Ivor and Olive Lewis on sun 30 jul 06
Dear Alisha Clarke,
I have been looking through my archives and came upon this little gem =
which I wrote 17.06.2001
"Having reviewed such relevant literature as I can find and conducted =
some simple experiments I have formed an opinion that Carbon dioxide, =
released as a free gas along shear planes (caused by pug milling, =
wedging and kneading) in flock structures, provides a trigger mechanism =
which initiates cracks under conditions of high differential drying =
stress."
I could have added "Throwing" to the list.
I will send you a picture of the flow structures that form in clay when =
it is worked by hand (I do not have a pug mill or ready access to one). =
This specimen Pug, which was water etched to reveal structure, was =
prepared by spiral kneading. Seems like a familiar image to me.
Unless we are made aware of some of the properties of the materials we =
use we remain ignorant of some of their potential behaviour. I think =
that, to a large extent, people forget about the water we mix with clay. =
When you realise that a cubic metre of Water at 15 deg Celsius will have =
dissolved in it a cubic metre of Carbon dioxide, it becomes influential. =
This volume of gas has to be released from solution as water evaporates =
from the clay because Carbon dioxide partial pressure will rise above =
the pressure that keeps it in solution. So it must be considered as an =
important factor causing internal stress in drying clay.=20
I just regard fractured pots as being defective and recycle the clay or =
if they are fired, scrap them.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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