What Happens to Clay in the Kiln?
Firing clay is where the magic happens — and the science.
As heat builds, your lump of earthy material becomes something permanent, functional, and often beautiful.
But what’s actually going on inside that kiln?
Here’s a step-by-step look at how clay transforms as the temperature rises.
Stage 1: Drying (Below 100°C)
Even if your clay feels dry, it still contains physical water — water that must evaporate slowly.
🔥 Why it matters:
If the kiln heats too fast at this stage, the water turns to steam too quickly, and your pot can crack or explode.
Stage 2: Burn-off (Around 350–600°C)
Now the clay releases chemical water and begins burning off organic materials like paper, grog additives, or leftover plant matter.
🔥 Why it matters:
This is the smelly part of a bisque firing. Proper venting is essential!
Stage 3: Quartz Inversion (573°C)
Quartz particles inside the clay change structure here — a known expansion point.
🔥 Why it matters:
Heat too fast through this stage, and you'll risk cracks, warping, or breakage.
Stage 4: Sintering Begins (700–900°C)
The clay particles start bonding slightly, forming a solid matrix.
It’s not yet waterproof — but it holds together.
🔥 Why it matters:
Your pot is now no longer just dried mud — it’s officially ceramic.
Stage 5: Vitrification (1000–1300°C, depending on clay)
This is where the real transformation happens.
The clay melts slightly, filling in gaps and becoming dense and strong.
This is when it becomes non-porous (if fired high enough).
🔥 Why it matters:
Each clay has its vitrification point. Firing too low = weak and absorbent.
Too high = warped, bloated, or melted.
So... What Are You Left With?
A porous but strong bisque if stopped early (Cone 06–04)
A vitrified, water-tight piece if glaze-fired at Cone 6 or 10
A completely changed material: from mud to stoneware, forever
Firing isn’t just heating up — it’s a whole chemical journey.
Understanding these stages helps you avoid cracks, control glaze fit, and appreciate just how incredible ceramics really is.
