Matt MacIntire on tue 22 may 01
What is it that makes some clays setup and stiffen just before enough water
has evaporated to really be leather hard?
If you slap the clay around a bit it gets plastic again - with no addition
of water. The water is still there in the clay, but the clay has stiffened
somehow. Does anyone know what this phenomenon is called and what actually
causes it?
I have also noticed that these clays also seem very easy to re-wet. Even if
they get black hard, I could usually re-wet them and coax them back to a
plastic stage, if I wanted. Occasionally I have done this even when the rim
is almost dry. In my experience these clays are typically fairly coarse
stoneware clays. Smoother clays don't setup as much, in my experience.
No big problem here, I just always wondered what was really going on.
Anybody know?
LOGAN OPLINGER on fri 25 may 01
Hello Ivor,
Interesting indeed! I know thixatropy. I understand thixatropy. I walk among thixatropic clays on our weekend hikes. Nasty stuff! Looks solid, feels solid under foot---until, enough pressure is applied and ZZZIIIPPP!!! your feet are anywhere but under you giving reassuring support.
Most of our clays here are earthenware or terracotta, with a high content of montmorillonite and another closely related mineral, haloysite (sp?). Some are primary, weathered directly from volcanic ash, and others are secondary; all the result of tropical weathering. Very dense, extremely fine grained, not nice to work with. I have found so far only the secondary clays can be made workable in the normal pottery sense.
To reduce the ammount of water necessary to develop proper plasticity, I add one to two percent by weight soda ash, disolving in water first, or I can use seawater (our public drinking water here has a high calcium content). Instead of about 50% water, I only need about 40% water. Still high though.
As you say, large amounts of sand or grog play havoc with the clay body structure during drying and firing. What I have found to work though is a generous 40 - 50 % addition of partially weathered volcanic tuff, 40 mesh and finer, to open up the clay. When exploring this problem, my reasoning was this, the partially weathered tuff has a clay component, so it expands/ contracts slightly when water is added/ removed. So far so good, but what about the firing process? The weathered tuff shrinks when fired (not like sand or fired clay grog), right along with the clay as it also shrinks during the firing. Thus internal stresses in the clay structure do not build up to a great extent during the firing.
This is as far as I have gotten with my investigation, but I hope this information is useful someone.
Logan Oplinger
Ivor wrote---
>...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.
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