Plates and Water
Clay is a hydrated aluminium silicate. Its properties come from the shape of its microscopic plates.
A film of water between plates lets them slide past one another. Surface tension holds them together. This creates plasticity: the material deforms under pressure and holds its shape.
Three Kinds of Water
| Water | Location | Evaporation Stage | Result |
|---|---|---|---|
| Shrinkage water | Between the plates | Room temperature drying | The body shrinks as plates close up |
| Pore water | Spaces between particles | Around 100–200 °C | No shrinkage; steam forms if heated too fast |
| Chemically combined water | Mineral structure | Roughly 350–600 °C | Irreversible chemical change; clay cannot be reclaimed |
Clay becomes ceramic during the chemical water release. Below this temperature, dry clay slakes in water and can be reclaimed. Above it, the mineral has changed forever.
Shrinkage
Shrinkage occurs twice: during drying and during firing.
A typical stoneware body loses 4 to 8 per cent at each stage. This gives 10 to 15 per cent total Shrinkage. These rates vary by body, so always measure test bars.
Planning for Shrinkage
- Build pieces larger than the target finished dimensions.
- Uneven drying causes cracking. A fast-drying section shrinks first and pulls against damp sections. Keep drying slow under plastic.
- Joins fail between pieces at different moisture levels due to uneven contraction. Join matching pieces with scoring and slip.
States of Clay
- Plastic: workable, holds shape, high moisture.
- Leather-hard: firm, carvable, ideal for joins and construction.
- Bone-dry: no free water, highly fragile, ready for the kiln.
- Bisque: fired once, porous, ready for glaze.
Wall Thickness
Trapped water turns to steam and can explode in the kiln. Keep wall thickness even, ideally under 20 mm. Always add a vent hole to any enclosed hollow form.
Common pitfall: blaming cracking on poor handling alone. Most cracks stem from differential shrinkage caused by uneven drying. Manage moisture carefully instead.