iandol on wed 23 may 01
Dear Matt MacIntire,
An interesting proposition! You will get several answers to be sure. One =
which may come forward is that the clay is thixotropic, that is, it =
responds to mechanical stress through a breakdown of the binding forces =
between the materials of which it is composed. Clay bodies with large =
additions of Montmorillonite tend to exhibit this behaviour because of =
the volume of water which accumulates between the t-o-t/t-o-t molecular =
layers. Under static conditions this water exhibits solid qualities, =
under dynamic conditions it fluidises, or so the theory goes.
Clays with a large proportions of sand, grog or other non plastic =
content provide pathways into the body because the plastic portion =
shrinks away from those substances as water evaporates leaving micro =
fissures. These form good capillary pathways, especially if they =
interconnect. So you may expect them to soak up water. Any residual =
stress from wedging, kneading or throwing will assert itself at these =
points with the usual disasterous concequences as drying proceeds.
Best regards,
Ivor Lewis, Redhill, South Australia.
Matt MacIntire on fri 25 may 01
Ivor,
I had heard of thixotropy, and wondered if that was related somehow to this
effect I notice. It sounds like thixotropy is involved.
I am struck that, quite often, we accept the nature of our materials without
any real scientific understanding of why they actually behave the way we
accept. Sometimes the full understanding is within our reach, sometimes it
is not. It is fun to at least aim for a thorough understanding.
In particular, I appreciate your explanation about the micro fissures
forming capillary pathways for water to re-infiltrate the clay body. That
explanation makes perfect sense and explains why my coarse clay re-wets
better than a smooth porcelain. I like knowing the WHY as much as the HOW.
At least as far as my understanding can take me...
my thanks for all your informative comments to the list.
Matt
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