Glay Pots: The Art Of Achieving The Perfect Fire Temperature

Clay pots are fired at extremely high temperatures, with the exact temperature depending on the type of clay and the desired finish. In a kiln, temperatures typically range from 1800°F to 2400°F (982°C to 1316°C). However, some methods, such as using a campfire, can reach temperatures of over 1000°F (538°C).

Firing clay involves heating it to the point where its molecular structure changes, and it becomes a ceramic material. This usually occurs at temperatures above 1000°F (538°C). The specific temperature required depends on the type of clay being used. For example, an earthenware clay body can fire to maturity at about 1830°F (1000°C) and melt at 2280°F (1250°C). On the other hand, a porcelain body made of pure kaolin might not mature until about 2500°F (1390°C) and not melt until over 3270°F (1800°C).

The process of firing clay involves several stages, each with its own temperature requirements. Firstly, the clay needs to be dried out completely before firing. This can be done in an oven at a temperature of 190°F to 194°F (88°C to 90°C).

The next stage is bisque firing, where the clay is turned from soft to hard ceramic. This typically occurs at temperatures between 1823°F and 1940°F (995°C and 1060°C). During bisque firing, chemically combined water is driven off from the clay body, and quartz inversion occurs, causing the pottery to increase in size by 2% and then decrease by 2% as it cools.

After bisque firing, the clay can be decorated with glazes, which are then fired. This glaze firing temperature depends on the type of glaze used but typically occurs between 1650°F and 2010°F (900°C and 1100°C).

Overall, the temperature at which clay pots are fired varies depending on the equipment, type of clay, and desired finish.

Clay pots can be fired in a kiln, grill, or pit

Kiln Firing

Firing clay in a kiln involves three stages: drying the clay, bisque firing, and then glazing or decorating. Before firing, it is important to ensure that the clay is completely dry to prevent it from exploding or breaking due to moisture trapped inside. This can be done by placing the clay in an oven at 194°F (90°C) for 30 minutes to an hour.

Kiln firing can be done using various types of kilns, such as microwave kilns, charcoal kilns, brick kilns, barrel kilns, raku kilns, and electric kilns. Each type of kiln has different features, temperature ranges, and firing processes. For example, microwave kilns are small, cylindrical containers that fit into a domestic microwave and reach temperatures of around 1650°F (900°C). On the other hand, charcoal kilns use burning charcoal as a heat source and can reach temperatures of over 2010°F (1100°C).

Grill Firing

Firing clay pots on a grill is a more accessible option for those who do not have a kiln. A charcoal grill can be used, and the process is similar to kiln firing. The grill is lit, and the pots are placed near the coals to heat up gradually, preventing thermal shock. Once the coals are hot, the pots are nestled among them, and the grill is covered with a lid left slightly askew to allow oxygen to fuel the fire. The pots can be left in the grill for 30 minutes to an hour or until the fire dies out.

Pit Firing

Pit firing is one of the oldest methods of firing clay, dating back to 29,000 BC. It involves digging a hole in the ground and filling it with combustible material such as dry leaves, twigs, coal, sawdust, or manure, and placing the pottery among this material. The fire is lit, and the pit is covered with dirt or sand to reduce oxygen and create a reducing atmosphere, adding colour and variation to the finished pieces. The fire is left to burn out, and the pots are removed the next day and cleaned.

Firing temperatures vary depending on the type of clay and glaze

For instance, earthenware clay is a softer type of clay that is fired at moderate temperatures, usually between 1,800°F to 2,100°F (982°C to 1,149°C). It is commonly used for manufacturing valuable wares such as bowls and containers. On the other hand, stoneware clays require high firing temperatures, typically between 2,100°F to 2,400°F (1,149°C to 1,316°C). Ceramics made from stoneware clays are very stable and diverse in terms of utility.

The type of porcelain clay known as kaolin clay is fired at the highest temperatures, ranging from 2,300°F to 2,600°F (1,260°C to 1,427°C). It is used to create exquisite and delicate porcelain products. Ball clays, which include plastic clays that are soft when wet, usually fall between 1,800°F to 2,400°F (982°C to 1,316°C) in terms of their firing temperature.

Low-fire clays, ideal for colourful and decorative pieces, are fired at lower temperatures of around 1,700°F to 2,000°F (927°C to 1,093°C). The first firing at lower temperatures, known as bisque firing, prepares the clay for subsequent glazing. This step is crucial as the wrong glaze choice can result in an unacceptable final appearance or functionality.

Glazes also have specific temperature requirements. If fired at too low a temperature, the glaze will not mature, and if the temperature is too high, the glaze will melt and run off the pottery surface. Thus, potters must be knowledgeable about the correct temperature range for their chosen glaze.

The firing temperature is a crucial factor that significantly impacts the final properties of a ceramic piece. It is essential to select the appropriate clay type and firing temperature to achieve the desired results in terms of both aesthetics and functionality.

Clay must be dried before firing to avoid cracking

Firing clay involves reaching extremely high temperatures, and there are several methods for doing so. Clay can be fired in a kiln, microwave kiln, charcoal grill, pit, barrel, or raku kiln, or using an electric kiln. However, before clay is fired, it must be dried to avoid cracking.

Clay drying is a delicate process and requires careful control to avoid mishaps. The key to crack-free drying starts before clay even leaves the worktable. It is important to only use smooth, consistent, well-conditioned clay that has been thoroughly wedged to remove air pockets. The clay should be pliable but not sticky, with no grog chunks or hard unincorporated bits, and should have a uniform thickness.

The drying process should be gradual, and pieces should be set to dry in a climate-controlled room at around 70°F. Direct sunlight, heaters, or drafts should be avoided, as they can cause the surface to desiccate too quickly. If the clay is drying too fast, it can be draped in plastic wrap or a damp cloth. Pieces should be turned regularly as they dry to prevent uneven stresses.

It is also important to learn to recognize the range of material states clay passes through as it dries. Clay can be wet, leather hard, or bone dry. Leather hard clay is ideal for smoothing surfaces and finishing details before bisque firing. If the clay gets overly dry, it can be re-wet by spraying or sealing in plastic.

As clay loses moisture, shrinkage stresses can develop, which can manifest as curling corners, sudden cracks, twisting or warping slabs, or spongy areas that are drying too fast. These issues can be addressed by adjusting humidity or supports.

The final stage of drying happens in the bisque kiln, where rapid heating or cooling can cause the clay to crack. To avoid this, the kiln should be programmed with gradual ramp rates between 60-120°F per hour, pausing at 180°F to allow the heat to equalize before proceeding.

Clay thickness is another important factor, as shrinkage can tear the clay apart if the thickness varies within a piece. Reinforcements such as extra clay, coil or slip trailers, scored scratch marks, or tiny grog inclusions can be added to vulnerable areas to taper thickness gradually and ease the transition in rigidity.

By following these steps and practices, potters can minimize cracks and warp and ensure their clay work survives intact to final firing.

Firing usually takes place in two stages: biscuit/bisque firing and glaze firing

Firing clay usually takes place in two stages: biscuit (or bisque) firing and glaze firing.

Biscuit/Bisque Firing

The first firing of pottery is called biscuit or bisque firing. Pottery is fired in a kiln without a ceramic glaze. This initial firing turns the clay into a ceramic material. The temperature is slowly increased to transform the object into a porous state for glazing. The temperature of a bisque firing typically ranges between 1700 and 1900 degrees Fahrenheit (around 926 to 1038 degrees Celsius).

Glaze Firing

The second stage is glaze firing. This is when the glaze is applied to the bisque-ware and then fired again to melt and fuse it to the clay body. The kiln is slowly brought up to the appropriate temperature for the silica in the glaze to melt and then slowly cooled. This makes the pottery strong, solid, and impervious to water and the elements. The temperature for glaze firing depends on the type of glaze used. Low-fire glazes are fired at 1845 degrees Fahrenheit, mid-fire glazes at 2192 degrees Fahrenheit, and high-fire glazes at 2305 degrees Fahrenheit.

After firing, the kiln must be cooled slowly to avoid thermal shock

Firing clay involves heating it to extremely high temperatures, and the cooling process is just as important as the firing process itself. After firing, the kiln must be cooled slowly to avoid thermal shock, which can cause cracking and damage to both the kiln and the pottery. Here are some detailed instructions and factors to consider for the cooling process:

Allow the Kiln to Cool Down Naturally:

The kiln should be turned off and unplugged after the firing is complete, and then allowed to cool down at its own pace. It is important to refrain from forcing the kiln to cool down faster, as this can be detrimental. The kiln should be left untouched until it reaches a safe temperature, which can take anywhere from 8 to 24 hours or even several days, depending on various factors.

Monitor the Temperature:

Use a kiln thermometer to regularly check the temperature of the kiln during the cooling process. This will help you track its progress and ensure it is cooling at an appropriate rate.

Adjust Settings for Automatic Cooling:

If your kiln has an automatic cooling cycle, set it to the appropriate cooling rate. This will help regulate the cooling process and prevent overheating.

Ventilation:

Place the kiln in a well-ventilated area to aid in the cooling process. However, be cautious not to create drafts or strong airflows, as this can affect the cooling process negatively.

Handle Pottery with Care:

Once the kiln has cooled down sufficiently, open the door completely and wait for it to reach room temperature. If the pottery is still warm, use heat-resistant gloves or insulation gloves to handle the pieces, as they can cause severe burns. Place the pieces on a heat-resistant surface and allow them to cool naturally.

Factors Affecting Cooling Time:

The cooling time of a kiln depends on several factors, including the size of the kiln, the temperature it was fired to, the type of kiln, and the surrounding environment. Electric kilns, for example, tend to take longer to cool than gas kilns. Additionally, a kiln fired to a higher temperature will require a longer cooling period.

Prevent Thermal Shock:

Allowing the kiln to cool down gradually helps to prevent thermal shock, which can cause significant damage. Thermal shock occurs when clay undergoes rapid temperature changes, causing it to crack or break. By cooling the kiln slowly, you reduce the risk of thermal shock and ensure the integrity of your pottery creations.

In summary, the cooling process of a kiln is a critical aspect of the pottery-making journey. By following the provided instructions and considerations, you can ensure the kiln and your pottery creations are properly cooled, preventing damage and ensuring the best possible results.

Frequently asked questions