GLASS (Gr. ὑαλίνη) a generally transparent or translucent, lustrous, hard and brittle substance which has passed from a fluid condition, at high temperature, to a solid condition with sufficient rapidity to prevent the formation of visible crystals. Glass is formed in nature in three main ways. Rarely a silica glass is formed in small amounts when lightning strikes sand. The concentrated heat fuses the sand. The melt immediately solidifies forming a fragile tube of silica glass sometimes several ft. long. Such tubes are called fulgarites and are generally found in desert environments. Rarely, also, natural glass is formed as the result of localized melting of rock in a large fault zone, the glass generally being black and referred to as pseudotachylite. The most common natural occurrence of glass is as obsidian, a glassy volcanic rock of acidic composition used by early man for making small implements. Early sources of obsidian for countries of the Middle E were the Aegean region and Ethiopia. Later obsidian was obtained from Armenia and possibly from Lipari and Pentellera.

Manufactured glass consists primarily of a combination of silicic acid and alkali (sodium or potassium). Probably it originated somewhere in the eastern Mediterranean lands, possibly in Egypt, with the Egyptians making small glass-covered objects from about 4000 b.c. The first glass vessels appear to have been made c. 1500 b.c. In Egypt and in other coastlands, like Syria, soda glass was produced. This contains sodium and calcium silicates and is made by fusing white pure sand (silicon dioxide) free from iron compounds, soda-ash (sodium carbonate) and limestone (calcium carbonate) or lime (calcium oxide) in fireclay pots or tanks at 1375^oC or above. The soda-ash was obtained from seaweed ash. Bohemian or potash glass contains potassium instead of sodium and was developed in well-wooded countries, particularly Germany, the potassium being derived from charcoal, particularly beech charcoal.

The ingredients of glass are rarely in a pure state, the sand generally containing some iron impurities. The green color they cause, due to ferrous silicate, may be neutralized by adding pyrolusite (manganese dioxide) (Gr. pyr “fire,” and luo “I wash”). The resultant glass may be clear and transparent (Rev 21:18) and look like still water (Rev 15:2). However, if there are numerous minute air bubbles left in the glass, it is translucent (Rev 21:21 NEB) and has the appearance of ice (Rev 4:6 NEB).

In modern times a mirror (cf. 2 Cor 3:18; James 1:23 NEB) is a sheet of polished glass silvered at the back. However, in ancient times it was a polished sheet of metal, called a speculum (cf. 1 Cor 13:12).

Bibliography J. R. Partington. A Textbook of Inorganic Chemistry, 6th ed. (1950), 760, 761; J. McNab, “Glassware,” Collier’s Encyclopedia, 11 (1964), 143-152.