Quicklime is made by putting limestone in a furnace while wearing gloves. If the player isn't wearing gloves, the limestone will corrode the player's skin and damage the player for 8 hp. Quicklime is used in the Regicide quest for making the barrel bomb.
With an empty pot in inventory, the quicklime is used with a pestle and mortar to make a pot of quicklime.
Limestone can be mined east of Varrock (near the house of the Odd Old Man) and in north-east Tirannwn.