How Glass Is Made Quiz
From sand to glass
Most glass starts as sand, which is mostly silica. On its own, silica melts only at a very high temperature, so glassmakers add soda ash to lower the melting point, and limestone to make the glass tough and stop it dissolving in water. This mix, called soda-lime glass, is used for most windows and bottles.
The ingredients are melted together in a furnace at about 1,500 °C. When the liquid cools, its atoms don't have time to line up in neat crystals. They freeze in a jumble, which is what makes glass clear and makes it a special kind of solid.
People were making glass objects, like beads, more than 5,000 years ago in Mesopotamia and Egypt.
Shaping glass
About 2,000 years ago, craftspeople on the eastern shores of the Mediterranean discovered glassblowing: blowing air through a pipe into a blob of hot glass to shape it. It made glass cheap and common across the Roman Empire.
Today, flat window glass is made by the float process, invented by Alastair Pilkington in the 1950s. Molten glass is poured onto a bath of molten tin, where it spreads into a perfectly flat, even sheet.
Glass can be recycled again and again without wearing out. Crushed old glass, called cullet, also melts more easily, which saves energy.
Special kinds of glass
Tempered glass is heated and then cooled very fast, which makes it much stronger. When it does break, it shatters into small, blunt pieces, so it's used in car side windows and shower doors.
Laminated glass has a layer of plastic sandwiched inside, so cracked pieces stay stuck together. Car windshields are made this way. Borosilicate glass, used in labs and oven dishes, can handle sudden changes in temperature.
Metals give glass color: cobalt makes it deep blue, and tiny amounts of gold make it ruby red. Nature makes glass too: obsidian forms from fast-cooling lava, and lightning striking sand can make tubes of glass called fulgurites.