Craig Martell on wed 5 feb 97
Bill Amsterlaw wrote on 2/4/97:
<5. Some people prefer to work with clay that is not de-aired ... and some
people like to wedge. My personal preference is to throw with de-aired clay
and wedge no more than necessary. Deaired clay seems dramatically more
plastic and it seems to resist slumping far better than non-deaired clay.>
Hi Bill: I'm "nitpicking" again, but what the hell, somebody has to do it
:). My understanding of pugmills, is that they do absolutely nothing to
promote plasticity. They greatly increase the workability of clays by
significantly "compressing" the clay as it exits through the nozzle. Venco
mills are designed to have a 4 to 1 compression ratio from the barrell,
through the nozzle. By de-airing, you are making the claymass denser and
more compressable as it is extited. This is all termed "compression
strength" by pugmill designers and other enthusiasts. Venco mills also have
that small valve next to the de-airing chamber that you can use to vary the
amount of vaccuum you are applying to the clay. You can thereby make the
clay more or less dense, depending on your requirements. I use my mill to
de-air a 6-t, Kaopaque 20 porcelain body and I need every bit of vaccuum the
pump can muster to de-air that stuff. A more "open" stoneware clay is
easier to de-air it seems.
Regards, Craig Martell-Oregon
SLPBM@cc.usu.edu on thu 6 feb 97
Plasticity has more to do with the amount of water surrounding the clay
particles (hopefully homogeneously) than to do with any compression
a pugmill can perform. The easiest, albeit most space consuming, way to make
plastic clay is to slake it, then dry in troughs, and then pug away!
Cant imagine a better way without a filter press. (dream on!)
Hope this adds.
Alex Soll
Martin Butt on sun 16 mar 97
Pardon my referring to parts of this thread from a while back, but I'm just
catching up on my Clayart reading (much less writing). First, some of the
more interesting comments, (to me) then my own two cents worth.
Craig Martell says (2\5\97) "My understanding of pugmills, is that they do
absolutely
nothing to promote plasticity. They greatly increase the workability of
clays by
significantly 'compressing' the clay as it exits through the nozzle...by
de-airing, you aremaking the claymass denser and more compressible as it is
exited."
Jonathan Kaplan says (2/6/97) "I have no doubt that a pug mill will aid in
helping to
make clay more useable. A hands down assertion that I am sure many of us
agree on, but what is not being considered is the wetting of the clay
particle. Optimum plasticity is achieved by the entire wetting of the clay
particle...That's why pug mills are notoriously poor clay mixers. Sure it
will mix, but it does not wet the clay particle. The 'water hull' concept in
clay chemistry/physics is the wetting of the entire clay particle, promoting
bonding to its neighboring clay particles...etc.etc. While pug mills do
offer compression of the clay particles, as well as deairing and all the
rest, it is the proper mixing of the clays, that quite frankly, none of us
really do unless we blunge our clays as a slip, sieve them, dewater them via
a filter press, and then and only then pug the clay."
Alex Soll has a slant similar to Jonathan's; (2/6/97) "Plasticity has more to
do with theamount of water surrounding the clay particles (hopefully
homogeneously) than to dowith any compression a pugmill can perform."
Bill Amsterlaw's observations (2/7/97) directly mirror my own: "Since I can
independently control the vacuum and the auger on my pugmill, I can easily
compare
clay that has been pugged with or without vacuum....When I have the vacuum
turned off, the clay that comes out is 'short'. If you take a coil of it and
bend it, the convex surface of the bend cracks open. If you stretch the
coil, it breaks easily. When you wedge this clay, it cracks any time it is
bent. You can wedge this clay for half an hour, and it will still be short!
If you then run the same clay through the pugmill with the vacuum turned on,
it changes dramatically. You can bend a coil of the deaired clay this way
and that without seeing any cracks."
Kevin Hulmes wrote (2/21/97) in a terrific post, mostly about bacterial
contributions to aging "Incidentally, to answer Ron's discussion point
comparing bacterial action with the rapid aging in a deairing pugmill, I
believe this aging occurs because the pug can achieve better compression on
deaired clay, shearing the agglomerates, distributing the clay constituents
more evenly, and increasing clay strength. If that is so, its not caused by
removing the air, but by the following mechanical action. This compares with
the mixing caused by bacterial movement rather than by any de-airing action."
Which brings me (FINALLY!) to the point I would like to propose: pugmill
compression, deaired or not, has practically nothing to do with increased
plasticity/workability, and vacuum deairing practically everything. Here is
what makes me think this.
1. I can compress the bejeeze out of a short clay, for instance by putting a
small
extrusion die on the end of a big pugmill, and it still comes out short.
2. I can take a handful of short clay fresh from the mixer, drop it into
the pugmill
vacuum chamber for a few seconds, take it out by hand (no mixing, no
compression), and ta-daa! Totally plastic clay.
How does this happen? I don't know, but here's my guess. I totally agree
with the water hull/wetted particle theory expounded above; I think that as
the vacuum sucks out the air,water is drawn in around each tiny clay particle
to replace it, thereby greatly increasing plasticity. Not quite as thorough
as blunging, but not bad, and a lot easier!
What do you think?
Martin Butt
Coyote Craft School, Albuquerque, New Mexico
email martinb888@aol.com
http://members.aol.com/martinb888
Elca Branman on mon 17 mar 97
I really appreciated your digesting, lining up the different opinions and
input on clay plasticity.
I tend to have my eyes start to glaze over when i read some of the
technical stuff, and indeed, my delete finger, with a mind of its own,
often kicks in halfway down..BUT it is really good to get the gist of the
matters all in one piece..my finger did nothing and my
attention stayed in place...Thanks Elca
Branman Potters elcab1@juno.com
in Stone Ridge ,N.Y.
in the Hudson Valley
Craig Martell on tue 18 mar 97
At 11:07 AM 3/16/97 EST, Martin Butt wrote:
>----------------------------Original message----------------------------
>Which brings me (FINALLY!) to the point I would like to propose: pugmill
>compression, deaired or not, has practically nothing to do with increased
>plasticity/workability, and vacuum deairing practically everything. Here is
>what makes me think this.
> 1. I can compress the bejeeze out of a short clay, for instance by putting a
>small
>extrusion die on the end of a big pugmill, and it still comes out short.
Hi Martin:
If you start with short clay, that's all your're going to get, short clay.
If you deair and compress a clay that has some degree of plasticity and
workability, the working properties of the clay will improve. Harry Davis
says in The Potter's Alternative that a deairing pugmill will increase the
workability of clays that do not have optimum plasticity. I think what he
says is: The pugmill will make the most of what workability is present in
the clay.
What you have stated about deairing without compression improving the
plasticity of clays is an interesting point but I still feel that deairing
plus compression will greatly improve the workability. Frank Hamer has
written a bit about deairing and compression and also feels that BOTH
improve the workability of clays that have some workability and plasticity
to begin with.
Regards, Craig Martell-Oregon
Gavin Stairs on wed 19 mar 97
Hi all,
I think that it is the high shear in a pug mill which improves its
workability. This is the same thing that happens when you wedge,
particularly spiral wedging. It means sliding the clay over itself, pulling
one way and pushing another at the same time. What this does is to line up
the clay platelets, so they can slip over each other easily. It also
mobilizes the water between the plates. You may not know that the water in
clay essentially freezes up when it is left alone for a time. Has to do
with the polarization of the water molecule lining up with the polarization
of the clay. That's why slips may gel, and so forth. It's a well known
fact of civil engineering that clay with a sufficient water content (like on
the hillsides in California after a rainstorm) is sensitive to vibration,
and may liquify if the vibration is strong enough. You can duplicate this
with some of your own clay body. Let it set up, and then vibrate it.
Maybe with a vibrator (?) (what's that?), or an electric razor, or an
etching tool. Anything that vibrates. Or you can just shake it with your
hands. I can't guarantee that it will liquify, but it should get softer.
Maybe I could invent an instant wedger: just a vibrating table. Just don't
sit on it.
The deairing probably doesn't do much to affect the handling, except that
you have to chop it up to deair it, and then shove it back together again,
which is going to wedge it pretty effectively. Vacuum deairing also
dewaters to some extent, so you have to make sure the water content is the
same after pugging/wedging.
Other reasons I can think of that may mean that pugged clay might be
different from hand wedged clay are temperature (both get hotter: the
mechanical work=heat from a pugger is greater than hand wedging, but
deairing will cool it, and hand wedging may heat it from body heat), the
rate of action (puggers are faster, and clay IS rate sensitive) and the
greater regularity of pugging. The pug comes out spiral pugged in a neat,
consistent way, which may give it some anisotropy in forming (do I throw it
round side up, or flat side up?). My guess is that sufficiently pugged clay
is probably more plastic, especially if you give it a couple of passes, and
the last one with the vacuum turned on, simply because the pugger works
harder at it than you possibly could hand wedging.
Gavin
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