How Is Glass Produced?

The basic ingredients of glass have remained unchanged for thousands of years. Silica sand, lime and soda still make about 90 percent of the world's glass, but the production scale has been considerably increased to meet modern demands.

Today's glassmakers scrupulously select tons of pure raw materials, weigh them to the ounce, thoroughly mix them and then feed the mixture, or batch, to gigantic tank furnaces that can hold more than 1,000 tons of molten glass. Although added cullet, or scrap glass of the same formula to be manufactured, speeds the melting process, the furnace must be heated to about 2,800 degrees Fahrenheit! So corrosive is molten glass at this temperature that the process has been compared to melting a block of ice in a container of sugar. The heat-resistant furnace walls must usually be replaced every two to four years.

However, once the process is in operation it continues day and night, week after week, until repairs or changes in formula force a shutdown. The batch is continuously fed into one end of the furnace while workable glass, mixed and refined to uniform viscosity, free of bubbles and impurities, is drawn from the other. The red-hot liquid is then ready to be drawn, pressed or blown into service.

The basic ingredients of glass have remained unchanged for thousands of years. Silica sand, lime and soda still make about 90 percent of the world's glass, but the production scale has been considerably increased to meet modern demands. Today's glassmakers scrupulously select tons of pure raw materials, weigh them to the ounce, thoroughly mix them and then feed the mixture, or batch, to gigantic tank furnaces that can hold more than 1,000 tons of molten glass. Although added cullet, or scrap glass of the same formula to be manufactured, speeds the melting process, the furnace must be heated to about 2,800 degrees Fahrenheit! So corrosive is molten glass at this temperature that the process has been compared to melting a block of ice in a container of sugar. The heat-resistant furnace walls must usually be replaced every two to four years.
However, once the process is in operation it continues day and night, week after week, until repairs or changes in formula force a shutdown. The batch is continuously fed into one end of the furnace while workable glass, mixed and refined to uniform viscosity, free of bubbles and impurities, is drawn from the other. The red-hot liquid is then ready to be drawn, pressed or blown into service.

Glass is made by melting silica, a chemical that comes from sand. At extremely high temperatures, silica melts and becomes liquid. Other substances are usually added to the melted silica to make the glass strong. In its liquid state, glass can be molded into different shapes.

Glass was first made at least 4,000 years ago. About 2,000 years ago, people in the Middle East discovered that molten (melted) glass could be blown into different shapes. They put a gob of glass on the end of a metal tube. Then they blew air through the tube. The glass at the end of the tube expanded, just like a balloon. Glass blowing remained the usual method for making glass vessels until the early 1900s. In 1903, an automatic glass blowing machine was invented.

Glass can also be made into sheets that are flat and smooth. It can be shaped in molds. Substances can be added to glass to give it color. And once glass is cold, it can be decorated by cutting and painting.

Glass can be recycled. It is sorted by color and then melted in big furnaces. The molten glass is formed into new shapes.

Glass is basically made by melting silica (SiO₂). To make processing easier by lowering the melting point of silica, sodium carbonate is often added. Thus silica melts at 1500^oC after adding sodium carbonate. Further, oxides of Calcium, Magnesium and Aluminum are added to increase the durability of the glass and prevent its solubility in water (a feature that comes about as a result of adding sodium carbonate). This type of glass may be known as soda lime glass.
Contemporary science has also made it possible for us to produce glass without silica, for example flouroaluminate glass and chalcogenide glass. Fiber optics is a widespread use example of silica free glass.

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