Wood railroad ties before (right) and after (left) infusion with<unk> , being transported by railcar at a facility of the Santa Fe Railroad, in Albuquerque, New Mexico, in March 1943. This U.S. wartime governmental photo reports that "The steaming black ties in the left of photo… have just come from the retort where they have been infused with<unk>  for eight hours." Ties are "made of pine and fir... seasoned for eight months" as seen in the untreated railcar load at right.<unk>  is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel. They are typically used as preservatives or antiseptics. Some<unk>  types were used historically as a treatment for components of seagoing and outdoor wood structures to prevent rot (e.g., bridgework and railroad ties, see image). Samples may be found commonly inside chimney flues, where the coal or wood burns under variable conditions, producing soot and tarry smoke.<unk> s are the principal chemicals responsible for the stability, scent, and flavor characteristic of smoked meat; the name is derived . The two main kinds recognized in industry are coal-tar<unk>  and wood-tar<unk> . The coal-tar variety, having stronger and more toxic properties, has chiefly been used as a preservative for wood; coal-tar<unk>  was also formerly used as an escharotic, to burn malignant skin tissue, and in dentistry, to prevent necrosis, before its carcinogenic properties became known. The wood-tar variety has been used for meat preservation, ship treatment, and such medical purposes as an anaesthetic, antiseptic, astringent, expectorant, and laxative, though these have mostly been replaced by modern formulations. Varieties of<unk>  have also been made from both oil shale and petroleum, and are known as oil-tar<unk>  when derived from oil tar, and as water-gas-tar<unk>  when derived from the tar of water gas.<unk>  also has been made from pre-coal formations such as lignite, yielding lignite-tar<unk> , and peat, yielding peat-tar<unk> .
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