II. Secondary clays.
(d) Refractory[16] secondary clays—as fireclays and some pipe clays.
(e) Pale-burning non-refractory clays—as pottery clays, ball clays and some shales.
(f) Vitrifiable clays—as stoneware clays, paving brick clays.
(g) Red-burning and non-refractory clays—as brick and terra-cotta clays and shales.
(h) Calcareous clays or marls, including all clays containing more than 5 per cent. of calcium carbonate.
III. Residual clays.
(i) Clays which have been formed by one of the foregoing actions and have been deposited along with calcareous or other matter but, on the latter being removed by subsequent solution, the clay has remained behind—as the white clays of the Derbyshire hills.
[16] A refractory clay is one which does not soften sufficiently to commence losing its shape at any temperature below that needed to bend Seger Cone 26 (approximately 1600°C.) (see [p. 116]).