Tar is a term used to describe a dark, viscous, oily substance, fundamentally a complex mixture of hydrocarbons and free carbon. This material is defined by its physical properties—its thickness and stickiness—rather than a single chemical formula. The word “tar” is non-specific and is applied to distinct substances arising from geological processes, industrial manufacturing, and simple combustion. Understanding its origins requires differentiating between naturally occurring, geologically formed residues and manufactured products derived from heating organic materials.
Natural Occurrence
Natural tar is technically known as bitumen or asphalt, a material that forms deep within the Earth over millions of years. Its origins trace back to the remains of ancient organic matter, such as marine plankton, which accumulated in sedimentary basins. When subjected to immense heat and pressure over geological time, this material underwent a transformation process similar to the creation of crude oil.
The resulting crude oil, a less dense fluid, then migrates upward through fissures and porous rock layers in the Earth’s crust. When this oil reaches the surface, the lighter and more volatile components, such as gasoline and kerosene, evaporate due to exposure to air and sunlight. This evaporation leaves behind the heavier, non-volatile hydrocarbon residue, which is the thick, sticky substance known as natural asphalt or bitumen. Famous examples, such as the La Brea Tar Pits, are large deposits where this dense, naturally-fractionated petroleum has pooled on the surface.
Industrial Creation from Coal
The most significant manufactured source is coal tar, a liquid byproduct of the industrial process known as destructive distillation or coking. This process involves heating coal to extremely high temperatures, often over 900 degrees Celsius, in the complete absence of oxygen. The primary purpose of this heating is to remove volatile components from the coal to produce a purer carbon residue called coke, which is used in steel manufacturing.
As the coal is heated, its complex organic molecules decompose and vaporize, forming a crude gas mixture. When this gas is cooled, it condenses into a thick, black liquid—the coal tar—and leaves behind coal gas. Coal tar is an exceptionally complex substance, a mixture of hundreds of organic chemical compounds.
Key components include numerous polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, anthracene, and phenanthrene, as well as aromatic compounds like benzene, toluene, and xylene. Historically, coal tar was a major raw material that was further refined to create dyes, drugs, explosives, and plastics.
Production from Wood and Other Biomass
Tar can also be intentionally produced from other carbonaceous materials, most commonly wood, through a process known as wood pyrolysis or destructive distillation. This technique involves heating wood, often pine, in a closed retort or kiln with limited or no air present, typically at lower temperatures than those used for coal coking. The heat causes the wood’s organic structure, primarily cellulose and lignin, to decompose.
This thermal decomposition yields three main products: a solid residue called charcoal, non-condensable gases, and a condensable liquid known as wood tar. The composition of wood tar is distinct from coal tar, containing compounds such as creosote, methanol, acetic acid, and various phenols. Historically, wood tar, particularly pine tar, was highly valued for its preservative qualities and was widely used for waterproofing wooden ships, ropes, and roofing materials.
Tar Formation in Tobacco Smoke
The most common contemporary reference to the term “tar” relates to the residue generated during the combustion of tobacco. This substance is not a raw material or an industrial byproduct but is formed in situ as the tobacco burns. When a cigarette is lit, the intense heat breaks down the complex organic compounds in the tobacco leaf.
These broken-down components recombine and condense into a dense aerosol of billions of semi-liquid and solid particles. This particulate matter, excluding water and nicotine, is what is collectively referred to as tobacco tar. The resulting residue contains thousands of chemical compounds, including over 70 known carcinogens.
Because it is a direct product of the combustion process, this tar is inhaled as a complex mixture of microscopic particles that deposit in the respiratory tract.