Tar

Tar is a sticky black liquid made of thick oil. It is a natural substance, oozing out of the ground in places like the La Brea tar pits. Usually it is made by heating coal inside a chemical apparatus.

Most tar is produced from coal as a byproduct of coke production, but it can also be produced from petroleum, peat or wood. The "destructive distillation" of a ton of coal can produce 700 kg of coke, 100 litres of liquor ammonia, 50 litres of coal tar and 400 m³ of coal gas.

It used to be one of the products of gasworks. Tar made from coal or petroleum is considered toxic. It causes cancer because of its high benzene content. In low concentrations, however, tar is used as a medicine on the skin.

Tar kilns are dry distillation ovens, used in Scandinavia for producing tar from wood. They were built close to the forest, from limestone or from more primitive holes in the ground. The bottom is sloped into an outlet hole, to allow the tar to come out. The wood is split to dimensions of a finger and stacked densely, and finally covered tight with dirt and moss to stop air entering. On top of this, a fire is stacked and lit. After a few hours, the tar starts to pour out, and continues to do so for a few days.