Jonathan Kaplan on sun 18 aug 96
As the original inquiry was from me for a colleague, I'll add some
information that might be useful.
Studio pug mills are valuable tools for us. They are of a "straight
through" design. By whatever configuration each manufacturer has chosen,
there are two bearings (or more) external to the chamber and shaft. The
clay enters perpendicular to the shaft and exits parallel with the shaft,
the the exception of the Walker machine. The deairing chamber is positioned
after a shreader screen that may be fixed on the shaft(the older
Bluebirds), or placed in the direction of clay movement, but removeable, in
the case of the new Bluebirds or the Venco. These shreader screens forced
the clay into small rabbit size pellet extrusions that are then passed
below a vacuum chamber that deairs them. As the clay continues its way down
the barrel, the blade configuration recompresses the clay into a dense
mass, The exit nozzle is of a smaller diameter than the barrel itself, or
it might be tapered, This further compresses and tightens up the extruded
pug. The action of the auger does indeed place a spiral into the pug which
must be reversed or negated when thrown by re-orienting the cut pug on the
wheel head so that the spiral is ninety degrees in oposition to the
shaft/wheel-head. These studio type pug mills do indeed get the job done.
If you take a very narrow "slice 'o pug" from a fresh extrusion, hold it by
one's thumb and first finger at the top and jiggle it rapidly, you can see
how good a job your machine does for you. The slice 'o pug should stretch
nicely without any cracking and should not delaminate.
The problem with the studio pug is that the deairing chamber is external to
the machine, and what little vacuum is able to enter the mixing chamber is
really not sufficient. But while it does indeed work, our industry
counterparts use a design that really produces well deaired clay.
These machines are essentially two separate puig mills connected by a drop
box that houses the vacuum chamber. The clay, usually coming from a pre-pug
or muller, enters the top section of the pug mill (which is horizontal in
configuration) where it is coarsely chopped and and combined. It exits this
section of the pug through a shreader screen and drops into a large vacuum
chamber( which is vertical in relationship to the first pug shaft and
barrel)in rabbit turd like pellets. Here it is deaired completely. It then
exits the vacuum chamber into another horizontal pug mill with a separate
auger than the first(horizontal in relationship to the vacuum chamber but
parallel with the first pug). The resulting extrusions are way dense and
extremely deairied. Ever notice how dense the "clay in a box" is? While it
is indeed possible to visually see the spiral in this clay, by performing
the "slice o' pug" test, this clay really stretches.
Clay that is not deaired will often delaminate upon firing, as evidenced by
the resulting spiral from the pug mill. Early pugmills had smooth interiors
in the barrels. Ths did not provide any "sticktion" whatsoever and the clay
just turned in the barrel on the auger. Bluebird now incorporates "rifling"
groves that are incorporated in the casting design of the barrel. I ran
stainless steel weld beads down the entire length of the interior of my
early Bluebird to help provide some tooth for the fine grained clay bodies
I was using. Groged bodies or coarse bodies might not have this problem.
Most studio pug mills are of the straight through design. Weiland makes a
very nice pre-pug machine and drop box configuration pugmill, all
stainless. Costly, but anyone mixing and using large amounts of clay would
benefit with an industrial pug. Clay in a box while convenient, over the
long haul is expensive. Even a clay mixer coupled with a straight through
pug in the studio, while yes, is labor intensive, if you have the space and
work force to do this, does indeed offer some payback, and does provide
very workable clay.
The ultimate clay mixing configuation that yields "z best" plastic clay
ever is to mix the component materials in a blunger as a slip. Pass it
through a Sweco or other vibratory screening device, and then dewater the
slip in a filter press to the moisture content necessary. Then pug it and
voila, clay the way it should be. But I would venture that few of us have
the space or the capital necessary for this type of clay preparation. So
what to do?
Find a clay body that works and either have it mixed, or use a stock body
from any of the many clay manufacturers. If you can, go visit them and see
how it is done, Ask questions. Do they test the incoming fireclays for
lime? Do them screen the fireclays? Are their clay bodies regularly
tested? eg....DTA tests on a dillatometer? Any simple tests for glaze
fit? Etc etc. etc. Have you found bits o' crud in your premade clay...nuts,
bolts, cigar buts, a comb, etc., I have, and this is indeed a downside of
the premade/prepared clay business. I sure would get bored and frustrated
if all I did all day is pug and bag clay.
I use some clays in a box for many of my contract clients because it is
convenient. I visited my main clay supplier in Denver, Mile High Ceramics,
and was overwhealmed by the system that they installed to produce blended
plastic clays. The same system also includes a shunt that allows the
blended dry materials to exit the pre-pug and be blown into bags. They
blend my casting body in this way. It is quite a technical marvel, and all
computer controlled. The plastic body can be made with any percentage of
water necessary, all accomplished by the computer metering out the water
into the prepugger. Very slick!!
Hope this sheds some light on clay preparation in the studio and the
machinery used.
Jonathan
Jonathan Kaplan
http://www.craftweb.com/org/jkaplan/cdg.shtml
(aka "Scooter)
jonathan@csn.net
Ceramic Design Group Ltd./Production Services Voice:
970-879-9139 POB 775112
FAXmodem: same
Steamboat Springs, Colorado 80477, USA CALL before faxing
"Arrive on time, tell the truth, be a good listener, and don't
be too attatched to the results. Above all, maintain a sense of
humor!"
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