If you spend any time in a ceramics studio, you will inevitably hear about cones: cone 04, cone 6, cone 10… These numbers might seem mysterious at first, but they are essential for understanding how clay and glaze firing works. Since I fire all of my pieces—and those I take on for my firing services—at cone 6, I would like to offer a brief overview of what that means.
What is a cone?
A cone, or pyrometric cone, is a small ceramic pyramid specifically designed to bend when it has absorbed a certain amount of heat. Unlike a simple thermometer, a cone does not just measure temperature, but "heatwork": a combination of the temperature reached and the duration of exposure. In other words, we don't rely solely on the number displayed on the kiln’s controller, but on what the heat has actually accomplished inside.
The cone system was developed in the late 19th century by the German chemist Hermann Seger, then standardized and popularized in North America by the Orton Foundation, which still manufactures the majority of the cones used in studios today.
Cone 6 in practice
Cone 6 corresponds to approximately 1222 °C (2232 °F) in an electric kiln, in an oxidation firing. This is known as a mid-range firing. It is the firing range I have chosen for my work, for several reasons. At this level, clay bodies achieve good vitrification: they become solid and non-porous, making them perfectly suited for daily use. The pieces are durable, sturdy, and safe for containing food or liquids.
Cone 6 is also an interesting compromise from an energy perspective: it requires less electricity than cone 10 (high-fire), while still providing access to a wide variety of glazes. It is, in fact, one of the most widely used ranges in the world of contemporary ceramics.
For those bringing pieces to my studio to be fired, it is important to verify that the clay and glazes chosen are indeed formulated for cone 6. Clay designed for cone 10, fired only at cone 6, will remain too porous and therefore lack durability. Conversely, a low-fire glaze (for example, cone 04) risks melting completely and running onto the kiln shelves, which can damage not only the piece, but also the equipment and the work of others.
Glazes and their formulation
A glaze is a thin layer of glass that melts and attaches to the surface of the clay during firing. For it to behave as expected, it must be formulated specifically for the target temperature range. A glaze is generally composed of three main elements:
- Silica, which forms the glass.
- Fluxes, which lower the melting point of the silica.
- Alumina (or clay), which stabilizes the mixture and prevents the glaze from running.
At low temperatures, glazes contain more powerful fluxes to melt quickly. At high temperatures, the heat is sufficient to melt the silica, so the recipe is different. Cone 6, in the mid-range, requires a specific balance of these ingredients.
This is why the same glaze can produce completely different results if fired at the wrong cone: too low, it remains matte and chalky; too high, it becomes runny, uncontrollable, and sometimes even dangerous.
Witness cones
Even though my kiln is digital and I can precisely program firing cycles, I always use witness cones. These small cones placed inside the kiln confirm what actually happened during the firing.
I usually arrange three of them: one below the target temperature (cone 5), one at the target temperature (cone 6), and one above (cone 7). By observing how they have bent, I can tell if the firing was perfectly accurate, a little too hot, or a little too cold. Witness cones allow me to ensure the consistency of my firings and the quality of the pieces that come out, whether they are my own or those entrusted to me by other ceramicists.
And that is why everything ends up at cone 6!
Cone 6 is not just a number on a label: it is a true language that ensures clay and glaze reach their maturity. For those wishing to use my firing services, it is essential to verify that your materials are compatible with cone 6. It is the guarantee that your pieces will come out of the kiln solid, safe, and true to your expectations, all while protecting the equipment and other people's work.
(See the Orton cone chart PDF below to get an idea of the different cones!)
