Ron Roy on tue 16 nov 99
As you all probably know - I have a dilatometer - for measuring the
expansion of clays and glazes as they are heated. There is an extra
increase in size due to free quartz in all the bodies I have have measured
(probably 150 by now) at 573C - but never a contraction afterwards. How
much the expansion is directly due to the amount of free quartz in the
I think it is very unlikely that cracking would occure at this stage on the
way up in any kind of normally fired potters glaze kiln. Much more likely
to occure on the way down in a bisque firing.
There is a reason for this - on the way down the outside of - lets say a
platter - when cooled faster than the inside tries to get smaller due to
the quartz inversion - the rest of the plate has not gone through the
inversion and won't allow the rim to get smaller - crack!
On the way up the rim would get hotter faster and would try to get bigger
at 573C - the middle holds it back - the clay is compressed at the rim and
is much less likely to crack.
>One piece of useful information - especially for large pots - when reaching the
>quartz inversion temperature on the way up (573 C) the pieces EXPAND as much as
>1%, then suddenly contract!
93 Pegasus Trail
Louis H.. Katz on thu 18 nov 99
I am in agreement. I think most cracking in bisque firing comes in the cooling.
you fire your kiln slow on the way up it might take 10 or more hours to go from
degrees F to 1800. On the way down however you loose the first 1000 degrees pret
fast. In some kilns this takes only 2 or three hours. If cracks are happening in
the bisque firing I suggest slowing the cooling by using better insulation, or b
turning the kiln back on low until it is below 950 degrees F.
Programable kilns.... If I had one of these I would slow the cooling down betwee
1150 and 950.