What Is Bisque Pottery?
Bisque firing is a crucial step in pottery making that involves firing bone-dry clay at a low temperature to convert it from a fragile state to a porous and durable state, known as bisque pottery.
Bisque pottery refers to ceramic ware that has been fired once at a low temperature without any glaze. The resulting surface is porous and matte, making it suitable for further decorating with glazes or other techniques. Bisque-fired pottery is typically strong and durable, and can be used for a variety of functional or decorative purposes.
What does “bisque” mean?
The term bisque in pottery normally refers to either bisqueware or bisque firing.
- Bisqueware are pots that have been fired once at a low temperature without a glaze. Bisqueware is normally strong, porous, and no longer dissolves in water.
- Bisque firing is the first firing of a clay pot and is usually at a temperature of between 1720 and 1835 degrees Fahrenheit. Bisque firing can sometimes also be referred to as “pre-firing”.
The difference between bisqueware and greenware
Although they are both different stages of clay, bisqueware should not be confused with greenware.
Bisqueware has been fired once and is hard, while greenware is dried but unfired pottery that is still soft and malleable.
Greenware can still be turned into soft clay by introducing water, or can be carved for decorating. This is not the case for the fired bisqueware, since the clay undergoes chemical changes at high temperatures that can’t be reversed.
Purpose of bisque firing
The purpose of bisque firing is to turn greenware into bisqueware, which changes the malleable clay into durable and porous ceramics.
- Removes moisture: The bisque firing process removes any remaining moisture from the clay, which is important because moisture can cause the clay to crack or explode when it is fired at higher temperatures.
- Strengthens the clay: Bisque firing also hardens the clay and makes it more durable. This is because the heat causes the clay particles to fuse together, creating a stronger, more solid material.
- Makes the clay more porous: During bisque firing, the clay also becomes more porous, which makes it easier to apply glazes or other surface treatments later on.
The bisque firing also prepares the bisqueware for the following glaze firing. Skipping the bisque firing can result into cracks during the glaze firing since the glaze and clay shrink at different rates. The water that still remains in the clay can also cause the glaze to bubble or create pinholes.
Moreover, the porosity of bisqueware makes it easy for the glaze to stick to the surface. If you apply a glaze directly to bone-dry clay it may slowly drip off the surface and ruin your pottery.
How to bisque fire pottery
Bisque firing is very similar to the normal firing process, except at lower temperatures. Follow these 7 steps to bisque your pottery:
- Dry your pottery: the clay needs to be completely bone-dry before firing to reduce the chance of explosions and cracks. This drying can take several days to complete.
- Choose a kiln: controlling the temperature and ramp rate is important, so an electric kiln is preferred over a fuel-burning one.
- Set the temperature: bisque firing normally requires temperatures between cone 08 and cone 04, depending on the clay, desired fragility and porosity of the bisqueware.
- Use a slow ramp rate: this avoids cracking and other damage to the pottery by slowly removing the water in the clay. This is particularly important at low temperatures. See the next section for more information.
- Regularly check on the kiln: if you hear any sounds coming from the kiln, this normally means the ramp rate is too high and you should lower the temperature.
- Cool the kiln: Once the firing is done, turn off the heat and let the kiln cool down at its own pace. Only open the door once the kilns has reached room temperature again. Cooling down the pottery too fast might lead to cracks.
Different ramps used in bisque firing
The bisque fire process normally consists of three temperature ranges during which the ramp rates differ. This gives better results than using the same ramp rate from room temperature all the way to the target firing temperature.
- The Low Ramp: The first ramp in bisque firing, where the kiln stays at a low temperature for a few hours to dry out the clay. The ramp rate needs to be very low to evenly and slowly remove the remaining water.
- The Second Ramp: The medium ramp, also a couple of hours long, where the kiln temperature slowly increases and might need to be extended for thicker clay pieces.
- The High Ramp: The final ramp where the kiln temperature increases until the clay reaches bisque temperature, which can take a few hours depending on the clay type. At this point, you can normally use the highest ramp rate available.
How long does it take to bisque fire?
As a rule of thumb, it takes roughly 10 hours to bisque fire a piece of pottery. However, there are many factors that can influence this such as the kiln you use, the clay you use, and the heating program.
The size, age, type, and make of your kiln can all affect how long it takes to bisque fire your pottery. A larger kiln will take longer to heat up and cool down than a smaller one, while an older kiln may not be as efficient as a newer one.
The temperature and time settings you use during the firing process can also impact how long it takes to bisque fire your pieces. Depending on the type of clay and desired outcome, you may need to adjust the firing schedule to ensure your pieces are fully cured.
The way you pack your kiln can also affect firing time. If your pieces are tightly packed together, it may take longer for the heat to penetrate through and evenly distribute. Conversely, if your pieces are spaced out too much, there may be more empty space that needs to be heated. Be sure to pack your kiln carefully and use a kiln of the right size.
Finally, using a preheat can lengthen the bisque firing time. Preheating involves heating your kiln up to a low temperature for a period of time before beginning the actual firing process. While this can help remove any remaining moisture from your clay and prevent it from cracking or exploding, it can also make the overall firing time longer.
Bisque firing vs glaze firing
Bisque firing and glaze firing are two stages in the pottery making process that serve different purposes.
Bisque firing is the first firing stage that takes place after the greenware has been shaped and dried. The temperature in the kiln is raised gradually to a point where the clay hardens and chemically bonded water molecules and organic gases are removed. This process creates the bisque ware, which is porous, hard, and water-resistant. Bisque ware can be left unglazed or it can be glazed with a liquid glaze in a separate glaze firing.
Glaze firing is the second firing stage that takes place after the bisque ware has been glazed. The glazed bisque ware is fired again in the kiln at a high temperature, typically hotter than the bisque firing stage. This causes the glaze to melt and fuse with the bisque ware, creating a smooth and glossy surface that is water-resistant and decorative.
The main difference between bisque firing and glaze firing is the purpose of the firing and the temperature at which it takes place. Bisque firing is a lower temperature firing that hardens the clay and prepares it for glazing, while glaze firing is a higher temperature firing that melts and fuses the glaze onto the bisque ware, creating a decorative and water-resistant surface.
There are cases where you may only want to bisque fire and not glaze fire pottery. Bisque firing can give pottery a desirable matte finish and make it porous, which can be useful for certain types of pottery.
For example, unglazed bisque-fired clay is often used for plant pots as the porous surface allows for water and air to circulate through the clay, promoting healthy plant growth. Bisque-fired pottery can also be painted with acrylic or other non-fired finishes for decorative purposes, without the need for a glaze firing.
You can find more information about glazing in this article.
Do you need to bisque fire pottery before glazing?
It’s generally recommended to bisque fire pottery before glazing. Bisque firing is an important stage in pottery making that hardens and stabilizes the clay, making it more resilient and water-resistant. It also burns out any chemically bonded water molecules and organic gases that could cause problems during the glaze firing stage.
Without bisque firing, the clay may not be strong enough to withstand the glaze firing temperatures and may crack or fall apart. Also, the porous quality of bisque-fired pottery is ideal for glaze application, as the glaze is absorbed into the clay, helping it adhere better.
While it is technically possible to skip the bisque firing stage and glaze fire pottery directly, it can be risky and may result in ruined pieces. So, it’s best to bisque fire pottery before glazing, to ensure a successful final product.
Single fire glazing
Single fire glazing is a pottery technique where the bisque firing stage is skipped, and the pottery is glazed and fired in one step. In this method, the clay body is soft and porous, and the glaze is applied directly to it before firing. The pottery is then fired once to achieve the final product.
The main advantage of single fire glazing is that it can save time and energy because it eliminates the bisque firing step. This method can also create unique effects on the pottery, as the glaze may interact differently with the clay body when it’s fired in one step.
However, there are also some drawbacks to single fire glazing. One of the major concerns is that the clay body may not be strong enough to withstand the high glaze firing temperatures, which can result in cracking or breaking. Additionally, because the clay body is not bisque fired, any moisture or organic materials that are still present in the clay can cause issues during the firing process, such as bubbling, blistering, or pinholing in the glaze.
Overall, single fire glazing can be a useful technique for some potters, but it requires careful consideration of the specific clay body and glaze being used, as well as close attention to the firing process.
Therefore, I would only recommend single fire glazing to advanced potters that really know what they are doing.