Various forming methods
Forming gives the ware its shape before firing. The method depends on the shape and how many pieces we need.
- Plastic forming (clay with about 20% water): hand building, wheel throwing, jiggering (a shaped tool presses clay in a spinning mold) and extrusion (clay is pushed through a die like noodles, used for bricks and pipes).
- Pressing (powder with a little water): powder is squeezed in a steel die. Dry pressing makes tiles and insulators fast. Isostatic pressing squeezes from all sides with liquid pressure for even density.
- Casting (a lot of water): slip casting pours runny clay (slip) into a plaster mold. The plaster soaks up water and a solid wall forms on the mold. The extra slip is poured out. Used for sinks, toilets and thin vases. Tape casting spreads slip into thin sheets for electronic parts.
- Injection moulding: powder mixed with a binder is injected into a mold, for small precise parts.
Drying
A formed piece is wet and soft. Drying removes the water before firing. If water is left, it turns to steam in the kiln and can burst the piece.
Drying has two stages. First, water at the surface leaves and the piece shrinks because the grains move closer. Then water inside the pores leaves and there is almost no more shrinkage.
Typical drying shrinkage is about 3 to 8%. Allow for it when you design a mold.
Drying defects and how to avoid them
The outside dries before the inside. Then the outside shrinks while the inside is still wet. This pulls the piece apart.
- Cracks: drying too fast or one part drying faster.
- Warping: uneven drying bends the piece.
To avoid them: dry slowly, turn the piece, keep wall thickness even, and use humid, warm air at first (humidity dryers) and then drier air.
Key formulas and definitions
- Moisture % (wet basis) = (wet mass − dry mass) ÷ wet mass × 100
- Drying shrinkage % = (original length − dried length) ÷ original length × 100
- Length before drying = length after ÷ (1 − shrinkage fraction)
- Slip casting: wall thickness grows in proportion to the square root of time
- Water removed = wet mass − final mass
Worked examples
1. A wet clay piece weighs 120 g. After drying it weighs 100 g. Find its moisture percent (wet basis).
Water = 120 − 100 = 20 g. Moisture = 20 ÷ 120 × 100 = 16.7%.
2. A tile is 20 cm long when wet and 19 cm when dry. Find the drying shrinkage.
Shrinkage = (20 − 19) ÷ 20 × 100 = 5%.
3. A tile must be 19 cm after drying and the drying shrinkage is 5%. What wet length should the mold make?
Wet length = 19 ÷ (1 − 0.05) = 19 ÷ 0.95 = 20 cm.
4. In slip casting a wall of 3 mm forms in 4 minutes. If wall thickness is proportional to the square root of time, how thick is it after 16 minutes?
Time is 4 times larger, so the wall is √4 = 2 times thicker: 2 × 3 = 6 mm.
5. 500 kg of clay body at 20% moisture (wet basis) is dried to 2% moisture. How much water is removed?
Dry mass = 0.80 × 500 = 400 kg. Final mass = 400 ÷ 0.98 = 408.2 kg. Water removed = 500 − 408.2 = 91.8 kg.
6. A factory must make 10,000 identical flat tiles a day. Which forming method fits best and why?
Dry pressing. A steel die gives the same size every time and is very fast.
Common mistakes
- Thinking the piece loses all its water by drying in the sun in one hour. Fast drying makes cracks.
- Forgetting shrinkage when making a mold. The mold must be a little bigger than the finished piece.
- Mixing up the methods. Plastic forming uses soft clay; pressing uses powder; casting uses slip.
- Thinking slip casting makes a wall from the inside. The wall grows on the mold surface, where plaster drinks water.