Ceramic Link

Process

Firing pottery: bisque and glaze firings, and what cones mean

Firing turns dried clay into ceramic, a hard material that no longer softens in water. Most pottery is fired twice: once to harden the clay and once to melt the glaze.

The firing stages on one scale

Firing ranges on one scaleApproximate temperatures. Cones measure heat over time, so the exact figure depends on how fast the kiln climbs.
  1. Last water leavesAbout 70 to 212°F (20 to 100°C)

    Damp clay turns to steam. Heat slowly here, or thick pieces can crack or burst.

  2. Burnout and bound waterAbout 570 to 1,470°F (300 to 800°C)

    Organic matter burns off and water locked in the clay minerals is driven out. Vent the kiln.

  3. Quartz changeAbout 1,063°F (573°C)

    Quartz in the clay changes size suddenly at about 1063°F (573°C), on the way up and on the way down.

  4. Bisque firingAbout 1,730 to 1,945°F (945 to 1,065°C)

    Clay turns hard and permanent but stays porous, so it can soak up glaze.

  5. Low-fire glazeAbout 1,830 to 1,945°F (1,000 to 1,065°C)

    Earthenware and many bright commercial glazes.

  6. Mid-fire glazeAbout 2,165 to 2,230°F (1,185 to 1,220°C)

    Mid-range stoneware and porcelain. A common choice for electric kilns.

  7. High-fire glazeAbout 2,300 to 2,345°F (1,260 to 1,285°C)

    High-fire stoneware and porcelain, often in gas or wood kilns.

The figures are rounded and approximate. They change with the heating rate, the kiln and the clay, so treat them as a map and use the cone maker's chart and your kiln manual for real firings.

What happens inside the kiln

As the kiln heats, the last water in the clay leaves as steam. This is why the start of a firing is slow, sometimes with the lid propped or the vents open. Later, organic matter in the clay burns away and water bound inside the clay minerals is driven off. At about 1063°F (573°C), quartz in the clay changes size suddenly. Pots handle this change best when the kiln passes through it steadily, both heating and cooling.

Higher still, the clay particles begin to fuse. At the top of a glaze firing, a stoneware or porcelain body becomes dense and close to watertight. This is called vitrification.

Bisque firing

The first firing, the bisque, is usually done at a relatively low temperature, often somewhere in the cone 08 to 04 range. Bisqueware is hard and can't be turned back into wet clay, but it is still porous. That porosity lets it soak up glaze like a sponge, which makes glazing far easier than on raw clay.

In a bisque firing, pots can touch and even nest inside each other, because there is no glaze to stick them together. Lids are usually fired in place so they keep fitting.

Glaze firing

The second firing goes to the temperature that the glaze and clay are made for. Glaze melts into glass, so glazed pots must not touch each other or the kiln shelf. Bottoms are wiped clean, and pots are spaced apart. Low-fire glazes on earthenware are sometimes fired on stilts so that the whole pot can be glazed, following the stilt maker's guidance.

What pyrometric cones measure

A pyrometric cone is a small slender pyramid made of ceramic materials that bend at a known point. Cones measure heat work, the combined effect of temperature and time. A kiln held at a temperature for longer does more work on the pots than one that only touches that temperature. That is why potters talk about cones more than degrees.

Cone numbers run in an odd order. Numbers with a leading zero, like 06 and 04, are cooler, and the bigger the number after the zero, the cooler the cone. Numbers without a zero, like 6 and 10, get hotter as they go up. So cone 06 is cooler than cone 04, which is cooler than cone 6.

Many electric kilns use a digital controller and may also have a device that shuts the kiln off when a small cone bends. Many potters still place witness cones on a shelf where they can see them through a peephole, as a check on what the controller reports.

Cooling and unloading

Cooling takes about as long as heating, often a day or more in total. Opening the kiln early can crack pots through sudden temperature change and is a burn risk. Follow the kiln manual and your studio's rules on when to open the lid and when to unload.

Kinds of kilns

  • Electric kilns are the most common in schools, community studios and homes. They fire in an oxidizing atmosphere, with plenty of oxygen.
  • Gas kilns can be fired with reduced oxygen, called reduction, which changes how some glazes and clays look.
  • Wood, soda and salt kilns add ash or vapor to the surface of the pots and need a crew and a lot of experience.
  • Raku is a fast, low-fire process in which pots are taken out of the kiln while red hot. It is done outdoors with special training and gear.

Before you run any kiln yourself, read the kiln safety basics and the maker's manual for your model.