Firing temperatures are among the most frequently asked questions. Here are some reference points to help you find your way. The golden rule is still the technical data sheet for your clay and your glaze: that's what counts.
The main types of clay
| Clay | Usual final firing | Key points |
|---|---|---|
| Earthenware | about 950 to 1,100 °C | stays porous after firing; the glaze makes it watertight |
| Stoneware | about 1,200 to 1,300 °C | vitrified, dense and strong |
| Porcelain | about 1,220 to 1,300 °C | white, fine, translucent when thin |
| Raku clay | about 900 to 1,000 °C | raku is a technique: the pieces are taken out of the kiln red-hot; they stay porous and fragile, for decorative use |
Ranges vary between manufacturers: some so-called “low-fire” stoneware fires at around 1,150 °C, some porcelain goes above 1,300 °C. Always check the range printed on the bag.
Bisque and glaze firing
Most pieces are fired twice:
- The bisque firing: a first firing that makes the piece strong but still porous, so it absorbs the glaze. For stoneware and porcelain, it's often done at around 950 to 1,000 °C; in that case it's also called a low bisque. For earthenware, it's often the other way round: the bisque is fired higher, then the glaze a little lower.
- The glaze firing (or final firing): at the temperature specified for the clay and for the glaze.
Pieces must be completely dry before the bisque firing: any water left in them turns to steam and can make the piece explode.
What are cones?
Pyrometric cones (Orton, Seger…) are small ceramic pyramids that bend under the effect of heat. They measure heatwork (temperature and time), not just an instantaneous temperature: that's why they're used to check a firing. Here are some reference points from the manufacturer Orton, for self-supporting cones and a rate of 60 °C per hour over the last 100 degrees:
| Cone | 06 | 04 | 6 | 8 | 10 |
|---|---|---|---|---|---|
| Temperature | 998 °C | 1,063 °C | 1,222 °C | 1,249 °C | 1,285 °C |
A hold at the end of the firing increases the heatwork: according to Orton, a one- to two-hour hold can be enough to bring down the next cone. Our Cones and temperatures tool gives the full chart.
One last tip
Never go above a clay's maximum temperature: it can warp, bloat or even melt and damage the kiln. If in doubt, test a small shard first, set on something that protects the kiln shelf.
