Corinne Null on wed 2 oct 96
We have had many wonderful discussions of glaze firings, for which I am most
grateful.
Now, however, I'm questioning my bisque schedule. I inherited it from
school, where it was designed for convenience and safety. The work was
quite uneven, and therefore slowness assured more success for everyone. I'm
firing 50F/hr to 300F, and then 100F/hr to 1785F. That ends up being about
a 19-20 hr bisque. Is there a better ramping schedule that would shorten
the time and yet not endanger the ware? What are the critical temperatures
and processes that occur in a bisque firing?
Thanks,
Corinne Null
Bedford, NH
cnull@mv.mv.com
Tom Buck on thu 3 oct 96
Corinne: If you read Pat Malone's 1975 MFA thesis, you will see how a
typical claybody behaves as it heats up and cools down. Part of Malone's
work was described by him in an article in Studio Potter 20 years ago.
See Studio Potter Book, pub 1979 by Van Nostrand Reinhold, Pages 28-36.
Malone presents a chart on P. 36 that summarizes the activities that occur
duting a typical firing to 1300 C, 2375 F.
He says the critical stages occur:
1) Loss of "mositure" -- room temperature to 200 C, 400 F.
2) Loss of chemically-combined water and formation of metakaolin,
resulting in a sudden volume increase of 1%, sometimes causing cracking if
it occurs too rapidly. The loss of OH- ions happens between 525 and 650 C
(980 - 1200 F). The volume change occurs suddenly at 573 C, 1063 F both on
heating and on cooling; at red heat this sudden increase may damage pots.
Malone suggest one slows the heat rate going up, and the heat loss going
down.
3) Change in crystal structure at 980 C (1800 F) resulting in a suddden
shrink in volume, followed by slower shrinkages (2) due to formation of
mullite and cristobalite.
4) If reduction conditions are induced, there may occur "iron-coring"
where Fe2O3 (red iron oxide) may be converted to FeO (black iron oxide),
somwetimes with impact on the glaze surface.
So, with this data in mind, a more likely program of heating is a
very slow heat rise until 400 F, then a 300 F/h heat rise til 950 F, slow
to 80-100 F/h until 1200 F, then 300 F/h again until 1750 F, back to
100F/h until 2150 F, and 300 F/h til finished.
If you are going to C6 (2250 F), you should be done in 10-12
hours. Good firings.
Cheers TomB Hamilton ON Canada URL http://digitalfire.com/magic/tombuck.htm
Debby Grant on fri 4 oct 96
HI AGAIN CORINNE,
YOU CAN KNOCK DOWN YOU FIRING SCHEDULE TO BETWEEN 8 AND 10 HOURS. GO SLOWLY
TO 1000 DEGREES F. OR YOU CAN USE BAILEY'S
PRE PROGRAMMED SCHEDULE INSTEAD OF USER. MY KILN IS NOT SO
SOPHISTICATED AND I FIRE ON LOW WITH DOOR CRACKED AND ONE
PEEP OPEN FOR ABOUT 4 HOURS, THEN ON MED. FOR ABOUT 3 HOURS
WITH LID DOWN BUT PEEP STILL OPEN, THEN ON HIGH WITH PEEP CLOSED
TILL CONE GOES DOWN.
GOOD LUCK, DEBBY
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