iandol on tue 23 may 00
Hello Anji,
Consider the following.
Although Stoneware is regarded as being fully vitrified, which it most =
likely is, there is always the suspicion that it has a degree of =
porosity which remains. About one percent. This is inevitable because =
during the sintering phase, residual air in the interior of the pot is =
sealed as the grains of clay, silica and felspar fuse. The volume of =
pottery decreases during firing because most of the air is squeezed out =
but that one percent remains. If this residual porosity remains as a =
slight continuous network water can be absorbed into the ceramic. This =
water changes to steam under influence of the radiation and it=92s =
volume increases a hundred fold or so. Eighteen grams of water can =
occupy twenty two litres of space, give or take a bubble or so. This can =
cause stress and make a glaze craze, very common in earthen ware used =
for baking. But microwave energy is a bit more instantaneous which may =
explain the Big Bang.
Iron coloured glazes do absorb microwave energy and this can cause a pot =
to feel hot. And those exposed unglazed areas would allow for greater =
adsorbtion of water.
There should be notes in the archives about tests for pots which are to =
be used in a microwave situation. I microwave our coffee mugs twice a =
day. Cone 8 Stoneware with a clear gloss glaze. No problems. Been in use =
for about eight years now.
Hope this helps,
Ivor.
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