Centrifuge vs. Vacuum

Jewelry Discussion
1

I have been doing my casting using my Neycraft centrifuge and have
had excellent results. However, it is hard for me to wind as the
spring is very tight. Therefore, I am thinking of using my little Rio
Vacuum casting/investing table, which I have been using just for
investing.

I have the Tim McCreight casting book,with excellent suggestions for
proper preparation of flasks for vacuum casting. However, I have
been told that vacuum casting often does not result in satisfactory
casts.

Is this the experience those of you who cast have had?

Also, another related question. Does one use the same burnout
schedule for vacuum as well as centrifugal casting? The schedule I
have been using for centrifugal casting is:

300 deg. 2 hours, 700 degrees, 2 hours, 1350 degrees 3 hours, 1000
deg. 2 hours, and holding flasks at l000.

I have had excellent casts, but for some reason the investment was
not white after burnout, but blackish.

Alma

2

The folks who say vacuum casting doesn’t result in satisfactory
castings aren’t doing it right. The large scale casting operations
that produce jewelry and findings rely on vacuum casting to produce
the majority of cast gold and silver jewelry and findings produced
in the world. The exceptions to this are the high temperature metals
like platinum and palladium where the centrifuge is still the best
way to cast them.

You will want the same burnout schedule for either method.

Your are not getting a complete burnout if you see black investment.
You need more time at the high end of your schedule and likely more
air flow through your kiln to get complete burnout. Keep extending
the time at high temperature till the investment is white.

James Binnion
James Binnion Metal Arts

3

To me the black is “showing” the burnout is not complete. Burnout is
somewhat kiln dependent as different kilns just heat up and burn out
differently. We do not ramp up, we get the kiln hot, put in the wet
flask and let her rip. 2 hours at 1350 should be good for a medium
flask, but again, it varies with the kiln and also the wax type. We
try not to have to hold a hot flask, when it is up to temp, for the
correct amount of time, we take it out, get it cool to the right
"COLOR" INSIDE THE FLASK, THEN POUR. CYNTHIA, MY WIFE, IS REALLY THE
jeweler (sorry for the caps, got new keyboard and the keys are
closer together and I keep hitting the key lock for an “a”) and she
is in England at this time, but she used a broken arm spring loaded
casting unit for decades. We then got a vacuum unit that also had a
casting side, and we now mostly cast with vacuum. We don’t have any
testing to prove this next statement, bout Cynthia feels that the
centrical/broken arm caster packs the metal into the flask
better/more that the vacuum unit. I feel that way more force would
be needed to pack the metal, but strange things happen… It is the
woo woo factor in melted metal work!!! (we NEVER cast if there is a
weather change occurring, we have had too many problems when we have
done so.

So the biggest concern is getting a proper, clean burnout of the
investment. Centrifugal or vacuum, it is up to you, again, in my
opinion. Good luck!!!

John Dach

4

Hi Alma, I use both, and both will give you equally good results. I=
think your burnout cycle and final flask temp looks good, but you
should have white investment around your sprue, if it’s dark around
the outer flaskedge that’s OK. I’d let it soak at 1350 an extra hour
if it were me. I also leave it for an hour at 450F, I read somewhere
that this is the temp that most wax ignites and you get a better
burnout if you soak at 450F, not sure if that improves things but it
can’t hurt. You really want to get the investment bone white to have
clean castings. You likely know all of this already but a few things
that might help you with vacuum casting = if you haven’t done much of
it are these. Assuming your setting your flasks on top of the silicon
pad rather than dropping in larger perforated flasks, You need to
have nice even edges on your flask, I mean flat like a cleanly cut
pipe. If your flasks are irregular they won’t want to seal. Fill your
flask all the way to the top with investment when investing. After it
dries, scrape it off level with the flask edge using your spatula.
It’s OK if you remove investment so it’s a little concave, but you
need a really flush surface to seal and create a vacuum. Do not only
fill 3/4 of the way with investment or the suction can break your
mold. I also think it’s much easier to melt and pour with a torch and
hand crucible rather than a electric hand furnace unless you are
pouring a couple of ounces or more. They both work, but a small pour
will sort of stick in the hand furnace, while the torch and crucible
method is much more fluid and pours exactly when you want. I think if
you’re accustomed to torch meltingin your centrifugal machine you
will be very comfortable torch melting with your vacuum. Other than
that, you just take out the hot flask, put it on your silicon pad
over the hole, turn on the vacuum, push down the flask and rotate if
it’s not sealing, light your torch, melt your metal and pour it in. I
leave it sit under vacuum until I’m sure it’s fully solidified.

Hope it works for you!
Mark

5

Thank you James for the about vacuum casting. I am glad
to hear that when done properly the castings are good. Also, I will
increase the time of burnout at the high temp level so that the
investment will turn white.

Alma

6 
The folks who say vacuum casting doesn't result in satisfactory
castings aren't doing it right.

Well, I’ll take Jim’s statement a step further - people who say that
vacuum (or centrifuge) gives inferior results are actually saying that
they don’t know much about casting. Next time you hear that from
someone just walk away. We use centrifuge mostly because it’s what I
started out with. I’ve worked at the bench on more pieces I can count
that were cast both ways, with no discernable difference in quality.
Now I FEEL safer with a centrifuge - somehow it seems more positive,
to me. But that’s my own prejudice and I know that it’s just a
prejudice because I’ve seen results… There are folks casting
12 inch flasks filled with hundreds of pieces out there…

And just to echo Jim - burnout is independant of the casting method.
And BTW our kiln clicks on at midnight with a mechanical timer. No
ramping, no nothing, just “On”. Works just fine.

7

Alma, I have used both kinds of casting methods and have found that
centrifigual casting works better for me. I started off using vacuum
casting, but living at 7300 ft. in elevations seemed to have some
sort of factor in my results. I was having alot of incomplete
castings with vacuum, but after switching to centrifigual, that was
resolved and I was very happy with the results. My dad is a jeweler
in the Florida Keys, and has vacuum cast for the last 25 years with
awesome results. I’m not sure if elevation makes a difference at
all. I have found with centrifigual casting, ( I have the small
Neycraft machine) the main thing is customizing your crucibles to
your own liking. Definitely need to season the crucible with borax
both in the main bowl, channel, and even on outside flange of
crucible (that goes into flask). I wonder if you custom made your own
handle for your machine if that might help, just a thought. As far as
burnout temperatures, I think I would want my holding temperature
higher. 2 reasons I say this, first My holding temp. for argentium
silver is 1350, and 1100 degrees for 14k gold. Second reason, and
this is just my personal opinion and experience, if you are going to
switch to vacuum casting I would want my holding temp higher because
as the flask sits on the casting table it is rapidly losing
temperature as you are melting your metal in crucible. That’s another
plus side of using the centrifigual casting, your torch is keeping
the cavity of your flask heated as you are melting metal. By the way,
do you flip over your flasks over an hour or so before casting? That
seems to help out with any blackness or wax residue that may exist. I
hope all this helps out, and I would like to hear any different
results anyone else has about this. Take care, Tom Tyma

8

Hi Alma,

Just a couple of comments to follow up on the generally excellent
advice that’s turned up so far.

Personally, I prefer centrifuges. They can hit 10Gs of force, while
vacuum tops out at 15psi. The extra oomph of the centrifuge seems to
help with very delicate details. (Champlive walls & cast bezels.
thin stuff.)

The big shops like the big perforated flask vacuums because they do
big runs. The centrifuges top out at about 10 ounces of silver (if
you’re sane.) A guy I used to work for had a homebuilt centrifuge
that could handle several pounds of bronze. Damned scary standing
next to pounds of spinning molten bronze that was just looking for a
way to give me a bath. (Yeah, it had a shield, but still…) Anyway, a
big vacuum is a much safer proposition when you’re talking big
pours.

That said, I taught casting for years, even the vacuum. Four more
tips:

(A) whoever said to pour the flask full and then cut it back a
little: not exactly. Leave about 1/8" (3mm) of empty headspace in the
top (not sprue base end) of the flask. It helps distribute the
vacuum. Make sure you adjust your top clearance of the model
accordingly. (No less than 1/2" (13mm) from where the top of the
investment’s going to end up. (so 5/8 (16mm) below the top of the
flask.)

(B) The next tip is to use heavy (6Ga) wax rods stuck to the sides
of the flask, starting at the top (open end) and going right down the
sides. Four (or so) of them. Melt them to the sides so they stay put,
but DO NOT let them touch either the model or the sprue base. The
idea is that they let the vacuum access deeper into the flask than
just the end. It’s critically important that these rods don’t even
get close to the model, or the bottom (sprue base) end of the flask.
If they do, they’ll vent the flask completely, and it’ll never work.
Rio’s ‘wax web’ is a more complete variation on this same idea. Read
their writeup for a longer explanation.

© The school shop where I taught had a concrete floor that looked
like sidewalk. (rough) Made a very handy flat abrasive for flattening
the top of the flasks. Just sit the flask down on the sidewalk, and
rub it carefully in a circular motion, until the entire upper rim of
the flask is an unbroken bright silver ring. That’ll help it seal
better. Absent sidewalk, try a sheet of 80-120 grit sandpaper stuck
to something flat.

(D) Take 6-8 sheets of newspaper, and tear out a square chunk of all
6 pages that’s bigger than your flask. Fold it roughly in quarters,
and then tear out the ‘center corner’. The idea is to make a hole
through the center of the pad of papers. (to let the vacuum thru…)
Soak the pad in your blowout bucket for a second or two to get it
really sopping wet, and then put it on top of your silicone pad.
Since your hand is wet anyway, rub your hand under the silicone
pad as well. The idea being to get water in there as a sealant. When
you put your hot flask on the sopping newspaper, it’s softer than the
silicone, and will give a bit, giving a better seal. (so long as you
make sure the hole through the newspaper lines up with the vacuum
port.) Yes, you do get a puff of steam, and the paper directly under
the flask rim does char, but in 12 years, I never had one catch
fire. The most that happens is the paper under the flask body
suddenly dries out. Also saves wear and tear on the silicone pads
which are vastly more expensive than old newspapers.

For whatever that’s all worth
Brian

9

Centrifuge vs. Vacuum & Melting Furnace

Hey Alma!

I have been teaching myself to cast for the last six months. I
inherited the equipment and it took me a while to figure out where
the gaskets/silicone pads went between the casting chamber adapters
and flasks. One pad for the vac table and one for the casting side.
No instructions, so no idea how it works. Had never seen anyone cast
before and had no idea how to light a torch.

I’ve learned from a lot of mistakes. Books, Utubes and help from
here on the forum and archives.

The most important thing I have found is the seal. Without a good
seal, the vacuum casting will fail. Be sure both sides of the
silicone pad are clean, soft/pliable and in good condition. I use a
little silicone tire dressing on both sides. A full size paper towel
folded in half or quarters with a hole in the center to fit over the
hole in the pad, and dipped in your bucket so it is wet, will create
a great seal, save wear and tear on the pad. Not dripping wet, but
saturated. If the flask is small it may not be heavy enough to seal
itself so don’t be afraid to push it down with the tongs and a gloved
hand.

Next, don’t pour until the vacuum is pulled down all the way. An
extra 20 seconds won’t hurt and will make a huge difference. If the
vac does not pull, don’t pour as there is a problem. You can put the
flask back in the oven and figure out what is wrong now, rather than
what went wrong later. I have found it never to be the pump, but the
seal.

Another thing I discovered by mistake is that there are two sprue
bases. One for centrifugal and one for vacuum casting. The
centrifugal base is rounded like an ice cream scoop. I suppose so
when the flask is set spinning the molten metal hits the bowl and
works its way down the hole. When I poured into this shape, I had
splash up and would have little balls of metal burning marks on the
table or base. The previous owner had marks like this too so it must
have been a common problem. Then I found vacuum sprue bases, they are
like the bottom of a funnel, steep tapered walls that gobble up the
molten as fast as you can pour. With no splash up.

You also questioned the furnace. I have to stand on a stool to do my
pours. The vac setup is on a tool box base, by the time I put a flask
on top it is about chest high and no way I can see down into the
flask, So I stand on a stool for the pour only. I have wondered about
the crucible falling out. I started using the tongs to hold the end
from falling out. Then I played with it when it was turned off and
pretended I was pouring and wanted to see how far I could turn it
before it would fall out. A long way! The first pour I did I waited
for it to hit the pour temp and poured immediately. Well, there was
probably a hundred degrees difference between the bottom of the
crucible and the top. The pour pretty much stopped at the end of the
crucible. So let it sit for a few minutes, maybe ten so the entire
crucible is red. That way the metal is not going to cool during the
pour. If you leverage yourself correctly when you pour, 12/13 pounds
will not seem like anything. It’s only about a 15 second arm wrestle!

If you are having any vacuum problems, be sure to check hoses. I
replaced all mine and added mini hose clamps to each connection. Auto
supply store will have them and it made a huge difference. I have the
drop down casting unit for perforated flasks and the furnace is about
six inches away, I put a silicone pad on the end of the table to
protect it from the heat. I also put one inside the unit on the
outside of the casting chamber to protect the hoses. A 4x6 flask at
1,100 degrees holds a lot of heat and gives it off to everything
inside.

To the experienced on here, this will already be second nature, to
someone entirely new to the field, every little hint helps.

Charlie

10

John Dach said…

Cynthia feels that the centrical/broken arm caster packs the metal
into the flask better/more that the vacuum unit....

I have noticed that when I put the rough castings in a magnetic
tumbler, the centrifugally cast castings come out less dinged up than
the vacuum cast castings do. Not scientific but an observation. Both
castings are great, with no problems, but slightly more dings on the
vacuum cast pieces.

Mark

11

I use both methods, vacuum for heavier things, centrifugal for
lighter things and platinum. In my case, it’s easier to judge
temperature of the melt using vacuum, as I can move the handled dish
style crucible around much more easily to see how the metal sticks
(or not). As soon as it moves freely, I pour. With the more or less
stationary centrifugal crucible, I can’t judge the temp by moving
the crucible as easily, so I end up blowing the metal around a bit
with the flame to see how it moves or use a quartz stirring rod to
push it around, both of which contribute to heat-soak. I tend to
over-heat it more often when using the centrifugal crucible than when
using a dish style crucible.

Heavier castings are usually more susceptible to porosity from
over-heating but less prone to non-fills, so vacuum works great.
Lighter castings are less vulnerable to shrinkage porosity but more
so to non-fills if the metal (or flask) is just a little bit too
cool. Centrifugal also gives that extra little oomph to the metal
that helps push it into thinner areas if it’s just a little on the
cool side. Centrifugal is much higher on the wow factor when
demonstrating casting to non-jewelry people too, fwiw.

Don’t get me wrong, I can get nice heavy castings with centrifugal
and very rarely have non-fills with vacuum, I just like to put the
odds in my favor as much as possible.

That’s my thought process on the pro’s and cons of both, arrived at
through decades of trial and error. Using both methods helps even out
results and minimizes the damage caused by my rather unscientific and
inaccurate temperature estimation abilities. Your mileage may vary.

Dave Phelps

12

Brian said…Leave about 1/8" (3mm) of empty headspace in the top
(not sprue base end) of the flask. It helps distribute the
vacuum… I felt the same way Brian, and did the same thing for the
same reason. But when using 2.5 x.2 inch flasks with smooth walls, I
had the investment crack twice and pull apart at the model cavity due
to the suction of the vacuum. Obviously that ruined those two flasks,
and it was back to the bench to recarve the models. After that I
began filling them all the way, then cutting them flush with my
spatula, then scooping the surface slightly concave to prevent the
breakage. The concavity allows the vacuum to reach a larger surface
area, but the outer edges of the investment are on the pad so no
chance of breakage. That said, I usually spin unless it’s a big pour
where I’ll useperforated flasks.

Mark

13

I want to thank everyone who kindly sent me so much valuable
on vacuum casting, as well as melting furnaces.

I have come to the following conclusions, based on the excellent
suggestions you have given me.

As I have done a lot of centrifugal casting, using the torch for
melting the metal is no problem. Therefore, I have decided not to get
a melting furnace, but to melt with my torch. My hands are steady and
making a nice smooth pour should be no problem. I will keep the flame
over the metal as it pours.

Also, I will hold the flasks at a higher temperature just to be sure
that they are the correct temperature to receive the metal.

Yes, indeed, I will be sure that I have a good seal. My flasks are
in excellent condition, but I will make sure the rims are perfectly
smooth and clean. I will use either the 6 ga. wax rods suggested by
Alberic, or insert some hanger wire as suggested in Tim McCreight’s
book–being careful to remove them before putting the flasks in the
kilns. Also, I will keep the investment below the level of the flask
rim.

My silicon pads are new. I will put some well saturated paper with a
hole in the center, over them to give them protection, and to get a
good seal.

Eager to give the vacuum its first run at casting.

Thank you one and all for your generous help. Alma

14 
whoever said to pour the flask full and then cut it back a little:
not exactly. Leave about 1/8" (3mm) of empty headspace in the top
(not sprue base end) of the flask

I once had a 2 hour conversation with the tech man from Kerr, about
certain things that are unimportant here and now. One of the things
he impressed upon me was the importance of scraping the investment
after it sets. When the investment sets up, at the top of the flask
it forms a skin. When you scrape away that skin you open up the
porosity of the investment again. Whichever method of casting you
use, there is a microscopic porosity to the investment that’s needed
for the movement of gases. He said that it’s so important it could be
the difference between good and bad casting, at times. Even if you do
as Brian says, reach down in there and skim off the top surface of
the investment after it sets.

15

Thanks Mark for the additional about preparing the
flasks. You mention scooping the surface slightly concave yet
leaving the edges flush so the edge of the investment is in contact
with the pad.

I will try it both ways and see what works best with my vacuum. I
figure the first few attempts at vacuum casting will be a learning
experience. I will prepare some waxes that are easy to carve and
sprue. If they turn out well, that will be fine. I will keep notes of
what works with my vacuum, and what does not.

Love my good old Neycraft centrifuge. It always gives me good casts,
but is just too hard for me to wind. So will give the vacuum a try.

Alma

16

I would be a little cautious about keeping the flasks at a higher
temperature.There is much that can and will go wrong with that too.
Actually, a colder flask is almost always better until you have a no
fill from it being too cold. The metal has to cool at the proper
rate so that micro porosity is not a problem. And shrinkage porosity
becomes more apparent with higher temps. A good general rule is find
the proper casting temperature for the average wax/casting thickness.
This means some trial and error with good notes taken as to
thicknesses of the model(s) and casting/flask temperatures. Run a
series of scrap loads just for testing. At least stuff you don’t have
to worry about so you will be at ease taking some chances. Once you
achieve perfect casts with “average” models, you can add about 50
degrees for slightly thinner castings, up to 200 degrees (F) for
ultra thin castings (butterfly wings, filigree, etc) and you should
lower the flask holding temp about 50 degrees F for "fat/solid
models. All the different gold alloys have their sweet spot too. I
have found over the years even with PLC temp control and
thermisters/thermocouples, RTDs and infrared readings, a change in
the kiln or movement of any of the sensors often requires a
recalibration of thought and some testing if I want to be sure of
success. Much less after 40 years though. Maybe I’m finally getting
the hang of it :slight_smile:

Johnny

17 
I once had a 2 hour conversation with the tech man from Kerr,
about certain things that are unimportant here and now. One of the
things he impressed upon me was the importance of scraping the
investment after it sets. When the investment sets up, at the top
of the flask it forms a skin. When you scrape away that skin you
open up the porosity of the investment again. Whichever method of
casting you use, there is a microscopic porosity to the investment
that's needed for the movement of gases.

I looked up the online direction for Kerr investment process. It
does not include scraping the top. I assume that if it was
important, it would be included.

Going by my own experience, production casting of over a million
pieces, I do not think I have had any significant porosity other than
the occasional piece, about on par with random voids, incomplete
pieces, ect. I did wholesale for other people, never had returns.

I do some casting of customers old gold, using totally 100% old
gold, and I rarely have any problems with porosity, and some of you
know the rumors about that.

Just to add an interesting point of reference, I am in Denver at
5280ft above sea level.

Mix water and powder for 1 1/2 minutes, vacuum 1 1/2 minutes, pour
into flask and vacuum 1 1/2 minutes.

Richard Hart G.G.
Denver, Co.

18

Thank you for that about scraping off the 1/8" of the
investment. I did not known this bit of I will add it
to my paper on investing.

Ken Moore

19

Hi Mark,

I have to think there was something screwy with the investment. I did
years of all sorts of flasks poured with headspace, and never had
that kind of a problem.

Even when using high-speed lab pumps that pulled hard. This may
also be one of those ‘ask six jewelers, get seven answers’ things
too, but my first suspicion is the investment mix.

FWIW,
Brian

20

All of the back-and-forth discussion on this topic is an example of
why the public nature of this forum is so important. I am a “know
nothing” when it comes to casting, but when I get ready to give it a
try, I’ll turn to the archives for lots of valuable
Thanks to everyone who participates.

Jamie