||The English used in this article may not be easy for everybody to understand. (October 2011)|
Tar is a viscous black liquid. It is made by the destructive distillation of organic matter. Most tar is produced from coal as a byproduct of coke production, but it can also be produced from petroleum, peat or wood.
Types of tar [change]
Term misuse [change]
The word "tar" is often used to describe several different substances which are not actually tar. Naturally occurring "tar pits" (e.g. the La Brea Tar Pits in Los Angeles) actually contain asphalt rather than tar. Tar sand deposits contain various mixtures of sand (or rock) with bitumen or heavy crude oil and not tar, as does the Tar Tunnel in Shropshire. "Rangoon tar", also known as "Burmese Oil" or "Burmese Naphtha", is actually petroleum. "Tar" and "pitch" are sometimes used interchangeably; however, pitch is considered more solid while tar is more liquid.
In English, German and French, "tar" is a substance primarily made from coal. 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, coal tar is used as a topical medicine. Coal and petroleum tar has a pungent odor.
In Northern Europe, the word "tar" refers primarily to a substance that is derived from the wood and roots of pine. In earlier times it was often used as a water repellent coating for boats, ships, and roofs. It is still used as an additive in the flavoring of candy, alcohol and other foods. Wood tar is microbicidal and has a pleasant odor. Producing tar from wood was known in ancient Greece, and has probably been used in Scandinavia since the Iron Age. For centuries, dating back at least to the 14th century, tar was among Sweden's most important exports. Sweden exported 13,000 barrels of tar in 1615 and 227,000 barrels in the peak year of 1863. Production nearly stopped in the early 20th century, when other chemicals replaced tar and wooden ships were replaced by steel ships.
The heating (dry distilling) of pine wood causes tar and pitch to drip away from the wood and leave behind charcoal. Birchbark is used to make particularly fine tar, known as "Russian oil", suitable for leather protection. The by-products of wood tar are turpentine and charcoal. When deciduous tree woods are subjected to destructive distillation the products are methanol (wood alcohol) and charcoal.
Tar kilns (Swedish: tjärdal, Norwegian: tjæremila) are dry destillation ovens, historically used in Scandinavia for producing tar from wood. They were built close to the forest, from limestone or from more primitve holes in the ground. The bottom is sloped into an outlet hole, to allow the tar to pour out. The wood is split to dimensions of a finger and stacked densely, and finally covered tight with dirt and moss. If oxygen can enter, the wood might catch fire, and the production would be ruined. 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.
Tar is used in treatment of the skin disease psoriasis, where coal tar is the most effective. Tar is also a general disinfectant. Petroleum tar was also used in ancient Egyptian mummification circa 1000 BC.
Tar was a vital component of the first sealed, or "tarmac", roads. The streets of Baghdad were the first to be paved with tar from the 8th century AD. It was also used as seal for roofing shingles and to seal the hulls of ships and boats. For millennia wood tar was used to waterproof sails and boats, but today sails made from inherently waterproof synthetic substances have negated the need for tar. Wood tar is still used to seal traditional wooden boats and the roofs of historical shingle-roofed churches, as well painting exterior walls of log buildings.
In Finland wood tar was once considered a panacea reputed to heal "even those cut in twain through their midriff". A Finnish proverb states that if sauna, vodka and tar won't help, the disease is fatal. Wood tar is used in traditional Finnish medicine because of its microbicidial properties.
Wood tar is also available diluted as tar water, which has numerous uses:
- As a flavoring for candies (e.g. Terva Leijona) and alcohol (Terva Viina)
- As a spice for food, like meat
- As a scent for saunas. Tar water is mixed into water that is turned to steam to the air
- As an anti-dandruff agent in shampoo
- As a component of cosmetics
Mixing tar with linseed oil varnish produces tar paint. Tar paint has a translucent brownish hue, and can be used to saturate and tone wood and protect it from weather. Tar paint can also be toned with various pigments, producing translucent colours and preserving the wood texture. Because of its paint-like properties, tar should not be touched with bare hands or feet, as it can dry to produce a permanent stain. However, in any case, paint thinner is used to remove it.