From Clay to Ceramic: What Really Happens Inside a Pottery Kiln

You’ve spent hours carefully shaping and molding a piece of clay, but the most dramatic transformation is yet to come. The kiln firing process can seem like magic, turning soft, fragile clay into a durable, lasting ceramic object. We will explore exactly what happens during this crucial stage, breaking down the science behind the heat.

The Critical First Step: Preparing for the Fire

Before a piece of hand-molded pottery ever sees the inside of a kiln, it must be completely dry. This state is known as bone dry. This isn’t just slightly dry to the touch; it means that as much atmospheric water as possible has evaporated from the clay particles. If you place a bone-dry piece against your cheek, it will feel room temperature, not cool. A cool feeling indicates there is still moisture evaporating.

Why is this so important? Any trapped moisture inside the clay will turn to steam when heated rapidly. If this steam cannot escape quickly enough, the pressure will build until the piece violently shatters or explodes inside the kiln. This can not only destroy your work but also damage other pieces and the kiln itself. Patience during the drying phase is the first and most vital part of a successful firing.

The First Transformation: The Bisque Firing

The initial firing is called a bisque or biscuit firing. The goal here is not to make the pottery waterproof but to turn it into a harder, more porous state that is easy to handle and ready to absorb glaze. A typical bisque firing for a mid-range stoneware clay body happens slowly, reaching a temperature of around 1800-1945°F (980-1060°C), often referred to as Cone 08 to Cone 04 in pottery terms.

Let’s look at what happens as the temperature rises.

Stage 1: Water Smoking (Up to 212°F / 100°C)

Even in bone-dry clay, there are still trace amounts of atmospheric water clinging to the clay particles. During this initial, slow heating phase, the kiln is essentially “smoking” this final water out as steam. The kiln must be heated very slowly through this stage to allow the steam to escape without causing damage. Many electric kilns are programmed to hold the temperature just below boiling for a period to ensure this process is complete.

Stage 2: Dehydration and Burn-Off (212°F to 900°F / 100°C to 482°C)

Once all the free water is gone, the heat begins to work on the clay itself. At around 660°F (350°C), the chemically bonded water within the clay’s molecular structure is driven off. This is a permanent change; after this point, the clay can no longer be broken down and recycled with water. It has begun its transformation into a ceramic material. As the temperature continues to rise, any organic materials in the clay, like paper fibers or microscopic plant matter, will burn away.

Stage 3: Quartz Inversion (Around 1063°F / 573°C)

This is a critical and stressful moment for the pottery. Clay contains silica, which is present in the form of quartz crystals. At exactly 1063°F (573°C), these crystals undergo a rapid expansion and change in their molecular structure. This is called the quartz inversion. The heating rate must be steady and controlled through this phase to prevent the piece from cracking under the stress of this sudden internal growth. The same process happens in reverse during cooling, where the crystals shrink, posing another risk for cracks.

Stage 4: Sintering (Approaching Peak Temperature)

As the kiln reaches its target bisque temperature, a process called sintering begins. The edges of the clay particles start to melt and fuse, bonding them together permanently. This is what gives the bisqueware its hardness and durability. However, the clay body does not fully vitrify, meaning it remains porous enough to accept a layer of glaze. Once the peak temperature is reached, the kiln begins its long, controlled cooling cycle.

The Second Transformation: The Glaze Firing

After the bisque firing, the porous ceramic is ready for glazing. Glaze is essentially a type of liquid glass formulated to melt at a specific temperature. Once the glaze is applied and dry, the pottery goes back into the kiln for a second, higher-temperature firing. For a common mid-fire stoneware, this could be up to Cone 6, or about 2232°F (1222°C).

The primary goals of the glaze firing are:

To melt the glaze into a smooth, glassy, and often colorful coating.
To cause the clay body to become fully vitrified, making it strong, dense, and waterproof.

During the glaze firing, the clay body goes through the same initial stages again, but this time it is much more stable. The real action happens at the higher temperatures. As the kiln approaches its peak temperature, the components in the glaze begin to melt and interact.

Fluxes, like feldspar or gerstley borate, act as melting agents, lowering the melting point of the main glass-former.
Silica is the primary glass-former, creating the final hard, glassy surface.
Alumina provides stiffness and stability, preventing the molten glaze from running right off the pot.

At peak temperature, the glaze has fully melted and fused with the now-vitrified clay body. The controlled cooling process is just as important here, as it allows the glaze to harden without cracking or developing flaws, a common issue known as “crazing.” After a day or two of cooling, the kiln can finally be opened to reveal the finished, transformed pottery.

Frequently Asked Questions

Why did my pottery explode in the kiln? The most common reason is that the piece was not bone dry before firing. Trapped water turned to steam and the pressure caused it to break apart. Another reason could be an air bubble trapped within a thick wall of the piece.

What is the difference between bisque firing and glaze firing? A bisque firing is the first firing at a lower temperature. Its purpose is to make the fragile clay hard and porous for easy glazing. A glaze firing is the second firing at a higher temperature. Its purpose is to melt the glaze and make the clay body dense and waterproof.

Can I fire hand-molded pottery in my home oven? No, a standard home oven cannot reach the extremely high temperatures required for the chemical and physical changes that turn clay into ceramic. The minimum temperatures needed are many times hotter than a kitchen oven’s maximum setting. Firing must be done in a proper pottery kiln.