Ceramic Raw Materials and Clay Bodies
Raw materials decide everything downstream: plasticity, shrinkage, vitrification and glaze fit. The exam tests whether you can name a material's function and predict what happens when its percentage in a body changes.
Three functional groups
- Plastic clay - kaolin (primary, refractory, low shrinkage, low plasticity) and ball clay (secondary, organic, very plastic, high shrinkage). Ball clay is added for workability, kaolin for whiteness and temperature resistance.
- Flux - feldspar, nepheline syenite, whiting, talc, frit. Lowers the temperature at which the body vitrifies; too much causes warping and bloating.
- Filler/opener - quartz (silica) and grog. Reduces shrinkage and drying cracks, improves thermal shock resistance; excess quartz promotes dunting through the 573 °C quartz inversion.
- Additives - bentonite 1–3 % for plasticity, barium carbonate to prevent lime popping, sodium silicate as a deflocculant.
Worked example: calculating shrinkage
A test bar is 12.0 cm long plastic, 11.4 cm dry, 11.0 cm fired. Drying shrinkage = (12.0 − 11.4) ÷ 12.0 × 100 = 5.0 %. Firing shrinkage = (11.4 − 11.0) ÷ 11.4 × 100 = 3.5 %. Total shrinkage = (12.0 − 11.0) ÷ 12.0 × 100 = 8.3 %. Note the denominators change: firing shrinkage is measured against the dry length, so you cannot just add the two percentages. Use total shrinkage to size moulds and to predict that a 20 cm mould will give a fired piece of about 18.3 cm.
Traps
- Calling ball clay a kaolin - they behave quite differently in the kiln.
- Assuming grog makes a body stronger; its main job is reducing shrinkage.
- Forgetting that iron-bearing clays fire darker in reduction.
- Quoting a shrinkage figure without stating whether it is plastic, dry or fired basis.