I.Lewis on sun 8 aug 99
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To: Subscribers clayart=40lsv.uky.edu
From: Ivor Lewis iandol=40pirie.mtx.net.au
Saturday, 7 August 1999
Subject: Throwing problem. Thixotropic clay
Though changes in pH may be a plausible way of explaining why some clays =
slump
easily when they are being thrown, this would be difficult to accept unless =
the
water used to lubricate the clay was excessively alkali or acid and freely
penetrated the clay right to the core. I opine that a change in pH, =
especially
towards the higher values, would cause the clay to deflocculate, even to =
become
a free flowing liquid.
Thixotopicity is the ability of a material to change from a jelly like solid=
to
a runny liquid when force is applied to it. This occurs in some paints when =
they
are shaken and stirred vigorously. I use a clay which kneads better if the =
bag
is slammed on the bench a couple of times. It also behaves in the same way =
as
experienced by Jeff Lawrence. It has a degree of thixotropicity. Thixotropic
muds are used in civil engineering projects and by oil drillers to seal =
fissures
and fractures in the strata through which they drill. The stuff flows freely
when pumped but sets once it stops moving through the cracks in the rocks.
Bentonite is a well known constituent of drilling muds. Bentonite also =
adsorbs
high volumes of water. Bentonite is also added to many compounded clays to
assist with plasticisation.
Perhaps throwing clays which squat contain excessive Bentonite. Under =
conditions
of extreme stress such a clay might change from a Sol to a Gel, from semi =
solid
to semi liquid. Such a change would decrease the compressive strength of =
that
clay body. Vibration from the motor, however slight, might be sufficient to =
make
this change happen. Once it occurs, it will persist until the stress is
eliminated or the clay develops sufficient strength to support its own =
weight
from above.
This may account for persistence of the effect if throwing is continued, =
making
it difficult to achieve and intended elevation for a tall pot.
An interesting experiment is to let such a pot dry leather hard and then =
section
it from top to bottom. Those with a mathematical bent may recognise the =
nature
of the curves they see in the cross section and speculate on the =
relationships
between height, mass, thickness and the Force of Gravity.
Ivor. At Redhill in South Oz where it is cold and threatening to rain.
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