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Tar
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Tar is modified resin produced from the wood and roots of pine by destructive distillation under pyrolysis. It is a viscous black liquid. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving and protecting 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.
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Encyclopedia
Tar is modified resin produced from the wood and roots of pine by destructive distillation under pyrolysis. It is a viscous black liquid. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving and protecting 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 tar is produced from coal as a byproduct of coke production.
Production
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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 -a sweet musky scent much like that of barbecque. 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.
Uses
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. 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 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.
Other types of tar
Term misuse
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, however, coal tar in low concentrations is used as a topical medicine. Coal and petroleum tar has a pungent odor.
Coal tar is listed at number 1999 in the United Nations list of dangerous goods.
See also
Sources
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