Difference between revisions of "Tar"

Latest revision as of 09:08, 20 September 2010

Tar is modified pitch (resin) produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest user was the Royal Navy. Demand for tar declined with the advent of iron and steel ships.

Tar-like products can also be produced from other forms of organic matter such as peat. Mineral products resembling tar can be produced from fossil hydrocarbons including petroleum. Coal tar is produced from coal as a byproduct of coke production. Bitumen is a term used for natural deposits of oil "tar" - such as at the La Brea Tar Pits.

Production

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 flavouring of candy, alcohol and other foods. Wood tar is microbicidal and has a pleasant odour — a sweet musky scent much like that of barbecue. 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, Finnish: tervamiilu) are dry distillation ovens, historically 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 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.

Uses

Tar was 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 as painting exterior walls of log buildings. Tar is also a general disinfectant.

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 which is turned into steam in the sauna
• 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.

Definition

The word "tar" is often used to describe several distinct 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.

Coal

In English, German, and French, "tar" is a substance primarily derived from coal. It was formerly one of the products of gasworks. Tar made from coal or petroleum is considered toxic and carcinogenic because of its high benzene content, though coal tar in low concentrations is used as a topical medicine. Coal and petroleum tar has a pungent odour.

Coal tar is listed at number 1999 in the United Nations list of dangerous goods.

See also

• Bitumen
• Coal tar
• Pine tar
• Pitch (resin)
• Pitch drop experiment
• Resin
• Tarring and feathering
• Tar Heels
• Tar pit
• Tarmac

References

Sources

• ^ "Geotimes - February 2005 - Mummy tar in ancient Egypt". Retrieved January 9, 2006. 
• Details history and uses of "Rangoon Tar"

ca:Quitrà cv:Тикĕт cs:Dehet da:Tjære de:Teer et:Tõrv el:Πίσσα es:Alquitrán eo:Gudro fa:قیر fr:Goudron gl:Alcatrán ko:타르 io:Gudro it:Catrame he:זפת lt:Degutas hu:Szurok nl:Teer ja:乾留液 no:Tjære pl:Smoła pt:Alcatrão ru:Дёготь simple:Tar sl:Gudron fi:Terva sv:Tjära tr:Katran uk:Гудрон vi:Hắc ín yi:פעך bat-smg:Smala zh:焦油