When it comes to pottery making, one of the most important steps is clay firing; this process changes the state of the clay and turns it into ceramic. In most cases, the firing process is in two stages: bisque firing and glaze firing. The first firing is often seen as very important, but what is it, and why does it matter?
The purpose of bisque firing is to transform greenware (unfired bone dry clay) from its fragile state to a more porous and durable state 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.
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 increases 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 higher 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:
- Increase the porosity of the ceramic to prepare it for glazing
- Ensure that the stains, glaze, and underglaze bonds 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.
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 air bubbles in the greenware that should be removed during bisque firing can cause the pottery to crack due to faster firing.
The Glazes and Underglazes may not adhere to its 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 or even explode 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 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 your speed and temperature you fire to. Then even longer to cool down.
You can also watch this Video on How to Load a Kiln.
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:
- The first stage is essentially moisture removal from the clay.
- In the second stage, organic matter and impurities are burnt away.
- In the third stage, chemically bound water is removed from the piece.
- Finally, the sulfur compounds and carbonates in the clay burn off and decompose, respectively. The pieces shrink and sinter during the firing.
What Temperature Is Pottery Fired?
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 maturation temperature.
- Firing at cone 06 causes the clay to shrink and sinter. Also, it turns the ware into ceramic, which is fragile by nature. At this temperature, the piece is porous and will easily accept glaze.
- Firing to cone 04 increases the pottery’s durability and makes it less fragile, 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 and 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?
Is the first firing of clay that transforms it into a ceramic. The process helps in ensuring that the glaze adheres to the surface of the piece. This process aims to completely dry the clay, stiffen it up, and give it the ceramic feel.
Is the second firing process, and 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. It also makes it less fragile and impervious to water and other liquids.
Also, in bisque firing, the ware is loaded into the kiln bone dry. In glaze firing, glazes and decorations are applied to the piece before placing in the kiln.
Other Tips for Firing Bisque
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.
Also read 21 Kiln Questions Answered for more helpful Kiln tips.
As you’ve seen in the article, bisque firing allows you to create better and stronger pottery. The process involves firing raw clay 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.