How quicklime is made

Quarry the limestone

Rock is blasted out of a quarry or mine. Makers pick stone by its chemistry, because the clay and magnesium inside it change how the finished lime behaves.

Crush and screen the stone

Crushers break the rock into fist-sized pieces and screens sort them by size. Pieces of similar size heat evenly, so a kiln loaded with them cooks the whole charge at the same rate.

Burn it in a kiln

The stone sits in a tall shaft or a long rotating kiln until it is hot enough to break apart. Carbon dioxide leaves with the exhaust, so the lumps that come out weigh far less than the stone that went in.

above 900 °C

Cool and draw off the lumps

Air blown through the bottom of the kiln cools the lime and carries heat back up to the stone above. The white lumps are drawn out and screened again.

Slake it with water

For many uses the lime is mixed with water, which makes it hiss and give off heat as it turns into calcium hydroxide. A little water leaves a dry powder. An excess of water leaves a thick putty.

Mill and bag the powder

Lumps are ground to a fine powder, sealed and shipped quickly, because the product slowly takes carbon dioxide and moisture back from the air.