What Is the Purpose of Bisque Firing?

Bisque Firing is one of the first terms you hear in pottery classes or the studio because it’s one of the most important steps in the pottery-making process.

The Purpose of Bisque Firing is to transform greenware (unfired bone-dry clay) from its fragile state to a porous and durable state called ceramic for the second stage of firing. The process allows you to safely do decorative work, apply Underglazes, and Glazes on the piece without damaging or cracking it.

Read on to learn more about the bisque firing process, its purpose, and what can happen if you skip it in the pottery-making process.

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What Is Bisque Firing?

Typically, the firing of ceramic occurs in two stages: bisque firing and glaze firing. In bisque firing, the clay is put in a kiln and heated slowly, and cooled slowly. This first stage gives the object’s porosity and water-absorbing properties and reduces the chances of damage during handling.

The bisque firing process causes permanent physical and chemical changes to the clay. Moisture is removed from the clay, and organic materials in it are burned out. The process also allows the clay particles to bond. The product is a stronger and harder ceramic with porosity for the application of glazes. 

The second stage, called glaze firing, is done after the potter has applied stains, underglazes, and glazes to the piece. This process usually takes longer than bisque firing, depending on the cone temperature you fire to.

In essence, the purpose of bisque firing is to:

• Turn the clay into ceramic, making it porous for glazing
• Ensure that the stains, glaze, and underglaze bond well to the bisque surface
• Remove residual moisture and burn off organic matter
• Make the ceramic stronger, so it doesn’t fall apart or crack
• Ensure the glaze bonds properly to the bisqueware.

The Bisque Firing Process 

Program the kiln to work a Cone 06, 05, or 04. Cone Fire program, Medium Speed, but Slow Speed is best. The bisque firing duration may not be the same every time.

The stages in bisque firing are the same, no matter the level of firing:

1. The first stage is essentially moisture removal from the clay.
2. In the second stage, organic matter and impurities are burnt away.
3. In the third stage, chemically bound water is removed from the piece.
4. Finally, the sulfur compounds and carbonates in the clay burn off and decompose, respectively. The pieces shrink and sinter during the firing. 

What Will Happen if You Skip the Bisque Firing Stage?

Technically, it’s possible to skip the bisque firing stage and start with glaze firing instead. However, this can be risky as the ceramic can crack or fall apart if not fired properly. Also, the chemically bonded water molecules and organic gases in the greenware that should be removed during bisque firing can cause the glaze to bubble or get pinholes.

The Glazes and Underglazes may not adhere to the surface properly. When this happens, the even textures and decorations you’re trying to create on the object will be ruined.

Skipping the bisque firing process can also cause the pottery to crack if the clay and glaze are not fired slow enough.

Because the ware wasn’t fired to maturity temperature in the bisque firing stage, some moisture will still be left in the clay. That’s why slow firing is essential when single firing.

In summary, bisque firing is better as it ensures that the glazes and decorations adhere perfectly to the object.

How to Load a Kiln for a Bisque Firing

The process of loading a kiln is quite easy if you follow the guidelines correctly. Make sure all the pieces are bone dry. Wet pieces can Crack or Blow up in the Kiln. 

• Mount the bottom shelf on a slit of at least 1 inch (2.5 cm) to aid circulation.
• The wares should also be at least 1 inch (2.5 cm) away from the elemental walls and 2 inches away from the thermocouple and Kiln Sitter.
• Leave up to 1 to 1.5 inches (2.5 to 3.8 cm) of space between the piece and the top shelf. The kiln cover should be 1.5 to 2 inches (3.8 to 5.1 cm) away from the nearest ware.
• You can stack the wares together in a bisque firing. 
• Separate your underglaze wares and ensure they don’t touch each other. As the underglaze will transfer onto the greenware, it touches.

• If you are using a half shelf, leave a 1/4 inches (0.6 cm) gap if they are side by side.
• Close the door of the kiln and begin the firing.

Firing the bisque kiln might take up to 12 hours, depending on the speed and temperature you fire to. Then even longer to cool down.

Here’s my Video on How I Load the Kiln for a bisque fire.

What Is The Best Temperature for Bisque Fire Clay?

There is no exact temperature requirement for bisque firing. However, the ideal range is usually between cone 06 to cone 04, regardless of your clay and glaze temperature. (low, mid, or high fire)

• Firing at cone 06 causes the clay to shrink and turn into a solid porous mass. Also, it turns the ware into ceramic. At this temperature, the piece is porous and will easily accept glaze. 
• Firing to cone 04 increases the pottery’s durability and ensures the gasses are eliminated from the bisque ware.

Generally, a higher firing temperature will result in a less porous ceramic. Suppose you want a piece with higher porosity that can easily absorb glaze. In that case, you should bisque fire it at a lower temperature. It is not recommended to fire hotter than cone 04.

What Is the Difference Between Bisque and Glaze Firing?

Bisque firing

In bisque firing, the Clay pieces are loaded into the kiln bone dry. This first firing causes a permanent chemical and physical change to the clay transforming it into a ceramic material. It is now porous, allowing the glaze to absorb into the pottery. This helps to ensure that the glaze adheres to the surface.

Glaze firing

The glaze firing is the second firing of the pottery. Glazes and decorations are applied to the porous surface before placing it back in the kiln. This process involves the coating, coloring, waterproofing, and decorating of the fired clay. Glaze firing gives the ceramic a fine finish with colors and an artistic design.

Other Tips for Bisque Firing Clay

Here are more helpful tips to make your bisque firing easier and prevent any injuries:

• Remove any flammable object on the lid of the kiln or nearby. 
• Avoid firing with cracked or damaged shelves as they can fall apart and damage the piece.
• Ensure the ventilation and all other aspects of the kiln are working properly.
• Turn off the kiln if you smell burning plastic. Look for the source and fix it before proceeding with the firing.
• Allow all the fired pieces to cool down before proceeding to glaze.
• In the bisque firing process, the kiln must have abundant oxygen to burn off organic and sulfur compounds that can affect the piece during the glaze firing.

Read 21 Kiln Questions Answered for more helpful Kiln tips.

Watch 21 Beginner Kiln Question Answered

Conclusion

As you’ve seen in the article, bisque firing allows you to create better and stronger pottery. The process involves firing Bone Dry Cclay in a kiln to make it stronger, increase its porosity, remove impurities and organic matter, and reduce residual moisture. 

In essence, bisque firing prepares the piece for the application of glazes and decorations, ensuring it absorbs the right amount of glaze without breaking or coming apart. 

Bisque firing is a fun process if you master it and follow the right techniques. If you want to produce wares with higher porosity, it’s recommended to use a low firing temperature.