Friday, 18 January 2013

Chemistry of Glass

Obsidian, a black volcanic glass, is probably the best known of the naturally occurring glasses. It was used by early man to form cutting tools, arrowheads and spearheads and is now used by modern man to make the sharpest surgical blades.

Synthetic glass was originally prepared by heating a mixture of sodium oxide (or sodium carbonate), calcium oxide and silicon dioxide (sand). If calcium oxide was not added to the melt, soda glass was obtained. Pure soda glass is not usable because of its high solubility in water. Soda lime glass has a large coefficient of expansion when heated and a low resistance to the effects of acids and bases. It usually has a green color due to the presence of iron oxide in the sand. It was later discovered that this color could be removed by adding manganese oxide to the melt when a colorless glass was desired. 

Manufactured glass is presumed to have been first used as a glaze for pottery. The earliest known glaze is on stone beads of the Badarian age of Egypt. These beads ranked in value with precious metals and stones at the time! The Egyptians first made vessels out of glass by the laborious process in which the glass was applied over a wooden or metal rod bit by bit. A cylinder of light blue glass made by this method dates back to the Akkad dynasty in 2600 B.C. Glass was first pressed into open molds in 1200 B.C. There is some evidence that Mesopotamia was the location where glass was first manufactured.

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Credit: Prof. Richard Banks,

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