The Coloration of Glass.

Manufacturer and builder ?, 1870

The French chemist, Pelouze, states that glass, even when produced with the best crude materials, (pure silica and sulphate or carbonate of soda,) shows always either a yellowish green or sea-green shade, which, according to this savant, originates from traces of iron that are always present in those materials. Window-glass, which contains more iron, is of a more greenish tinge than mirror-glass, but each of these kinds turns more or less yellow in the sunlight, and indeed the more so the less greenish they were originally. An exposure to the sun on a bright day is sufficient to produce this change in color, the thickest pieces becoming yellow throughout within a few weeks. A scratch upon window-glass appears nearly as yellow as salphur after being exposed to the sun for but a short while. It is true that, owing to the little depth to which it penetrates, this color can sometimes scarcely be recognized; but it will always be observed by thoroughly examining the specimen. If glass that has assumed a yellow tinge in the light is reheated to a dark-red heat, it assumes again its original greenish shade; but an exposure to the sun turns it yellow for a second time; though, if reheated, it will again partake of its original color. Indeed, this experiment may be repeated for an indefinite number of times, provided the glass be heated over 700° Fahr. In diffused daylight, for instance, in a room, no change of color appears to take place, and, if it does, it will only be noticed after many years. Pelouze had in his possession glass plates that had been preserved for upward of twenty years, without showing the least change in color. Glass that has been turned yellow by the rays of the sun contains protoxide of iron and sulphate of sod. By the action of the light, oxide of iron and sulphuret of sodium are formed; heating reproduces the original combinations. It is shown by chemical tests that glass that has turned yellow by the action of the sun contains traces of a sulphuret, while before the exposure not the least trace can over be ascertained. Carbon, silica, boron, phosphorus, and hydrogen turn glass yellow by reducing the sulphate to a sulphuret. Glass that is free from sulphate of soda is not changed. As regards the philosophy of these phenomena, it may be condensed in the following In the glass that has turned yellow by reduction of the sulphate at a high temperature, the iron exists in the form of a protoxide, which is without action upon sulphurets. On the other hand, in the glass turned yellow by the sun, the iron is present as an oxide, capable of being converted into a sulphate.

Besides these, other not less remarkable changes in the color of glass are known. Faraday, for instance, states that certain English kinds of window-glass, when exposed to the sun, assume a purplish tinge. This is not unknown to glass-makers; it appears in glass that contains oxide of iron and manganese together. This shade also disappears ander the action of heat, which fact may be accounted for by the circumstance that the oxide of manganese yields a part of its oxygen to the iron, which then forms with it a colorless oxide of iron.