Why potters fire twice
Greenware — unfired clay — is fragile and water-soluble. Glazing it directly is possible (single firing) but difficult, because the glaze has to survive the same chemical changes the clay is going through. Firing twice separates the two jobs and makes each one controllable.
The bisque firing turns clay into ceramic: chemically bound water is driven off, organic matter burns out, and the body becomes strong enough to handle and to take glaze. The glaze firing then melts the applied glaze and bonds it to the now-stable body.
- Bisque temperatureCone 06–04 (999–1060 °C)
- Bisque purposeStrength, water solubility, burnout
- Glaze firing temperatureCone 6 (1222 °C) or cone 10 (1300 °C)
- Glaze firing purposeMelt the glaze, mature the body
- CoolingNatural, lid closed, no crash cooling
The bisque schedule
The early part of a bisque is where pots explode. Between 100 °C and 600 °C, water and organic matter leave the body as steam and gas. If the ramp is too fast, steam pressure builds faster than it can escape and the pot fails.
Candle at 100 °C
Hold for 30–60 minutes to drive off mechanical water slowly, especially for thick or damp work.
Ramp to 600 °C at 60–80 °C/hour
This is the critical window. Slow and steady prevents steam explosions and allows organic burnout.
Ramp to top temperature at 100–150 °C/hour
Once the chemically bound water is gone, the ramp can be faster.
Soak and cool
Hold at the top for 10–20 minutes for evenness, then cool naturally with the lid closed.
If a pot feels cool to the cheek, it is still losing moisture. Give it another day, or candle for two hours.
Loading the kiln
Loading decides whether a firing is even. Heat rises, so the top of the kiln runs hotter than the bottom unless you compensate with spacing or a slower schedule. A well-loaded kiln is filled to a similar density at every level.
- Leave at least 2 cm between pots and 5 cm from the elements.
- Put taller, heavier work low; smaller work on top.
- Group similar thicknesses together — a thin-walled bowl next to a solid sculpture will not fire evenly.
- Never let a pot touch the kiln wall or an element.
- Leave the peephole clear so you can see the cones.
The glaze firing
The glaze firing is simpler in schedule than the bisque — no bound water remains — but more consequential, because this is where the surface is decided. Two things matter: reaching the right heat work, and cooling at the right rate.
| Segment | Rate | Purpose |
|---|---|---|
| Room → 600 °C | 100–150 °C/hour | Burnout, no water risk |
| 600 → top temp | 100–150 °C/hour | Glaze melt begins |
| Soak at top | 10–30 minutes | Even melt, heal pinholes |
| Cooling | Natural, lid closed | Avoid crazing and stress |
Opening the kiln early to speed things up puts the glaze in tension against the body, which is a reliable way to produce crazing. Let it cool with the lid closed, even if it takes all night.
Verifying the firing with cones
A controller tells you what it intended to do. A cone tells you what actually happened inside the kiln. Always use both. Place a guide, target and guard cone in a plaque visible through the peephole, and check them at the end of the firing.
- Guide cone (one number cooler) should be fully down.
- Target cone should be bent to about 90°.
- Guard cone (one number hotter) should still be standing.
- Log the time each cone went down alongside the controller reading.
A firing log — schedule, load, cones, results — is what turns firing from a gamble into a process. Every good studio keeps one.
Ceramic work involves respirable crystalline silica, kiln temperatures up to 1300 °C and glaze materials that can be toxic in powder form. This guide is an educational reference — it is not a substitute for the instructions supplied with your equipment. Always ventilate, wear appropriate respiratory protection when handling dry materials, and read our studio safety guide.
