Kinds of GLASS. The manufactured glass now in use may be divided into three general kinds; white transparent glass, coloured glass, and common green or bottle glass. Of the first kind there is a great variety; as the flint glass, as it is called with us, and the German crystal glass, which are applied to the same uses; the glass for plates, for mirrors, or looking glasses; the glass for windows and other lights; and the glass for phials and small vessels. And these again differ in the substances employed as fluxes in forming them, as well as in the coarseness or fineness of such as are used for their body. The flint and crystal, mirror and best window glass, not only require such purity in the fluxes, as may render it practicable to free the glass perfectly from all colour; but for the same reason likewise, either the white Lynn sand, calcined flints, or white pebbles, should be used. The others do not demand the same nicety in the choice of the materials;
though the second kind of window glass, and the best kind of phial, will not be so clear as they ought, if either too brown sand or impure salts be suffered to enter into their composition.
Of coloured glass there is a great variety of sorts, differing in their colour or other properties according to the occasions for which they are wanted. The differences in the latter kind depend on the accidental preparation and management of the artists by whom they are manufactured, as will be afterwards explained.