What Happens to Clay During Drying and Firing? The Stages of Making Ceramics
Shaping an object from clay is only the beginning of the ceramic-making process. The soft, pliable material must first dry and then be fired in a ceramic kiln. At each stage, its properties change, including its moisture content, strength, volume, and structure. This is why a vessel that looks finished immediately after shaping is not yet suitable for everyday use.
Understanding the drying and firing processes helps prevent cracks, warping, and other problems that can occur even in carefully made pieces. It also makes it easier to plan wall thickness, decide how to join separate components, and estimate the time needed to complete an object.
How Does Clay Change After Shaping?
Fresh clay contains water, which allows it to be kneaded, stretched, and shaped. Tiny particles of clay minerals move relative to one another, allowing the material to remain pliable and retain the shape given to it.
Once shaping is complete, the water gradually begins to evaporate. The clay becomes firmer and reaches a stage known as leather-hard. At this point, the surface can still be smoothed, edges trimmed, decorations added, and properly prepared components attached.
This is an important stage when learning ceramics. A form that is too soft may become distorted under the pressure of a tool, while clay that has dried too much can crumble during finishing. Learning to recognize the right consistency comes primarily through hands-on experience with the material.
Why Does Clay Shrink During Drying?
As water evaporates, the clay particles move closer together, reducing the overall dimensions of the piece. This phenomenon is known as drying shrinkage. The amount of shrinkage depends on the type of clay body, its water content, and how the material was prepared.
The greatest problems occur when different parts of a piece dry at different rates. The thin rim of a bowl may lose moisture much faster than its thick base. The resulting stresses can sometimes cause cracks or deformation.
For this reason, freshly shaped pieces should not be placed directly next to a radiator or exposed to intense sunlight. Slow, even drying is safer, initially under a loosely draped plastic cover. Particular attention should be paid to joined sections and areas where the clay is thicker.
When Is Ceramic Clay Dry Enough to Fire?
Before the first firing, the clay must become completely dry. This stage is known as bone dry. The piece becomes lighter in color, firm, and noticeably lighter than it was immediately after shaping. However, it remains fragile and can easily be damaged when handled.
A dry surface does not necessarily mean that all the moisture has disappeared from the inside. Thick walls can retain water much longer than thin decorative elements. Drying may take anywhere from a few days to several weeks, depending on the size of the piece, environmental conditions, and its construction.
Placing clay in the kiln too early is risky. As the kiln heats up, any remaining water turns into steam. A rapid increase in pressure inside the material can cause the piece to crack or even burst apart.
The First Firing - What Happens Inside the Clay?
The first firing, known as bisque firing, transforms dried clay into a durable ceramic material. In many studios, it takes place at temperatures of around 900–1000°C, although the appropriate range depends on the clay body and the techniques being used.
At the beginning of the heating process, any remaining moisture is removed. During the following stages, organic matter burns away and clay minerals undergo chemical transformations. At approximately 450–650°C, dehydroxylation occurs, among other processes. This involves the removal of chemically bound water from the structure of the minerals.
Unlike ordinary drying, the changes that take place during firing are irreversible. Fired ceramics cannot be mixed with water again and reshaped in the same way as raw clay.
Once the piece has cooled, it becomes bisque — a hard but still porous material. Its surface is well suited to glazing because it can absorb water from the liquid glaze mixture.
Glazing and the Second Firing
Glaze is a mixture of carefully selected mineral ingredients that forms a glass-like layer on the ceramic surface during firing. It can give a piece color, shine, a matte finish, or a particular texture.
Glaze is most often applied to clean bisque by dipping, pouring, or brushing. The thickness of the coating matters: a layer that is too thin may produce uneven coverage, while an excessively thick layer may run during firing.
The temperature of the second firing depends on the clay body and the glaze being used. Low-fire ceramics are often fired at around 1000–1100°C, whereas stoneware generally requires higher temperatures, frequently around 1200–1300°C.
Under heat, the glaze melts and forms a durable coating as it cools. At the same time, the ceramic body may become denser. However, not every glaze or clay body is suitable for contact with food. This requires the appropriate selection of materials and firing conditions.
Why Does Finished Ceramics Sometimes Crack or Change Shape?
Cracks are not always caused by the firing process itself. The underlying problem may have developed much earlier, during shaping, joining separate pieces, or drying.
Uneven wall thickness, poorly joined components, and trapped moisture all increase the risk of damage. During firing, the heating and cooling rates also matter, as does the compatibility between the thermal expansion of the glaze and that of the ceramic body.
Beginners should pay particular attention to forming walls of even thickness and joining components carefully with ceramic slip. When making enclosed forms, it is also important to provide adequate ventilation and ensure that the inside of the piece dries thoroughly.
What Can the Drying and Firing Process Teach Us About Ceramics?
Ceramic work requires planning for the next stages from the moment shaping begins. It is important to account for material shrinkage, allow enough time for drying, and remember that the appearance of a piece may change after firing.
Working with clay also teaches us to recognize the properties of the material. Over time, it becomes easier to judge when a surface is ready for decoration, how thick the walls should be, and why drying should not be rushed.
The most important thing to understand is that a finished ceramic object is created through the combined effects of shaping, drying, and heat. Each stage influences the next, and careful control of the entire process makes it possible to create pieces that are more durable, predictable, and refined.
