The Cone System Explained
Pyrometric cones measure heat work — the combination of temperature and time — not just temperature alone. A kiln that heats slowly does more work at the same peak temperature than one that heats quickly. That's why potters use cones instead of just a thermometer.
Cones are numbered from low to high. Low-fire cones use a zero prefix (cone 06 is lower than cone 04), while mid-fire and high-fire cones go from cone 1 upward. The higher the number, the higher the temperature.
Bisque Firing vs Glaze Firing
Bisque Firing (First Firing)
Bisque firing transforms fragile, bone-dry clay into a durable, porous ceramic. Most potters bisque fire to cone 06 (about 1828°F / 998°C), regardless of what clay body they use. The goal is to burn out organic materials, drive off chemical water, and create a porous surface that absorbs glaze well.
- Fire slowly through the first 500°F to drive off moisture without cracking
- Leave the kiln lid cracked (peephole open) until 1000°F to let steam escape
- Total firing time: 8–12 hours depending on kiln size and load density
Glaze Firing (Second Firing)
Glaze firing melts the applied glaze into a glassy coating and matures the clay body to its final strength. The temperature depends on your clay and glaze — it must match both. This is typically a higher temperature than the bisque firing.
Temperature Ranges by Clay Type
Earthenware — Cone 06 to 04 (1828–1940°F / 998–1060°C)
The oldest type of pottery. Terra cotta, majolica, and most brightly colored commercial pottery is earthenware. It remains porous after firing, so it needs a glaze to hold water. Ideal for decorative pieces and tiles.
Pros: Bright glaze colors, lower energy costs, accessible kiln temperatures.
Cons: Porous without glaze, less durable than stoneware, chips more easily.
Stoneware — Cone 6 to 8 (2232–2280°F / 1222–1249°C)
The workhorse of studio pottery. Stoneware vitrifies (becomes non-porous) at these temperatures, making it durable, food-safe, and chip-resistant. Most functional pottery — mugs, bowls, plates — is stoneware fired to cone 6.
Pros: Durable, food-safe, wide glaze variety, works in electric kilns.
Cons: Fewer bright glaze colors than low-fire, higher energy costs.
Porcelain — Cone 8 to 12 (2280–2383°F / 1249–1306°C)
The finest and most demanding clay body. Porcelain becomes translucent, extremely hard, and rings when tapped. It's challenging to throw — it has less plasticity and is prone to warping — but the results are stunning.
Pros: Translucent, extremely strong, elegant white body, prized in fine ceramics.
Cons: Difficult to throw, warps easily, requires high-temperature kiln.
Types of Kilns
Electric Kilns
The most common kiln for home and studio potters. Electric kilns are clean, easy to control with digital controllers, and fire reliably to cone 6–8. They create an oxidation atmosphere, which affects glaze colors.
Gas Kilns
Fueled by natural gas or propane, gas kilns can reach higher temperatures and create a reduction atmosphere — where oxygen is limited — producing unique glaze effects like copper reds and celadons that electric kilns cannot achieve.
Wood-Fired Kilns
The traditional method. Wood ash from the fuel settles on pots and melts into natural ash glazes. Firings take 12–72 hours and require constant stoking. The results — with their natural, unpredictable surfaces — are highly prized.
Common Firing Mistakes
- Firing wet clay: Even slightly damp clay explodes in the kiln. All work must be completely bone-dry before bisque firing.
- Firing too fast: Rapid temperature increases cause thermal shock, cracking pieces. Slow ramps under 200°F/hour in the early stages are safest.
- Mismatched cone: Using a glaze rated for cone 6 on a pot fired to cone 10 results in a runny, melted mess. Always match glaze and firing temperature.
- Opening too early: Kiln-cracked pots are almost always caused by opening the kiln before it cools below 200°F. Wait at least 12 hours after the firing completes.
- Overloading: Packing the kiln too tightly prevents even heat distribution. Leave at least 1 inch between pieces and between pieces and elements.
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