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Tar is a viscous black liquid derived from 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
The word "tar" is used to describe several distinct substances. Naturally occurring "tar pits" (e.g. the La Brea Tar Pits in Los Angeles) actually contain asphalt, not tar, and are more accurately known as asphalt pits. Tar sand deposits contain various mixtures of sand (or rock) with bitumen or heavy crude oil rather than tar, as does the Tar Tunnel in Shropshire. "Rangoon tar", also known as "Burmese Oil" or "Burmese Naphtha", is actually petroleum. "Tar" and "pitch (resin)" are sometimes used interchangeably; however, pitch is considered more solid while tar is more liquid.
In English and French, "tar" is a substance primarily derived from coal. It was formerly one of the products of a gasworks. Tar made from coal or petroleum is considered toxic and carcinogenic because of its high benzene content, however, coal tar in low concentrations 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 derived from wood, which is used even as an additive in the flavoring of candy and other foods. Wood tar is microbicidial and has a pleasant odor.
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 (tökötti). The by-products of wood tar are turpentine and charcoal. When deciduous tree woods are subjected to destructive distillation the by-products are methanol (wood alcohol) and charcoal.
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. 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:
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, wet tar should not be touched with bare skin, as it can dry to produce a stain, though paint thinner is effective in removing it.
Coal tar is listed at number 1999 in the United Nations list of dangerous goods.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Tar". A list of authors is available in Wikipedia.|