Understanding Kiln Firing: From Bisque to Glaze Firing Explained
Updated: Sep 16
Kiln firing transforms clay through a series of permanent physical and chemical changes. Most functional pottery is fired at least twice: first in a bisque firing, which turns dry clay into a porous ceramic form that can be handled and glazed, and then in a glaze firing, where the clay and glaze mature at a higher temperature.
At Throw Clay LA, we fire pottery in three Skutt electric kilns and a 40-cubic-foot Geil gas kiln. Most student work is fired electrically to Cone 6, while the gas kiln gives members and more experienced students opportunities to explore different firing atmospheres and surfaces.
Understanding what happens during bisque and glaze firing helps explain why clay changes so dramatically between the worktable and the finished ceramic piece.
What Is a Cone?
A cone, more formally called a pyrometric cone, is a small piece of specially formulated ceramic material used to measure heatwork inside a kiln. As the firing progresses, the cone softens and bends after absorbing a specific combination of temperature and time.
Potters use cones to confirm what the pottery actually experienced during the firing, rather than relying on temperature alone.
For a deeper explanation of cones, firing ranges and why Throw Clay LA primarily fires to Cone 6, read What Does Cone 6 Mean?
The Bisque Firing: Preparing Clay for Glazing
Bisque firing is the first firing in the ceramic process. It transforms completely dry clay into a permanent ceramic material while leaving it porous enough to accept glaze. Bisque firing takes place at a lower heatwork than the final glaze firing.
What Happens During Bisque Firing?
Water Evaporation:
Any remaining physical water is driven from the clay during the early stages of firing. Slow heating during this period helps moisture escape safely and reduces the risk of damage from steam.
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
After bisque firing, the pottery is much stronger but remains porous. When liquid glaze is applied, the bisque absorbs water from the glaze suspension, leaving a layer of glaze materials on the surface. That combination of strength and porosity makes bisque ware easier to handle, decorate and glaze before the final firing.
The Glaze Firing: Maturing Clay and Glaze
During the glaze firing, bisque-fired pottery returns to the kiln after glaze has been applied. As heatwork increases, the clay body continues to mature while the glaze materials melt, interact and form a bonded ceramic surface.
At Throw Clay LA, most student work is glaze fired to Cone 6.
Cleaning the foot after dip glazing. Brush glazing a textured mug. Test tiles - always test your clay/glaze combinations for the best results.
What Happens During Glaze Firing?
Glaze Melting
As the kiln heats, glaze materials begin to react, melt and flow, eventually forming a bonded ceramic surface. Different glaze materials begin reacting at different points during the firing, so there isn't a single temperature at which a glaze simply “melts.”
Clay Body Maturation
During the glaze firing, the clay body continues to sinter and vitrify. At Cone 6, a properly matched clay body reaches its intended maturity, becoming stronger and less porous.
Color and Surface Development
Oxides and other glaze materials react during the firing, producing changes in color, texture and surface. Firing atmosphere also matters. In our Geil gas kiln, reduction firing allows us to control the amount of oxygen available during parts of the firing, producing effects that can differ significantly from an electric oxidation firing.
Cooling
Cooling is also part of the firing cycle. As the kiln cools, molten glaze solidifies and some glazes develop crystals or other surface characteristics. Cooling too quickly can also place thermal stress on ceramic work.
How Glaze Firing Affects the Final Piece
Glaze firing determines much of the final character of a ceramic piece. Depending on the clay body, glaze, firing atmosphere and firing schedule, the finished surface may be glossy, matte, textured, translucent or highly varied in color.
Glaze application before firing also matters. Excess glaze near the foot of a piece can melt onto the kiln shelf, damaging both the pottery and the shelf. At Throw Clay LA, glaze must be kept clear of surfaces that will come into contact with the kiln shelf.
Successful Kiln Firings
Once a piece reaches the kiln room, our studio technicians take responsibility for loading and firing it. But successful firing begins well before the kiln is loaded.
Potters can reduce problems by using clay bodies and glazes compatible with the studio’s firing range, allowing work to dry completely, applying glaze carefully, and keeping glaze clear of surfaces that will come into contact with the kiln shelf.
As potters begin experimenting with different clay bodies and glaze combinations, testing and recordkeeping become increasingly useful. The same glaze can behave differently on different clay bodies or under different firing conditions, so keeping track of materials and results makes it easier to understand what worked so you can repeat it.
40 Cubic Foot Geil Gas Kiln on the left. Two of our 3 Scutt electric kilns ready to unload. Our new glaze station at Throw Clay LA.
About Throw Clay LA
Whether you’re curious to try pottery for the first time or ready to develop your skills, Throw Clay LA offers one-time pottery experiences, six-week Build Skills courses, and six-week Refine & Explore courses in wheel throwing and handbuilding at our studio in Downtown Los Angeles.
We also offer pottery workshops and private pottery events for groups, while studio memberships provide experienced potters with dedicated space and time to pursue their own work, along with access to studio equipment, glazes, firing services and free weekly Member Clinics.
