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.