Production Process » Clay Production
The process begins with an industrial clay mixer that I constructed mostly from scrap metals and surplus transmission components. The one pictured was just completed in January of 2008 and can mix up to 500 pound batches, which is double the capacity of the first mixer I built.
As soon as I mixed my first batch I knew I would never purchase pre-mixed clay again: it costs nearly twice as much to buy and gives a potter very little flexibility. Many commerically-produced clay bodies are forumulated for student use because the academic institutions that purchase them comprise their main customer base.
So I took a close look at a $5000 commercial
clay mixer built for
studio potters, and I built a slightly more powerful one for about one-tenth the cost. The picture on the right shows the inside of the mixer
with blades that revolve at about 20 rpm and mix a batch of clay in about 10 minutes.
I then started researching the process of formulating a stoneware clay body. I started with a simple stoneware formula and did some tests to see how well suited it was to the temperature range I work in, about 2400°F, and then gradually developed the clay body.
I wanted something that would
be nice to work with on the wheel, dry and fire
well with minimal cracking, and, most importantly, be able to stand up to its intended use as functional ware, including use in an oven. That
is simply too much to ask of a commercially made clay, but easily within reach with the right mixing equipment.
The picture on the left shows about 100 lbs of wet clay scraps dumped into the mixer, which is how I usually start the process of mixing clay. These scraps are reclaimed from the throwing and trimming process.
To this I will add another
100 lbs of dry materials, including
four different types of clays and some feldspar to help the clay body fuse during the firing process and dissolve traces of silica, which is
the main cause of oven ware failure if not dealt with properly.
Mixing clay this way easily solves the problem of what to do with all the reclaim that tends to accumulate fairly quickly when doing a lot of production work. It also helps the newly mixed clay to age faster than it does when mixing entirely from dry materials. Clay which has aged for a while is always easier to work with, and I find myself making nicer pots if the clay has at least a week or two to sit before use.
The last step before making pots is to run
the clay through the machine
pictured on the left, a “de-airing pug mill”. It is made from a section of 8" pipe and has an auger that revolves inside,
grinding and mixing the clay as it pushes it forward through a set of stainless steel screens that shred it like spaghetti. As the clay comes
out of the screens it passes through a vacuum chamber, controlled by a small vacuum pump, before congealing and being pushed out the other
end of the machine, ready for use. One of the most useful machines a potter can have, the de-airing pug mill uses the vacuum pump to boost
the plasticity of the clay, making it easier to work with.
The vacuum also eliminates most of the microscopic air pockets
found throughout the tiny clay particles that were just recently mixed together and are still absorbing moisture. More water can be added
before running it through if the batch came out stiffer than needed. The final product is a very dense, plastic clay that is almost always
just perfect for throwing.
I built this machine mostly from scrap metals and surplus power transmission components, including a five-horsepower motor and heavy duty gear reducer. To the right is a picture of the auger with the casing removed, showing the configuration of the blades that push the clay through the pug mill.
Next step: Glazing >>
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