Hanne Bjorklund on thu 8 jul 99
No, I was not talking about crawling or shivering. Crawling has a fat soft
undulating edge. I don't know much about shivering, except that I believe
the glaze flies off during the firing.
Crazing is in more or less straight lines, sometimes almost invisible, with
directional changes where the cracks meet. The top edges look 'sharp' and
well defined, as a result of the glaze opening up because it is shrinking
more than the clay during the cooling stage. The reverse is like covering a
half inflated balloon with slip, and then, when the slip is dry, inflating
the balloon further. The slip cannot stretch, so it cracks.
After a long drought the pond on my land dries up, and the surface cracks
just like a crazed glaze.
The situation where a crazed glaze eventually left the pot, was on a low
fired soap-dish, by a well known potter. My guess is that the clay was not
fired to it's vitrification temperature and was therefore still porous.
Water seeped into the clay through the crazing, the clay absorbed the
water, expanded, and the glaze, because it was unable to expand, lifted off
the soapdish.
HANNE
Ray Aldridge on fri 9 jul 99
At 08:23 AM 7/8/99 EDT, you wrote:
>----------------------------Original message----------------------------
>
>
>The situation where a crazed glaze eventually left the pot, was on a low
>fired soap-dish, by a well known potter. My guess is that the clay was not
>fired to it's vitrification temperature and was therefore still porous.
>Water seeped into the clay through the crazing, the clay absorbed the
>water, expanded, and the glaze, because it was unable to expand, lifted off
>the soapdish.
>
How interesting. Still, I wonder why, if the glaze were able to craze to
relieve the initial poor fit, it couldn't continue to subdivide to relieve
the additional expansion. Also, I'm not sure that fired clay actually
expands much when wet. Does it really? This seems to run contrary to my
understanding of the nature of porous bodies. On the other hand... did the
soap dish go through a hot dishwasher?
Maybe this was a case of shivering. Did it chip off the edges first? Did
any of the body come with it? As someone more knowlegeable than I pointed
out recently, it's hard to tell mild crazing from mild shivering.
Ray
| |
|
