How kiln firing schedules work

A firing schedule is a list of segments. Each segment has a ramp rate in degrees per hour, a target temperature, and a hold. Together they control how much heat-work the clay and glaze receive, which is what actually matures a piece, not temperature alone.

A schedule is a list of segments

Every programmable kiln runs a sequence of segments. Each one holds three numbers: a ramp rate in degrees per hour, a target temperature, and a hold in minutes once it reaches that target. Run them in order and you have drawn a temperature curve from room temperature up to the peak, and sometimes back down in a controlled way.

A simple glaze program might read: climb at 400 F per hour to 1000 F, slow to 180 F per hour through 1150 F, push to within 250 F of the peak, then finish at 120 F per hour to the top and hold ten minutes. Each line has a job.

The stages of a firing

A typical firing passes through a handful of zones, and good schedules respect each one:

• Candle and water-smoking (up to about 250 F / 120 C). Mechanical water in the clay turns to steam. Go slow, or trapped steam cracks or bursts the piece. This matters most for bisque and thick work.
• Organic burnout (roughly 600 to 1300 F). Carbon and organic matter burn off. Keep a vent open so the smoke leaves the kiln rather than carbon-coring the clay.
• Quartz inversion (around 1063 F / 573 C). Silica in the clay suddenly changes volume. Rushing through it, up or down, can dunt the ware with cracks. Schedules ease across this band.
• Maturity (the final climb to the cone). The last segment decides the heat-work, so it runs slower and steadier to land the cone exactly.
• Cooling. Most firings cool naturally with the lid closed. Some glaze effects want a controlled slow cool, programmed as negative-rate segments.

Bisque and glaze schedules differ

A bisque schedule starts slower, because it is the first firing and the clay still holds water and organics. A glaze schedule can climb a little faster early, since the bisqued body is already dry and burned clean, then add a short hold near the top so the glaze melt smooths over. You can see both side by side on the bisque vs glaze firing page.

Let the builder do the arithmetic

Once you know your cone and whether you are firing bisque or glaze, the schedule builder picks the ramps and holds for you, reads the peak from the Orton chart at the right rate, and prints a clean sheet to tape to the kiln. You stay in control of speed and the optional top hold.

Sources

• Orton pyrometric cone temperature chart, ortonceramic.com.
• Techno File: Kiln Firing Schedules, Ceramic Arts Network (ceramicartsnetwork.org).
• Skutt KilnMaster and Bartlett controller cone-fire programming manuals.