Bisque Firing vs Glaze Firing: What's the Difference? | ProKilnSupply

Bisque Firing vs. Glaze Firing: What Every Ceramic Artist Needs to Know

If you're new to ceramics, the two-firing process can feel unnecessarily complicated. Why can't you just glaze the raw clay and fire it once? The answer has everything to do with physics, chemistry, and the practical realities of working with molten glass (which is essentially what glaze is).

Here's a clear breakdown of both firings and why they each matter.

What Is a Bisque Firing?

Bisque (also spelled "biscuit") firing is the first firing that transforms raw, dried clay into a hardened ceramic material. Before bisque firing, dried clay is still technically just dried mud — it will dissolve back into slip if it gets wet. After bisque firing, the clay has undergone permanent chemical changes and can never return to its plastic state.

During bisque firing, several important things happen:

Mechanical water (the water trapped between clay particles during drying) drives off between room temperature and 212°F. If this happens too fast, steam can crack your pieces — which is why preheating slowly is essential.

Chemically bonded water (water that is part of the clay mineral's molecular structure) releases between 900°F and 1100°F. This is a significant change that makes the transformation permanent.

Organic materials and carbon in the clay body burn out between 572°F and 1652°F. Pieces need to be in an oxygen-rich environment during this phase — which is why electric kilns are well-suited for bisque.

Bisque firings typically go to cone 06 (1828°F) — hot enough to complete all these changes while leaving the clay slightly porous so it can absorb glaze in the next step.

Why Bisque Before Glazing?

The porosity of bisqueware is exactly what makes it ideal for glaze application. Bisqueware absorbs water from a liquid glaze suspension, pulling glaze particles onto the surface and creating an even coating that dries quickly and holds well. Raw (greenware) clay absorbs glaze too aggressively and unevenly, making application difficult to control.

Bisqueware is also much more robust than greenware — it's easier to handle, less likely to chip or crack during glazing, and won't dissolve if you need to wipe off a glaze mistake and reapply.

What Is a Glaze Firing?

Glaze firing is the second firing, and it's where the magic happens. The bisqueware, now coated in glaze, is loaded back into the kiln and fired to a higher temperature — typically cone 06 for low fire, cone 6 for mid-fire stoneware, or cone 10 for high-fire work.

During the glaze firing, the glaze materials melt and flow across the surface of the piece, creating a glassy coating that is decorative, waterproof, and (when properly formulated) food-safe. The clay body also continues to mature — becoming denser and more vitrified as temperature increases.

Can You Skip Bisque and Single-Fire?

Yes — single firing (also called raw glazing or once-firing) is a legitimate technique used by experienced potters. It requires careful formulation of both your clay body and glaze, a very controlled firing schedule, and some experience reading how raw clay responds to glaze. For beginners, the two-fire process is strongly recommended for more predictable, consistent results.