Gordana Ricijas on thu 21 mar 02
I test plates and cups for crazing on temperature 200/20°C. The pieces are heated to 200°C and the test pieces are dropped onto a bucket of water (20°C). This is reapeted 5x. Plates don't craze, but cups (and bowls) sometimes craze near the feet. What can couse this crazing on cups and bowls?
- glaze coefficients is 5-10% less than of the body,
- I made test with thicker and thinner application of glaze, but both (and thinner and thicker glaze coat) less or more craze on thermal shock.
It is interesting that plates don't craze, only cups and bowls! What do you think about that.
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Eleanor on thu 26 nov 09
And speaking of Physics..............
Back in the Stone Age (early 50's) my ceramics instructor, the late
Hui Ka Kwong, was asked about the safety of using our ^4 fired ware in
He replied that since the ware was fired at about 2000F there was no
reason to fear that it wouldn't survive an oven temperature of, say,
Now, 60 some-odd years later, we speak of thermal shock. Please,
gurus, in words of one syllable, explain.
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John Hesselberth on thu 26 nov 09
On Nov 26, 2009, at 10:18 AM, Eleanor wrote:
> He replied that since the ware was fired at about 2000F there was no
> reason to fear that it wouldn't survive an oven temperature of, say,
> Now, 60 some-odd years later, we speak of thermal shock. Please,
> gurus, in words of one syllable, explain.
He was right, of course. The simple definition of thermal shock is =3D
exposing a pot to a temperature far different from its current =3D
temperature. The problem comes (and what we often refer to as being a =3D
problem of thermal shock) when the piece is heated rapidly and/or =3D
unevenly. If the inside of the pot has frozen food in it and the outside =
is exposed to a 450F oven there can be a problem. Even then if the glaze =
and clay body are well matched to each other the pot might survive. But =3D
if you can heat/cool the pot evenly on all sides it is far less likely =3D
to break from rapid heating or cooling (or thermal shock)