offcntr | Captain's log (Reply)

I've been keeping firing logs forever; if I dig far enough into my filing cabinet, I can probably find logs from my Craft Center days, possibly even graduate school. They're a good way to keep track of how often I fire, how much gas I use, what firing strategies. Sometimes they're even useful in predicting what I should do next time.

Sometimes.

Our big gas kiln is weirdly unpredictable. I don't just mean the oxidation spot, which seems to move around from firing to firing. Firing strategies that work for months, even years, will suddenly crash, for no perceptible reason. For the longest time, I had a schedule that fired reliably at eight hours overnight pre-heating, twelve to thirteen hours more the next day to reach cone 10, with the kiln about half a cone hotter on the bottom. I could shut down at cone 10/9.5 and rely on heat rising to drop that last half cone on top on carry-over.

Jon couldn't match my experience. No matter what he tried, even if he tried the exact same settings (possible after I installed gas gauges on all the burners), he'd always fire hot on top, cooler bottom. I privately felt a bit smug, thinking I had a better handle on firing this kiln than he did.

And then, about six months ago, it abruptly switched on me as well. The bottom was staying as much as two cones cooler than the top, necessitating a lot of fussing with the damper, trying to slow down the top long enough for the bottom to catch up, wasting time, wasting gas.

This is where the logbook comes in handy: trouble-shooting. I have a record of a series of attempts, what helped, what didn't. What to try next time. I record the times of adjustments, gas, primary air, damper. Pyrometer reading, and notes on what I'm trying to accomplish at any given point.

Gas pressure is read on gauges on the individual burners, as mentioned. There are valves on each burner, plus a big emergency shut-off valve where the pipe comes in from the meter, in case we have to shut off everything at once. Pressure is measured in water-column inches; I generally pre-heat overnight at 1.6", then turn up to 2" (or more recently, 1.8") for the day's firing. For now, I keep pressure the same at all burners, though I suppose I wouldn't have to.

Primary air is the air going directly into the burners. Speeding gas pulls in air via the venturi effect. The amount of air is effected by the constricted shape of the burner, the speed/pressure of the gas, and the size of the opening permitting in the air. We can't control the first--burners are cast iron--and already talked about the second, above. The third is controlled by a disk-shaped cover, threaded over the gas pipe at the back of the venturi. By spinning the disk, you can move it closer or further from the opening, changing the amount of primary air. I generally fire with the air open 2 turns overnight, move it to 2.5 in the morning when I turn on the gas.

Secondary air is what comes in around the burners, through the burner ports, peepholes, gaps in the brick. Hot gases in rising in the chimney create draft, which pulls in this secondary air. By opening and closing the damper, a piece of kiln shelf that slides in and out of a slot in the chimney, we control the cross-section of flue, and hence the secondary air. Body reduction and the neutral-to-reducing atmosphere later in the firing are generally controlled with the damper.

Londonderry Air is something else entirely.

The pyrometer is a digital read-out attached to a bi-metallic thermocouple that's slid into a ceramic sleeve, protruding into the kiln. Heat from the kiln causes a current in the metals that's then converted into temperature in degrees Fahrenheit or Celsius. It's not actually that accurate, which is why we use pyrometric cones we can watch through the peepholes to determine when we've hit temperature. However, the pyrometer, like an instant-read thermometer in cooking, can give us a snapshot of the firing, tell us whether we're still gaining heat or have stalled. Like the time, it can also be a handy reference when you're comparing different firings.

My notes are usually things like "cone 08 is down, start body reduction" or "tweak the damper for flames in the chimney" (a way to gauge kiln atmosphere), and occasionally "why won't the bottom heat up, dammit!" Descriptions of what's happening, as the other columns give the when and why.