Joseph Herbert on sun 25 may 97
My contribution to the S-crack discussion is the general observation that all
ceramic cracks are attributible to differences in volume changes. If the
volume of one part of a pot changes more than the volume of the rest of the
pot, the resulting stresses cause a crack.
An example is a pot bottom that has a greater water content than the walls of
the pot. Greater water content, greater shrinkage, more possibility of
cracking. "Compressing" the bottom with a rib or sponge probably has the
effect of getting rid of the clay that has gained excess water. That clay on
the pot's surface would transmit the excess water, by diffusion, into the
rest of the bottom as the rest of the pot starts drying. Eventually, the
bottom drys out and shrinks more than the rest of the pot that started out
dryer and cracks.
Another example of differences in volume changes causing cracks is in the
quartz inversions during cooling. If one part of the pot cools past the
inversion point, and shrinks, while some other (thicker) part is still above
the inversion point and has not shrunk, a crack will develop.
Finally, the cracks in a glaze are a result of the volume of the glaze
changing more than the volume of the vessel. These cracks will vary in
number and width depending on the amount of difference in the volume change.
So, when you are thinking of cracks, think of the things you can do the make
the volume changes that your clay object undergoes similar for all the parts
of the object. No differences, no stresses, no cracks.
I attempted to say this artistically but fear I am doomed to deletion.
Another techno-nerd invading the sacred domaine of the ceramic artist.
Joseph "Junk Email" Herbert
JJHerb@aol.com
Gavin Stairs on mon 26 may 97
At 12:36 PM 25/05/97 EDT, you wrote:
>----------------------------Original message----------------------------
>My contribution to the S-crack discussion is the general observation that all
>ceramic cracks are attributible to differences in volume changes. If the
>volume of one part of a pot changes more than the volume of the rest of the
>pot, the resulting stresses cause a crack.
This is certainly true, but the change in volume may be such as to
contribute to a bending rather than an axial push or pull. This requires a
very much lesser change in volume to induce a relatively large motion. For
example, when you throw a thin wall vessel, it has a tendency to unwind
during drying. And when you throw a strong vertical curve, it has a
tendency to pull the lip back from the thrown form. The same sort of thing
must occur on the bottoms of pots, and at strong changes in curvature.
This can induce large tensile stresses at these points, and may result in
tears and s-cracks during drying. During drying, the diimensional changes
which resulted in relatively small forces in plastic clay may induce larger
forces in leather hard clay. Uneven water content and drying can of course
contribute as well. My guess is that the vast majority of tensile failures
can be corrected by even, slow drying. But those that result from these
geometric strains from formation during throwing, for example, probably
require the assistance of compression, to realign the residual stresses,
and to avoid the magnification of the problem by sharp bends leading to the
dreaded s-crack.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
ClayMaven on tue 3 jun 97
Two magic words Particle Orientation. There are even some good pictures
showing this on the Ceramic Web. I'm sure using a rib at the bottom of a
pot helps to eliminate over wet surface clay. And constant sponging will
help keep water from soaking into the bottom of the pot. But surely the
act of compressing down on the clay with the rib helps orient the
particles. Keeping the Surface more locked from picking up new water.
After all the wheel is spinning and the clay is Moving and under the rib
aligning.
Suzanne Kraman
Richmond VA
ClayMaven@aol.com
Suzanne Kraman
ClayMaven@aol.com
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