Jonathan on wed 30 sep 98
Hi Everyone:
I'm posting this question for a good friend who is not on the list.
Susan - my friend - fires to cone 10 reduction in a down draft, sprung
arch soft brick kiln. When the firings are good they are very very
good, but when they are bad...well, you know.
She cannot consistently control the atmosphere and get heat rise in the
^8 - ^10 range. Often the choice is reduction or heat rise. Following
is information about her kiln:
- two burners at the back, natural gas with blowers
- hardbrick floor to top of arch = 48.5"
- rise of arch = 8.75"
- width = 44"
- back to front wall = 35.3"
- bagwall is three bricks high and fairly tight
- fire box is 5.5" wide
- ports are 4.75" by 5.125" each
- flue is 8.375" by 7.5"
- door is bricked in then "sealed" with a kaolwool blanket
- flue channel is two lines of k23s on their sides
Not sure what the burners are rated at, but they are Cutter burners.
Nor are we sure how much gas pressure is available at the kiln site, and
delivery from the city is inconsistent (this being DC and all...).
However, she doesn't think that gas pressure is a problem.
Susan candles the kiln all night with pilots on, starts the burners and
blowers, then starts bridging to body reduction around cone 012. This
takes about 40 minutes, goes into body reduction for about an hour, then
she puts it in light reduction and has normal to slow temperature rise
up until cone 9, and which point she can spend 4 - 6 hours (!!!!)
staying in reduction to reach cone 10. If she does not stay in
reduction her celedon type glazes look flat and dead. Often she can not
turn the gas up because the damper is opened full and there is already
too much back pressure. Turn up causes temp. drop. Average firings are
16 hours long, with bad firings taking up to 18 - 20 hours.
Any ides? She is wondering about the relationship between port space
and exit space, but please respond if you see anything else that should
be responded to. I will pass these on to her and respond if there is
info missing that would be helpful.
Thanks,
Jonathan in DC
Vince Pitelka on fri 2 oct 98
Jonathan -
This sounds almost identical to a kiln we have here at the Craft Center.
Ours is entirely IFBs, except for hardbrick in the floor, the fireboxes,and
around the flue.
We fire ours about the same way in the early stages, but with the main
burners on very low overnight. We turn the kiln up first thing in the
morning, and get to body reduction several hours later. We usually do a
light partial reduction the rest of the way up. I am curious why Susan is
even trying to climb during glaze reduction? Why not leave it in partial
reduction from the end of body reduction until cone 10 (which likely will
take at least four to six hours), and then if she needs a glaze reduction
she can do one with the burners turned down a fair bit to make sure the kiln
does not continue to climb. After 30 to 45 minutes of glaze reduction, She
could do a ten to fifteen minute oxidation cleanup before shutting off the
kiln, to help heal any bubbles/blisters from outgassing during reduction.
I do a lot of slip decorating on stoneware, and I do not do a regular glaze
reduction at all. I get much better glaze colors, and less problems with
high-iron slips. At cone 10 I do a short oxidizing cleanup, and shut it
down. For many glazes a heavier reduction is required, but I think that
many people overdo glaze reduction.
It is distinctly possible that Susan's kiln is underpowered. Get ahold of a
WCI gauge (one which measures low pressure gas in water column inches). She
should have at least seven WCI, as that is minimum standard house pressure.
If Susan has the standard 7", you might call your gas company and see if
they can boost the pressure up to 10 or 12 WCI. If she has other gas
appliances on the line they will not change it, but if she has a separate
metered system for the kiln (as she should) they will likely be glad to
boost the pressure a bit.
Also, it concerns me that you say the damper is open full and there is still
too much back pressure. Either the stack is inadequate, the flue opening is
too small, or there some another restriction.
Good luck -
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Arturo M DeVitalis on sat 3 oct 98
To add more info regarding firing time...John Glick fires for 30
hours...very slow the last four cones until cone 11 is reached....so for
me the issue of glaze firing time is a personal decision arrived at by
comparing the costs involved vs the the look and the market value of the
resulting pots. Simple as that!
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