Thonas C. Curran on fri 30 jul 99
OK, I still have my little portable kiln on the patio, firing all sorts
of nice reduction glazes (except those finicky copper reds). Last
night I brought home results of firings from Skidmore College's lovely
big kilns, and needless to say glazes were smoother than those fired at
home due to soaking and slow cooling in the giant kilns.
Question: Directions on the 20 year old kiln call for firing to
maturity without reduction (unless you want body reduction) and then
adding reduction to the last 30 minutes of the firing. Problem is lack
of soaking time, unless you count the 30 minutes of reduction. The kiln
cools very quickly (turn off at midnight, you can still open 'er up the
next morning). Some glazes need extra beauty sleep or else they have
blemishes, and has anyone any experience with small updraft gas kilns?
I hate to waste $10 worth of propane and a whole firing cycle in order
to find a way to soak the glazes without losing the reduction effects.
I did lengthen the recommended reduction time from 30 to about 45
minutes, but there was no apparent improvement. I assume (should I?)
that the reduction at the end does soak the glazes at the same time, and
maybe next time I'll try again with longer reduction period. Any
comments? Carolyn AKA cnc
John Post on sun 8 aug 99
Hi Carolyn,
It sounds like you need to control the rate at which your kiln cools.
Recently I began testing a small updraft electric-reduction kiln. The kiln
is fired using only the electric elements up to cone 8. During the firing,
at 1800 degrees F, I use a small propane burner to create a reduction
atmosphere in the kiln. When I hit cone 8, I turn off the burner and use
only the elements to fire the kiln down.
Based on my experience with controlled cooling in both oxidation and
reduction I have found that it is important for the kiln to cool slowly
from its top temperature down to about 1300 degrees F.
After the kiln hits temperature I slow cool at 250 degrees an hour down to
1800. Then I slow cool at 75-100 degrees an hour down to 1300. At 1300, I
let it cool on its own. On my larger electric kiln I use a computer to
control the rate at which the kiln cools. On my smaller electric-reduction
kiln I use the switches to control the rate of cooling. In your case
you'll use the burner to fire down the kiln.
If you have a pyrometer you could use this and the burner to control the
rate at which your kiln cools. I bet that if you watched how fast your
kiln cools in the first hour without a controlled cooling you would find
that it easily drops 400 - 500 degrees. This fast cooling doesn't help the
glazes to develop, especially the matte glazes. By controlling how much
heat your burner puts into the kiln you should be able to control the rate
at which your kiln cools. Watch your pyrometer and try to let the kiln
fall at only 250 degrees an hour for the first hour or two. Then let it
cool at only 100 degrees an hour down to 1300.
The controlled cooling should take about 6-8 hours. It will change the
glazes dramatically. It's well worth the time and effort it takes.
As far as a soaking goes, I would try to control the cooling first and then
see if a soak at or near your top temperature is necessary.
Reduction effects: My little electric-reduction kiln only has the burner
on for 2 hours. The rest of the time it is fired up and down with the
electric elements. I get beautiful claybody reduction and the glazes look
great too. All this with only two hours in reduction. The firing down
won't cause you to lose the reduction effects, it will actually help them
to develop more fully.
If you have any questions let me know.
regards...
John Post
Sterling Heights, Michigan
temmoku@actionwebs.net
>OK, I still have my little portable kiln ...firing all sorts...of nice
reduction glazes
>The kiln cools very quickly (turn off at midnight, you can still open 'er
up the
>next morning).
Some glazes need extra beauty sleep ...and has anyone any experience with
small updraft gas kilns?
>I hate to waste $10 worth of propane...to find a way to soak the glazes
without losing the reduction effects.
>I did lengthen the recommended reduction time...to about 45 minutes...no
apparent improvement...next time I'll try again with longer reduction
period. Any
>comments? Carolyn AKA cnc
John Post
Sterling Heights, Michigan
temmoku@actionwebs.net
Thonas C. Curran on mon 9 aug 99
Dear John, Thanks so much for the great advice. I guess I've been too
impatient about the cooling and not doing anything to retard the natural
cooling.
What kind of updraft electric reduction kiln are you using? That
sounds intriguing! Or is this something you are working on yourself,
not a commercial kiln that's on the market? At any rate, I am grateful
for the advice and hints. Cheers from Carolyn aka CNC
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