Clay used to be an intense topic of discussion among stovebuilders,
since it is a traditional material in the trade that dates back to
antiquity.
I'm sure that traditional wattle'n'daubers weren't learning everything
from scratch on every house they built, and would know where the local
"good" clay was.
We used to experiment with clay quite a bit before we switched to a
different system. Although the application is different from cob, I'm
sure there are a lot of similarities as well - we look for flexibility
and ability to take movement in a clay mortar. I've used commercial
preparations and also some local clays with long-term success.
Silt is definitely what you want to avoid. Sand doesn't do any harm - it
is merely a filler or aggregate. The main quality that we look for is
plasticity. In stovebuilding mortar, which is the same as adobe, this
translates into how much sand the mortar can carry and still remain
workable. A commercial clay that works very well is Redart or Goldart,
both of them very plastic earthenware clays from Ohio. Goldart can take
carry around 5 parts sand and still give a very nice trowellable mortar.
When looking for local clays, the first place to look would be for local
brickmaking clays. Generally, the earthenware (low fire) clays tend to
be a lot more plastic than the fireclays, so that you are looking for
local clay that was used for making soft bricks. Most of the old brick
houses up and down the Ottawa Valley are made from the blue leda clay
that is found next to the Ottawa River. We once built a heater using
clay that we dug up outside the front door.
Nothing beats running a few simple tests first. In addition to the ones
you describe, it is also useful to roll the clay up into a pencil size
coil and see how far you can bend it before it cracks. Also, you can
measure it to see how much shrinkage there is on drying.
Best........Norbert
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Norbert Senf email: mheat@mha-net.org
Masonry Stove Builders mheat@hookup.net
RR 5, Shawville website: http://mha-net.org/msb
Quebec J0X 2Y0 fax: 819.647.6082
voice: 819.647.5092
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