Calcium oxide

Calcium oxide, better known as quicklime or just lime, is a white, caustic solid made of calcium and oxygen (CaO). People have made it for thousands of years for mortar and plaster.

It is made from calcium carbonate, in the form of limestone. Lumps of limestone are fed into a tall kiln and heated above about 900 °C. The heat drives off carbon dioxide gas (CaCO₃ → CaO + CO₂), and what is left is lime, lighter and crumblier than the rock it came from. This step, called calcining, is a big source of industrial carbon dioxide, because the gas comes from the rock itself as well as from the fuel.

In glass, lime is a stabilizer. In soda-lime glass it stops the glass from dissolving in water. In display glass it helps keep the glass stable as it is formed and cooled, and in glass fiber it helps the mix melt. Often the glassmaker adds limestone directly and lets the furnace do the calcining. Add water to quicklime and it heats up fiercely as it becomes slaked lime. Heated in a flame, it glows bright white: the original limelight of theaters.