Mike Bailey on sat 17 jul 99
As I understand the situation, a glaze should be under compression
(rather than tension). You ask about the ratio:
In some tests that I did with David Hewitt we found that:
A clay with a glaze whose RTE was 1/3rd that of the clay; shivered
A clay with a glaze whose RTE = the clay; fitted
A clay with a glaze whose RTE was 3 times the clay; crazed.
There seems to be a reasonable 'window' in which to operate!
However, there are many things going on in a glaze that aren't accounted
for in the mathematics. For example, we recently had a discussion on
CLAYART about the colouring oxides - they are great at cutting down on
crazing - an effect out of all proportion to the small change in the RTE
that the calculations would have you believe.