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Making Pottery Clay : Processing Clay with a Pug Mill

OK, so in this segment we’re going to take
a look at some of the mechanical devices that we’re going to use, to continue to process
our clay, so that it’s as homogeneous, as possible, and a good consistency to use it.
Now, behind me you see two different pieces of equipment here, that both serve the same
function. They’re called pug mills. They both operate under the same principal; one is just
smaller than the other. The clay is fed in through the hopper, here, with this, and packed
in with a tamper, and inside this tube is an auger, like a big screw. And, that churns
the clay up, and really works out all of the lumps and inconsistencies, and packs it together,
and then the clay is extruded out the end, in a long, thick coil, which we cut off, and
bag, and store for use. Now, like I said, they’re both pretty much the same piece of
equipment, this one’s bigger, and has a large, much larger hopper, because this one is also
designed to do some of the mixing of the materials that we did by hand. But, again, this is an
expensive piece of equipment, and may be hard to justify the expense, if you’re only mixing
a little bit of clay at a time. So, I have my clay that I pulled off of our plaster bat,
it’s just about the right consistency for use. It’s important to note, however, that
when I took it off the plaster, I was really careful to ensure that I didn’t get any chunks
of plaster in the clay itself. Plaster is a hydrous material; water’s locked up, chemically
bonded in that plaster material, and chunks of plaster inside the clay. If you make something,
you know, a pot on the wheel or something, and a chunk of plaster ends up inside the
wall of your pot, the moisture locked up in that plaster will expand rapidly in the kiln,
when the piece is fired, and cause the piece to “explode”. It just means that there will
be a little steam expansion in there, and a little chunk of your pot will be blown off
the side of the pot. So, be careful not to get any chunks of plaster in your clay. So,
I’m going to take some of this clay here, and I’m going to fill the hopper of the mixer
with the clay, and turn the machine on. This one, I’m going to turn to mix, for just a
second, and then, I’m going to turn it to pug. And, this does all the work for me here,
mixing and churning that clay, and as you can see, here it comes right out the end,
in a nice usable chunk. Now, some of these pieces of equipment are more sophisticated
than others. A lot of models have a vacuum pump that attaches to it, attaches to this
area in the, on the tube here. And, that vacuum pump will suck out all of the air, air bubbles
from the clay, and really compact the clay as tightly as possible, because, again, you
don’t want air bubbles in your clay, when you’re trying to use it. It makes it difficult
to throw on the wheel, because of different consistencies, and also, can cause, again,
“explosion” problems in the kiln. These two don’t have that, so they’re going to have
to, we’re going to have to prepare the clay, by wedging it by hand, before we can use it.
And, if you don’t have one of these pieces of equipment, you can wedge the clay by hand,
once you’ve taken it off the plaster, and you don’t have to have the pug mill, you can
just go straight to the wedging. And, that’s what I’m going to show you how to do, next.


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