Published: 9 February, 2000

Glass-making process has evolved

How is glass made?

Glass is used to make marbles, windows, milk bottles, cooking pots, mirrors, big doors for department stores, the picture tube in your television, light bulbs, car safety windows, computer screens and many other things.

A recent application is to use glass fibers to transmit phone calls and to connect computer centers. There are many different kinds of glass and they are made in different manners.

However, the main ingredient is sand. An ancient Egyptian recipe to make glass is to mix sand, lime and soda. This mixture is heated until it becomes a liquid.

The Romans discovered in the first century BC that you can form thin bulbs of glass by gathering a blob of glass on the end of a long metal tube and then blowing into the tube, just like blowing bubble gum.

This discovery was a big step towards making glass windows. You still can see glass blowers at work when you visit the Corning Museum of Glass. They use this method to make beautiful vases, bowls, and glass sculptures.

When hot glass cools, it becomes stiffer and stiffer. In that regard, liquid glass is not like the liquids you know.

For example, water does not get noticeably stiffer when you cool it, but at 32 degrees Fahrenheit, suddenly turns to ice. Glass just continuously becomes more jelled.

At room temperature, glass is so stiff that it behaves in all applications like a very hard and brittle solid.

Tour guides in Europe like to tell the tourists that the old stained glass windows (and some are almost 1,000 years old) are thicker at the bottom edge because the glass has flowed a bit over the centuries.

But scientists, who have studied this issue, believe the glass was made that way originally.