Kiln firing is the magical process that transforms soft, fragile clay into durable, functional ceramics. At Throw Clay LA, we rely on our three Scutt electric kilns and a 40-cubic-foot Geil gas kiln to bring our creations to life. As a cone 6 studio, we embrace the versatility and beauty that mid-range firing offers. In this post, we’ll break down the two main stages of kiln firing—bisque firing and glaze firing—and explain what happens during each stage to help with your understanding of kilns.
What Is a Cone?
A "cone" is a small, pyramid-shaped piece of ceramic material formulated to bend at a specific temperature. Cones are used as visual indicators to monitor and measure the heat work (a combination of time and temperature) inside a kiln during firing. Each cone number corresponds to a precise temperature range. For example, cone 6, which we use at Throw Clay LA, represents a temperature of approximately 2232°F. The bending of the cone signals that the desired heat work has been achieved, ensuring the proper maturation of the clay body and glaze. Cones are an essential tool for achieving consistency and accuracy in ceramic firing.
The Bisque Firing: Preparing Clay for Glazing
Understanding Kilns: What Is Bisque Firing?
Bisque firing is the first firing in the ceramic process. It’s designed to harden the clay body and make it porous enough to absorb glaze. During this stage, the kiln gradually heats the clay to a lower temperature than the glaze firing, typically between Cone 06 (1830°F) and Cone 04 (1940°F).
What Happens During Bisque Firing?
Water Evaporation:
Free water and residual moisture evaporate from the clay as the temperature rises above 212°F.
Slow heating is critical at this stage to prevent steam from causing cracks or explosions.
Burning Organic Material:
Between 570°F and 1470°F, organic materials and carbonates burn out of the clay, leaving behind a cleaner body.
Proper ventilation ensures these materials exit the kiln safely.
Sintering:
The clay particles begin to bond at around 1650°F, giving the piece its initial strength.
At this point, the clay is no longer soft but remains porous, ready to absorb glaze.
How Bisque Firing Affects the Final Piece
Bisque firing ensures that your pieces can be handled and glazed without breaking. The process also prevents raw glaze from absorbing unevenly, leading to a smoother and more predictable glaze application.
The Glaze Firing: Adding Functionality and Beauty
Understanding Kilns: What Is Glaze Firing?
Glaze firing is the second firing, where the bisqued pieces are coated with glaze and fired to their final temperature. At our cone 6 studio, this means heating the kiln to approximately 2232°F.
What Happens During Glaze Firing?
Glaze Melting:
As the temperature rises, the glaze ingredients melt and form a glass-like surface.
The melting begins around 1830°F and continues until the desired cone temperature is reached.
Clay Body Maturation:
At cone 6, the clay body vitrifies (becomes glass-like) but retains slight porosity, making it ideal for functional ware.
Color Development:
Oxides and other glaze components react to the heat, creating vibrant or subtle finishes.
In our Geil gas kiln, we can achieve unique effects through reduction firing, where oxygen levels are controlled to influence glaze outcomes.
Cooling:
Controlled cooling is crucial to avoid thermal shock and to allow crystalline structures to develop in certain glazes.
A slow cool often enhances depth and complexity in glaze surfaces.
How Glaze Firing Affects the Final Piece
Glaze firing transforms your work into its final form. The surface becomes glossy, matte, or textured, depending on the glaze used, and the piece becomes waterproof and functional. The interaction between glaze and clay body during this firing gives ceramics their unique character. Knowing how drippy your glaze is can help prevent mishaps during this stage. Cleaning the foot of your piece thoroughly and using stilts when needed are essential steps to prevent glaze from sticking to kiln shelves, which can cause damage to both your piece and the kiln itself. These last points are particularly important in understanding kilns for successful results.
Successful Kiln Firings
Understand Your Materials:
Different clays and glazes respond differently to firing temperatures and atmospheres. Always test new materials and their combinations before committing to your final clay/glaze combination on important pieces.
Know We Pay Attention to Loading:
Proper spacing between pieces ensures even heating and prevents accidental sticking. At Throw Clay LA our staff is responsible for loading kilns properly.
We Monitor the Kiln:
Whether using our reliable Scutt electric kilns or the powerful Geil gas kiln, we carefully monitor and set up proper kiln programming which is key to our consistent results.
Keep a Firing Log:
Documenting your firing materials and resulting outcomes helps you refine your process over time. Take pictures and write comments about your pieces - the clay body and glaze used, what went right and what you would do differently next time. You'd be surprised how easy it is to forget what went into your favorite combination!
Conclusion
Kiln firing is both an art and a science. By understanding the stages of bisque and glaze firing, you can unlock the full potential of your ceramics. At Throw Clay LA, we’re committed to mastering this transformative process and sharing its magic with our community. Whether you’re new to ceramics or a seasoned potter, appreciating the nuances of firing will help you create pieces that are as durable as they are beautiful.
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