Lewis on wed 22 jul 98
Dear Gary,
One could easily devote an entire book to answering your questions, and
indeed someone has: Harry Davis, who wrote The Potter's Alternative (1987,
Chilton Book Co., ISBN 0-80198006-2). Davis worked in Nigeria, Peru, and
New Zealand, running potteries that used only local materials for their
clay bodies and glazes. The book is out of print, but I found it for sale
at ABE books: http://www.abebooks.com/
It's listed at $40, and I highly recommend you buy it. The first two
chapters deal with mining clays and minerals for bodies and glazes,
followed by chapters on building and using jaw crushers and roller mills,
ball mills, blungers, and pug mills. There are also chapters on de-watering
slip, making saggars and firebricks, presses, various tools, etc. You will
need a geological map of your area; this will help you enormously. Digging
clay is fairly easy provided the bed is close to the surface; the bed I
mine is very clean, and the clay is ready to throw right out of the ground.
Davis wrote several pages on identifying and testing clay samples. You will
also find similiar information described in Clay and Glazes for the Potter
by Daniel Rhodes.
Here in Sweden, the only stoneware available lies at a depth of 60 meters
or so; I purchase this clay from a mining company. There are two common
earthenware clays, and I dig one of these at a depth of about 1 meter. They
are easily found using a geological map--creek beds are good for
identifying a nearby clay bed, but don`t mine directly from creeks. There
is often a lot of organic material present, and the clay contains more
water than in the rest of the bed. Creeks and rivers can be good for
collecting samples of geological formations found upsteam. You'll want to
mine from an area that is close to a road. In addition to a map, I use a
compass, pick, shovel, wheelbarrow and truck when prospecting, and
sometimes hydrochloric acid to identify the presence of calcium.
Davis crushed local rocks to make his glazes and designed a formula he
called 8:1:1--80% granite, 10% calcium carbonate, 10% clay. This will
usually make some sort of celadon in reduction. You can also do line blends
of various ashes and rock. Start with 100% rock and 0% ash, then 90%-10%,
and so on until you have 0% rock and 100% ash (11 samples in all). Continue
testing the ones that look promising. Glazes with large amounts of ash
often need a suspension agent. I always wash the ash I use, but other
people don't. Glazes made with unwashed ash are caustic, and are hard on
the hands.
At present I work mostly with slip glazes made from earthenware clay and
washed ash. My local clay fired to cone 10 oxidation makes a smooth glaze
saturated with iron. By adding a flux such as 5-10% ash or calcium, the
oxides are subdued and a brown-black glaze results. Additional flux makes
syrupy brown greens, greens and then pale glazes. The advantage of these
slip glazes is that they don't require milling of any sort, and only need
to sieved through the proper screen. Slip glazes usually require raw
glazing.
There are a thousand more details to describe, such as how to mix wet and
dry ingredients, but work beckons. Sorry that I have to cut this off so
abruptly--let me know if you have any questions.
Good luck,
Colin Lewis
mc,lewis@swipnet.se
>From: Gary Huddleston
>Subject: Clay mining in Mexico - Help!
>
>I want to explore the ancient art of pottery making by digging
> my own clay (for high fire wood kilns), making my own kiln
> bricks, and formulating ash glazes. I am retiring to the
>mountains of rural central Mexico, and commercial clays
>will not be available, nor will refined minerals and other
>chemicals. Measuring by volumn will be the rule. How do
>I identify suitable clays. I understand creek beds and washouts
>are a good place to start looking.
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