Tar

Tar is a dark or dark-brown solid substance with a texture resembling wax when touched. It is produced when wood, coal or crude oil is heated to high temperature with little or no air present. This process is known as pyrolysis, meaning that natural matter breaks down and releases gas, tar and hard residue.

Tar is not a single pure chemical compound, but rather a mixture of hundreds of different hydrocarbons. This is why its properties—such as viscosity, color and odor—vary depending on the source material and the temperature used.

Types and production

The oldest type is wood tar, usually produced from coniferous trees. Wood is placed in a pit or sealed oven, and fire is lit at the top or side; because of low oxygen, the wood does not burn completely, and tar drips down and collects below. Another type is coal tar, a byproduct released when coal is converted to coke in steel mills. The third type, most widely used today, comes from the heaviest residue left after crude oil refining.

Historical uses

For centuries, tar was used to waterproof wooden ships and vessels, preventing water penetration and protecting the hull. Ropes on ships were also coated with it. It was also used as a folk remedy for throat and mouth diseases, because tar has antibacterial properties. Wood tar trade was an important commercial activity in northern European countries through the medieval period.

Modern uses

Today the most common use of tar is from crude oil, applied in road construction. When mixed with crushed stone, it produces a durable road surface. Coal tar was once a major source of chemicals used in textile dyes, medicines and preservatives; many of these are now produced from crude oil instead. Tar is also used for roof waterproofing and wood preservation.

Health and environment

Tar contains compounds known as polycyclic aromatic hydrocarbons (PAH), some of which have been associated with cancer from long-term exposure. Health authorities therefore restrict certain uses of coal tar, particularly in products applied to skin and in some road applications. Tar can also contaminate groundwater at former industrial sites.