Dan Dermer on wed 19 jul 06
I'm now looking into the more *unusual* possibilities for resolving issues
with temperature un-evenness in my updraft kiln (fires hotter on bottom).
I'm wondering whether distance from burner to burner port might be an issue.
I came across this old thread by searching the archives
(http://www.potters.org/subject01300.htm), which seemed to say that, "The
rule of thumb is to back off the burner 1/4-inch for every inch of flame
tube diameter.... so a two inch pipe on a burner requires a 1/2-inch back
When I checked my kiln, I measured 1.5" from the burner to burner port
(brick face), with diameter of the burner = 2.5", inside diameter = 1.75".
I believe they are venturi MR750s.
Based on the rule of thumb above, shouldn't they be closer in? Maybe a
distance of only .5" to .75" or so?
Would this make any difference -- maybe send a more efficient flame higher
up into the kiln -- helping to solve my heat un-evenness problem??? :-) I
kinda realize this is a stretch but wonder whether it might be worth trying.
Some further background... the kiln is a 16 cf updraft, with 8 burners total
(2 on each side of the 24" x 24" stack) facing up from beneath the kiln.
From the onset of reduction (cone 010), I'm at 4.75" - 5.0" water column
pressure through the end of the firing, which takes another 4-5 hours.
That's about 9-10 hours total to get cone 10 over on top, and probably cone
11 on bottom.
I have decent firings and get the results I'm looking for mostly, but still
have cone 10 over on the bottom ~45 min. ahead of cone 10 going over on top.
So I have to be mindful when stacking the kiln of which glazes go where,
since the bottom will get more heat work.
Meanwhile, I've already tried the usual variables -- changing the stack
(looser on top, tighter on bottom), higher gas pressure to bring flame tip
higher (bottom just gets hotter faster), oxidation/reduction, and other
ideas from gurus on Clayart. I even coated the inside of the kiln with ITC,
which did reduce my overall firing time significantly. Still, at the end of
the firing, when cone 10 gets to 3 o'clock on bottom, I'm usually switching
to more of an a neutral atmosphere (from reduction), waiting 30-45 minutes,
and watching cone 10 go over on top. Last coupla firings, it's taken a
little longer -- sometimes 60 minutes.
Thanks for any advice-
p.s. for all you kiln guru puzzlers... pix of the burners and the kiln may
be seen here:
http://photos.yahoo.com/dbdermer (click on kiln-Q album)
William & Susan Schran User on thu 20 jul 06
On 7/19/06 11:58 PM, "Dan Dermer" wrote:
> I'm usually switching
> to more of an a neutral atmosphere (from reduction), waiting 30-45 minutes,
> and watching cone 10 go over on top.
Beyond stacking and gas pressure, the next is back pressure controlled by
You've been opening the damper to move from reduction to neutral, but
perhaps the damper is still closed down too much.
I'd suggest opening the damper even more, in very small increments, until
you get no flame from the bottom spy hole. Flame from the bottom spy hole
would indicate back pressure and a reduction atmosphere.
-- William "Bill" Schran
Bruce Girrell on thu 20 jul 06
Cone 11 on the bottom and cone 10 on the top of a 16 cu ft updraft?
Perhaps you might count your lucky stars. One cone difference in an updraft
ain't too bad. I can't imagine it doing much better than that, though you do
have a chance with your up-firing burners.
My experience with an updraft is that it is very easy to choke heat at the
bottom by the placement of kiln shelves or ware. Make sure that there is
plenty of space between the shelves and the wall.
Bruce "just my experience though" Girrell
Snail Scott on thu 20 jul 06
At 08:58 PM 7/19/2006 -0700, you wrote:
>I'm now looking into the more *unusual* possibilities for resolving issues
>with temperature un-evenness in my updraft kiln (fires hotter on bottom)...
>Meanwhile, I've already tried the usual variables -- changing the stack
>(looser on top, tighter on bottom)...
Tighter on the bottom can cool off that area
of an updraft because it doesn't allow the heat
to circulate well, but it only works to a point.
I generally have better luck keeping the bottom
nice and open, and tight on top. Not just the
stack, but the damper too, all working together
to cause just enough of a restriction on top.
The restriction seems to cause a little bit of
turbulence at the top, the clay cooks evenly,
and it keeps the heat in. Of course some outflow
is needed, or there's be no heat, but you can't
let it go wandering out too quickly. That's
what lets the top stay too cool: not enough
Every kiln is different, but I have always had
good results in updraft firings when I stack
nice and tight on top, and have the damper at
the proper setting. It's gotta be both, so I'd
modify your stacking pattern, then play with
damper settings. Just my 2 cents.