Coloration of Glass by Sunlight.

Scientific American 17, 27.4.1867

Mr. Thomas Glaffield, at a recent meeting of the Massachusetts Institute of Technology, made a communication on the action of sunlight in changing the color of window glass, giving the results of experiments now extending over more than three years. Isolated cases of such change had long been known, but they were attributed to some defect in the manufacture. His recent experiments confirm his first results, viz., that almost all kinds of plate, crown, and sheet glass undergo a change of color from the influence of the sunlight, the white glass becoming first yellowish, then brownish, and then pink. That the color permeates the body of the glass, and is not confined to the surface, nor produced by reflection therefrom, has been proved by grinding off about one sixteenth of an inch from both surfaces, and the four edges of a duplicate exposed specimen, which, after repolishing, exhibited the same color. Really colored glasses, so manufactured, as far as his experiments go, do not change except in the purplish tints becoming darker.

The cause of the change has been referred to the presence of oxide of manganese, the oxide of iron, sulphur, etc. He thinks it is due to the presence of oxide of manganese, which is used to give glass a white color. If the materials were pure this substance would be unnecessary, and the change would not occur; the glass made from the very pure Berkshire sand very seldom changes. No change is observed in lead glass, unless manganese is present. This change most not be confounded with what is celled "rust" in glass, which is a mere mechanical disintergration of the surface, from the washing out by the rain of the soda contained in the glass. He exhibited an extensive series of specimens, showing the effect of exposure, to be seen for periods varying from one month to more than three years. Fuller details on this interesting subject will be found in the next number of the American Journal of Science and Arts.

- K.

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