Rimas VisGirda on sat 25 sep 10
I base the following hypothesis (I don't have enough data for it to be a th=
eory) on observation and assumption over 30+ years of teaching and working =
with clay; many different types of clay over the years in different places,=
mainly high fire (cone 9-11) stonewares and porcelains. I bisque fire as f=
ast as I can and I cool as fast as I can; I attribute (students') cracks in=
a bisque to mishandling pieces in the green state, particularily when bone=
dry by bumping a rim or from placing a (slightly) uneven rim on a flat whe=
elhead for trimming... I cannot remember any of my work cracking from a bis=
que. I high fire as fast as I can and cool as fast as I can, I also attribu=
te (students') cracks to mishandling bisqueware, clinking a lip against a h=
ard object, improperly bisque firing and getting an un-noticeable crack sta=
rted by stacking pots in pots without enough leeway, firing tiles in a hori=
zontal stack, etc. I don't recall any of my work coming out of a
high fire cracked... In my work I typically multi-fire in the following se=
quence: bisque (010-04, sometimes 2-3 times), high fire (8-13, sometimes 2-=
3 times), low fire (05 sometimes 0-2 times), luster/og fire (019-016, somet=
imes 1-6 times). I've only had (usually porcelain) plates and large tiles c=
rack at the second to fourth luster firing; I attributed this to quartz inv=
ersion. That was back in the days when my kilns were controlled by a kiln s=
itter and I didn't have (or want) a pyrometer. If I elevated the flat stuff=
off the shelf or it was such that I could fire it vertically that seemed t=
o help, but didn't solve the problem. Mostly I quit making flat stuff. Unti=
l I got my first computer kiln which solved the problem when I programed it=
to go super slow on the way up and on the way down about 7-10 F on either =
side of quartz inversion. I don't know a thing about cristobolite. I used t=
o build center draft downdraft hardbrick kilns; occasionally we
would do a dog and pony show by tearing down the (stack-up) door at cone 1=
0. It was pretty beautiful to see a whole chamber glowing and the front pot=
s turning black silhouttes against the glowing interior of the chamber. I c=
an't remember any pots, posts or shelves cracking... Just lucky all these y=
ears I guess.
Respectfully submitted without any scientific backing.
Regards, -Rimas
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