Firing is the part of pottery that most newcomers find intimidating, and understandably so. You have spent hours wedging, throwing and trimming, and now the whole lot goes into a machine that will reach temperatures hot enough to melt copper. Get it right and you end up with a pot that rings when you tap it. Get it wrong and you are sweeping up shards. The good news is that the two firings most potters use — bisque and glaze — follow predictable rules, and once you understand what each one is for, the numbers stop looking arbitrary.

The two firings at a glance

Most studio potters fire twice. The first firing, the bisque (often called biscuit in the UK), turns fragile greenware into a hard, porous, handleable body. The second, the glaze firing, melts a layer of glass over that body and, at stoneware temperatures, vitrifies the clay itself. A few potters fire once — the raw glazing approach — but it demands a slow, careful schedule and is unforgiving of thick walls. For everyone else, two firings give you a chance to sand, wax and glaze without the pot collapsing in your hands.

What bisque firing actually does

The bisque firing does three jobs, in this order:

• Water smoking, roughly 100–200°C. Free and chemically combined water leaves the clay. Ramp slowly here; steam forms faster than it can escape and pots crack or explode.
• Organic burn-out, roughly 300–600°C. Carbon and sulphur burn away. This needs oxygen, so keep the kiln venting and do not overpack the shelves.
• Ceramic change, roughly 600–1000°C. The clay minerals break down and re-form; the pot becomes permanently hard and no longer slakes down in water.

A typical bisque schedule reaches cone 06 to 04 — about 1000°C to 1060°C — at a ramp of 80–120°C per hour, with a gentle start to protect the work. Bisque firings are almost always done in oxidation: dampers open, plenty of air, no reduction.

Choosing your bisque temperature

Bisque is not just a warm-up; the temperature you choose changes how the pot behaves. Too low and the body stays soft and dusty, so glaze crawls off the rim. Too high and the clay begins to close up, so it will not absorb water from the glaze bucket. You want a body porous enough to drink up glaze but sturdy enough to survive a second firing.

• Earthenware bodies: bisque around cone 06–05 (1000–1040°C).
• Stoneware and porcelain: bisque around cone 04–03 (1060–1100°C), then glaze fire hotter.
• Keep bisque at least two cone numbers below your glaze firing, so the body still has somewhere to go.
• Record every firing — peak temperature, ramp, soak and results. A simple notebook will teach you more than any chart.

The glaze firing: where the surface melts

The glaze firing is the real event. Here the glaze — a suspension of silica, fluxes, clay and colouring oxides — melts, flows a little, and fuses to the pot. Temperatures vary hugely by clay type:

• Earthenware: cone 06–02, roughly 1000–1120°C. Glazes stay glossy and bright, colours are easy, and the body remains porous.
• Mid-range stoneware: cone 6, around 1220°C. The most popular range for electric kilns — durable, food-safe and forgiving.
• High-fired stoneware and porcelain: cone 9–10, roughly 1280–1300°C. The body vitrifies, the glaze gains depth, and reduction gives those elusive copper reds and celadon greens.

Match glaze to clay. A cone 6 glaze fired in a cone 10 kiln will run off the pot and onto your shelf; a cone 10 glaze fired at cone 6 sits there as a dry, powdery crust.

Why cones beat the digital controller

A modern controller tells you the temperature at the tip of a thermocouple. That is useful, but it is not the whole story, because clay and glaze respond to heatwork — the combined effect of temperature and time. A kiln that reaches 1220°C over six hours does a different job from one that arrives in three. Pyrometric cones are the honest witness: they bend when they have received a specific amount of heatwork, not merely when the air is hot.

Place a small cone on each shelf — a set of cone 5, 6 and 7 on a clay patty — and a matching large cone in the kiln sitter if you use one. If cone 6 only leans, you are close; if cone 7 has gone down, you overshot, and you should adjust your firing schedule rather than your glaze recipe.

Soak time and the cooling curve

A soak, or hold, is a pause at the top of the firing. Ten to thirty minutes at peak evens out the kiln, lets glaze smooth over, and helps pinholing and blisters heal. Longer soaks add heatwork without pushing the peak higher — useful when your kiln struggles to climb to temperature.

Cooling deserves equal attention. Quartz inversion around 573°C is where many pots crack through the base, so a controlled cool through that band — or simply leaving the kiln closed overnight rather than cracking the lid at 300°C — saves a lot of heartbreak. Patience at the end of a firing is the cheapest insurance in the studio.