Quicklime is made by putting limestone in a furnace while wearing gloves. If the player isn't wearing gloves, the limestone will just explode and damage them. 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.
Trivia
- Before the update restoring players' health once leaving a Player Owned House, a scam existed where somebody would ask a player whose health had been reduced by fighting in a boxing ring to help them smelt limestone, giving the impression that they don't have the required smithing level. The target of the scam would be damaged and often die from the resultant explosion of limestone, through having already lowered health.