Humans have used glass for thousands of years, for weapons and decorative objects. Its uses have expanded over the centuries, but the process for making glass has remained largely the same. The main component of glass is sand — specifically, a type of quartz sand known as silica sand. When heated to nearly 2 000 degrees Celsius, silica sand turns into glass. Silica sand is often mixed with other ingredients, which changes its properties.
Sodium carbonate, for instance, lowers the melting point of sand. But glass made out of just silica sand and sodium carbonate is not very durable and dissolves in water. This is not ideal, if you don’t want your windows to dissolve as soon as it rains! So glassmakers often add calcium carbonate — limestone — to the mixture, too. Glass made out of these three ingredients: sodium carbonate, limestone, and silica sand, is known as soda-lime-silica glass.
It is hard, durable, and transparent, and is the most commonly used type of glass. Because of its chemical composition and the way it’s manufactured, glass is sometimes classified as a ceramic material. Glass shares many properties with ceramics: it is solid, and hard, although brittle. It is also corrosion-resistant and heat-resistant. But one major difference between glass and other ceramics is the way its atoms and molecules are arranged.
Most ceramics have atoms arranged in a regular pattern — a crystalline structure. The atoms in glass, on the other hand, are not organised in any regular pattern. We say glass is non-crystalline, or amorphous. The amorphous structure causes glass to harden gradually as it cools down. This allows molten glass to remain viscous and malleable for longer than crystalline materials; which means, glass can be formed into many different shapes.
One common technique for shaping glass that takes advantage of this property is glassblowing. In glass blowing, molten glass is inflated through a special tube — a blowpipe. This way, glass stretches into a hollow bubble that can be shaped with the help of tools and moulds, to form objects such as bottles, or lightbulbs. The properties of glass can be changed by adjusting the ingredients used to make it and the proportions in which they are mixed. This makes glass a very versatile material that has many applications in everyday life.
For instance, glass can be made transparent for windows or screens. Or, by mixing in ingredients such as metal oxides, glassmakers can obtain glass in different colours. Coloured glass can be used to make anything from intricate stained-glass windows, to dark-coloured bottles for drinks or medicine. The properties of glass can also be manipulated by the manufacturing process. For example, heating glass to a very specific temperature, then suddenly chilling it, increases its strength and its resistance to temperature changes.
This process is known as tempering. Tempered glass doesn’t break as easily as regular glass. And if it does break, it shatters into small, blunt, relatively harmless pieces. This makes it useful for tabletops and oven doors. To sum up, glass is one of the oldest materials made by humans.
The basic ingredient of glass is silica sand, which needs to be heated to very high temperatures in order to melt and become glass. Glass shares many properties with ceramic materials, but unlike most ceramics, glass has an amorphous structure. By mixing silica with other ingredients, or by manipulating the manufacturing process, some of the properties of glass can be changed, for example its durability, colour, or melting point. Since glass is very versatile, it has so many applications in our everyday lives.