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Bubbles on Castings

hi all you casting mavens,

i’ve been experiencing bubbles on my castings and have trouble
shooted enough to ask for your help. i’ve had bubbles prior to
all the conditions i’m about to describe. i still have them, but
less of them. first i assumed that my mixer was introducing too
much air into the ivestment, so i bought a kitchen-aid (made by
hobart, model k5)super heavy duty mixer and mix with flat
spatula (no wisk!) or the dough hook. though i needed a new
mixer, this did not help much. after i bought the mixer, the
stainless bowl didn’t fit into my old bell jar so… i bought
a larger 10" bell jar, (the i.d of the old bell jar was about
8’’) then everything fit nicely. i put the stainless bowl under
the bell jar and debubble. but i still have bubbles.

my vacuum machine gauge indicates 27 inches of mercury, after i
tap it a few times, (thats all the vacuum is capable of, we are
at sea level) then i play punching bag with the investing table
for about 1 1/2 minutes. since the indicator gauge is pretty old
(over 20 years???), i didn’t think it was accurate, so i was
assuming the vacuum unit was fine. it is only about 9 years old,
a 3cfm. the next thing i did was to connect a direct line only
to the investing table. i got what seemed like a more vigorous
boiling, but… still bubbles on my castings, but not as bad.
all the bubbles are located on the inside of shanks at the 12
o’clock position, even though i tilt all my waxes a bit. i may
have short changed the time a bit on this batch. of course i
vacuum the flasks again after i pour the investment in them.
after i finish the second vacuum, i swirl the investment around
in the flask to hopefully dislodge any bubbles.

without investing anything i created a vacuum in the bell jar to
see how long it would hold the vacuum. without any indicator i
figured this would be inconclusive without comparitive data. but
it did hold for the few minutes it took for me to figure that
out. so my next step is to put a new gauge in line with the
direct line. this will tell me exactly what vacuum i’m drawing.
finally here are my questions:

  1. do surface tension reducers get old? i pour the s.t.d. back
    into the bottle after i’ve used it. i’ve had the same quart
    bottle for about 2-3 years.

  2. what is surface tension reducer? is it just denatured alcohol
    with pink dye?

  3. i’ve always read that 27 was an adequate vacuum. is 27 too

  4. i put my old gauge indicator directly on the vacuum pump and
    only got a 26 indication. assuming i got a good seal, does one
    need an evacuation chamber (bell jar) to get an accuarate

  5. i’m aware that the larger bell jar means more air space for
    my 3cfm to remove air from. assuming the vacuum unit is good, am
    i asking too much from this size vacuum unit.

  6. if a 3cfm unit isn’t adequate enough, what could you guys
    suggest that would be a cheap alternative to a 5 to 8 cfm vacuum
    unit? i know about old car air conditioners working, but i
    assume that they are more comparable to a 3cfm unit. perhaps a
    large home air conditoner compressor would work?

  7. has anyone tried to reverse the fittings on a standard air
    compressor so it sucks instead of blows? i sort of put my finger
    over the intakes of my compressor, WOW!, did it suck! i’m happy
    to still have my fingers unbroken. no easy way to put a hose on
    this particular compressor tho.

  8. could some one explain ‘negative inches of mercury’ to me?

many thanks to all who respond to any of these questions! i know
it is asking a lot.

best regards,

geo fox

Hi George,

I never even suspected that you people who don’t use mixers like
the Whip-Mix vacu-vestor combination unit go through all this
incredible rigmarole to invest! Maybe you need to look up your
local friendly shaman and see if they can hustle you up a
talisman for good luck:)!

Surface tension reducer is cheap and may get old. Why not treat
yourself to a new bottle? You would, at least, assure yourself
that this wasn’t the problem.

Tilt your rings in the flask so that the bubbles have the
ability to slide off.



                                  Skip Meister
                                NRA Endowment and

Back when I was casting, I mixed the investment in a rubber bowl
with a spatula, banged the bowl against a table a few times to
release air bubbles and poured it down the inside of the flask.
Occasionally I would paint the waxes with debubblizer and a layer
of investment. I got very good castings, usually.

Richard D. Hamilton

Fabricated 14k, 18k, and platinum Jewelry
wax carving, modelmaking, jewelry photography

George, not trying to avoid your direct questions but, have you
tried using a little more water in your investment? I used to use
a 40/100 ratio of water and was having the same problem with
bubbles - especially on rings with hollow undergarments. I
started using a 41/100 ratio and the bubbles dissapeared. (could
be the investment too) Ken

G’day, George et al: First let me explain how a vacuum is

You seal a glass tube at one end of at least a metre in length
and fill it completely with mercury, stick your thumb over the
open end and place it under the surface of a small bowl of
mercury. When you release your thumb the height of the mercury
in the tube will fall, and top of it will be at such a height
that the atmospheric pressure all around is sufficient to hold it
up. Above the mercury is a vacuum. If the atmospheric pressure
rises, the mercury will rise, and vice versa; That’s how you make
a barometer. The average height of a barometer at normal
temperature (20C) and pressure is 760mm or 29.9 inches (They
called this NTP - Normal Temperature and Pressure - in physics
classes 60 years ago) Now instead of having the open end of
your tube in a bowl, you have it in the mercury pool in a jar
which is sealed, but has a tube to which you can attach your
pump or whatever you are evacuating. When you start pumping the
mercury will fall to the height of whatever air pressure is left
in the jar. So now you have a vacuum gauge. If your mercury
falls to a height of say, three inches, you have a vacuum of
30"-3" = 27" inches of vacuum. So there can be no such
measurement of so many ‘negative inches of vacuum’ as you would
have less than nothing. (DOES NOT COMPUTE!! as R2D2 once said)
However, vacuum gauges based on a barometer are only used in
certain laboratories - old fashioned ones usually, and the usual
but less accurate gauge uses a flattened brass tube which bends
according to the pressure differential between the outside and
the inside. There are several other methods of measuring vacua -
or very low pressures to be pedantic, and include the loss of
heat by conduction from a hot wire, the slowing of a vibrating
quartz fibre, and others, but these are for extreme low
pressures, like the pressure in a synchrotron - ten to the minus
fifteen of a centimetre. But interestingly, all these things
have to be calibrated against a mercury barometric device. One
type being a glass McLeod gauge - I used to make 'em! Sorry to go
on like that, but I couldn’t think of a shorter clear explanation
of ‘negative inches of mercury!’ So here endeth the lesson.

       / /    John Burgess, 
      / /
     / //\    @John_Burgess2
    / / \ \
   / (___) \
1. do surface tension reducers get old? i pour the s.t.d. back
into the bottle after i've used it. i've had the same quart
bottle for about 2-3 years. 

George, I dont know what you are using but you might try
switching to R&R ultravest. I never put anything on my waxes and
never have bubbles. Its supposed to have a wetting agent that
eliminates the need for “debubblizers”. I am not as technical as
some of these guys, so I don’t know how it works, I just know it
does. They may send you a free sample, especially if you tell
them you are switching from Kerr (arch rival). Anyway it works
for me. Good luck. Mark P

Mr. Fox We do a lot of casting 3to4 times of year. we got a lot
of bubbles in side and a tool rep sold me some waxofilm and it
works well just spray it on let set and mix your investment . do
you vac. 1 or 2 times? this too might be why you have bubbles. we
went to a class swest put on 15 years ago and we got a lot out of
it and if swest still has a class you might want to go. you can
get waxofilm from swest.




Here is what i have to say: A good vacume system must have a good
vacume pump strong enough to produce a vacume that wll permit
water at room temp (72) to boil at room temerture. This means
that the vacume created under the bell jar will be strong enough
to lower the normal vapor presure of water (760 millimeter or 30
inches of mercury) at room temp to a vapor presure below 19
millimeters or .75 inches of mercury at which vapor presure water
will boil at 72 deg F.

A standard size will produce a reduction of air pressure between
25-29 inches of mercury (depending on altitude) in approx. 45
sec under a 10 inch X 8 inch bell jar. the investment should
bubble violently with in 60 sec, if all is right.

So you can try doing this with water and see if it boils in under
a minuite.

I also agree with the other persons point on the angle of the
wax’s. At work we do not even use a bubble relese and have no
problem what so ever with bubbles. So im my humble opionion the
bubbles that are on your casting can not excape.

Let them excape :slight_smile:

Rick in KC

hi john b.,

thankyou for the wonderful explanation! now do i have it
straight? when one says that a vacuum unit ‘draws 29’’ of
mercury’ there would be .9’’ of mercury left in the tube. is
that right?

i have another question about vacuum measurement. a catalog
description says about their vacuum unit: ’ producing a vacuum
of 25 microns’. how does that relate to inches of mercury?

i got the ‘negative inches of mercury’ from another catalog that
sells vacuum units. i don’t read many science books tho i
should. i have seen star wars several times instead of reading
those science books and i would like to tell you that R2D2
doesn’t talk. it was C3PO who said ‘does not compute’ several
times. at least i have my 'droids straight.

thanks again

best regards,

geo fox

 thank you for the wonderful explanation! now do i have it
straight? when one says that a vacuum unit 'draws 29'' of
mercury' there would be .9'' of mercury left in the tube. is
that right?

G’day, George; Yes, that’s right - meaning there is enough gas
pressure in the system to hold only 0.9 inches of mercury up in
our vacuum gauge/barometer.

       a catalog description says about their vacuum unit: '
producing a vacuum of 25 microns'. how does that relate to
inches of mercury?

That would be a really good pump, meaning that the gas in the
system would hold up a mercury column only 25 microns or 25
millionths of a metre high: twenty five times ten to the minus
five of a metre, which is how scientists usually refer to vacua -
or very low pressures. At those kind of pressures mechanical
pumps sort of give up, and a device called a vapour diffusion
pump with no moving parts (other than vapours) takes over at
around 0.1mm of mercury. =

If one needs to go lower than one micron, there is a modern pump
which appears to me to work by magic, using only electricity.
In the very distant past right after WW2 I built a system which
would pump down to 0.1 micron - but I had to spend a few weeks
getting the leaks and bugs out; At that kind of low pressure even
heavy-walled rubber tubing is deemed to be porous! I had to coat
lenses with a known-thickness film of gold or aluminium then a
very, very thin coat of silica. You couldn’t buy anything much
then, so I even had to make a mercury diffusion pump blown in
glass. Yes, I really enjoyed my work in those days.

  i got the 'negative inches of mercury' from another catalog
that sells vacuum units.

Well, I won’t be dogmatic about it, for dogs sometimes bite when
you aren’t expecting it. But I am very skeptical about ‘negative
inches of mercury.’

i have seen star wars several times instead of reading those
science books and i would like to tell you that R2D2 doesn't
talk. it was C3PO who said 'does not compute' several times. at
least i have my 'droids straight.

George - it is around 20 years ago since I saw Star Wars (and
about the last time I went to the movies!) so I sit corrected -
my son agrees it was C3PO. I have however, been reading Science
Fiction ever since I was ten and still have every copy of ANALOG
since 1966; I read it when it was ASTOUNDING SF magazine. But
like everything else, SF has begun going down the gurgler ever

Thanks, George for your questions. Cheers,

John Burgess,

Hi. To the original question about bubbles on casting. About 15
years ago I went to Kerr Rep.s talk on casting problems. And
bubbles came up. :^) His suggestion was that there two thing to
do. On watch your timing on when you pour the investment into
the mold. There is a point called by there tech. guy’s called
gloss off. This is when the water on the top of newly pored
mold goes away or is soaked up the water thirsty investment. On
fresh new investments its supposed to happen at about seven
minutes. The older the investment the higher the time. He said
"try to poor the investment near gloss off, 5-7 minutes with new
investment." His second suggestion was to “keep mixing and
don’t stop as soon as the lump are out.” “The more you mix the
less air in mix.” I vacuum in the bowl so I can get more
vibration to dislodge bubbles. And a second time in the flask for
trapped air. Use vacuum film but let it dry or you’ll get
water marks (looks like mold). Hope this helps. Jim


I just started casting, but I may have a semi-simple solution to
your problem. A way to avoid bubbles is to paint the investment
on. I mix up about a tablespoon of investment at a time (make
sure consistency still “gloves” on your fingers) and I paint it
on the pieces. Once you have about a 1/2 inch thick coating on
it, invest it like normal. It is frustrating and time consuming,
but the results have been worth it. Good Luck! Daryn

I do not know if you have gotten the answer to your questions
about bubbles on your castings but I might be able to help. first
: take the time to clean or replace your air suction lines to
your vacum bench; They some times get cloged ao partualy cloged
and this will reduce the volume of vacume you can drad to the
vacume chamber. yur gauge might still show a good vacume even tho
you realy arn’t getting one to the chamber.

Second; the best way I have found to totaly elimate boubles on
my castings , is to make a contraption that allows you to mix
your investment in one chamber then draw avacume in it and then
release a valve and allow the investment to flow into your flask
while totaly under a vacume. wala!!!no more bubles you can make
one yourself or I think that Rio Grande sells a unit called
the"St.Louis Mixer" it costs about $865.00 .

Thrid: a solution of alcohol and a liquid soap will do fine for
a deboublelizer, then add the red dye!

a Forth thing: you can paint the investment on to your molel
before investing and also eliminate the boubles, this is ok if
you only have one or two mkodels on a sprew to deal with.

The size of your vacume machine should be fine. I’ve cast 75
models on one sprew using a 6"x8" flask with out any problems.
at sea level you should beable to draw a vacume of 29-30 with a
good seal. but 27 should be enough. If you are having a problem
getting enough vacume quick enough with the size of bell jar you
are using you might try a fairly cheep trick. buy a portable tire
inflater tank at your local auto parts store. splice it between
your vacume pump and the vacume table with a valve between it and
the table , then pull a vacume on it , leave your pump running
then when you are ready open the valve to the table and you have
a very high volume vacume quick.

Good luck

I do not know if you have gotten the answer to your questions

hi vernon,

thankyou very much for your reply. i’ve already acted on several
of your suggestions. replaced all the hoses, cleaned all the
investment out of the connections and when i get time, i’ll add
that extra drawing chamber.

i just bought a nice orbital mixer (hobart K5) so i’m sorta
commited to mixing outside a vacuum for now. unless there is a
great trick to adapting that monster to mix in a vacuum?

how much soap do you put in a given amount of alcohol?

best regards,

geo fox

I’m glad I could help. To answerone of your questions
…yes>>>you can use your mixer with my contraption. you mix the
investment outside the vacume chamber then pour it in and then
pull your vacume on the top chamber with the investment in it.
after it boils you then can slowly open the value between the
top chamber and the botom chamber where you have your flask and
model which should already have a vacume pulled on them , and
nothing will happen because you could not know what I left
out.!!! on the top chamber mount a value on the cover (I use a
1/2" thick plaxigless cover with a ruber seal for the top) do
not conect this volue to anything you will crack this value to
allow the top chamber to loose its vacume , when this is done
the vacume on the bottom chamber will suck the investment into
the flask. thus filling it to the top. a word of caution here
if you release the presure on the top chamber too fast the force
of the investment being sucked into the bottom flask might
dislodge the wax from the sprue base. With this rig I never
needed to coat my waxes .

now for your next question … just a very small amount of soap
to the alcohol a half tea spoon to a pint should do.

Hi all,

I really liked all of the different way folks have found to
eliminate bubbles in castings. And John B. and other techies,
thanks for the REAL inside dope on this stuff. Now I just wish I
could figure out how to get some of it to get filed in the old
gray matter in such a way as to be findable later when I need

We mix with a little electric hand mixer with just ONE MIXER
BLADE in it rather than the 2 that could be used. Much less air
incorporation. CAREFULLY measure both the water and investment,
not relying on the eye for proper consistance. The instructions
on R&R’s ultra vest state (approximate quote)" add WATER TO
INVESTMENT and mix for 2-3 minutes. (I split the difference and
mix for 2 1/2 minutes). I vacuum for 30 seconds AFTER the
investment settles from the “boil”. Pour down the side of the
flask (use RIO’s pink debubblizer in a perfume mister on the wax
early enough so that it is dry by the time I am investing… uses
the debubblizer up slowly but you get a very even coat and a
gallon lasts and lasts… ). Re vacuum for at least 1 minute
after the boil settles (not much boil here as most gas has
already been removed in the first vacuum).

If you had investment in the vacuum lines (when you cleaned
them) you might consider adding some sort of a line filter to the
line going to the vacuum pump. Investment dust will raise heck
with the pump vanes or pistons or valves, deminishing the
capabilities of the pump dramatically, more so the greater the

If the bell is really fitting the table seal, you should be able
to hold a vacuum for a L O N G time but the valve closing
the bell from the rest of the system should be a good one and
between the bell and the pump, meaning, don’t rely on the pump
to hold the vacuum.

I forgot who commented about changing the water/investment ratio
to a bit more water but I think I am going to try it. Sounds
reasonable to me, just be aware of potential investment strength

Lastly, R&R has a wonderfully helpful technical support
department so if all else fails, give them a call for some of
their ideas. (800-800-7496)

John and Cynthia/MidLife Crisis Enterprises
Maiden Metals/C. T. Designs/ Bloomin’ Wax Works. etc.

PO Bx 44, Philo
CA 95466
Ph 707-895-2635 FAX 707-895-9332

If your’re headed in the right direction,
each step, no matter how small, is getting you closer to your goal.