Quicklime, more formally known as calcium oxide (CaO), is a caustic alkaline substance that is produced by heating limestone in specially designed kilns. There are a wide range of uses for this substance, ranging from mortar to flux, and it has been used by humans for centuries. Many companies produce and sell it, sometimes with specific chemical impurities that 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 its generation may be one of the oldest chemical reactions known to man. People have certainly been using the material all over the world for thousands of years; in Mesoamerica, for example, people treated corn with it in a process known as nixtamalization, while in India, it 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 it, 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 compound can be ground into a powder and packaged for sale.
Numerous things can impact the quality of the resulting material, 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 that 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 carbon dioxide 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, it can be immensely useful for things like mixing strong mortar, acting as a flux in smelting, and treating wastewater.