Charcoal furnaces can be found on the southern coast of Hosidius. They are used to burn up to 60 Juniper logs per furnace into Juniper charcoal, which is one of the required ingredients for making Dynamite.
There are two furnaces, and players have to pay 600 coins to Smoggy as fee for taking care of a furnace for them. The process of turning logs into charcoal takes about 18.5 hours.
Locations in Great Kourend