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burner distance to port- updraft

updated sat 22 jul 06


Dale Neese on thu 20 jul 06

Hello Dan,
It is my understanding that all updrafts typically fire hotter in the bottom
than the top. How much hotter is the problem. I have fired a 24 Olsen
updraft for over 15 years and cut my teeth on Alpine updrafts at the
university and the top always lags behind the bottom in heat. I believe when
I first built my kiln, Fred's recommendations for burner placement was 1
inch from burner tip to port. Any closer and the burner tips are at subject
to heat damage. I have adjusted my firing schedule to help bring the top of
the kiln to cone 10 normally within 15 to 30 minutes of the bottom.
I also tightly stack porcelain pieces in the bottom of the kiln to take
advantage of the heat that porcelain likes. And as you also mentioned,
smaller lower posts for larger mass of shelving in the bottom. More open
stack in the upper regions. Try not to restrict heat flow by setting pots up
into the arch area near the exit flue as this will keep heat down in the
bottom also.
I start reduction at cone 010 to 09 depending on the clay I am using. Black
Mountain clay doesn't like early heavy reduction because of the high iron
content and will bloat slightly. I start reduction at 09 and keep a few
Black Mountain pieces in the top. With all lighter stoneware clays and
porcelain I reduce at cone 010. I have noticed with the help of a oxy-probe
that reduction will trail off as the temperature climbs. I obtain a good 2
inch smoky flame at the lower peep hole at the point of reduction with all
10 burners at full turn up. 4 water column inch pressure on 8 burners, 2
water column inches on 2 burners under the center shelf. I fire with propane
from a 250 gallon tank. One thing I don't do is mess with the damper or
burners once reduction has started. My damper is marked for the same
reduction position for each firing.
When cone 7 is at 3 o'clock in the bottom, I pull out the damper 1/2 inch.
This slight adjustment allows more heat into the top earlier. I haven't
noticed much if little difference in the reduction or quality of the glazes.
Then let the kiln go on up. Usually cone 10 in top and bottom are within a
half cone difference of each other. If not I pull the damper another 1/2
inch. Still in slight reduction. Once cone 10 is down in the bottom I never
look at the bottom cones again. Not anything you can do anyway. I have cone
10 go down in the top shortly after. It seems like an eternity but you have
to realize that the temperature differences are only a few degrees but it
takes time to get the heat work up to the top. When I mean cone 10 down I
mean almost flat. Then turn the burners down, push in the damper slightly
for a short clearing and soak. Less than 5 minutes. This helps even out the
kiln also. Turn burners off and close the damper tight. Layers of refractory
insulation fiber over the top of the kiln keeps the kiln from cooling as
quick. I don't unload until I can hold my finger in the peep hole, unloading
without gloves.
Hope this helps.

Dale Tex
"across the alley from the Alamo"
San Antonio, Texas USA

Snail Scott on fri 21 jul 06

At 08:53 AM 7/20/2006 -0500, Dale wrote:
>It is my understanding that all updrafts typically fire hotter in the bottom
>than the top...
>...cut my teeth on Alpine updrafts at the
>university and the top always lags behind the bottom in heat...

I've fired quite a few of these, and
found that they can be hot on top just
as readily as on the bottom. Stacking
and damper are very important and work
together to control heat flow.


Dale Neese on fri 21 jul 06

I really haven't come across an updraft that ends up the firing with the top
being hotter than the bottom. I would hate to struggle with that beast! Only
with the damper fully open at start up to reduction, does the top of the
updraft heat up and cones 010-09 go down first, ahead of the bottom.....that
is.. if you don't have a tightly stacked top shelf cutting down on heat flow
to the exit flue prior to reduction. Stacking is important.
Close the damper at reduction and the bottom heats up ahead of the top. My
reasoning that a mass of ware in the hotter bottom part of the kiln is
better to withstand slightly more heat over firing time so that the bottom
(in my kiln) doesn't get so far ahead of the top of the kiln. My porcelains
and Shino's are better there than the top 1/3 of the kiln. Open the damper
towards the end of the firing to bring the heat higher in the stack. Damper
is important.

Dale Tex
"across the alley from the Alamo"
San Antonio, Texas USA

Dan Dermer on fri 21 jul 06

Dear all,
Thanks so much for the discussion and tips.... some things I'm going to try

1. making extra sure there's plenty of space between shelves and wall. in
recent firings, I've been more aggressive at having pieces hang 'out' over
the edges of shelves in order to get more stacking space out of my kiln...
I had also moved the bottom two 12x24 shelves apart by a distance of maybe
3/8", in order to squeeze a little more stacking space out of it, and with
the thought that a little space between shelves in the middle of the kiln
might improve circulation (perhaps it didn't....)

2. I might try moving my burners closer to the ports so there's a 1-inch
gap, instead of 1.5". I'm not sure how to do this yet, so any suggestions

3. I plan to add some kaowool refractory blanket to the outside/top of the
kiln, and keep it in place with some old broken kiln shelves. maybe this
extra bit of insulation will keep heat in the top a little better.

4. I had been waiting until cone 10 was bending on bottom (and cone 8 barely
starting on top!) before easing into neutral/oxidation. Per Dale's
suggestion, I might try starting that a little sooner.

My only concern with #4 is that I'd worry about the effect of less reduction
on certain glazes toward the end of the firing. Plus, while I used to think
that oxidation pulled heat to the top in an updraft, but I'm not sure that
it really does. In my kiln, the temperature top-bottom is out of whack even
on the way up toward initial reduction, when the kiln is most definitely in
oxidation (according to my oxy probe). Cone 010 falls on bottom at least 30
minutes before it falls on top -- without fail.

So maybe that points to more stack-related issues, or just plain ol' kiln

I actually thought about doing a firing where I use two oxy probes (have
access to a second one) inserted into the upper/lower peep holes of the
kiln, in order to get a better idea of differential temps all along the way.
That way, there would be immediate feedback in temperature and atmosphere
from changes in damper/gas-pressure/etc -- you could watch both probes

But, I don't know if it might harm the probe (thermal shock?) to
remove/replace it during the end of the firing in order to look at the
cones.... anybody?

At any rate, I will let y'all know if the next firing is improved!

Warm regards-