Tar is a dark brown or black liquid, with a heavy, thick, oil-like consistency. This dark, viscous liquid is obtained by the chemical process of intensely heating organic material in the presence of very limited air (oxygen), This process is known as destructive distillation, during which large molecules are broken up or cracked. Wood tar is formed by high temperature carbonization of wood in anoxic conditions (dry distilling) causing tar to drip away from the wood.