So I visited a friend’s workshop today to see his casting procedures.
He’s a dental mechano, so he is using wax, investment, burn-out and a
centrifugal casting machine. No surprises here (after all, we stole
this technique from them).
What struck me most, and I had quite some strokes about similarities
and differences between jewellery and teeth, was his pressure pot.
Let me explain:
He uses a vacuum mixing system, then pours into the cylinder, holds
the cylinder briefly against a vibrator (orbital sander in a plastic
bag) and then puts the cylinder into a modified pressure cooking pot
and throws 2 bars into it.
So: The investment cures under pressure. He arguments that any
remaining bubbles will be much smaller and that the investment is
much more compact, giving an even smoother surface.
We played around with all his toys; Boy, what a day! But on my way
home I kept thinking “pressure pot”. Anybody has experiences with
pp’s in jewellery casting? Or general insights and opinions?
It would work fine, I’ve seen pressure casting used in the sculpture
industry while casting resin for a couple of reasons. One, it’s
easier and cheaper to buy/build a pressure pot than a vacuum bell.
Two, when you vacuum resin, air bubbles will boil out of the mold
making a mess, under pressure, no boiling, so the resin stays put.
We played around with all his toys; Boy, what a day! But on my way
home I kept thinking "pressure pot". Anybody has experiences with
pp's in jewellery casting? Or general insights and opinions?
He’s right, of course, in principal. Just as vacuuming the
investment causes bubbles to expand, and thus rise to the surface,
later pressure on the investment will cause any remaining tiny
bubbles to become tinier. Given the precision fit dental castings
must have, this added slight improvement may be worth the trouble,
but for most jewelry casters, the vacuum process is enough.
By the way, did you happen to ask what sort of metal and investment
he’s using? Many dental alloys are quite different from jewelry
metals, often melting much higher, and needing investments that take
higher temps than do normal jewelry investments. Also, keep in mind
that dental casting often requires an exacting control to eliminate
any shrinkage between wax and final casting, so things fit right.
This is quite different from jewelry casting, where models are
usually just made large enough to compensate for casting shrinkage.
The dentists can use slightly different higher expansion investments
to handle this problem as well. The combination of different, high
expansion and/or high temperature investments may well change the
whole story about getting bubbles out with just vacuum, so his
process, though it may look the same in terms of investing, may
indeed require the pressurized cure cycle for best results. Also,
keep in mind the different shapes/geometries of what we cast, versus
what the dentists cast. Most dental casting are fairly uniform in
shape from one casting to another (crowns, bridges, they’re all teeth
shaped and sized, which is usually smaller than jewelry castings),
which might allow more refining of the process to optimize it for the
things being cast. Dental casting is usually done with smaller
flasks, for example. These various factors might mean a denser
investment would be a benefit for them. With a lot of jewelry
casting, though, larger metal volumes and often more delicate and
varied geometries can mean that gas permiability through the
investment becomes important to get a good fill. After all, if you
want denser investment, use less water, and that’s what you get, but
when you do that, you may find problems in casting. While denser
investment might help sometimes, at other times it would hinder us,
not help.
So: The investment cures under pressure. He arguments that any
remaining bubbles will be much smaller and that the investment is
much more compact, giving an even smoother surface.
While he is right about making the bubbles smaller but I think you
will find he is he is way off base on the idea that somehow this
will result in “more compact” investment. It will only be more dense
due to smaller bubble size but the density of the investment itself
will remain the same.
James Binnion @James_Binnion
James Binnion Metal Arts
i suppose some sort of control experiment to determine & compare
the ratio of weight to volume would settle this claim
Or, just remember your old physics classes. Water is not
elastic/compressable, and neither would be the particles of silica or
other minerals making up the investment powder. So pressure won’t
compress either one. The mix of the two forms a slurry, or emulsion,
and the end density of the investment would depend on how much water
is spacing those particles apart when they bond to each other, as
well as the degree to which, before it sets up, the particles settle
out, leaving liquid water on top. I can think of no reason why air
pressure would increase that settling rate, especially given the
speed with which most investments set up. So if it doesn’t settle out
more, and we already know the volume of water doesn’t change with
pressure, then the volume of the set investment would remain the
same, and thus the density wouldn’t change.
Speaking as a dentist I would like to add something to this:
It would be typical to us a vacuum mixer for the investment. There
probably would be little additional benefit in finishing the setting
of the investment under preasure. If the investment was just
mechanically mixed without a vacuum, then preasure curing might be
useful. I have never read about this process.
In order to achieve an excellent fit of the casting on the tooth,
many steps and materials have to be balanced to arrive at just the
right amount of expansion. One of the techniques was hygroscopic
investing. The investment would be vacuum mixed and then the casting
ring would be allowed to finish its setting while submerged in water.
I have not used this method. The differences in dentistry to balance
expansion and contraction would be much much less than a ring size.
That is why we pay more for materials from impression to wax to
investment to metals. The whole system of materials and techniques
have to work together.
Well..... what about the black holes that power the center of the
galaxy are you saying there is no water in existence compressed
there ?
That’s just about right. At the ‘bottom’ of a black hole the
gravitational pressures are incomprehensible. We are not even talking
in terms of trillions of pounds per square inch; we are talking of a
‘place’ where atoms and molecules don’t even exist- just a sort of
soup of pure energy which was once atoms and molecules. The ultimate
statement in science says that atoms cannot be either created or
destroyed, but only transformed from one form of energy to another.
Example: when light strikes a real black surface, it doesn’t just
vanish; it converts to a lower frequency - heat; which can be
detected with instruments but not our eyes.
what about the black holes that power the center of the galaxy are
you saying there is no water in existence compressed there ?
I didn’t catch the original post from which the quote was taken…
JohnB. is right, but he also tempered it a bit, I think. First off,
black holes (theoretically, of course) don’t power the galaxy, they
are slowly devouring the galaxy - more like the drain in a bathtub.
The gravity pressures are not simply incomprehensible (though that
is an accessable statement) it actually alters the space-time
continuum - it alters not just things affected by gravity, and light
also, but also time itself. An intriguing quote: “So, to put it
succinctly, the reason you cannot escape a black hole is because you
cannot move backwards in time (or faster than the speed of light).”
And part of the problem with them is that we do not have the
brainpower or science to even conceive of what is inside, or on “the
other side” of one. It’s like a nightmare where nothing makes sense,
actually. Fascinating stuff, that.
what about the black holes that power the center of the galaxy are
you saying there is no water in existence compressed there?
Might be wrong, but I believe the answer would be no. Water would,
at the least, be broken down to H and O. Probably more action. Like
being stripped of electrons or possibly forming into some sort of
plasma soup.
I meant to write "_MATTER_" and not atoms although that is
partly true too.
I’m not sure this is still completely accepted truth. Watching too
many science channel and discovery channel shows can fill one’s head
with info I don’t normally need, and one such show involved the
so-called “Hawking paradox”, a series of calculations by Stephen
Hawking involving the theory that over time, black holes evaporate
away and are lost. This implied that matter/energy itself was
destroyed. What bothered physics folks about this wasn’t the
destruction of matter or energy, but rather the destruction of the
"it contained. After decades of wrestling with this
paradox, One physicist concluded, if I remember the show right,
concluded that the was not lost, because during the
process of matter originally falling into the black hole, it’s
would be imprinted on the event horizon, and thus
preserved. Hawking himself, I think, resolved this by noting that the
math pretty much requires multiple paralell universes, and for every
one in which a black hole disappeared, there’d be others in which the
same black hole never formed, so the lost in one paralell
universe would be preserved in another. Very dizzying stuff indeed
for this old non-math type… But this implies that the matter or
energy itself, even if it’s is preserved, CAN be
destroyed, at least within the limits of our own universe…
Like I said. Too many episodes of discovery channel… (grin)
Let me put this one to bed. If you are not mixing under vacuum the
pressure pot will absolutely make certain dental investments
significantly more dense by greatly reducing the size of air bubbles
in the slurry- less air, more investment in a given volume. We used
to use one ourselves until we bought the more expensive vacuum
mixers. For gypsum investments there is not a big advantage.
If you need stronger investment there are three basic thing you can
do: 1. lower your water to powder ratio 2. Use warmer water 3. Allow
the investment cure for several hours prior to burnout to increase
green strength. Mike Stover at Ransom and Randolph has a wealth of
knowledge in this area.
You probably don’ know this about me, but I am a cosmology nut.
Cosmologists now know that black holes are integral to galaxy
formation. Not only is there a black hole at the center of every
galaxy, but there is a predictable relationship between the size of
the galaxy and its black hole and the speed of the stars on the
outer edge of the system that are now too far away to be affected by
the gravity of the singularity at the center.
Might be wrong, but I believe the answer would be no. Water would,
at the least, be broken down to H and O. Probably more action.
Like being stripped of electrons or possibly forming into some sort
of plasma soup.
This has nothing to do with jewelry making, of course. And I’m not a
black hole expert, either. But it’s interesting…First off, the
SUN is plasma soup - peanuts.
To paraphrase the part of Einstein’s Special Theory of Relativity
that applies:
Space and time are actually one thing, called “spacetime”, often
called “The Space-time Continuum”. An object in spacetime cannot
exceed the speed of light, and also it must always move forward in
time (so the theory goes).
Mass distorts the structure of spacetime. The presence of mass tends
to tilt the direction of time towards the mass - not a definition of
gravity, exactly, but we experience it as such. As you get nearer to
a mass, time leans towards the mass (paraphrased). One object must
move towards another because it is forward in time. As one nears an
infinite mass, all time leads to the mass, and the effect is
inexorable. Once you pass the “event horizon” (point of no return),
you cannot excape the black hole because you cannot exceed the speed
of light and you cannot go backwards in time.
The effect of all that is that (always remember this is all
theoretical) what happens around a black hole has more to do with
warping the very fabric of the universe than merely making plasma out
of H2O or something like, “There’s a lot of pressure in there.” It’s
my thought, not necessarily science’s, that it could be something
like the reversal of the big bang. Everything around a black hole
gets sucked inside, but the essential question is, where does it go
and/or what does it become? These things are cutting edge physics and
mathematics, and even string theory doesn’t adequately address the
questions…
in my way of thinking which has not been shaped or refined by a
college Degree in astro physics.
let us say a large comet composed of mostly ice along with other
detritus and stuff found though out the cosmos gets pulled into a
black hole at the center of a galaxy, The comet goes in as mainly
frozen water, for the sake of this discussion entering the black
hole as water in frozen form is what should " matter " ( HA). what
that ice becomes when it is inside the black hole and changed by the
effects of all that gravity is merely a process of what has to
happen to the water ice of the comet for that "matter " to go
through the compression process .
For the more practical purposes of presure curing investment i am
unsure, if water cannot be compressed at the pressure being used for
this process. i am quite confident that some sort of expeiment could
be devised to prove if the process of presure curing investmest has
an effect to produce improved castings or not, which was my point in
my original comment reply to the thread and not the theory that water
can or can not be compressed
Hawking himself, I think, resolved this by noting that the math
pretty much requires multiple paralell universes,
I’ve seen Steven Hawking, too, and read a couple of his books. Very
accessable, very good at breaking things down for the intellegent lay
person. I have relative who is a high-level physicist. In that
capacity he’s not a “mathematician” like a specialist who does math,
but for all intents and purposes he is that, too. He told me he read
a scientific paper by Hawking generally about this very topic -
cosmology, string theory (now m-theory), and the like. He said that
after the introductory paragraph he was completely lost, the math was
far beyond him from the first words… Quite the man,
Hawking…Me, I’m like many here - fascinated and interested but
not able to generate mathematical alternate universes in my sleep…
Somebody wrote me offlist today about a couple of things. One of the
things was whether a discussion of cosmology belongs on a forum
about jewelry making - not really a complaint, more like “Well,
that’s a bit out there.” Well, I essentially agree with that, and
have no argument - I also replied offlist, already. I would like to
point out two things, though. First is, as I said to him, that I for
one enjoy it when topics evolve into other things which are more or
less related. Part of that enjoyment is that it becomes more of a
conversation between all of the people here, and not merely a
dictionary and trouble-shooting spot. I like hearing Helen talk
about her glass beads (BTW, if you’re reading, resolution?), and I
enjoy Rick talking about Colorado, which I’ve visited many times.
That’s not to say that getting all chatty about things unrelated is
good, but I think we can all relax a bit, too.
The other aspect is more meaty. I would dub it a “Da Vinci syndrome”
that would be that an artist can never know too much about the
world. Or any person, for that matter. While it’s certainly true
that it’s probably not a good idea to discuss such things at great
length, I don’t think it hurts to learn about anything, either.
Perhaps someone is making a “black hole pendant” even as I write.
Nature is vast, nature is great, it is that which we describe in art
and literature, even if it’s not literal. Science gives us a greater
understanding of it, and perhaps art gives us a greater insight into
it, if done well.
I’m not saying these things because justification is needed, it’s to
express an appreciation of what Orchid is, and what it can be…
I like hearing Helen talk about her glass beads (BTW, if you're
reading, resolution?)
Thank you and sorry. I’ve let people (including my M-I-L) down. It
sort of got pushed further down my list of priorities as that piece
is surprisingly one she’s not nagged me about. The enameling
experiments too have taken a back seat much to my chagrin.
I decided it was high time I actually tried to make some money so
I’m in the middle of making a range of pieces to sell, but my “to do”
list is slowly being crossed off one by one as I accomplish them and
I will report back on the projects - they’re not forgotten.