Quicklime, more formally known as calcium oxide (CaO), is a caustic alkaline substance which is produced by heating limestone in specially designed kilns. There are a wide range of uses for quicklime, ranging from mortar to flux, and the substance has been used by humans for centuries. Many companies produce and sell quicklime, sometimes with specific chemical impurities which make it especially suitable to certain applications.

Humans have been aware of the steps needed to create quicklime for a very long time, and chemists believe that the generation of quicklime may be one of the oldest chemical reactions known to man. People have certainly been using quicklime all over the world for thousands of years; in Mesoamerica, for example, people treated corn with quicklime in a process known as nixtamalization, while in India quicklime was used in a mixture designed to waterproof boats. Today, quicklime is used in many industrial processes, some of which were developed hundreds of years ago.

Quicklime is also known as burnt lime, a reference to its manufacturing process, or simply lime. To make lime, limestone (CaCO3) is broken up and shoveled into a kiln, which is heated to very high temperatures. The high temperatures release carbon dioxide (CO2) from the stone, turning it into calcium oxide. After it is cooled, the calcium oxide can be ground into a power and packaged for sale.

Numerous things can impact the quality of the resulting quicklime, ranging from the temperature of the kilns to natural impurities in the stone. Because limestone is a natural product, it can sometimes be hard to control these impurities; as a result, companies which produce quicklime tend to test their product regularly, to ensure that is of high quality. The specialized kilns are also closely monitored to ensure that the limestone heats and cools at an appropriate rate.

Quicklime requires careful handling. As it sits, it can acquire CO2 from the air, reverting to its original form. This means that it needs to be used quickly, especially once it has been mixed with water in a process known as slaking. Lime is also extremely caustic; it can burn the skin and cause other damages. When used responsibly, quicklime can be immensely useful for things like mixing strong mortar, acting as a flux in smelting, and treating wastewater, among many other things.