Clay Preparation
Clay straight from the ground is variable: lumps, stones, roots, uneven moisture. Preparation makes it consistent enough to throw, press or cast, and the exam tests the sequence plus the water chemistry of slips.
Sequence for a plastic body
- Slaking / blunging - soak or machine-mix the clay with water.
- Sieving - 60–80 mesh for plastic bodies, finer for casting slip; removes stones and unsoaked lumps.
- Magnetic separation - pulls iron particles that would cause specking or rust spots.
- Filter pressing - squeezes out excess water to a plastic consistency.
- Pugging and de-airing - a vacuum pug mill extrudes a homogeneous, air-free column.
- Ageing / souring - days to weeks of storage lets water distribute and organic action improve plasticity.
Deflocculation and slip density
Casting slip needs high solids with low viscosity: sodium silicate and soda ash (often about 2:1) give the particles a negative charge so they repel. Target specific gravity 1.75–1.85.
Worked example
100 kg of dry body (particle density 2.6 g/cm³) is to be made into slip of SG 1.75. Per 100 g of slip, solids by weight w: w/2.6 + (100 − w)/1 = 100/1.75 = 57.1 mL, so w = 69.6 %. Water per 100 kg dry clay = 30.4 ÷ 69.6 × 100 = 43.7 kg, i.e. about 30 % water by total weight.
Traps
- Over-deflocculating: the slip becomes thixotropic and casts unevenly.
- Adding water to thick slip without checking SG; this drops density and increases shrinkage cracks.
- Skipping the sieve after reclaiming scrap.
- Using hard water, which consumes deflocculant and changes the set.