Cryolite Opaline Glass

CRYOLITE OPALINE GLASS.

CRYOLITE was discovered toward the end of the last century in a bay in Arksut Fiord, West Greenland, where it constitutes a large bed or vein in the gneiss of about three hundred feet in length and eighty feet in thickness. The name is derived from two Greek words meaning "ice" and "stone," and is applied because of the fusibility of the mineral in the flame of a candle. It was supposed to be sulphate of barytes until examined by Abildgaard, who found it to contain fluoric acid. Subsequently Klaproth detected soda in its composition. It was not, however, until 1850, when Jules Thomson discovered that the mineral could be easily decomposed either by the dry or wet way with lime and the calcareous salts, that it came into industrial use. In appearance, cryolite is snow white, partially transparent, of vitreous lustre, and brittle texture. Its hardness is 2.5, specific gravity 3; and it cleaves in three directions, two of which are rectangular.

From cryolite, aluminum, alum, caustic soda, and glass of a peculiar quality are obtained. About six thousand tuns of the mineral are yearly brought to this country for soda manufacture. The glass is produced in Philadelphia under the name of "hot cast porcelain," and, when made of pure cryolite, is milky white in hue, and slightly transparent. Impure cryolite yields an opaque glass, closely resembling marble. The mixture for milky glass consists of—

Oxide of zinc ….. 1 part

Cryolite ... ... ... .. 4 parts

Sand ... ... ... … 10 “

This is melted in pipe-clay pots, which are not attacked by the fluo-silicic acid disengaged. The glass is very hard, remarkably solid, and is not attacked by strong acids, even when pulverized. These properties are doubtless due to the presence of undecomposed cryolite. With a small quantity of the mineral, the glass is brilliant, and refracts light strongly; with a greater quantity it becomes opalescent; and, finally, on more cryolite being added, the glass turns opaque, and closely resembles porcelain. — Scientific American.