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firing results (long)

updated sat 31 aug 96


Jonathan Kaplan on sun 4 aug 96

I thought I would share with the list the results of my gas kiln firing
with all cone 5 oxidation glazes. I did post an inquiry a few weeks back
about what I could expect.

I recently purchased a 40 cuft fiber car kiln through Laguna here in
Denver. I'll not burden you with some of the resulting problems, but I
would guess that from my posts, you might know that I do have certain very
sound ideas of how things need to be in ceramics. I was very specific with
the builder and as you might expect, some of my sepcifications were not
followed. However, the kiln is built like a Mack Truck, a Volvo, or a
Hummer. he steel shell is extremely overbuilt and will probably outlive me.
The fiber installation is good, but the hearth and firebox areas all 2600
degree brick will require some attention. The burners are essentially
modified Alfred types with forced air ala Harry Dedell. These required alot
of retrofitting by me and now are acceptable. The kiln runs on a very
sophisicated controller and it does indeed perform for me.

I retrofitted the burners to include heavy duty pilots from Pyronics and
limiting orfice valves from North American Combustion. The Honeywell flame
rectification system, spark system and valves all provide flame supervision
that has an immediate response. I also added variable speed controls to the
blowers. The system now has a very sensitive turndown and meets my

The key to all of this is an AIC oxygen probe, purchased from Nils Lou many
years ago that has been in storage up to last month. It works like a charm,
is very sensitive to any change in damper setting, and the thermocouple
section of the probe is right on the money. In fact, the numbers are so
exact that I now have re-programmed the controller's ambient settings so
that both the probe thermocouple (platinum) and kiln controller
thermocouple(type K) are within an acceptable range. I have no financial
interest in these probes, but I would urge those of you on the list with
gas kilns to buy one, despite the cost. These puppies will pay for
themselves quickly and provide you with reference numbers that are accurate
and will enable you to reproduce results in subsequent firings.

All the glazes that I had fired for years in the electric kiln were exactly
the same in the gas kiln. No glaze defects, no nothing. All of my glazes
are in the cone 3-5 range. I'm pleased. But they key is watching the probe
and keeping the burners in excess air (and yes I know its not stochiometric
combustion) towards the neutral range. According to the probe, I was
running at .04 to .02 on the probe, which according to AIC.s excellent
documentation, is in the range of 14-12% excess air or so. Slight changes
in the damper setting produced remarkable changes in the probe setting. And
this is the key, IMHO, between firing by the seat of your pants and having
information of substance. Pottery kilns, gas kilns that we have all built,
used, etc., have never contained really sophisticated combustion, and as
such, (and this is only within recent years that Marc Ward, Jim Bailey,
etc, have made improvements) the burners are the weak link in the system. I
know many potters who have gone to industrial systems, albeit costly, but
nonethe less have excellent combustion, efficient, cost effective, etc.,
and have all the bells and whistles to provide combustion from high excess
air to richly reducing, if necessary.

Here is an example. I have a client that has been having trouble with
producing anything but seconds in his gas kiln. I produce a some product
for him. We have worked together on tests, variables, run blind "taste
tests" etc etc. While I know for a fact that there may be a glitch in the
casting body that my customer uses, the weak link are the burners that are
firing towards the end of the neutral range, towards a light reduction. No
probe, no way of monitoring the pressure in kiln, hence, what is going on
with the atmosphere? All the pots are usualy accompanied with glaze
defects, notably crawling and pin holing. The body is responsible for
these, I am convinced, in that the same defects occur when these are fired
in an electric kiln. We use a terra cotta casting body for these pots, and
my client uses a white casting body. The glazes are the same. Interesting.
I fired some of the white body pots in this last kiln, and half of them
were OK, and the other half were full of glaze defects. The terra cotta
casting body pots were fine. The glazes smooth, the colors fine. ALl in
all, very successful and I am indeed, a happy camper now that I can avoid
those killer electric bills once a month. Even at $0.06 a KW hour, it does
add up. Plus, I can fit 4 times the quantity of ware, at less of a firing
cost. I calcuated the gas costs, bith by the gas company's statement as
well as reading the meter: a 40 cubic foot load to cone 5 costs $9.00.

SO there you have it. Thanks to those who provided me with information
before the firing, and I would be glad to reciprocate in any way I can.


Jonathan Kaplan

(aka "Scooter)
Ceramic Design Group Ltd./Production Services Voice:
970-879-9139 POB 775112
FAXmodem: same
Steamboat Springs, Colorado 80477, USA CALL before faxing

"Arrive on time, tell the truth, be a good listener, and don't
be too attatched to the results. Above all, maintain a sense of