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Later, as the art advanced, the value of certain materials in regulating the fusible and refractory qualities of the natural clay were recognised. There is in clays the play of melting and non-melting substances. Flint and alumina are hard, and uphold the clay in the fierce fire of the kiln; potash and soda make it soft and fusible. It is believed that the value of ground flint in pottery was not discovered until late in the seventeenth century. The story is that a certain potter travelling on horseback was delayed on account of an inflammation of his horse’s eyes. An ostler, wishing to help him out of his difficulty, took a flint from the road, and, after putting it in the fire until it was red-hot, threw it into cold water. It broke into pieces, pure white, which were easily powdered and applied to the animal’s eyes. Delighted as the potter was with the cure of his horse, the discovery of a white, infusible material for his pottery pleased him still more.

The combination of iron and lime in a clay forms a natural flux. For example: Fire-clay, which is quite refractory but which contains a small per cent. of iron, will, by the addition of a proportion of whiting (lime), become more fusible. Iron and lime in larger quantities are found in the clay which is made into red ware—flower-pots, red bricks, and other common articles. The large per cent. of iron gives it its red colour. It does not make as strong a ware as other clays. This red ware, which fires at the lowest heat, we will classify as Group I. To Group II. belongs the yellow or Rockingham ware. This fires at a light-orange heat, from 1,800 to 2,000 degrees Fahrenheit. It is often finished with a dark-brown glaze. Mixing-bowls and other kitchen ware come under this group. Group III. contains the buff or cream ware, usually made of fire-clays. This hardens at an intense heat, from 2,200 to 2,500 degrees Fahrenheit. In this group is the fine pottery. Art pottery comes under this head. Group IV. is the buff or cream ware, fired at white heat and beyond. This is the stoneware. On account of the great heat at which it is fired, it is possible to use only certain colours in the glazes. They are limited to gray, brown, green and blue. Ox-blood has been obtained in this ware, but it is uncertain. Claret-and vinegar-jugs and butter-crocks come in this group. Group V. is the white or tableware, and Group VI. the translucent ware or porcelain, made of kaolin, the purest grade of clay. Felspar makes it meltable and translucent, and, to give it plasticity, a small proportion of ball or fire-clay is added.