iandol on thu 21 mar 02
Dear Craig Martell,
The tables available to me give the following values for forms of =
Silica. Quartz // to the axis 8*10 -6. Perp to the axis 14*10 -6, Fused =
(glassy state) 0.4*10 -6. Which of these would be chosen for inclusion =
in a glaze calcs program? Values for mature ceramic bodies seem to vary =
between 2.0*10 -6 and 6.0*10 -6. Since glazes are prediminantly vitreous =
silica there seems to be potential for trouble between glaze and body =
even before devitrification happens.
The formula I gave is for fully vitreous substances. Once =
devitrification starts it becomes a can of worms. You say you have =
pyroxenes. Have you identified them? Since Pyroxenes seem not to have =
cubic lattice systems it is, perhaps, a fair assumption that they behave =
like quartz and have different values along each axis. This brings =
separate internal problems within the glaze as well as antagonism with =
the body on which it rests.
These are problems which will not be solved by inventing an algorithm =
based on assumed values for substances which do not exist in the =
material we are dealing with. The answer will come through testing, =
making specimens and heating and cooling them while measuring the degree =
of expansion or following the empirical trial and error pathway.
It is my firm belief that taking a given recipe and attempting to adjust =
it to fit any and every clay body may lead to disappointment. If it was =
my intention to make a microcrystalline glaze, then I would anticipate a =
change in the Coeff.of Exp. and make pre-emptive adjustments =
accordingly. I would also anticipate that there would be some degree of =
defect development and take steps to ameliorate the potential for =
catastrophe from either fast or fatigue cracks.