The varieties of clay used in the manufacture of terra-cotta are the blue, buff, and red clays of Cornwall, Devon, and Dorset, red London clay, and many others. Some varieties of Leeds clays are also employed. These are plastic clays, containing a moderate but variable quantity of oxide of iron - from 1 1/2 to more than 11 per cent. The clay is treated in several ways. In some districts it is ground in the dry condition, and then mixed in pug mills; in others it is ground wet to a " slip," which is dried to the proper consistency for working on the "slip" kiln. It is usual, especially for large objects, to mix the clay with a moderate proportion of ground-baked clay, old pots, ground flint, sand, or Cornish stone, in order to prevent excessive shrinking and warping, and it is essential to allow the tempered clay to stand for some time before working. The ordinary terra-cotta bricks, facing blocks, ornamental tiles, etc., are machine-pressed, but fine objects are pressed in plaster moulds, and the larger objects are often built up and modelled by hand.

Blue bricks are usually made by incorporating " mill cinder" or "iron scales " with the clay, the bricks being burnt at a very high temperature.