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Cement-like gunk on castings


#1

Please help. I’ve been casting for about seven years with someone
else’s oxy-acetylene set up with absolutely no problems and finally,
this summer, got my own centrifugal casting set up. I opted for an
oxy-propane torch since I was told it is cleaner. I’ve had nothing
but problems since, most of which I will not bore you with, but now
I use distilled water, with no change in the investment, waxes, or
burn-out cycle. Recently I started to get these cement-like deposits
all over my castings. It polishes out pretty easily with a 120 grit
wheel but I lose alot of fine detail. Any ideas?

Thanks.
Peg


#2

Peg it sounds like you are using to much borax cut back to a very
small pinch


#3

Peg;

Recently I started to get these cement-like deposits all over my
castings. It polishes out pretty easily with a 120 grit wheel but I
lose alot of fine detail. Any ideas? 

It sounds like your investment is breaking down, possibly due to
age, Some time when investment has become stale I(for lack of a
better term you can get spaulling at least that’s what I was told it
is called. You can also develop finned castings, this usually happens
when the investment has absorbed moisture investment has a short
shelf life in humid areas. I personally don’t like Propane /Oxy
torches due to their lack of temperature, it is cleaner burning but a
rich Oxy/Acetylene mixture is hotter and can also dissipate or burn
off excess oxygen giving less porosity resulting in less waste Hope I
have been of some help, R&R Ransom & Randolph have a good trouble
shooting area on their web site, I think Kerr does as well.

Kenneth Ferrell


#4

Well, to start with, I wonder if your apparent suspicion that the
torch is the culprit may be too quick, since for my part, I cannot
think of any way in which the torch might do such a thing. So
perhaps you can start by telling us quite a bit more about what
you’re doing. At the moment, all we know is you’re using an
oxy/propane torch, and doing lost wax casting. Tell us, for starters,
what metal alloys you’re casting. Continue with what type of
investment you use, what sort of burnout oven you use, describe in
detail your investing and burnout and casting procedure. And then,
perhaps you’d be able to describe the deposit a bit better.
“Cement-like” is somewhat vague. What color is it? What have you used
in trying to remove it? Is it a foreign material, or is it a rough
surface to the metal itself? Etc. Etc…

Peter Rowe


#5

Sounds like you may be overheating the investment and it is breaking
down and fluxing. This is something I have seen on high temperature
metals like bronze, nickel white gold or palladium white gold
castings when there is just too much heat for the standard gypsum
bonded investment to deal with. What metal are you casting?

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#6

More was called for so here are all the gory details of
how I achieve the lovely cement-like look on my recent castings.

I use plain old sterling casting grain and scraps from old castings
(buttons, etc.) to cast organics coated with sticky wax for
thickener/stiffener. I use Satin Cast 20 and distilled water (my tap
water is VERY hard) vacuumed at 23 psi to remove bubbles. That’s the
max that will draw at the altitude here. I burn out at ramp of 300
degrees and hold for one hour at each stop of 300, 600, 900 in a new
programable kiln; spend 3 hours at 1250; and cast at 1100. I use
borax for flux and a reducing flame. My castings tend to be large,
treed structures with 100 or more grams of metal. None of these
parameters are any different from what I used before the "gunk"
appeared.

Now to the gunk. It is grey and appears as both bubbles and large
surface areas on different pieces within the same casting. Sometimes
but not always there is fire scale beneath the gunk. The gunk doesn’t
scratch with a finger nail but polishes off with 120 grit. It doesn’t
pickle off or come off in an ultrasonic cleaner.

I’m wondering if dirty metal combined with the new torch could be
the problem. I often don’t follow the rule of half-old to half-new
metal, although I’ve not had any problem using old metal before. The
reason that I suspect that the torch is a factor is that, because the
torch is hotter than I am used to, I believe that impurities in/on
the metal may be melting in before I can identify them as glowing
spots and fish them out. Obviously I need to try a run with
exclusively new grain which will be my next step.

Thanks for any help.
Peg


#7

Peggy, You have an improper burnout compounded with dirty metal. The
top of your burnout cycle should be 1350 and the investment should
be chalky white before casting. It sounds like your casting
temperature could be too high also. Are you trying to cast wings with
thick bodies or leaves? Organic material needs a longer hold at 1350
to clean out the residue. Try changing your burnout cycle and
cleaning the metal before casting.

Good Luck!!
Shannon


#8

Here’s the disclaimer. I have been casting dozens of flasks weekly
since 1971, but am constantly challenged by the Orchid experts. My
two cents…

Students in our casting class experience this many times. There are
a few reasons in my observations. I don’t have all your details about
process, but here are a few thoughts.

Incomplete burnout. Peak temperature should be 1350 f. No grey color
in the investment upon quenching will show you this. Larger loads
need more time at hottest temperature. This assures the flask will
breathe when the molten metal goes in. The air must travel out.

Some foreign matter gets cast into the flask. If the metal looks
good in the melt (real shiny smooth & clean surface) cast as quickly
as possible. A pinch of borax flux is plenty on most melts. 20 mule
team borax works fine and is cheap. (Laundry detergent & almost pure
borax) Stir when molten to mix metal as well as skim the crud off the
molten metal.

Make sure any old investment is removed from a re-melt. Deox metals
work the best and cast a second and third time with little problems.
Make sure you have a decent spru button. Not enough metal will have
the tail end of the pour go into the flask. This can be a problem.

Sequence is- melt, flux, stir & cast as quickly as you can. The
metal when stirred should feel fluid. Vac casting or spin casting? If
vac cast pour the metal through the flame as it goes into the flask.
Vac flasks will cast better if there is vent at the inner flask edge
(spur wax works fine).

Use a neutral flame for melting. Not reducing or oxidizing. Propane
oxygen is the best fuel combination with a larger torch. Bigger than
the bench torch. Not the Little Torch.

Old investment? Bottom of the barrel is usually the worst casting
unless you mix the unused investment once in a while.

Be safe with proper gloves, goggles for melting and vent/mask for
any investment contact.

Kindest Regards,

Todd Hawkinson
Jewelry Dept
Mpls Comm & Tech College


#9

Hi Peg, Thanks for the added info. Here are some things that to mind,
as it does sound like a fluxing is might be at work. Are you using a
clean carbon rod to clean up your metal? Move the junk off to the
side? Is there too much flux present either added? Possibly a dirty
crucible?Any of these things may effect as you have explained. In my
experience this cement stuff does’t get to the detail areas. more
likely on the shank or in the tree. The added heat of the torch alone
may just not be the culprate but it could be that the new torch is
heating things differently. Bringing other issues to light. If this
sounds simular let us know.


#10

I am not a jeweler or a caster, but wonder if the change to propane
may have caused a little sulfur to get into the metal from the
propane flame. Acetylene does not have any, but propane, being a
fossil fuel is likely to contain small amounts of sulfur from the
crude oil, and the odorant is a mercaptan which will contain some.
Just speculation, I am not a chemist, nor do I play one on TV.


#11

Peggy,

Sounds like your max burn out temp is too high, well above the
programmed 1250. For fat (or thick) castings I generally cast with a
flask temp of 800 or lower with silver. I don’t think that your
problems stem from using too much ‘old’ metal, I’ve been guilty of
that and haven’t had your problems… too hot a turnout or flask cast
temp and it is a reproducible problem.

Please realise that while I have been doing this work for a long
time and the frequency of big mistakes is getting smaller the
magnitude compensates :slight_smile:

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


#12

Overheating a flask can cause investment to get cement like.


#13

Peg,

There’s one other thing that I’ve encountered that could be an
issue, and I haven’t seen it mentioned by any of the other posters.

You mention that you’re casting organics, but not what type. I have
found some types of plant materials, in particular, that react with
the investment to create a hard, cement-like coating that is almost
impossible to remove. Yes, you can polish it off with sandpaper, but
that’s about it. Those same castings don’t quench well (they quench
in temperature, but the investment certainly doesn’t break away from
the metal as you’d expect). One of the biggest culprits has been
poinsettia leaves (I’ve tried a variety of things completely without
any success).

If you adjust the burnout temps and lower your casting temp and
still have the problem, try an experiment to see if the type of
organic you’re using could be causing it. Do 2 flasks, one with plain
waxes and the other with the organic. Mix them identically (and
simultaneously) and burnout/cast them simultaneously. See if there’s
a difference.

Hope this helps!

Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry


#14
I am not a jeweler or a caster, but wonder if the change to
propane may have caused a little sulfur to get into the metal from
the propane flame. Acetylene does not have any, but propane, being
a fossil fuel is likely to contain small amounts of sulfur from the
crude oil, and the odorant is a mercaptan which will contain some.
Just speculation, I am not a chemist, nor do I play one on TV. 

Not likely a factor. Sulphur, for one thing, is not a volatile gas,
so most sulphur compounds in crude oil or coal stay in the solid or
liquid mass until it’s burned. If sulphur were in propane, it would
likely be a sulphur dioxide gas, which you’d smell.

sPropane generally has a reputation, like natural gas, of being
clean burning, causing fewer problems with metal oxidation or
contamination than does acetylene.

And in this case, if sulphur were the problem, it much more likely
would be from the investment itself. it’s gypsum based, with is a
sulphur compound. It’s one reason why investment shouldn’t generally
be heated above 1300 or at most 1350 during burnout. Above that, it
starts to break down the gypsum, and the released sulphur compounds
then DO cause problems, with black and rougher castings. Doesn’t
generally cause a yellowish cement like coating.

Most of the time I’ve seen anything like that, it was from burning
out models made from various plastics, or even rubbers, instead of
wax.

Peter