ivor & olive lewis on tue 13 oct 09
Dear Marci,
I recently tested the effectiveness of several substances recommended as
thickening agents for clay slip. My sample slip was prepared from powdered
porcelain clay using those proportions of water and clay that would
represent their proportions in a plastic clay. To ensure I had a fluid the
water used contained Dispex, a proprietary deflocculant. Measured samples
were tested
My observations match your experience and I quote "I can take porcelain
slurry and add a little vinegar and it thickens right up".
Apart from the few drops of flocculating substance neither the volume or
mass of clay nor the volume or mass of water changed.
Since clay particles are solid and water is a liquid the only way I can
explain the effect is to say that the Water changes from being a Liquid to
being a Solid. This seems to defy Common Sense because Water only Exists as
a Solid if its Temperature falls below its Freezing Point.
Best regards,
Ivor Lewis,
Redhill,
South Australia
ivor & olive lewis on tue 20 oct 09
Dear friends,
F. H. Norton, in his chapter on Forming Plastic Masses in "Fine Ceramics.
Technology and Applications", illustrates the idea with two examples, one
where the thickness of the water film is 0.03 microns and one where the
water thickness between the two crystals is 0.005 microns. The calculated
force needed to separate the kaolin crystals separated with the thicker
layer of water is 0.0006 dynes, and by the thinner layer of water, 0.05
dynes.
Hence, less water, stiffer clay !
Depending on the specific nature of a particular clay, the mass of water
needed to generate a mass that can be shaped without fracture and retain th=
e
form it has been given can vary between 20% and 30% (W. E. Brownell)
In the samples Mel has kindly shown on his Clayart Page, I allowed 30% by
mass (30 grams) of Water to be adsorbed by 70% by mass (70 grams) of Kaolin=
.
Using an RD of 2.6 for Kaolin and 1.0 for water I calculated the volume of
mineral as though it were one solid piece and then obtained the Volumetric
ratio of the two substances. This is the upper limit of the range given by
Brownell.
Current "Plasticity Theory" used by our contemporaries may be well and trul=
y
out of date. For more than thirty years industrial ceramists have been
informed that in plastic clay, water is solid not fluid.
Best regards,
Ivor Lewis,
Redhill,
South Australia
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