The Most Dangerous Mistake in Firing
Firing a pot that still contains moisture is one of the most destructive things you can do in a kiln. The consequences are not just a ruined pot; they are an exploded pot, with shrapnel that can destroy every other piece in the load.
What Happens to Moisture in the Kiln
When clay heats up, any water trapped inside turns to steam. Steam expands rapidly, far more than the surrounding clay can accommodate. If the water cannot escape fast enough as vapor through the porous clay body, it forces its way out violently, shattering the pot from the inside.
This is not rare. It happens regularly to potters who rush drying or who try to fire pieces that feel dry on the outside but still hold moisture deeper inside.
What "Bone Dry" Means
Bone dry clay has zero chemically free water remaining. It:
- Feels room temperature everywhere: no cool spots that indicate remaining moisture.
- Looks uniformly light and chalky in colour: no darker patches.
- Sounds hollow and crisp when tapped.
- Is brittle: it chips and crumbles easily under pressure (handle with extreme care).
Dryness Verification Checklist
Before kiln loading:
- Check temperature at rim and base
- Inspect for darker moisture zones
- Confirm no cool spots on thick sections
Do not load if any indicator fails.
The transition from leather hard to bone dry can take anywhere from a few days to several weeks depending on clay body, wall thickness, humidity, and room temperature.
Down the Rabbit Hole
Understanding what happens to water inside clay is closely tied to the science of sintering, the process by which clay particles fuse during firing. The Wikipedia article on vitrification explains how continued heating beyond the bone-dry stage eventually transforms porous clay into a dense, waterproof ceramic.
