Why Pottery Needs Two Firings

Every pot you make from raw clay must go through the kiln at least once. Most studio potters fire twice: a bisque firing, then a glaze firing. The bisque firing transforms fragile greenware into durable, porous ceramic that can be handled and glazed. The glaze firing melts the glaze onto that surface and vitrifies the clay body further. Understanding the differences in temperature, soak time and cooling rate is the key to reliable results, whether you are making mugs for the kitchen or frost-proof planters for the garden.

The Bisque Firing: Building Strength Without Melting

Bisque firing (often called biscuit firing in the UK) is a slow, gentle process. Its goal is to drive off chemical water and burn out organic matter, not to melt the clay. For most earthenware and mid-range stoneware, aim for a top temperature between 950°C and 1000°C (cone 08 to cone 06). Porcelain and high-fire stoneware may be bisqued higher, around 1000–1060°C, to give a stronger body before glazing.

The ramp up matters as much as the top temperature. Water smoking – the stage between 100°C and 200°C – is when remaining moisture turns to steam. If you climb too quickly, trapped steam can explode your pots. A safe schedule is 60–80°C per hour up to 600°C, then 100–120°C per hour to the top. Soak at peak for 10–15 minutes to even out the kiln. Cooling should be slow, especially through 573°C where quartz inversion causes a sudden volume change. Leave the kiln closed until it drops below 100°C.

The Glaze Firing: Where Surface Magic Happens

The glaze firing takes your bisqued pot to a higher temperature, melting the glaze into a glassy layer and further densifying the clay. Temperatures vary widely. Earthenware glazes typically mature between 1000°C and 1150°C. Stoneware glazes need 1200–1280°C. Porcelain often goes to 1300°C or slightly above. Always match your glaze to your clay body – a low-fire glaze on a high-fire stoneware will crawl or blister.

Soak time is critical here. Once the kiln reaches top temperature, hold it for 15–30 minutes. This allows the glaze to melt evenly, heal pinholes and release trapped bubbles. A longer soak can make glazes run off the pot; a shorter one leaves them dry and rough. For crystalline or matte glazes, the cooling rate is just as important as the soak – slow cooling encourages crystal growth and a softer surface.

Soak Times: Why Holding Temperature Matters

A soak is simply a pause at the top of the firing. It is not about adding heat; it is about giving the kiln time to equalise. Without a soak, the top of the kiln may be hotter than the bottom, and glazes can look different on different shelves.

• Bisque firing: 10–15 minutes soak at 950–1000°C. This evens out residual carbon and ensures all organic material is burned away.
• Glaze firing (earthenware): 15–20 minutes at 1050–1100°C. Enough to smooth the glaze without it running.
• Glaze firing (stoneware/porcelain): 20–30 minutes at 1220–1300°C. Longer soaks help vitrify the body and clear bubbles.

Use cones or a digital controller to verify your soak. If your kiln has hot and cold spots, place a cone pack on each shelf and adjust your soak time until they bend evenly.

Cooling Rates: The Stage Most Potters Rush

Cooling is not passive. Quartz inversion at around 573°C and cristobalite inversion near 220°C both cause the clay to contract sharply. If you cool too fast – by opening the lid, peepholes or the door – you risk dunting (cracking) and shivering (glaze flaking off). A controlled cool of 100–150°C per hour down to 800°C is gentle enough for most work. Below 800°C, you can let the kiln cool naturally with the lid closed.

For specific glaze effects, cooling rate changes everything. Fast cooling through the glaze’s softening range gives bright, glossy surfaces. Slow cooling – say 50°C per hour between 1100°C and 900°C – encourages matte finishes, speckles and crystalline growth. Experiment with your controller if you have one; if not, simply resist the urge to peek early.

Putting It Together: A Practical Starting Schedule

Here is a reliable two-fire schedule for a mid-range stoneware clay body in a small electric kiln. Adjust to suit your own materials and test tiles.

• Bisque: Ramp 80°C/hr to 600°C; 120°C/hr to 1000°C; soak 15 minutes; cool naturally with lid closed.
• Glaze: Ramp 150°C/hr to 1100°C; 100°C/hr to 1220°C; soak 25 minutes; controlled cool 150°C/hr to 800°C; then natural cooling.

Keep a firing log. Note top temperature, soak time, cooling rate and how the pots look. Over a few cycles you will learn your kiln’s personality. Whether you are firing a batch of garden planters or a set of breakfast bowls, respecting the bisque and glaze cycles – and above all, being patient during cooling – will reward you with ceramics that last.