Preparation of Clay

The clay selected for terra-cotta should contain about 5–8% of iron oxide and about 1% of lime. Sometimes clay with high alumina content is used. To these selected clays, sand, ground glass, old terra-cotta or pottery are added. The addition of these materials provides strength and rigidity to the terra-cotta and also prevents shrinking while drying.

Such a mixed clay should not have grit, pebbles, organic matter, etc. It is mixed with water, shovelled and kept for several days under damp conditions. It is pugged in a pug mill before being taken to the moulding section. At this stage, the required quantity of colouring substance is added to get the desired shade of colour after burning.