Drying
Drying is the cheapest and most neglected stage of production; most cracks, warps and lost kiln loads begin here. The exam tests the stages of water loss, shrinkage arithmetic and the controls that prevent cracking.
Water in a clay body
- Pore water - between particles, leaves first, causes most of the shrinkage.
- Adsorbed and interlayer water - needs more energy, leaves between about 100 and 200 °C in the kiln.
- Stages: plastic → leather hard → bone dry. Leather hard holds its shape but can still be trimmed and joined; bone dry means no free water remains.
Rules for even drying
- Dry slowly at first - cover with plastic sheeting with a small vent.
- Keep thickness even; thick bases and thin rims dry at different rates and shear.
- Avoid draughts and direct sun on one side of the ware.
- A drying room at 30–40 °C with moving air is faster and safer than hot air on wet ware.
- Dry fully before firing: residual water turns to steam and spalls the pot.
Worked example
A plate is thrown 30.0 cm in diameter and the body has 6 % drying shrinkage. Dry diameter = 30.0 × (1 − 0.06) = 28.2 cm. If the design calls for a 30 cm finished plate, throw at 30 ÷ (0.94 × 0.92) ≈ 34.7 cm using 6 % drying plus 8 % firing shrinkage as combined factors.
Traps
- Judging dryness by touch only; use weight loss or a warm, dry base.
- Firing a load with one still-damp thick piece.
- Drying ware on a cold concrete floor - the base stays wet and cracks.
- Assuming shrinkage is identical in every direction regardless of body homogeneity.