Tar comes from dozens of sources, but they all share one thing in common: organic matter subjected to heat, pressure, or both, with limited oxygen. The word “tar” gets applied loosely to thick, dark, sticky substances ranging from petroleum crude bubbling out of the seafloor to the residue condensing inside a cigarette filter. Some tar forms over millions of years deep underground, some is manufactured in industrial ovens in a matter of hours, and some appears as an unwanted byproduct that engineers spend considerable effort trying to eliminate. The differences between these substances matter more than their shared name might suggest.
Tar That Seeps From the Earth
Long before humans learned to make tar, the planet was producing it on its own. Natural petroleum seeps occur where underground reservoirs of crude oil find cracks or faults that reach the surface. Off the coast of southern California, for example, seafloor seeps have been releasing viscous, heavily biodegraded hydrocarbon residues for thousands of years. These natural seeps are the primary source of the tarballs that wash up on California beaches, with minimal contribution from human-caused spills or drilling activity.1Academia.edu. Biomarker Chemistry and Flux Quantification Methods for Natural Petroleum Seeps and Produced Oils, Offshore Southern California The oil that seeps out has often spent so long underground that bacteria have already broken down many of the lighter compounds, leaving behind a thick, tar-like residue rich in heavy hydrocarbons.
Perhaps the most famous natural tar deposit is the Rancho La Brea Tar Pits in Los Angeles, where asphalt has been seeping to the surface for tens of thousands of years. Radiometric dating of bones recovered from Pit 91 documents at least two major episodes of animal entrapment, one stretching from roughly 45,000 to 35,000 years ago and another shorter window from about 26,500 to 23,000 years ago.2GeoScienceWorld (PALAIOS). Chronology and Spatial Distribution of Large Mammal Bones in Pit 91, Rancho La Brea Animals that wandered into the sticky surface became trapped, drawing in predators who got stuck themselves. The asphalt preserved their bones in remarkable detail.
What makes La Brea especially interesting to scientists is that the tar is not sterile. Researchers studying the pits discovered diverse communities of bacteria living in the asphalt, organisms that have been exposed to complex hydrocarbons for millennia and have evolved enzymes capable of breaking them down.3PubMed Central. Microbial diversity in natural asphalts of the Rancho La Brea Tar Pits These microbes offer a window into how life adapts to extreme chemical environments and could someday inform strategies for cleaning up hydrocarbon pollution.
Birch Bark Tar and Prehistoric Engineering
Tar is among the oldest manufactured materials in human history, and it was not Homo sapiens who invented the process. European Neanderthals were intentionally producing birch bark tar as early as 190,000 years ago, making it one of the earliest known examples of synthetic material production by any hominin species.4Scientific Reports. Identifying Palaeolithic birch tar production techniques They used it primarily as an adhesive, hafting stone tools to wooden handles. The fact that Neanderthals could produce this material has reshaped how archaeologists think about their cognitive and technological capabilities, since making birch tar requires controlled heating in a low-oxygen environment and an understanding that bark transforms into something entirely new under those conditions.
The process itself is deceptively simple in concept but finicky in practice. Birch bark must be heated to several hundred degrees while sealed away from open air. If too much oxygen reaches the bark, it simply burns. If the temperature is too low, the tar never forms. If it runs too hot, the tar breaks down into useless char. Experimental archaeology has shown that even small variations in technique produce chemically distinct tars, which makes identifying how ancient peoples actually did it a genuine puzzle for researchers trying to match archaeological residues to specific production methods.
Wood Tar From Northern Forests
By the medieval period, wood tar production was a major industry across Scandinavia, the Baltic states, and Russia. The raw material of choice was pine, specifically the resinous heartwood of old Scots pine stumps left in the ground for decades after the tree was felled. By the time producers harvested them, the stumps consisted almost entirely of resin-rich wood, since the softer sapwood had long since rotted away.5Journal of Analytical and Applied Pyrolysis. Characterisation of traditionally kiln produced pine tar by gas chromatography-mass spectrometry
The traditional kiln process involved stacking these wood pieces into a large earthen mound or pit, lighting the pile, and then sealing it to starve the fire of oxygen. This destructive distillation slowly cooked the wood, driving off volatile compounds while the heavier resinous fractions condensed and dripped downward. Tar was drained from the bottom of the kiln into barrels, sometimes in batches of 10 to 20 liters at a time.5Journal of Analytical and Applied Pyrolysis. Characterisation of traditionally kiln produced pine tar by gas chromatography-mass spectrometry The resulting product was a dark, strongly scented liquid used to waterproof ships, preserve rope, and protect wooden structures from rot. For centuries, pine tar was so economically important to maritime nations that it was considered a strategic commodity. Finland and Sweden were major exporters, and access to tar supplies influenced colonial trade policy.
Pine tar is chemically quite different from coal or petroleum tar. It is rich in phenolic compounds and resin acids derived from the original wood, which give it antimicrobial properties. That is why it worked so well as a preservative and why some formulations are still used today in traditional wood care, veterinary products, and even specialty soaps.
Coal Tar and the Coking Oven
The tar most people think of when they hear the word entered the scene during the Industrial Revolution, when coal became the dominant fuel. Coal tar is a byproduct of the coking process, where coal is heated to extremely high temperatures (typically above 1,000°C) in the absence of air to drive off volatile compounds and leave behind coke, the solid carbon used in steelmaking. The volatile gases that boil off are cooled and condensed, yielding three main chemical products: coke oven gas, coal tar, and crude benzene.6Journal of Coal Chemistry. Features of chemical product formation during the coking of stamp-charged coal blends
The composition of coal tar is staggeringly complex, containing thousands of individual chemical compounds. Among the most significant are polycyclic aromatic hydrocarbons, or PAHs, a family of molecules made up of fused carbon rings. Coal tar also contains phenols, cresols, naphthalene, and dozens of nitrogen- and sulfur-containing compounds. For much of the 19th and 20th centuries, coal tar was the feedstock for entire branches of the chemical industry. Synthetic dyes, pharmaceuticals, explosives, and early plastics all trace their chemical ancestry to compounds first isolated from coal tar distillation.
One of coal tar’s more surprising modern roles is in medicine. Crude coal tar has been used to treat psoriasis for well over a century, and it remains an active ingredient in some topical treatments today. Researchers believe the therapeutic effect comes from polycyclic aryl hydrocarbons in the tar, specifically a compound called carbazole, though the exact mechanism is still not fully characterized.7PubMed. Review of the mechanism of action of coal tar in psoriasis It is a striking example of a substance that is simultaneously a known carcinogen and a useful medicine, depending entirely on the dose and application.
Petroleum Tar and Bitumen
When crude oil is refined, the heaviest fraction left behind after vacuum distillation is called vacuum residue or, colloquially, tar. This residue is the starting material for petroleum bitumen, the black binder used in asphalt roads worldwide. Three main techniques convert this residue into usable bitumen: oxidation (blowing air through the hot residue), cracking through vacuum distillation, and compounding, which blends different fractions to achieve desired properties.8Construction and Building Materials. Processing of High-Paraffinic vacuum residues by thermocatalytic methods to obtain bitumen
Petroleum-derived bitumen is the largest volume tar product in the modern economy by a wide margin. Nearly every paved road contains it. It is also used in roofing materials, waterproof coatings, and pipe linings. The stuff is distinct from coal tar chemically, with a higher proportion of long-chain hydrocarbons and asphaltenes (very large, complex molecules) and a lower concentration of the lighter aromatic compounds found in coal tar. This distinction matters for health and environmental regulation, because the two products pose different risks despite looking almost identical to the naked eye.
Cigarette “Tar” Is Not Really Tar
When public health agencies warn about “tar” in cigarettes, they are using the word loosely. Cigarette tar is not a pre-existing substance in the tobacco leaf. It forms during combustion, when the burning tip of a cigarette reaches temperatures high enough to break down organic molecules in the tobacco into a complex vapor. As that superheated vapor rapidly cools within a few milliseconds, it condenses into the aerosol particles that make up visible smoke.9Progress in Energy and Combustion Science. Smoke generation inside a burning cigarette: Modifying combustion to develop cigarettes that may be less hazardous to health What collects on a filter or in the lungs is a condensate of thousands of compounds, including PAHs, nitrosamines, phenols, and heavy metals. This residue is what gets measured and reported as “tar” on cigarette packaging in countries that still require it.
The health significance of this combustion-derived tar overlaps with coal tar in one key respect: both are rich in PAHs. In mouse studies comparing the cancer-causing potential of various PAH mixtures, coal tar extract and cigarette smoke condensate both produced skin tumors at rates comparable to pure benzo[a]pyrene, one of the most studied carcinogenic PAHs. Adding cigarette smoke condensate to coal tar extract did not worsen the tumor response beyond what coal tar alone produced, suggesting their carcinogenic PAH profiles overlap substantially.10PubMed Central. Polycyclic aromatic hydrocarbons as skin carcinogens: comparison of benzo[a]pyrene, dibenzo[def,p]chrysene and three environmental mixtures in the FVB/N mouse
When Tar Meets Water
Coal tar causes some of its most widespread environmental damage in a form most people never think about: pavement sealcoat. In much of North America east of the Continental Divide, parking lots and driveways are sealed with coal-tar-based products that are heavily enriched in PAHs. When it rains, those compounds wash off into storm drains and eventually into streams and rivers. Research has found that coal tar sealcoat runoff is acutely lethal to some fish species and causes a range of cardiovascular abnormalities in developing zebrafish embryos.11PubMed. Severe Coal Tar Sealcoat Runoff Toxicity to Fish Is Prevented by Bioretention Filtration
The problem extends beyond fish. Laboratory experiments exposing freshwater macroinvertebrates (the insects, snails, and crustaceans that form the base of stream food webs) to coal-tar sealant flakes found a clear dose-dependent decline in both the abundance and the diversity of these organisms.12PubMed. Coal-tar based pavement sealant toxicity to freshwater macroinvertebrates The PAHs in the sealant are bioavailable, meaning they dissolve into water at concentrations high enough for organisms to absorb them. Several U.S. cities and counties have banned coal-tar sealants in response to this research, though they remain legal and widely used in many areas. Asphalt-based sealants serve as an alternative, with substantially lower PAH content.
Biomass Gasification and the Tar Nobody Wants
In the push toward renewable energy, biomass gasification has emerged as a promising way to convert plant material into hydrogen-rich fuel gas and syngas. The process heats biomass (wood chips, agricultural waste, or similar feedstock) in a low-oxygen environment, breaking down the organic material into useful gases. But it also produces tar, and in this context, tar is purely a headache. The sticky condensate blocks pipes, fouls filters, corrodes equipment surfaces, and reduces the overall efficiency of the gasification system.13Industrial & Engineering Chemistry Research. Catalytic Cracking of Biomass-Derived Hydrocarbon Tars or Model Compounds To Form Biobased Benzene, Toluene, and Xylene Isomer Mixtures
Engineers have developed catalytic cracking methods to deal with this tar, either inside the gasifier itself or in a secondary reactor downstream. Catalysts break the large tar molecules into smaller, useful compounds through reforming, cracking, and selective oxidation. This can be done at temperatures close to the gasifier outlet, which makes it energy-efficient.14PubMed Central. Tar Formation in Gasification Systems: A Holistic Review of Remediation Approaches and Removal Methods Some researchers are going further, using catalysts to convert biomass tar directly into valuable industrial chemicals like benzene, toluene, and xylene, turning a waste product into a feedstock.13Industrial & Engineering Chemistry Research. Catalytic Cracking of Biomass-Derived Hydrocarbon Tars or Model Compounds To Form Biobased Benzene, Toluene, and Xylene Isomer Mixtures The broader effort in biomass gasification research involves testing natural mineral catalysts, alkali metals, nickel-based compounds, zeolites, and rare-earth composites to maximize hydrogen output and syngas quality while minimizing tar formation.15Materials Reports: Energy. Advancements in biomass gasification and catalytic tar-cracking technologies
Coal Tar Pitch in Aluminum and the Search for Alternatives
One of the less visible but industrially critical uses of tar is in aluminum production. The Hall-Héroult process, which has been the standard method for smelting aluminum for over a century, requires carbon anodes that are consumed during electrolysis. These anodes are manufactured using coal tar pitch as a binder, mixed with calcined petroleum coke and baked into solid blocks. The aluminum industry is a significant source of greenhouse gas emissions partly because of this reliance on fossil-derived carbon, and researchers are actively investigating bio-pitch, derived from renewable biomass, as a sustainable replacement for conventional coal tar pitch binders.16Next Materials. Current status and future potential of bio-pitch as binder in carbon anodes for aluminum production
Replacing coal tar pitch is not straightforward. The binder needs to have specific softening points, coking values, and wetting characteristics to produce anodes that perform reliably in an electrolysis cell at roughly 960°C. Bio-pitch made from sources like wood pyrolysis or lignin processing does not yet match all of these properties. But the research is motivated by a genuine industrial need: as coking capacity declines in some regions and environmental regulations tighten, the supply of high-quality coal tar pitch is becoming less certain.
Tar-Like Substances Beyond Earth
Tar is not unique to our planet. Throughout the outer solar system, dark, complex organic solids coat the surfaces of moons and other bodies. Scientists broadly classify these materials as “tholins,” from the Greek word for “muddy.” Tholins form when simple gas mixtures like methane and nitrogen, or ices, are exposed to ultraviolet radiation from the Sun or to energetic charged particles trapped in planetary magnetospheres.17PubMed. Solid organic matter in the atmosphere and on the surface of outer Solar System bodies The result is a reddish-brown to black gunk that shares some chemical properties with terrestrial tar: it is composed of large, complex organic molecules, is insoluble in water, and defies easy chemical characterization.
Titan, Saturn’s largest moon, is perhaps the best-known example. Its thick, hazy atmosphere is rich in organic aerosols that settle onto the surface, and laboratory analogs of these materials look and behave remarkably like a kind of extraterrestrial tar. Similar tholin-like deposits are thought to explain the dark patches on Pluto, the reddish coloring of some Kuiper Belt objects, and the dark material on certain icy moons. None of this material formed from biological processes. It is the product of simple chemistry driven by radiation, a reminder that the universe is quite good at making sticky, dark, complex organic gunk without any help from living things.
Why “Tar” Means So Many Different Things
Part of the confusion around tar is that the word has no single chemical definition. A chemist would never use “tar” as a precise term. Coal tar, wood tar, petroleum bitumen, cigarette smoke condensate, biomass gasification tar, and extraterrestrial tholins are all chemically distinct materials with different compositions, toxicities, and properties. What they share is a set of physical characteristics that humans noticed long before anyone understood the underlying chemistry: they are dark, viscous or semi-solid at room temperature, sticky, and derived from organic matter.
This linguistic looseness creates real confusion in practical contexts. Coal-tar sealant and asphalt-based sealant look similar but have wildly different PAH levels and environmental impacts. “Tar” on a cigarette label is a regulatory measurement of total particulate matter minus nicotine and water, not an ingredient list. The “tar” in tar pits is really natural asphalt. And the “tar” that plagues biomass gasification engineers is a class of condensable organic vapors that would be unrecognizable to someone scraping pine tar out of a Scandinavian kiln. When you encounter the word, the single most useful question is always: tar from what? The source material and the process that created it determine everything that matters about the substance, from whether it can waterproof a boat to whether it can give you cancer.