. It reaches full
strength in just an hour or two if properly mixed.

HTH
Peter Rowe


#14

Thanks Peter,

As I have been reading these replies I have been educated in this
process. I think I might place the flasks in a zip plastic bag if I
have to let them set for more that several hours before burning out.
Normally we don’t have the time to dunk them let set to dry then burn
out. I will be casting in the near future so I will try out not
letting the invested flask to fully dry out.

I try to learn something every day and with the help of you and the
other Orcidians I manage to do that.

Thanks to all,

Ken Moore
www.kenworx.com


#15

What I usually do is mix my investment and let it dry for about 30-
45 mins. then pop the rubber bases off and put them in the oven. I
then turn on my over but set the timer to come on at 3:00 am and when
I come in the next morning, they are at the highest temp. I then turn
it down to let the temp cool down to casting temp and let it hold
there for about 2hrs to equalize the oven temp and the flask temp.
Then I cast the pieces. This system has worked for me the last 10
yrs. and so far, not 1 problem. I use a regular household timer so
that my oven kicks on in the early AM so I don’t get stuck in there
with all the wax fumes and it’s ready the first thing in the morning.
This may help for you.

Steve Cowan
Arista Designs
www.aristadesigns.net


#16
I have not had any problems steam dewaxing and putting flasks in
the oven on a time delay for 7 hours before the kiln came on. 

While more efficient heat transer to the interior of the flask
during initial heating is perhaps useful, the main advantage to a
properly hydrated flask is in the action of generated steam helping
drive the wax out, rather than letting it soak into the investment,
which then requires more time at burnout temp to remove all the
carbon. But you’re steam dewaxing already, so you’ve already got a
pretty clean flask.

But yes, you’re also right. most of the time, especially if the
investment is mixed properly to the right water/powder ratios, it
will be quite strong enough to withstand any slight additional heat
shock it might get from some of the situations you mention,
including being bone dry and cold before continuing a failed burnout.
Actually, in that situation, where the burnout started then failed,
and the investment has already been heated a good bit, enough to cure
it and remove most wax, I’d avoid rehydrating since at some point,
the investment is altered by heating to that final form where a dunk
in water starts to break it down… Even in cases where unfired
flasks have dried, problems with not rehydrating are rare. But in
school situations, where one has multiple students occasionally
making investing mistakes and sometimes water/powder ratios can be a
bit variable, one can reduce the already fairly rare instances of
failure (along with the associated student anguish) by rehydrating
flasks that have sat long enough to become dry before burnout.
Rather than an absolute necesity, it’s more along the lines of an
abundance of caution. This helps to foil that Murphy fellow, who
seems to think his rules should apply, especially if you’re in a
hurry, Christmas is almost here, and the job HAS to be done on
time…

Peter Rowe


#17

My experience is in casting mostly gold for crowns and inlays: I use
a small casting ring (capacity of about one ounce of investment) and
usually cast only one or 2 items in a ring using, in the past,
asbestos expansion liners and cristobalite investment.

When you stick to the recommended water/powder ratio you get a
chemical setting reaction rather than a drying. Within 30 minutes
you can go into a cold oven and ramp up as fast as you can to first
melt out the wax and then to reach final temperature for casting in a
second ramp up. The fastest I cast anything was probably 75 minutes
from investment mix to casting, and it fit properly.

After casting I would let the red glow of the button disappear and
quench while still really hot in cold water. Cristobalite would
clean off very nicely with a toothbrush. Hi temp investments for
higher melting alloys would be much harder to clean out.

Charles Friedman DDS
Ventura by the Sea


#18

There are also different requirements of different waxes as far as
wet vs dry flask on burnout. To me, the harder the wax, the more the
wetness is needed. We cast a lot of Filo wax (very high plastic
content) and a '“wet” flask is essential, as well as a hot kiln to
put the flask into. Dry investment and/or a cold start up almost
guarantees cracked investment. Softer, more fluid and lower melting
point injection waxes are not as “demanding” and highly plasticized
waxes.

I know “your not really supposed to cast Filo wax” but we do as/when
needed and it works fine if the wet flask is put into a HOT kiln.

John Dach


#19

Investment should be moist (not wet) for burnout. The moisture turns
to steam and helps push the wax away from the walls of the pattern
cavity. A dry flask will act as a sponge and absorb the wax making a
rough pattern. I came in on this late, what kind of investment are
you using?

Shannon Calloway
TIJT


#20

Hi Alma,

I always advise anyone to use the mixing instructions from the
manufacturer of the Investment. In your case it is Satin cast. Here
is the link to the instructions needed.

http://tinyurl.com/ybr9vt6

If you are in the mountains… high elevations or in a hotter are
you may have to adjust your ratios slightly.

If you have any problems with their materials, call their technical
people. Sometimes there may be a bad batch floating around and they
can tell from the batch number on the Investment Barrel. This is rare.

Kerr and R&R have an excellent technical department that can give
you answers to any issues you may be having. Most investing issues are
caused by having faulty equipment or operator error.

Hope this is helpful.
Daniel Grandi
http://www.racecarjewelry.com