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TAR
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TAR , a product of the destructive See also:distillation of organic sub-stances. It is a highly complex material, varying in its See also:composition according to the nature of the See also:body from which it is distilled, different products, moreover, being obtained ac-cording to the temperature at which the See also:process of distillation is carried on. As commercial products there are two See also:principal classes of tar in use—(1) See also:wood tar, the product of the See also:special distillation of several varieties of wood, and (2) See also:coal tar (q.v.), which is primarily a by-product of the distillation of coal during the manufacture of See also:gas for See also:illuminating purposes. These tars are intimately related to See also:bitumen, See also:asphalt, See also:mineral See also:pitch and See also:petroleum. Wood Tar.—Wood tar, known also as See also:Stockholm and as See also:Archangel tar, is principally prepared in the See also:great See also:pine forests of central and See also:northern See also:Russia, See also:Finland and See also:Sweden. The material chiefly employed is the resinous stools and roots of the Scotch See also:fir (Pines sylvestris) and the Siberian See also:larch (Larix sibirica), with other less See also:common fir-See also:tree roots. A large amount of tar is also prepared from the roots of the swamp pine (P. australis) in See also:North and See also:South Carolina, See also:Georgia and See also:Alabama, in the See also:United States. In the distillation of wood a See also:series of products, including gas, tar, pyroligneous See also:acid, See also:acetone, wood spirit (see METHYL See also:ALCOHOL) and See also:charcoal may be obtained, and any of these may be the See also:primary See also:object of the operation. The carbonization of wood can be effected in two ways: (1) by stacking and firing as in the manufacture of charcoal: this method is very wasteful as it is impossible to recover the valuable by-products; and (2) by distilling from retorts, ovens or kilns (after the manner of See also:coke See also:production from coal): this method is more economical as it leads to the See also:isolation of all the by-products. The retorts may be See also:horizontal or See also:vertical and the See also:heating effected by any available See also:fuel, or by the inflammable gases and less valuable grades of tar obtained in previous operations. The condensing plant is also of variable See also:design; a common See also:pattern consists of a connected series of slightly inclined See also:copper pipes contained in a rectangular tank of See also:water (see COAL TAR). After settling the distillate separates into three layers: the lowest consists chiefly of tar and See also:creosote See also:oils with a little acetic acid; the See also:middle layer consists of water, containing pyroligneous acid, wood spirit, acetone with a little tarry See also:matter; whilst the upper consists of See also:light hydro-carbons. The tarry layer is run off by means of a See also:cock near the See also:base of the tank, and is then distilled from retorts resembling coal tar stills. At first, between to° and 12o° C., water and acetic acid comes over; then, between 120°—230° C., the heavy or creosote oils; the See also:residue in the still is wood pitch, which finds application in making briquettes, artificial asphalts, certain varnishes, &c. The crude tar and pitch are also largely used as protective coatings for woodwork exposed to atmospheric conditions. The heavy oils on further fractional distillation yield more acetic acid, and then mixtures of carbolic acid, creosols, &c.
Wood tar is a semi-fluid substance, of a dark See also: Externally tar is a valuable stimulating dressing in scaly skin diseases, such as See also:psoriasis and chronic See also:eczema. Internally wood tar is a popular remedy as an expectorant in subacute and chronic See also:bronchitis. It is usually given as tar water, 1 See also:part of wood tar being stirred into 4 parts of water and filtered. Given internally tar is likely to upset the digestion; taken in large quantities it causes See also:pain and vomiting and dark urine, symptoms similar to carbolic acid poisoning. Coal tar is used in medicine as Pix liquida preparara. From it is made Liquor picis carbonis, prepared with See also:tincture of quillaia. Coal tar is rarely prescribed for See also:internal use. Its See also:external use is similar to that of wood tar: the Liquor carbonis detergens, a proprietary preparation, owes its properties chiefly to the contained phenol. It is used in water as a lotion for skin diseases, and also in an inhaler in the treatment of whooping-cough, See also:croup and bronchitis. Additional information and CommentsThere are no comments yet for this article.
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