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Friday, January 6, 2012

Fire is Life - The Firing Part I

Long before wood-fired kilns, which have been used since classical times, ceramic works were fired by heaping wares with burning fuel in an open pit. The first kilns contained the heat by covering the mixture of wares and fuel with heaped-up earth. Air holes were created and the whole affair was lit. The burning grass inside heated the ware. There was no stoking. When the fire burned out the clay objects had hardened enough to become useable.

The modern gas kiln, or it's electric powered relative, is a complicated affair, filled with electronics and safety features, designed to protect the user and the architecture that contains it, from an outbreak of fire, or leakage of poisonous fumes.

It is a device designed to create, contain, and control fire, but not let it get out of hand.

The kiln I use at my studio is not unlike many installed in schools around the country. It's a rebuilt Alpine updraft with two burners, with a set of controls. In order to pass inspection, the vents in my studio were hard-wired to the kiln itself, so that it would be impossible to start firing without obtaining fresh air from the outside.

So while the flame that courses though a gas kiln, is part of the same fire that moves through the sun, or the other stars in our galaxy, or a furnace or stove-range at home, it is in this instance a heavily controlled apparatus designed to minimize danger.

The wood-kiln that I fire with in Cold Spring New York sits outside and has effectively no moving parts other than hand adjustable vents and stoking ports. The stoking is done by human effort, one or two people remembering to put a log or two in every one to five minutes. The only electronics are a few pyrometers stuck in to various parts of the firing chambers.

That act of feeding a gas fire, is done automatically. No effort is required on the part of the user to bring fuel from below the earth to the burner ports. No effort other than installing this insanely complex piece of technology, maintaining it, passing inspection, and paying the gas bill!

It's all part of an immensely complex machine for delivering the power of petro-fuels to end users in our modern society.

Wood-firing releases the blasts of energy from quadrillions of photons that struck one of the trees nearby that got cut up for firewood. Photosynthesis, links sugars to form cellulose, and traps the energy of the sun. Natural gas is the same energy, but trapped and transformed in the earth's crust.

It's all solar energy.

I usually manually control my gas-firings. So while my kiln has an auto-pilot, that can be called upon to fly a certain course and pattern, i.e. a 'ramp' up temperature at a certain rate, I've learned that my firings turn out best when I make those adjustments myself. 

I have my hand on the tiller at all times once the main burners are lit.

I busied myself with early preparations:

a) Prepare a place to snooze. Sheets and blankets brought over. Hauled a small mattress out from under a table.

b) Bring milk, tea, and snacks to keep me energized.

c) Start a kiln log to record kiln temperatures from two thermocouples, one located at the top of the kiln, the other at the bottom.

d) A welding shield for staring in at the cones at high temperature.

e) A Hair Dryer. This is necessary to blow the fumes out of the spy hole so that it's possible to look in at the cones. 

Cones? Potters please indulge:

A very bright fellow named Seger came up with the idea of creating small cones of different clays that would melt at different temperatures, to use as a gauge when firing. This was an old idea, reworked. Dr Edward Orton standardized the manufacture around the turn of the century. His cones are still in use today.

The genius of pyrometric cones is that they indicate total "heat work", rather than just temperature. A cone will bend from a low heat for a very long time, or a very high heat for a short time. Since cones are the average thickness of a typical pot, this means any clay or glazes formulated with similar stuff will bend or melt at the same time.

The most important indication that cones provide to a high-fire potter is not determining when to end the firing (in this illustration the firing ended at Cone 10, with 8 and 9 flat and 11 on the right tipping), but rather in determining when to put the fire into reduction, from oxidation. This process, called body reduction, occurs between Cone 012 thru 05 with another set of cones that are calibrated for lower temperatures.

To give an idea of what those temperatures are . . a Cone 10 firing will terminate with the kiln at something like 2300 degrees F. Body reduction often will start at around 1600 or a little later.

Not all high-fire potters reduce their ware and glazes, but many still do. In recent years, Cone 6 glazes and clays have taken off in use, and most of this work is fired in schools, or by small studio potters in electric kilns. Electric kilns, unless contorted with the addition of some burnable fuel, always oxidize the work. While some glazes thrive in oxidation, such as Temoku and Oribe, most high fire glazes need reduction to develop colors. Many earth oxides remain a white or bland color until giving up their oxygen, this requires reduction. Without a reducing environment they look terrible.

The ceramics world abounds with exceptions. Bright colors are rife through today's ceramics media and most of that work is oxidized. The formulation of high temperature mason-stains, and compounds resistant to oxidation,  have aided the oxidation potter in keeping colors bright.

To me, unless some fuel is involved, the resulting colors 'pinch the eye' as one artist in India put it.

The key with reduction is knowing when to initiate it, for how long, and how intensely. For timing, I use cones, and I glance at my pyrometers. For the duration, I again use my cones, since invariably I'm reducing but also slowly increasing the temperature, and for the intensity I use my eyes, to examine the color of the flame exiting the kiln, and through the spy-holes the quality of atmosphere that is circling the wares. Other potters use a probe designed to measure the levels of carbon monoxide, since a reduction atmosphere is just that, incomplete burning of fuel, from gasses that are starved for oxygen.

My countdown continues:

a) Shut kiln door tight. Cones all visible through spy-holes? YES.

b) Flammables away from Kiln  (boxes, buckets, etc.) - YES.

c) Vent to Roof Open (take the insulation off!) - YES. (At this moment I feel my studio starting to become cold as the warm air rushes up the enormous stovepipe above the kiln.).

d) Main Gas Switch - ON.

e) Enter Firing Program - (To fire my automatic kiln manually you have to enter a program anyway, but rather one that asks the kiln to do the impossible in an impossibly short period of time. My program had only one step. Get to 2375 degrees in one minute. The kiln would try to do this and leave me to 'steer' the valves and vents, thus 'flying' the kiln atmosphere myself without automatic intervention. DONE.

f) Alarm Shutoff Programmed to 2375 degrees. (In no event would the kiln get over 2375. Once that temperature was reached, or if the fire went out completely an alarm would sound. My kiln has a very loud bell on it!! DONE.

f) Reset Gas Solenoid Switches - (two buttons on top of the two burner solenoids that open the gas to the kiln). CHECK.

g) Mode Switch to "Soak". DONE.

'Soak' reminds me of a tired laborer soaking in a hot-tub. The pots are allowed to heat up during a 'soak' cycle, or a 'soak' at the end of a firing simply means 'maintain temperature'. It's another way of saying "Go on vacation. Take a hike. Give it a rest!'

    Don't mess with me,
     I'm firing manually.

h) Turn Kiln ON - I turned the key and flicked the switch to start the Pilots.

Pilots. Berth. Refreshments. Check-in. Closing the door. All rituals. Main engine start. Countdown.

The 'Fire-Eyes' went through a series of sequences to light both pilots (Pilot and Co-Pilot!) . . joking, actually the pilot for the Left Burner and the Pilot for the Right, same thing. . .  The Fire-Eyes are safety devices that make sure the pilots are burning. If not, they shut the whole thing down. The main burners, i.e. the engines, may be on or off, but the pilots must never go out!

It had been ten years since I brought this rusted hulk of metal back to New Haven with me from Lawrence Kansas. It barely functioned, but I fired it a dozen times or so during those years. Stephen Rodriguez, God bless him, used to bring over bits of hardware in the middle of the night to help me keep things going.

After River Street, I moved my kiln to New York City, where she lived with me behind my Park Slope Brooklyn apartment. And then I moved her back to the New Haven area, to my present studio. My good friend Joe Catenzaro sold me a new set of controls. We rebuilt the entire burner system.

The kiln sputtered to life. It took a few moments for the gas lines to clear themselves of nearly sixteen months of non-use during which time I rebuilt most of the kiln. During that time I made a lot of pottery, bisqued it. . . and wrote poems. But no glaze firings other than the ones I did in Tony's wood kiln.

I heard the sequence of relays first convey gas to Pilot 1, a spark flashed to ignite it. The pilots were lit.

All lights were green. Only one switch remained to be flicked to start the main burners.

It was 6:00 PM, Tuesday January 3rd.

Overnight, the warmth of the small flames alone would bring the kiln slowly to just over boiling temperature.

I went home! It was my wife's and my 25th Wedding Anniversary. She prepared a delicious venison roast, which we washed down with a wonderful bottle of wine.

Late that night I drove back to check. The pilots hissed like two baby dragons coiled in a feather nest.

I returned home, exhausted, and slept until eleven the following day, then I awoke with a start. It was already late. Today I would fire. The burners would light, and I would fly her across the Atlantic.

Continued . . . Part II


[Those reading this may wonder why all of the concern and preparations being devoted to this particular firing. I after all have done this many times before. The reason quite simply is that I recently rebuilt a good portion of my kiln. I replaced the refractory insulation at the top and repaired major damage from age to the sides. I also rewired a good portion of the electronics. ]

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