GREG MARSHALL on sat 10 jun 00
Mark, here's some information about firing rates that I got from Jim =
McKinnell at a workshop years ago:
"These suggestions apply to kilns of all sizes and particularly to kilns =
smaller than 15 cubic feet which can be fired too rapidly.
1. Unless kiln contains green ware - or very large bowls and platters, =
the temperaature can be advanced about 300 degrees F per hour up to cone =
012 to 08, where partial reduction can be imposed in some manner.
2. In order to get both good reduction and good, unhurried glaze =
melting (starts about cone 3 or slightly lower) allow two hours between =
cone 012 and cone 1. Keep kiln in partial reduction from cone 08 to =
cone 9.
3. Allow 2 1/2 to 3 hours between cone 1 and cone 5. Slow kiln down a =
little if necessary at cone 3 to insure sufficient time and prevent =
temperature jump - which many kilns will do if not retarded.=20
4. Allow 3 1/2 to 4 1/2 hours - or longer - between cone 5 and 10. =
Most kilns jump between cone 5 and cone 7 due to exothermic reactions in =
the clay body. Very wise to slow kiln down again when cone 5 is over to =
prevent this sudden jump and to give more time to slow melting and =
bubbling of glazes - at the same time keep the kiln in partial =
reduction. That is, the interior of the kiln should be slightly to =
definitely hazy and there should be some orangish yellow or purple flame =
coming out the peep hole. Two or three inches of flame is generally a =
minimum. Each kiln has its own partial reduction appearance and must be =
learned by trial and error.
5. Slow melting between cone 5 and 10 permits growth of mullite and =
anorthite (calcium feldspar) crystals. Slow cooling under slight =
oxidation is also necessary if kiln cools 200 plus degrees F per hour =
(which small kilns will do). It may be necessary to "fire the kiln =
down" for 2 or 3 hours to prevent drops greater than 120 degrees F per =
hour.
6. Heavy reduction and oxidation at end - optional. More important in =
smallish kilns to keep in some partial reduction all the way from cone =
08 onwards."
I hope this offers some helpfull guidelines. Thanks to Jim =
McKinnell.=20
Greg Marshall
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