How to Fire Glazed Ceramics: Cones, Kiln Loading, and Cooling

Firing is the step that turns powdery glaze into smooth glass bonded to your piece. It's also where timing and loading matter most. Here is how glaze firing works, in plain terms.

Bisque firing vs. glaze firing

Bisqueware has been fired once, usually at a lower temperature, to harden the clay. Glaze firing is the second firing, after you apply glaze. Always match your glaze to your clay body and fire to the cone on the label (see low-fire vs. high-fire glazes).

What is a cone?

A cone is a small pyramid of ceramic material that bends at a specific combination of temperature and time. Kilns measure heat, but cones measure heat work, which is what actually matters to glaze. You'll see numbers like cone 06 (low fire) or cone 6 (mid fire). A number with a leading zero is cooler than the same number without one, so cone 06 is much cooler than cone 6.

Witness cones

Placing a small cone in view of the peephole lets you confirm the firing reached the right point. Our kiln room supplies include witness cones in a range of numbers. They're inexpensive insurance against a ruined load.

Loading the kiln

• Glazed pieces must not touch each other or the kiln walls. Glaze melts and will fuse pieces together.
• Wipe the foot clean of glaze, or use stilts, so nothing sticks to the shelf.
• Use kiln wash on shelves to protect them from drips (see kiln furniture basics).
• Leave airflow space around the load so heat distributes evenly.
• Check that every piece is fully dry before loading.

Choosing a firing speed

Most programmable kilns offer slow, medium, and fast schedules. Slow schedules are gentler on thick pieces and large loads and tend to give more even glaze results. Faster schedules save time on thin, simple pieces. Follow the instructions for your kiln and your glaze.

Cooling

Let the kiln cool completely before opening it. Opening a hot kiln can crack pieces from thermal shock. A good rule is to wait until the kiln is well below about 200°F before you unload, and longer for large or thick items.

Safety

• Fire only in a ventilated space and consider a kiln vent (see our kiln vents).
• Never open a hot kiln or touch hot pieces without proper protection.
• Follow your kiln manufacturer's instructions for wiring, clearances, and operation.

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