Tom Buck on wed 16 dec 98
Yes, you got me rightly, if you are using a typical midfire or
highfire Stoneware claybody: a glaze with a coefficient of thermal linear
expansion (calculated) of 7x10-6 (or 70x10-7) will probably fit most SW
bodies (say, ones with a COE of 6+x10-6) and provide a proper "squeeze" to
the pot and thereby increase its physical strength. However, a Porcelain
likely would have a smaller COE than that of a stoneware, and therefore a
glaze COE of 7.0x10-6 may mean the glaze would craze.
And further, to make things more difficult, although undocumented
there is certainly interaction between claybody and glaze layer; but what?
and how does the interaction affect glaze fit? Since there are no easy
answers, one does the best one can: calculate the COE using known oxide
data, and if the answer is 7 +/- 0.2 x 10-6 then proceed with test
pieces/pots placed in the kiln, fired, and examine the results. If crazing
occurs, the Seger formula would need adjusting to lower the COE, opening
up options to use less KNaO, or use more silica/alumina and sometimes add
small amounts of boria.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).