What is Glass?

By Lyle Hill

Glass is one of the most versatile – and most misunderstood – materials used in the world. Glass can be used for everything from eyeglasses to bottles, windows to the ‘glassphalt’ you find on the road. Many products called “glass” are actually ceramics and have entirely different manufacturing process than the glass we discuss in this article. Glass that is put into buildings or automobiles, in windows or table tops is usually called flat-, float-, window or plate glass.

The ancient Romans made flat glass by rolling out hot glass on a smooth surface. The resulting glass was neither clear nor even, but it was good enough to use in windows of the day. In fact, glass was quite a luxury at the time and only the nobility could afford it. By 1668, the French company Saint-Gobain had perfected a “broad glass” method of manufacture that involved blowing long glass cylinders, slitting them and unrolling them to form an almost-flat rectangle. This plate glass was then ground and polished on both sides.

By the late 1800s, glass was being made by blowing a very large cylinder and allowing it to cool before it was cut with a diamond. After being reheated in a special oven, it was flattened and affixed to piece of polished glass which preserved its surface. In 1871, a gentleman named William Pilkington invented a machine that allowed larger sheets of glass to be made. It was the first of a number of marvelous glass-related inventions to come from the Pilkington family.

By the early 1900s most glass was manufactured using the sheet glass method—through which a ribbon of glass was drawn from a tank furnace between cooled rollers. It produced a less expensive, albeit imperfect window.

Manufacturing processes did not change much until 1959 when another Pilkington by the name of Sir Alistar invented the float glass process. It changed glass manufacturing forever. In the float glass process, a continuous strip of molten glass at approximately 1000 degrees centigrade is poured continuously from the furnace onto a large shallow bath of molten metal, usually tin. The glass floats and cools on the tin and spreads out to form a flat surface. The speed at which the controlling glass ribbon is drawn determines the thickness of the glass. Glass is much less expensive to produce via the flat process than any other type of process. Today more than 90 percent of the world’s flat glass is made via the float process.

Float glass is very versatile as well. Add silver backing to it and you have created a mirror, heat and then cool the glass very quickly in a special oven and you have made tempered glass. Your windshield is really a “glass sandwich” called laminate glass, made of two pieces of float glass with a plastic interlayer between them. And if you seal two pieces of glass with an airspace between them, you have created insulating glass.