Tar is modified
resin produced primarily 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 is produced from
coal as a byproduct of
coke production.
Production
Tar kiln at Trollskogen in
Öland, Sweden.
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 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 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 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 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
A tar-like substance can be produced from corn stalks by heating in a microwave. This process is known as
pyrolysis.
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
