Brian Molanphy on mon 15 apr 02
greetings,
mel counselled me via email about our soda kiln. this was to be a note to
him, but i thought it might be of general interest.
this is a hard brick sprung arch downdraft kiln, with about 16 ft3 stacking
space. we try to fire to ^10, but sometimes only get to ^8.
(before the march firing, i removed the bag walls, which were three bricks
high (stacked on the long narrow edge), with gaps spaced in the top row. i
left one brick on each side, parallel to the direction of the flame, in the
hope that the brick would keep the flame from going up the flue before
making its tour of the kiln. there are two homemade burners (no blowers) at
the back of the kiln. during that firing, ^11 was soft on the bottom, but
only ^6 dropped on top. when i looked into the kiln throught the burner
ports, i could see that the flames were immediately turning to go up the
flue. they didn't appear to circulate up to the top of the kiln and then
down and out. that's what it looked like anyway. so for this last firing, i
replaced the bag walls.)
the april firing reached ^8, cooler in some spots. backpressure flame out
top peep was 6" for the first half of the firing (that is, from ^08), then
8" for the second half. ^6 fell a bit sooner than usual, then stalled. so i
moved the damper around. during the first half, it was closed 3" of a total
of 8". tried it at 4" and 5" for awhile without much cone change, just more
backpressure. i could see through a thin gap in the chimney that the tip of
the flame was four or five feet up from the flue. mel pointed out that if i
can see through a gap in the chimney, holes in the kiln fabric may be the
cause of problems in reaching temp.
so after waiting hours for a change, i put soda in, 1.25 kg instead of usual
1 kg.
color of slips and body is great. liner glazes are glossy, but not mature, i
guess, given the cones.
some flat bottomed pots were set on the SiC shelves, some on starbursts. of
course, since the kiln wasn't hot enough, this evidence may not count for
much. one jar on the shelf cracked at bottom, a larger one on wads didn't.
the largest on wads has cracklines on the inside, in a kind of starburst
pattern that matches the wad marks on the outside. i think the real problem
lies with compression/tension in the forming. wadding may help prevent the
flaw from showing, but only slightly.
before the firing, i ground off the alumina/kaolin wash from the shelves.
they have warped, so i coated the other side with itc 100, and set the pots
on that side. itc was thinner with 40% water by volume. it seemed really
thin. i applied it with a brush, one coat. after the firing, the pots did
not stick. the areas under the pots are colored dark orange. the exposed
parts of the shelves, whether coated with itc or not, were covered with the
usual salt/soda build up. this rubs off easily with a piece of SiC. i had
expected the itc coated areas to resist this build up. did i apply the itc
too thinly? the shelves were not *new*. i ground them clean before brushing
on itc, but still there must have been stuff in the crevices. would this
interfere with itc efficacy? or the fact that i neglected to dampen the
shelves with water before brushing on the itc?
yikes, surely my longest post. i would love to read your comments...
brian
| |
|
