Chess Denman on sat 9 oct 99
I have a home built 20 cu ft down draft kiln with four propane burners
firing into paired front and back burner ports that lead through into side
gutters with a bag wall. Flue at the back (set to Niels Lou's regulation
area)
I measure the temperature in the top of the kiln near the back and in the
front (through the door) Here I also have an oxyprobe mounted.
Pots from my last biscuit firing fired to 1015 (measured at the ceiling)
and 950 measured at the door could be broken apart. The body is a whitish
craft crank with lots and lots and lots of grog.
Previous biscuit firings have not had this problem.
My guess is that I have some effect like "carbon coring" (but there is no
black at the center of the clay body) In cross section the clay looked
pinkish on the outer faces and dead white in the middle.
The difference this time was that I fired trying to use the oxyprobe to get
a neutral flame from 600 up.
At 600, pushing in the damper to leave about 6"of exit flue area left gave
me hot air at the peep (on the door) some evening of temperature between
door and kiln and an oxyprobe reading of .035 or so. Pushing in the damper
further produced flame at the peep and varying oxyprobe readings and flame
coming out of my burner ports doing my thermocouples no good at all.
To reduce safely for glazes I must cut off primary air to my burners.
Muffling the kiln with the damper wont do it safely and keep my burners
intact.
Now, my questions are these.
1 any ideas about my disasterous biscuit firing?
2 am I reducing in my kiln at 600 whatever the oxyprobe says?
3 should I resite the probe (say to the roof where the thermocouple is)
OR
4 is the door thermocouple right and am I underfiring
5 and the door oxyprobe right and no reduction occuring because my burners
are set to let in too much primary air so that they blast air into the kiln
irrespective of the damper setting. (a sort of forced air effect)
Before you say it I have not done cones at biscuit temps but I have done
them at glaze temps and after the long reduction I do cone nine goes down
with the top thermocouple reading 1280 or so. It seems to be down over most
of the kiln though more down at the top than at the bottom.
Chess.
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