There are a number of substances that can be used instead of glass. In some cases a substitute is better than glass but generally they are used because they are cheaper.

Mica

This mineral, which is also but wrongly called isinglass, is found in Farther India; it is a silicate and can be split into thin sheets; in color it ranges from colorless to a jet black and from the transparent to the translucent. It is useful in many ways because it is fireproof.

Gelatine

Gelatine is obtained from the skins, hoofs and horns of animals. Isinglass is a nearly pure gelatine and is a white, tough, partly transparent substance which is obtained chiefly from the air-bladders of fish.

To make sheets of either gelatine, or isinglass, dissolve some of the finest glue, or isinglass - the latter is the best - in enough hot water so that it will form a flexible solid sheet when it is cold.

While it is still hot strain it through a piece of cheese cloth; this done, grease a clean sheet of glass and build up the edges with some putty; warm the glass and pour on the gelatine to a thickness of 1/8 inch. Now lay another greased and hot sheet of glass on top of the gelatine and let it stay there until it is cold.

The sheets of gelatine can be given any color by adding a little aniline to the gelatine while it is hot.