A unique substance with many purposes, glass seems hard to the touch, but it is not actually a solid. Its chemical makeup is similar to that of a liquid. Glass usually allows light to pass through it. It is easily breakable, but it can be treated to become very strong. It resists most chemicals and can be formed into all kinds of shapes. Humans have used glass for thousands of years in their homes, at work, and as a form of art.

Properties and uses

Although glass looks like a solid, it is built like a liquid. The tiny particles that make up true solids are arranged in a specific order. The particles in glass, however, are arranged randomly, or in no order, as in a liquid.

Glass is very durable, meaning that it does not wear away easily. It is fragile but can be made very strong through a special heating and cooling process. Liquids and air cannot pass through glass. Light, however, can shine through most types of glass.

When glass is in a melted state, it can be molded into almost any shape or size. Glass fibers can be as small as a millionth of an inch in diameter. Some mirrors for telescopes are more than 16 feet (5 meters) across.

People use glass in countless ways. Glass windows allow light to pass through and also keep out cold and rain. Electric light bulbs and television picture tubes are made of glass. Glass bottles and jars allow people to see what is inside. Glass containers can store dangerous chemicals. Glass mirrors reflect images. Lenses made of high-quality glass are used in microscopes and telescopes. Very strong glass is used for windows in spacecraft and undersea vehicles.

How glass is made

Glass is not expensive to make. The materials that make up glass are widely available and easy to obtain. Modern machines can quickly and easily create huge numbers of glass items, such as bottles.

The main ingredient for glass is pure silica, or sand. It takes very high temperatures to make glass from sand. Long ago people discovered that by adding other chemicals to the silica, the process needed much less heat. Certain chemicals can also make the glass stronger or color it red, blue, or green.

The silica and other chemicals are called the batch. To begin, glassmakers add to the batch some glass that has already been made. That scrap glass helps the silica to melt. Glassmakers then heat the batch until it melts. To melt on its own, silica must be heated to 3,600°  F (1,982° C).

Once the batch is melted, glassmakers remove any bubbles or streaks. Then the molten (melted) glass can be molded into shapes. It can be rolled into long, flat sheets for windows. It can be poured into molds. It can also be blown into the shapes of bottles and jars.

To blow glass, glassmakers use a long, hollow tube with a mold of the desired shape at one end. They put molten glass in the mold. They then blow into the tube. The air going into the molten glass pushes it outward against the sides of the mold. When the glass cools, it becomes hard and holds the shape of the mold. Without using a mold, the glassblower can also shape the bubble of molten glass by hand. Now there are machines that do glassblowing. Human glassblowers today are usually artists.

History

The oldest known glass is more than 4,000 years old. Ancient Egyptians made glass beads in about 2500 BC. The Egyptians also made glass jars by pouring molten glass over a center of sand. When the glass cooled, they took out the sand and the glass was hollow.

The practice of glassblowing probably began with glassmakers in Syria more than 2,000 years ago. With glassblowing, it became easy to make glass containers. The ancient Romans made glass for windows, but they were not able to make clear glass. After the breakup of the Roman Empire, glassmaking faded in importance in Europe. It continued in the Middle East, however.

After the 12th century glassmaking regained importance in Europe. One of the most important uses was stained glass for church windows. In stained-glass windows, strips of lead hold sections of colored glass together. At the end of the 13th century in Venice (in what is now Italy), glassmakers began to make thin, clear glass called cristallo. Venetian glass became famous for its lightness and beauty. By the 17th century Bohemia (in what is now the Czech Republic) had developed into another important center for glassmaking.

People in the American colonies made glass starting in 1608. In 1825 a U.S. company in Pittsburgh, Pennsylvania, invented a glass-pressing machine. The machine helped to make glass in huge amounts. Pittsburgh became the glassmaking center of the United States.

In the 20th century there were many advances in glass technology. Glass became easier to make, less expensive, and stronger. By this time glass windows and containers were everyday features of most homes. People also discovered new uses for glass. By the 1990s tiny pipes made of glass, called fiber optics, could carry light signals over long distances. Fiber-optic cables are used for long-distance telephone service and computer networks.