Bonita Cohn on thu 15 apr 99
You have not described the kiln size and burner setup. If it is like the old
one we just scrapped at Ruby's Clay Studio here in San Francisco, I can tell
you a few things about cone 10 reduction. For more than 20 yrs., I've been
firing this old 12 cu. ft. stacking updraft, 2 forced air burners, Alpine.
We replaced Ruby's original with another old "silver bullet" that Jade Snow
Wong donated it to us in 1987. She had it since the 50's.
It's a simple little box that holds heat. You must control how much
goes in and how much leaves. Basically, it's a balance between damper, fuel,
and air intake. Have 2 sets of cones: body reduction 08, 5, 7, and 8,9,10.
Pack tall to medium stuff on the bottom (5 inches or more), for circulation.
Have the bag walls in place. (We had 20"x9" x .5" kiln shelves set on their
side). To start, over night, I set the damper at two fingers and air at 7/8
closed, with small gas amount (1/2"). Let it sit for an hour, then turn on
the blowers. (For a bisque I use smallest gas increments, and wait until red
heat to turn on the blowers). Gradually open the air with each gas
increase, to maximum 1/2 air open, over the course of the firing. (The
manufacturer's guidelines for oxidation had the maximum air open 1/2 with 3"
of pressure.) This is your reserve "adjustability" . Use the air flaps to
control the rate of burn and quality of flame. I usually don't go past 2.5
inches of natural gas pressure. Opening the damper will heat the top, and
closing the air will slow down the hotter bottom. Are you doing glaze
reduction? If so, make sure there's back pressure from the bottom peep
consistently after body reduction (010 to 06, depending on your glazes). This
is achieved by tapping in damper until small back pressure happens. The
intermediate cones 5 and 7 will reveal uneven conditions so you can make
adjustments. Go more slowly; a 12 hr reduction firing from red-heat is not
unusual. Too much air intake heats the bottom too fast. Too much gas stalls
it and you'll be there forever. And the color of the flame out the top tells
you a lot. READ THE FLAME. As it gets past Cone 5, the flame is greenish
yellow with an orange core. Orange/lavender earlier is reduction, open damper
some but be sure to allow back pressure with a cleaner flame out the top. At
the end I soak, by turning down the gas to 2" for 12-20 min, then close the
damper, first turn off the main gas valve, then the 2 burners. Watch the
Dragon go to sleep.
I'll leave it at that for now, as you have not stated the temp and atmosphere
you are trying to achieve. It was a great little kiln and I will miss it. It
was too small for most of us, but it was a good size for an individual to
fire often, therefore getting a lot of tests thru on a regular basis. And a
great bisk kiln for those heavier big pieces that get blown up in the
electric kilns.
Another way to deal with a kiln that is consistently uneven, is to know your
zones and use the glazes that work in those zones and stop sweatin' it!
E-mail me off list if you like, for further info. Good luck! Bonita in
Sunny San Francisco.
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