Tar is a general term for a complex, viscous mixture of hydrocarbons resulting from the thermal decomposition of organic materials. It is a black or dark brown liquid byproduct composed of hundreds of different chemical species, not a single compound. Tar formation involves heating carbon-rich substances under limited or no oxygen, causing the material to break down into gases, solids, and a condensable liquid fraction. This process occurs in controlled industrial settings, natural geological events, and during uncontrolled burning, leading to chemically distinct types of tar.
Formation Through Pyrolysis and Destructive Distillation
The most common industrial method for producing tars, such as coal tar and wood tar, is pyrolysis or destructive distillation. Pyrolysis is the thermal decomposition of organic materials at elevated temperatures in the absence of oxygen, which prevents combustion. This controlled heating breaks down large, complex organic polymers within the source material into smaller, volatile molecules.
For instance, in the production of coke for the steel industry, coking coal is heated in airless ovens, often between 1,000°C and 1,100°C. This process yields coke as the solid product and coal tar as a liquid byproduct. The high heat causes the coal’s organic structure to crack, releasing vapors that are cooled to condense into coal tar, a dense, black liquid rich in complex aromatic hydrocarbons. Lower-temperature destructive distillation, below 700°C, yields a coal tar containing more phenolic compounds and alkyl derivatives.
Similarly, heating wood or biomass in a retort vessel, a process historically used to make charcoal, yields wood tar, charcoal, and non-condensable gases. The resulting tar’s composition depends heavily on the maximum temperature reached and the specific organic source material used. Chemical reactions involve the initial breakdown of cellulose and lignin, followed by secondary reactions where primary vapors recombine or further decompose. This eventually forms the characteristic complex, ring-structured molecules found in the final tar product.
Natural Geological Formation
Naturally occurring tar, often referred to as asphalt or bitumen, is geologically distinct from industrial pyrolysis products. This substance is a highly viscous or solid form of petroleum originating from the decay of ancient organic matter, primarily marine microorganisms and plants. This matter has been subjected to immense pressure and heat underground over millions of years. It is essentially the non-volatile residue left behind after crude oil has undergone a long-duration alteration process.
The formation of natural asphalt involves the upward migration of lighter crude oil through fractures or porous rock layers until it reaches the surface or a shallow reservoir. Once exposed, the lighter, more volatile hydrocarbon components of the oil begin to evaporate. This process can be accelerated by water washing and bacterial degradation, which consume the less viscous parts of the oil.
The remaining material is the heavy, sticky, black residue known as bitumen or asphalt, composed of the largest, non-evaporating molecules. Notable examples, such as the La Brea Tar Pits, illustrate this low-temperature, long-term process. Although often colloquially called “tar,” this geological product is chemically defined as a naturally occurring petroleum residue. This differentiates it from the coal or wood tars created by high-heat industrial decomposition.
Tar Formation in Incomplete Combustion
Tar is also an unintended byproduct of incomplete combustion, a rapid, uncontrolled process occurring when organic material burns without sufficient oxygen. This lack of oxygen prevents the full oxidation of the fuel into carbon dioxide and water vapor. Instead, the organic matter undergoes rapid, localized pyrolysis, where molecules break down and quickly recombine.
The resulting sticky residue is rich in complex molecules known as polycyclic aromatic hydrocarbons (PAHs). These are formed as smaller hydrocarbon fragments, such as acetylene, combine to create stable, multiple-ring structures. PAHs condense upon cooling to form the dark, viscous material seen in chimney soot, bonfire residue, or smoke. For instance, in the burning tip of a tobacco product, high temperatures generate compounds that condense into the particulate matter known as “tar” in the mainstream smoke.
This process is a lower-yield, less controlled version of pyrolysis, where PAH formation depends on the interplay between temperature and available oxygen. The concentration of these tar compounds is highest in smoke from smoldering fires. In these fires, the temperature is high enough to break down the material but too low for the fragments to fully oxidize. These rapidly formed, uncontrolled tars are chemically complex and contain many toxic and carcinogenic PAHs that pose environmental and health concerns.