Kiln Firing Schedule Builder: Bisque and Glaze Schedules

How the kiln firing schedule builder works

The builder starts from the built-in slow and fast programs of Bartlett controllers, which run many electric kilns, as published by Digitalfire. A medium speed sits between them. For bisque, there are five segments: a slow climb to 121 °C to dry the ware, a faster climb to 537 °C while organic matter burns out, a slow stretch through quartz inversion to 593 °C, a steady climb, and the final approach to the cone.

Glaze firings are simpler, because the ware has already been bisque fired: an optional drying segment, a fast climb and the final approach. If you choose drop, hold and slow cool, the builder adds Digitalfire’s C6DHSC cooling: a quick drop of about 56 °C, a 30 minute hold, then a controlled cool at 83 °C an hour to 760 °C.

The final segment always runs at 60 °C an hour over the last 139 °C. That’s the heating rate Orton uses for the middle column of its cone chart, so the end temperature matches the cone you chose. It’s slower than some fast programs, which is why a fast glaze here takes a little longer than Bartlett’s own.

Bisque firing schedules: slow at the start

A bisque firing has two jobs: to drive every last trace of water out of the clay, and to burn out organic matter, without cracking or exploding the work. Both happen at the bottom of the firing, which is why bisque schedules start slowly.

The preheat, or candling, hold keeps the kiln just below boiling so water can escape gently. Skutt’s preheat runs at 80 °F an hour to 180 °F and holds for one to two hours for most work. Thick or slightly damp pieces need longer, and work that’s bone dry can manage with less. Our pottery fault finder explains why trapped water, not air, causes most kiln explosions.

Quartz inversion at 573 °C is the other slow stretch. Quartz in the clay changes form suddenly at that temperature and expands slightly, so heating through it more slowly helps large or thick work survive. Plainsman notes that it hasn’t found this a problem in its own bisque firings, but slowing down costs little and adds a margin of safety.

The slow and fast bisque options differ mostly in these early stages. A slow bisque suits thick, large or closed forms, and anything you’re not sure is fully dry. A fast bisque suits thin, small, bone-dry work in a kiln you know well. When in doubt, choose slow: a few extra hours of electricity cost far less than a load of cracked or exploded pots.

Glaze firing schedules and holds

Glaze firings can climb faster through the middle, because the ware has no water or organic matter left to drive off. What matters most is the end: the last hundred degrees or so decide how the glaze melts and how the cone bends.

A hold at the top lets the heat even out through the kiln and gives glazes time to smooth over. Digitalfire’s cone 6 schedule holds for 10 minutes at the peak, and many potters use 5 to 15 minutes. Longer holds add heatwork, so a long hold can over-fire a load as surely as a higher temperature can. If your witness cones bend too far, shorten the hold before you lower the temperature.

Some glazes need an extra step to cure pinholes. Digitalfire’s schedules use a drop-and-hold: falling a little below the peak and holding, which lets bubbles heal over while the glaze is still fluid.

Slow cooling for better glaze surfaces

Most electric kilns cool quickly once they switch off, which suits glossy glazes. Matte and reactive glazes often look better when cooled slowly, because crystals have time to grow as the glaze sets. Digitalfire credits a controlled cool from about 1150 °C to 760 °C with noticeably better surfaces on its cone 6 glazes.

The builder’s slow cool option follows that schedule. It isn’t right for every glaze: it can dull glossy glazes, and Digitalfire notes that slow cools can shorten the life of the relays in electric kilns. Try it on test tiles first.

For very large or thick pieces, a slower cool through quartz inversion on the way down also reduces the risk of cooling cracks, or dunting. Digitalfire’s quartz inversion cracking avoider schedule slows to about 41 °C an hour through that range in both directions.

Check the schedule with witness cones

A controller measures temperature at one spot, and thermocouples drift with age. Cones measure heatwork where your pots actually are, so they’re the only reliable check that a schedule is doing what you think.

Place a witness cone pack on a middle shelf, and ideally one on the top and bottom shelves too. After the firing, compare the bend with the cone you aimed for. If the cone is underbent, add a few degrees or a longer hold next time; if it’s overbent, take some off. Our pyrometric cone chart gives the temperature of every cone at three heating rates.

Write down every firing: the schedule, the load and the cones. After a few firings you’ll know exactly how your kiln behaves, and you can fine-tune the schedule with confidence.

Programming the schedule into your controller

Most electric kiln controllers have a ramp-and-hold or custom program mode. You enter the number of segments, then for each one a rate, a target temperature and a hold time, exactly as the table shows. Check whether your controller expects Celsius or Fahrenheit and switch the builder to match, because the rates change as well as the temperatures.

The drop segment in a slow cool is entered as a very fast rate, often the controller’s maximum, or as a rate it calls full. A cooling segment needs the controller to support controlled cooling, and some older models don’t. If you’d rather leave firing to someone else, a kiln firing service fires work to the right schedule for you.