The different varieties of clay possess the following common characters:—
1. They are readily diffusible through water, and are capable of forming with it a plastic ductile mass, which may be kneaded by hand into any shape. This plasticity exists, however, in very different degrees in the different clays.
2. They concrete into a hard mass upon being dried, and assume, upon exposure to the heat of ignition, a degree of hardness sometimes so great as to give sparks by collision with hardened steel. In this state they are no longer plastic with water, even when pulverised. Tolerably pure clays, though infusible in the furnace, become readily so by the admixture of lime, iron, manganese, &c.
3. All clays, even when previously freed from moisture, shrink in the fire in virtue of the reciprocal affinity of their particles; they are very absorbent of water in their dry state, and adhere strongly to the tongue.
4. Ochrey, impure clays emit a disagreeable earthy smell when breathed upon.
Brongniart distributes the clays into:—
1. Fire-clays, (argiles apyres, Fr.; feuerfeste, Germ.)
2. Fusible, (schmelzbare, Germ.)
3. Effervescing (brausende, Germ.), from the presence of chalk.
4. Ochrey (ocreuses, Fr.; ockrige, Germ.)