Ray Carlton on sun 2 nov 97
hello all
I have just fired my 65 cubic foot brick kiln for the 1st time and it was a
disaster. if any body has some advice or words of encouragement it would be
gratefully received. The kiln is hi temp insulators inside and low temp
insulators outside with the flue and firebox areas are super duty
solids..it is roughly a cube but slightly higher and longer than wide with
a sprung arch roof...
it has 6 x 1 1/2" long venturis 3 per side fed by 1/2" gas lines. it is
based on the daniel rhodes downdraught designs in his kiln book.
I have fired fibre kilns for 20 years but had no experience with this type
of kiln. The firing advanced ok and all seemed well as i could see plenty
of the usual green flame from the spyholes and even from the five meter
[18foot] stack at certain times/// i tried to duplicate the reduction
cycles i have used for many years in the fibre kiln but alas and alack much
of the work was underfired and under reduced [mainly the top 1/2 of the
setting].. the rest was overfired and the 2nd bottom shelf was radically
overfired. I have a permeable bagwall and i realise i can even out the
distribution of heat by closing up the holes and raising it etc etc.
my main question for you is, how much reduction do i need for good copper
reds?
Is it necessary to adjust the air controls on the venturis or can you do it
from the damper? {ijust did it with the damper) what color flame should i
be looking for? should i be able to see it from the top of the stacK? or do
i need a hole at the back of the flue? MMMM many questions
the damper at full pressure [7psi] was open half way
we were going for 13 hours until^13 was down, i was taking my time i didn't
want to rush
the spyholes are lower front and upper side [both sides] and reduction
seemed even enough nice green [flame] but died in the front spyhole at times.
the worst affected was the copper glaze the chun's and celadons seemed to
handle it ok
i apologise for the lenght of this but a lot of work will go under the
hammer and i want too avoid it happening to the same extent next time
thanks in advance for any advice or suggestions
:-((
raycarlt@valylink.net.au
Ray Carlton
McMahons Creek Victoria Australia 3799
Jack Troy on mon 3 nov 97
You'll get lots of responses to your query, Roy, and you can sort through them
at your leisure, but a couple of things occur to me when I read what you'd
written:
#1. Green flames usually signify oxidization where I live, and oxidisation,
where you live. I usually associate such flames with very high pressure, also,
which is confirmed by your mentioning flame out the stack. (Flame out the stack
serves mainly to heat the air just above the top of the chimney, delighting the
gas company, but short-changing the pots you're firing).
#2. The lower portion of the kiln was overfired. Though you didn't mention it,
I assume the burners come in perpendicular to the sidewalls, rather than
parallel to them. Again, excess pressure could push heat through the bagwall
directly into the lower half of the kiln, over-firing that portion of the load.
#3. Reduction woes: My experience with copper red glazes is that they benefit
from early reduction, before the glaze melts and seals the copper away,
(oxidized, oxidised) in an impermeable glassy layer. Also, direct
flame-impingement seems to burn out the copper. All those gorgeous Chinese
copper reds were fired in saggers. So were the ones that weren't quite so
gorgeous.
SO:
A. Reduce the gas pressure at least so none is ever visible out the stack when
you're firing above red heat. (God forbid it would ever be visible below red
heat). You may find the kiln climbs well with only 1/2 as much pressure as you
used.
B. Adjust the burner-flanges to produce a lazy, diaphanous yellow flame within
the kiln when you reduce. Our kilns fire evenly and best when no flame comes
out the bottom spy-hole, level with the first shelf, about 2" above the floor.
While some potters prefer to govern reduction with the damper alone, others
combine the effects of both the damper and primary air (flange adjustment),
while still others use the primaries primarily. You might also want to check
the size of the burner-ports. If they are more than about 1/2" larger than the
end of the burner, the burner may be entraining too much secondary air,
contributing to reduction woes.
C. Consider tightening the lower 1/2 of the bagwall to boost heat into the
crown of the arch.
D. There may well be additional considerations, but I'll leave them to others.
The rain has stopped here, and I have 800 daffodil bulbs to plant, along with 36
Asiatic lily bulbs and whole passel of iris to transplant.
Good forms, good firings,
Jack Troy
Donald G. Goldsobel on mon 3 nov 97
One of the easiest cures for circulation problems lies with how the kiln is
stacked. Leave enough room between shelves to allow the flame to be drawn
down through the setting.Try closing the holes in the bag wall, so the
flame is fully directed uprds and then pulled down by the force of the flue.
As to the question of how much reduction: When the copper turns the flame
green and the flame is about 12 to 18 inches long at the flue and 6-8
inches at the spy hole, I have the required reduction for my smaller brick
kiln. As my kiln has aged, it has changed due to more air entering from
the door's sill and cracks. New kiln is like a new spouse, there is a
period of adjustment.
Good luck.
Donald in the 100 degree San fernando Valley. I'm roasting!
>----------------------------Original message----------------------------
>hello all
>
>I have just fired my 65 cubic foot brick kiln for the 1st time and it was a
>disaster. if any body has some advice or words of encouragement it would be
>gratefully received. The kiln is hi temp insulators inside and low temp
>insulators outside with the flue and firebox areas are super duty
>solids..it is roughly a cube but slightly higher and longer than wide with
>a sprung arch roof...
>it has 6 x 1 1/2" long venturis 3 per side fed by 1/2" gas lines. it is
>based on the daniel rhodes downdraught designs in his kiln book.
>I have fired fibre kilns for 20 years but had no experience with this type
>of kiln. The firing advanced ok and all seemed well as i could see plenty
>of the usual green flame from the spyholes and even from the five meter
>[18foot] stack at certain times/// i tried to duplicate the reduction
>cycles i have used for many years in the fibre kiln but alas and alack much
>of the work was underfired and under reduced [mainly the top 1/2 of the
>setting].. the rest was overfired and the 2nd bottom shelf was radically
>overfired. I have a permeable bagwall and i realise i can even out the
>distribution of heat by closing up the holes and raising it etc etc.
>my main question for you is, how much reduction do i need for good copper
>reds?
>Is it necessary to adjust the air controls on the venturis or can you do it
>from the damper? {ijust did it with the damper) what color flame should i
>be looking for? should i be able to see it from the top of the stacK? or do
>i need a hole at the back of the flue? MMMM many questions
>the damper at full pressure [7psi] was open half way
>we were going for 13 hours until^13 was down, i was taking my time i didn't
>want to rush
>the spyholes are lower front and upper side [both sides] and reduction
>seemed even enough nice green [flame] but died in the front spyhole at times.
>
>the worst affected was the copper glaze the chun's and celadons seemed to
>handle it ok
>i apologise for the lenght of this but a lot of work will go under the
>hammer and i want too avoid it happening to the same extent next time
>
>
>thanks in advance for any advice or suggestions
>
>:-((
>
>
>raycarlt@valylink.net.au
>
>Ray Carlton
>McMahons Creek Victoria Australia 3799
>
| |
|
