TARS, LOW TEMPERATURE, the oily distillates ob tained by the destructive distillation of organic substances such as wood, peat, lignite and bituminous coal at temperatures not exceeding 650°C.
Hardwood Tars. The condensed volatile product of wood distil lation is known as crude pyroligneous acid, which has the average composition: water 81%, methyl alcohol 3-4%, acetic acid 6-8%, tar 7%. After settling, the major portion of the tar sepa rates from the aqueous acid although the latter always retains some tar in solution. The chemical constituents of wood tar form a very complex mixture comprising the "fatty" acids, esters, ketones, alcohols, phenols (usually polyhydric) and their methyl ethers, together with waxes. The most important representatives of these groups present in the tar are formic and acetic acids, with their methyl esters, acetone, methyl and allyl alcohol, guaiacol, catechol, and esters of pyrogallol.
Beech-wood tar is practically the only wood tar subjected to a complete straight distillation. Three fractions are usually col lected, first the light oils and water containing acetic acid and methyl alcohol up to i80°C., then the heavy oils up to 240°C. and finally the pitch. The important fraction is the "heavy oil" distillate from which beech-wood-tar creosote is obtained (see CREOSOTE). The oils from wood-tar distillation receive some application in flotation methods of purification, whilst the pitch is used in briquetting and in insulating compositions.
Resinous Wood Tars are almost wholly represented by pine wood tar, which is commonly termed "Stockholm" or "Archangel" tar and is made extensively in the forests of Russia, Finland and Sweden. It is an important commercial product and differs from hard wood tar in containing the pleasant smelling mixture of ter penes known as turpentine. Pine-wood tar is the residue left after
the turpentine has been distilled, usually with the aid of steam. It is used widely in the manufacture of cordage, e.g., tarred ropes and twine, and for impregnation of hemp-fibre for oakum. It is used to a slight extent in pharmacy as a component of some oint ments and antiseptics. Distillates of pine-wood tar, particularly the creosote fraction, are used in froth flotation processes.
Peat Tar, a jet black, semi-solid oil, lighter than water, which can be dehydrated by heating at 00°C. On distillation up to 360°C. a hard pitch is obtained along with an oily distillate, the latter consisting of a neutral fraction, containing the solid waxes, phenols and only traces of basic compounds. The most striking characteristic of the neutral oils is their high degree of unsatura tion as manifested by absorption of atmospheric oxygen. The waxes, which resemble the montan wax of lignite, melt at about The acidic (phenolic) constituents of the tar consist of phenol and its homologues, the cresols and xylenols in small quan tity, mixed with tar acids boiling at 250-360°C. The latter show a carbolic acid coefficient of 31, i.e., they are 31 times more powerful than phenol as bactericides. Among the phenolic constituents of peat tar, guaiacol, methylguaiacol and a methyl ether of pyro gallol have been identified. These ethers are absent from coal tar, but are found in wood-tar creosote, of which they form the major portion. Peat tar may therefore be looked on as one of the tran sition steps from wood tar to coal tar. Although the products which can be isolated from peat tar are of undoubted value, the difficulty of winning and drying peat militates against the carboni zation of this fuel on an industrial scale.
Lignite and Brown Coal Tars resemble peat tar in many respects. They are usually black oils of a buttery consistency owing to the presence of paraffin wax. Of an average density of 0.95, they retain a high percentage of water but solidify com pletely at 6-8°C. In contradistinction to tars from bituminous coal, they are almost completely soluble in petrol, a residue of about 5% only being usually left.