There’s a window above my desk through I’m watching a cold wind blowing leaves down the street. It’s not blowing in my face, however, thanks to a very special material: glass.
Glass is an “amorphous solid”– its molecules don’t form a strict pattern, like the molecules of steel or granite, but are jumbled together like more like the molecules in a fluid. You might think of glass as a very stiff liquid.
Glass consists primarily of sand, heated with other materials such as soda and limestone to about 1,300 degrees Celsius. The additional materials lower the melting point of pure sand to a temperature achievable with a wood fire.
Since the principle requirements for making glass are sand and fire, it’s not surprising the ancient Egyptians, who had plenty of both, started using glass (as a decorative glaze) around 3,000 B.C. Over the next few centuries they also began using it for plates and vessels, but what really shifted glassmaking into high gear was the discovery of glassblowing, probably about 50 B.C. in Phoenicia, which made possible many new glass objects (bottles, for instance). Glassware became common and relatively cheap throughout the Roman Empire.
In medieval times the Venetians created new compositions, colours and techniques. In the late 1600s the English invented “crystal” specifically to compete with a kind of Venetian glass called “cristallo.” Very pure raw materials and added lead oxide gave crystal a unique sparkle: lead oxide makes crystal refract light more sharply than ordinary glass, resulting in rampant rainbows.
Yet clear, distortion-free windows remained a challenge. Window glass was first made by blowing a gob of glass into a globe, flattening one side, then attaching a solid iron rod to the flat part, reheating the globe and rotating it until it formed a flat disk about one meter in diameter from which panes of glass were cut.
Better methods followed, such as squeezing glass out between two rollers, but distortion and discoloration remained a problem until the development in the 1950s of the float method, in which a continuous strip of glass floats onto a molten metal, usually tin. The flat surface of the metal gives the glass a smooth, undistorted surface.
Glass containers such as bottles and jars are made blowing hot glass into molds. Light bulbs are made much the same way. A modern light-bulb glass-blowing machine can produce 2,000 bulbs an hour, which would have astonished the Phoenicians (though not nearly as much as the fact we’d want to make so many ridiculously fragile and tiny wine bottles).
Most commercial glass (90 percent) is soda-lime glass, made from ordinary sand and (you guessed it!) soda and lime. Soda-lime glass can be worked at a relatively low temperature (700 degrees Celsius) and is inexpensive.
A distant second in commercial importance is borosilicant glass, such as Pyrex. It can still be worked at a relatively low temperature (about 820 degrees) but it doesn’t crack as easily when subjected to temperature extremes (because it doesn’t expand or contract as violently when heated or cooled), which makes it ideal for use in the kitchen and laboratory and in automobile headlights.
Glass is brittle because it has many microscopic flaws in its surface. Putting stress on the glass causes these flaws to grow at the tip until the glass shatters (you see this in slow motion when a crack spreads across your windshield from a tiny nick). “Tempered” glass is more resistant to breaking: its surface is cooled first, becoming rigid, so that as the interior then cools and shrinks, the whole piece of glass is compressed.
Glass can be colored by adding particular compounds such as copper and cobalt for blue and manganese for purple. The common green of most wine bottles results from the oxidized iron (rust), and the brown typical of beer bottles comes from rust and sulfur. Red glass can be created by adding tiny particles of gold, or by adding copper or selenium.
Glass continues to be used at the cutting edge (so to speak) of technology. Very pure glass can be stretched into long fibers that absorb very little light, even over kilometres. Information can be transmitted along these fibers as pulses of light, faster and more reliable than electrical pulses along metal wires.
But as I look out my window at dead leaves being chased by a howling northwest wind, it seems perfectly clear to me: the best use of glass is keeping Saskatchewan’s chilly outdoors from coming indoors.
