Tar

Tar is a dark, viscous liquid or semisolid produced by heating organic material, wood, resin, or coal, at high temperature with little or no oxygen, a process called dry distillation or pyrolysis. The heat cracks large molecules in the feedstock and yields a complex mixture of acids, phenols, and aromatic hydrocarbons that differs markedly in composition from the original material^{1 • 2}. The word covers several distinct products: wood tars such as pine tar and birch bark tar, coal tar from coke ovens, and, in geology, the "tar" of tar sands, which is actually natural bitumen, a degraded petroleum rather than a pyrolysis product³.

Wood tar

Traditional production. Wood tar has been made since prehistoric times from resinous woods, valued for its adhesive, water-repellent, and medicinal properties⁵. Throughout history it has served to glue materials, waterproof surfaces, and conserve wood, and today it also appears in medicines and cosmetics; countries including Norway and Morocco obtain tar from endemic trees¹. A review of production species found tar can be drawn from 18 tree taxonomic families, notably the cypress family (Cupressaceae), the pine family (Pinaceae), and the birch family (Betulaceae)¹.

Methods. Two production methods are distinguished: per ascensum and per descensum, each taking different forms¹. In traditional kiln production of pine tar, the tar is drained continuously, or in portions of 10 to 20 liters, from the bottom of the kiln into wooden barrels; tar collected in the first barrel is distilled at lower temperatures than tar in later barrels, giving diverging chemical composition and physical properties⁶. Before purpose-built tar kilns, funnel-shaped tar pits were used, built close to the forest as primitive holes in the ground without an outlet pipe².

Composition. The composition of wood tar depends on the pyrolysis method, duration, temperature, and the type of wood used, including the age and growing conditions of the trees. Wood tar mainly consists of aromatic compounds and small oxygenated compounds from wood pyrolysis; tars from resinous woods also contain products of the aromatisation, demethylation, and decarboxylation of terpenoids in the resin, in some cases a significant portion of the material². Wood tar can be separated by fractional distillation over the full range from the initial evaporating point to the coking point, 110 to 300 °C⁵. Pitch obtained by refining wood tar is called wood tar pitch, with the botanical source specified where possible, for example pine tar or birch bark tar².

Coal tar

Origin. Coal tar is a by-product of the destructive distillation (carbonization) of coal to produce coke or gas. In a coke oven, prepared coal is heated in an oxygen-free atmosphere until volatile components evaporate; cooling the coke oven gas condenses a black product, coal tar, recovered after decantation^{7 • 8}. Coal tars are black or almost-black viscous liquids or semisolids with a characteristic naphthalene-like odor, slightly soluble in water and soluble in benzene and nitrobenzene; their composition depends primarily on the carbonization temperature and, to a lesser extent, on the coal used¹².

Products and uses. Distillation of crude coal tar yields creosote, coal-tar pitch, and crude naphthalene and anthracene oils¹². Creosote, derived from the distillation of tar from wood or coal, has been used as a wood preservative since the mid-1800s; pesticide products containing it protect outdoor wood such as railroad ties and utility poles against termites, fungi, mites, and other pests, applied by certified applicators in wood-preserving facilities⁹. Coal-tar pitch serves primarily as the binder for aluminum-smelting electrodes, and also in roofing materials, refractory brick, pipe-coating enamels, and antifouling paints¹². Coal tar has also long been used in preparations for skin conditions such as eczema, psoriasis, seborrheic dermatitis, and dandruff¹².

Health and environmental concerns

Carcinogenicity. Coal tars and coal-tar pitches are known to be human carcinogens based on sufficient evidence from studies in humans. Occupational exposure is associated with skin cancer, including scrotal cancer; a 1946 United Kingdom study found patent-fuel workers were 500 times as likely as other workers to die of scrotal cancer. Excess lung cancer has been found in workers exposed to coal-tar fumes in coal gasification and coke production and to pitch fumes in aluminum production¹². Both coal tars and pitches contain known and potential carcinogens including benzene, naphthalene, and other polycyclic aromatic hydrocarbons (PAHs)¹². NIOSH considers coal tar products (coal tar, coal tar pitch, and creosote) to be potential occupational carcinogens, with a recommended exposure limit of 0.1 mg/m³ as a time-weighted average of the cyclohexane-extractable fraction; the OSHA permissible exposure limit is 0.2 mg/m³ as the benzene-soluble fraction⁴.

Sealcoat runoff. Coal-tar sealcoat, a pavement sealant that typically contains 20 to 35% coal tar pitch, a byproduct of steel manufacturing that is 50% or more PAHs by weight, is a major PAH source in stormwater. One study found PAHs in stormwater runoff 65 times higher from parking lots sealed with coal-tar sealant than from unsealed lots, and another found coal-tar sealcoat to be the largest source of PAHs to urban lakes¹⁰. In response, cities and states have restricted the product: Austin, Texas banned the sale and use of coal-tar-containing pavement sealants in 2005, Washington State banned sale on January 1, 2012 and use after July 1, 2013, and the District of Columbia banned sale and use in 2009¹⁰.

Tar sands: a different "tar"

In petroleum geology, "tar" appears in the term tar sands, a generic name used for several decades for natural bitumen accumulations, petroleum-bearing rocks exposed at the Earth's surface; other terms include oil sand, asphaltic sand, and bituminous rock³. Natural bitumen is defined as petroleum with a gas-free viscosity greater than 10,000 centipoises at original reservoir temperature, distinguishing it from less viscous crude oils³. These rocks have been mined since antiquity for use as paving¹¹. This material is a degraded crude oil, not a pyrolysis product, so the geological "tar" of tar sands is chemically distinct from wood tar and coal tar despite the shared name.

References

1. Ninich, O. et al. (2022). Plant sources, techniques of production and uses of tar: A review. Journal of Ethnopharmacology.
2. Characterising terpenic plant resins, wood tars and pitches in heritage science. npj Heritage Science. / Coal Tars and Coal-Tar Pitches, Report on Carcinogens, 15th Edition, NTP.
3. Natural Bitumen Resources of the United States, USGS Fact Sheet 2006-3133.
4. Coal tar pitch volatiles, IDLH documentation, NIOSH.
5. Plant sources, techniques of production and uses of tar: A review, ScienceDirect
6. Characterisation of traditionally kiln produced pine tar by gas chromatography-mass spectrometry.
7. Tar, coke plant by-products, ArcelorMittal Europe.
8. Coke Production, AP-42 Section 12.2, US EPA.
9. Creosote, US EPA.
10. Coal-Tar Sealcoat, Polycyclic Aromatic Hydrocarbons, and Stormwater Pollution, US EPA.
11. Meyer, R.F. and Attanasi, E. (2004). Natural bitumen and extra-heavy oil, USGS.
12. ntp.niehs.nih.gov page

Topic: Encyclopedia › Technology and the built world › Energy technology › Fuels and conversion technology › Titles In to W › Titles Ro to W

Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 15, 2026

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