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Tar

Tar is a dark brown or black bituminous, odorous, viscous liquid obtained by the of organic materials such as , , or . It is characterized by its heavy, oily consistency and high carbon content, distinguishing it from related substances like or . Produced through —intense heating of in the absence of oxygen—tar has been derived from diverse sources including trees, , bones, and residues for thousands of years. Common types include wood tar, obtained from resinous woods like pine ( family) or birch (), often containing and ; coal tar, a byproduct of during coke and gas production; and plant tars from species in families such as (e.g., and ). Historically, tar served practical purposes such as gluing materials, boats and ropes, preserving wood against , and even decoration in ancient cultures across regions like , , and . In modern applications, is primarily refined into chemicals, for wood treatment, and for binders in road paving, roofing, and pavement sealants, though its use has declined due to environmental concerns, including bans on coal-tar based sealants in several jurisdictions such as , , and , with ongoing proposals in others as of 2025. Wood tar and its find roles in antiseptics, ointments for skin conditions like , and manufacturing plastics, insulating materials, and caulking compounds. Plant-derived tars retain ethnopharmacological value in and for treating dermal diseases in both human and veterinary contexts. Tar's chemical complexity—a mixture of aromatic hydrocarbons, phenols, and acids—underpins its versatility but also its hazards; and its derivatives are classified as carcinogenic due to polycyclic aromatic hydrocarbons (PAHs). Significant production occurs in areas like for wood tar and industrial coke facilities worldwide for , with ongoing research into sustainable plant-based alternatives.

Definition and Terminology

Etymology and Nomenclature

The word "tar" derives from teoru, referring to the pitch or obtained from . This term traces back to Proto-Germanic terwō and ultimately to the deru-, which conveys notions of firmness, solidity, and or , reflecting tar's origins in tree and saps. Cognates appear across , such as tjara and teer, underscoring its ancient association with viscous, tree-derived substances used for sealing and preservation. In , tar is distinguished from related terms based on form and source; for instance, "" denotes a denser, semi-solid residue derived from the of tar, often used interchangeably but technically more solid and less fluid than liquid tar. Specific variants include " tar," a high-quality produced from of pine wood, named for the Swedish city of due to a historical royal on its , which ensured its and reputation in and cordage. These distinctions help tars by production method and application, with wood-derived forms like Stockholm tar differentiated from coal or sources. Regional naming variations highlight cultural and linguistic adaptations; in , "brea" specifically means tar, as seen in place names like , where natural asphalt seeps were known to and early settlers. In French, "goudron" refers to tar generally, with "goudron de houille" denoting , a term rooted in industrial contexts for the byproduct of . Such terms reflect localized uses, from road surfacing to medicinal applications, while maintaining the core concept of a dark, viscous residue.

Physical and Chemical Properties

Tar exhibits a dark to black coloration and exists as a or semisolid at , depending on its specific composition and source material. Its typically ranges from 1.0 to 1.2 g/cm³, reflecting the dense packing of its components. The of tar components spans a broad range, generally from 150°C to 400°C, as it is a complex mixture rather than a single , allowing over varying temperatures. A defining of tar is its high , which can reach up to 10,000 or more, contributing to its flow resistance and utility in applications requiring . Tar demonstrates in organic solvents such as , , , and acetone, but it is insoluble in , which underscores its hydrophobic nature derived from non-polar hydrocarbons. This profile, combined with its viscous consistency, imparts strong qualities, primarily due to the presence of polymeric hydrocarbons that enable binding to surfaces. Chemically, tar shows basic stability with resistance to mild oxidation under ambient conditions, making it suitable for preservative uses, though it can undergo when exposed to elevated temperatures, leading to increased rigidity and the formation of pitch-like residues. This thermal polymerization arises from reactions among its aromatic and aliphatic components, altering its molecular structure without complete decomposition.

History

Ancient and Pre-Industrial Uses

Evidence of tar use dates back to prehistoric times, with archaeological findings indicating its application in mummification processes in around 3500 BCE. Analysis of a naturally mummified body from the predynastic period reveals the presence of complex balms containing resins and tars, likely sourced from , used to preserve soft tissues and inhibit . This practice evolved over millennia, though was not universally employed and became more prominent in later periods. In , particularly , birch-bark tar served as a versatile and agent, applied to canoes and boats constructed from wood or animal skins. Produced by heating birch bark in low-oxygen conditions, this tar was used to mend , haft tools, and vessels, facilitating maritime activities such as fishing and trade across the around 5000–2500 BCE. Archaeological residues on flint blades and from sites in confirm its role in wooden during this era. Among ancient civilizations, tar found widespread use in maritime and applications. In and societies, pitch derived from pine resin or natural was applied to ship hulls and coat bottoms, preventing water ingress and on wooden vessels. The Romans also employed for sealing aqueducts, reservoirs, and early road surfaces, drawing from terminology "asphaltos" meaning "secure." Biblical texts in the reference tar similarly; 6:14 instructs the use of "pitch" (zepheth, denoting a bituminous substance) to waterproof inside and out, reflecting common Near Eastern waterproofing techniques of the period. Before 1800, pre-industrial extraction of tar in and relied on rudimentary methods, including collection from natural seeps and of pine resin. In the and parts of , communities gathered viscous from surface seeps for use in building and sealing, a practice documented in Mesopotamian records and continuing into times. In and , was produced by slow of wood in earthen kilns or pits, yielding a dark, sticky byproduct for local trade and ship maintenance as early as the . In , traditional production from black involved heating resinous wood, supporting regional economies through the .

Development of Industrial Production

The development of industrial tar production began in the 18th and 19th centuries, building on ancient uses of tar for preservation and as precursors to scaled . In , particularly , emerged as a significant byproduct during the early 1800s from the of for manufactured gas used in urban lighting. The first commercial gas works, established in in 1813, produced as a waste material initially discarded into rivers, but by the 1830s, its potential for industrial applications prompted collection and basic processing. Concurrently, wood tar production industrialized in during the 18th and 19th centuries to supply naval stores for shipbuilding, utilizing kiln-based methods by small landowners in northern regions. These kilns, often constructed from earth and wood, processed pine resin to yield tar essential for caulking and preservation, with dominating exports to the Royal Navy until the mid-18th century when colonial sources began competing. Key innovations in the mid-19th century included the expansion of coke oven operations around the 1850s, which generated substantial volumes as byproducts during iron and production; this led to the establishment of dedicated coal tar refineries, such as the 1843 facility at Crew’s Hole in , focused on extracting and for timber treatment. In the , particularly post-World War II, tar production shifted toward sources, with refined and derivatives supplanting for applications like road paving due to the booming and wartime advancements in . This transition reflected broader energy trends, as output surged to meet demands, reducing reliance on coal-derived tars. Economically, tar's role fueled the boom, enabling dyes, pharmaceuticals, and synthetics; by 1900, global reached approximately 3 million tons annually, scaling from artisanal levels and contributing to the second industrial revolution's growth in organic chemicals.

Types of Tar

Wood Tar

Wood tar is derived from the of resin-rich woods, primarily (Pinus spp.), (Fagus sylvatica), and (Betula spp.), where the wood is heated in low-oxygen conditions to yield a viscous distillate. This process, known as , breaks down the lignocellulosic components into condensable vapors that form the tar, along with byproducts like char and gases. Historical production was centered in , where stumps and roots—naturally high in —were preferred raw materials, and in , where similar methods supported large-scale output for export. Distinct from coal tar's heavier, black composition, wood tar exhibits a lighter golden to dark brown color and a characteristic smoky, empyreumatic aroma due to its and components. Its higher content, including , , and oils, contributes to properties, making it suitable for binding materials in traditional applications like stone repair and . Key variants include , produced via kiln-burning of pine wood, which is notably rich in and volatile oils, enhancing its fluidity and qualities. tar ( spp.), obtained similarly from juniper wood or , has been employed in folk medicine for its and effects, treating skin conditions and wounds historically across and beyond.

Coal Tar

Coal tar is a viscous, black liquid obtained as a during the of , primarily in the of for and . This process involves heating in the absence of air, known as , which yields as the main product while condenses from the volatile matter released. Historically, was first isolated in significant quantities in the from , where was carbonized to produce town gas for lighting and heating urban areas. Characterized by its thick, black appearance and pungent naphthalene-like odor, is a complex mixture dominated by high concentrations of aromatic hydrocarbons, including polycyclic aromatic compounds that distinguish it from other tars. Upon , it yields valuable sub-products such as oil, used for , and , a key intermediate for . These aromatic-rich fractions contribute to its industrial utility, though they also impart potential health risks due to carcinogenic components. Coal tar played a pivotal historical role as the foundation for the synthetic organic chemicals industry, particularly through its derivatives like . In 1856, discovered , the first synthetic dye, while experimenting with derived from in an attempt to synthesize . This serendipitous breakthrough, conducted in Perkin's home laboratory, transformed from a mere waste product of into a vital of dyes, pharmaceuticals, and other compounds, sparking the .

Petroleum and Bitumen Derivatives

Petroleum tars, also known as asphaltic tars or , are primarily derived from the of crude residues following atmospheric . These heavy fractions consist of complex hydrocarbons with carbon numbers predominantly greater than 40, forming a semi-solid or viscous material at ambient temperatures. Natural deposits, such as Trinidad Lake Asphalt, represent another key source, occurring as a naturally formed, semi-solid of and mineral matter in lake-like formations. These tars are characterized by high asphaltene content, typically comprising 5-25% of the composition, which contributes to their structural complexity and colloidal . , being polar, high-molecular-weight polycyclic aromatic hydrocarbons with heteroatoms, enhance the material's resistance to and provide greater thermal and oxidative compared to more volatile tars from other origins. Their lower volatility stems from the predominance of high-boiling-point aliphatic and naphthenic components, making them suitable for durable, long-lasting applications. Common variants include road tars, formulated for pavement binding with penetration grades typically between 60/70 and 200/300, and roofing asphalts, which are oxidized to achieve higher softening points around 85-105°C for weather resistance. These petroleum-derived products are aliphatic-rich, emphasizing saturated hydrocarbons over aromatics, which supports their use in structural paving rather than chemical feedstocks. Global production of petroleum bitumen occurs in major refining regions, with Middle Eastern countries such as and being significant producers alongside others like and those in , which accounts for the largest share. As of , global production was approximately 128 million metric tons. This reflects the scalability of processes in oil-rich areas since the mid-20th century, shifting from earlier sources like .

Production Methods

Destructive Distillation Processes

is a process in which materials such as or are heated in low-oxygen retorts, resulting in the of tar, combustible gases, and solid as primary outputs. For , temperatures typically range from 350–500°C, while for , primary occurs at 400–600°C, though the overall coking process reaches 900–1100°C.) This method relies on , where the absence of sufficient air prevents complete and instead promotes the breakdown of complex structures into volatile condensable liquids, including tar. Historically, the process was conducted in sealed vessels or to capture the evolving vapors, which condense to form the viscous tar fraction. The process unfolds in distinct stages, beginning with primary , during which the —such as lignins and in or bituminous components in —undergoes initial . This stage produces primary and volatile compounds, including , , and initial hydrocarbons, as the large molecular structures break down. Secondary cracking follows as these volatiles are further heated, leading to the formation of aromatic compounds and heavier tar precursors through recombination and dehydration reactions. The resulting tar is collected by condensing the vapors in cooled chambers, separating it from non-condensable gases. Efficiency in destructive distillation varies with feedstock and conditions, with typical tar yields ranging from 10 to 20% of the dry weight under historical batch processes, though actual outputs depend on factors like heating rate and moisture content. For , yields are lower, around 3 to 5% by weight, due to its higher carbon content and denser structure. Pre-1900 equipment, such as cylindrical iron retorts or for and horizontal retorts in for , facilitated controlled heating, while beehive ovens were employed in early processing to produce with incidental tar recovery. These methods allowed for scaling in industrial settings, particularly during the when demand for tar in preservation and chemicals grew.

Modern Refining Techniques

Modern refining techniques for tar production prioritize continuous, high-efficiency processes that enhance product purity and minimize environmental impact, primarily through integration with and operations. , commonly employed in refineries, processes heavy residues such as bitumen-derived tar under reduced pressure to lower boiling points and prevent , enabling the separation of valuable fractions like vacuum gas oil and while achieving high purity levels. This method is particularly effective for bitumen, where it facilitates the isolation of tar components at temperatures below 400°C, improving yield and reducing compared to atmospheric . Hydrocracking represents another key advancement, involving catalytic under high pressure and temperature to break down complex tar fractions into lighter hydrocarbons, such as and precursors, with conversion efficiencies exceeding 90% for heavy feeds. In this process, bifunctional catalysts like nickel-molybdenum on alumina promote hydrogen addition and cracking, effectively separating and upgrading polycyclic aromatic structures prevalent in tar, resulting in reduced and enhanced stability of the output. For specifically, hydrocracking operates at 350-450°C and 10-20 , yielding up to 70% middle distillates while minimizing formation. Fluidized bed pyrolysis has emerged as a preferred for biomass-derived tar, utilizing a of inert particles like sand to ensure uniform heating and rapid vapor release at 450-550°C, producing tar yields of 30-50% by weight under optimized conditions. The fluidized state enhances heat and , allowing for shorter residence times (seconds) that preserve volatile tar components, with bio-oil outputs rich in suitable for further refining. Catalysts such as zeolites integrated into the can further boost selectivity toward desired tar fractions by promoting . Since the post-1950s, in operations, including systems and advanced , has significantly reduced emissions in tar production by optimizing energy use and minimizing waste, with reported decreases of up to 20-30% in releases through precise temperature and pressure regulation. The use of catalysts in these modern es has improved overall yields to 30-50% for tar fractions, particularly in and applications, by enhancing cracking efficiency and reducing side reactions. Global annual tar output exceeds 200 million tons as of 2024, driven largely by production of approximately 180 million tons and global production of around 20–25 million tons, alongside outputs from . Building briefly on the legacy of as a foundational approach, contemporary methods emphasize scalable, catalyst-enhanced operations for sustainable tar generation.

Chemical Composition

Hydrocarbon Components

Tar consists primarily of compounds, which form the backbone of its organic composition, derived from the or of organic materials such as , , or residues. The major classes of hydrocarbons in tar include aliphatics (primarily paraffins and olefins), aromatics (such as benzenes and naphthalenes), and polycyclic aromatic hydrocarbons (PAHs). Aliphatics typically account for 2-10% of the hydrocarbon fraction in low-temperature tars, and less in high-temperature varieties, contributing to the saturated and unsaturated chain structures, while aromatics dominate at 40-60%, providing the characteristic stability and viscosity of tar. Olefins, as unsaturated aliphatics, are present in smaller amounts, often linked to incomplete processes. Among the aromatics, PAHs are particularly prominent, featuring fused ring structures like anthracene (C_{14}H_{10}), naphthalene (C_{10}H_{8}), and phenanthrene, which arise from the thermal decomposition of complex precursors. These compounds exhibit average molecular weights ranging from 200 to 1000 Da, influencing tar's solubility and reactivity; for instance, lighter PAHs (e.g., naphthalene at 128 Da) distill at lower temperatures, while heavier ones contribute to the pitch-like residues. The structural diversity, including alkyl-substituted and oxygenated variants, underscores the heterogeneity of tar as a mixture rather than a single entity. Variations in hydrocarbon composition occur depending on the source material and production method. In , particularly from high-temperature , PAHs can constitute up to 70% of the total s, with comprising 8-12% and 1.5-1.8% by weight, reflecting the aromatic-rich nature of precursors. Conversely, features a higher proportion of alicyclics, such as and cyclic paraffins, which form from the resinous components of lignocellulosic materials; these can reach significant levels alongside lower PAH contents (e.g., 5% aromatics in organic-soluble fractions). Petroleum-derived tars, or derivatives, show intermediate profiles with more aliphatic chains (10-20%) balanced by aromatic cores. These differences affect tar's physical traits, such as the darker color in due to extended PAH conjugation.

Non-Hydrocarbon Elements and Impurities

Tar, derived from sources such as , , or , incorporates non-hydrocarbon elements that alter its chemical reactivity, , and suitability for applications. is a prominent in , with concentrations typically ranging from 0.5% to 2% by weight, often existing in thiophenic and sulfidic forms that enhance the material's but also promote corrosiveness through the formation of sulfur oxides during heating or . appears at levels of 0.1% to 1.5%, predominantly in pyridinic and pyrrolic structures, which can interfere with downstream processes by poisoning catalysts. Oxygen content is higher in low-temperature tars, largely bound in hydroxyl groups, contributing to the tar's acidic nature with values around 6.5. Trace metals represent key impurities, particularly in and bitumen-derived tars, where concentrations can reach 350 to 2500 ppm, originating from complexes in crude oils and leading to deactivation in . Other metals like may co-occur at similar levels, exacerbating processing challenges. Ash residues, consisting of inorganic minerals such as silica and alumina, comprise 0.1% to 1% in refined tars, though higher in crude forms up to several percent, resulting from incomplete separation during production. , often emulsified and ranging from 0.5% to 5% in raw tar, promotes instability and microbial growth if not addressed. These elements and impurities impact tar's performance: sulfur and nitrogen compounds increase corrosiveness and environmental emissions, while phenolic oxygen functionalities enhance solubility in alkaline media but lower pH, affecting storage and handling. Purification via effectively separates heteroatomic fractions, removing up to 90% of sulfur in some oxidative-distillation hybrids and substantially reducing ash and water to below 0.1% and 0.5%, respectively, yielding higher-purity products for applications like production.

Applications and Uses

Industrial and Construction Uses

Tar plays a pivotal role in , particularly in road surfacing where —a petroleum-derived tar blend—forms the primary for durable pavements. In the United States, approximately 94% of the 2.7 million miles of paved roads are surfaced with , providing a flexible and cost-effective for high-traffic . This widespread adoption stems from asphalt's ability to withstand heavy loads and weather variations, with modern formulations incorporating tar binders to enhance adhesion between aggregates. Historically, tar-based mixtures like were used in early 19th-century paving to bind stones and reduce dust on roads. Beyond roadways, tar derivatives are essential in roofing and waterproofing applications, where roofing felts or serve as underlayments to protect structures from moisture infiltration. These materials, typically composed of or paper saturated with or , create a secondary barrier beneath or membranes, preventing leaks and extending longevity in residential and . -impregnated felts are valued for their tear resistance and impermeability, commonly applied in flat or low-slope roofs to ensure long-term performance. In industrial applications, coal tar pitch functions as a critical in carbon anodes for aluminum , where it is mixed with and baked to form electrodes used in electrolytic processes. This ensures structural integrity and electrical conductivity, accounting for a significant portion of global aluminum output through the Hall-Héroult method. Similarly, coal tar pitch serves as an effective in coal briquettes, enhancing cohesion during compression and to produce high-strength fuel products from fine coal particles, particularly in regions with abundant coal byproducts. Tar derivatives also contribute to wood preservation, with creosote—a distillate from —applied to treat timber for outdoor and marine uses, such as railway ties, utility poles, and bridge components. Creosote penetrates wood fibers to repel , fungi, and decay, providing decades of protection in harsh environments and remaining a standard in industrial timber treatment. However, its use is restricted to industrial and commercial applications under regulations such as those from the U.S. EPA, due to health concerns. The global market for tar and its derivatives was valued at approximately USD 68 billion in 2025, driven primarily by petroleum-based products like and , which dominate over 70% of industrial and applications due to their scalability and versatility in projects. Coal tar variants, while essential in niche sectors like production, represent a smaller but stable segment, with the overall market reflecting tar's indispensable role in modern manufacturing and building.

Medical and Traditional Applications

Tar has a long history in , particularly wood tar derived from , which across incorporated into salves and ointments to treat infections and inflammatory conditions such as wounds and rashes due to its and soothing properties. In the , emerged as a key component in antiseptics, with its derivative carbolic acid (phenol) revolutionizing surgical practices by reducing post-operative infections when applied to wounds and instruments, as pioneered by . In modern medicine, remains a standard topical treatment for , applied in shampoos, creams, or solutions typically containing 2-10% to reduce scaling, itching, and inflammation by slowing skin cell growth. Similarly, is commonly used in soaps and bath additives for managing eczema, leveraging its , antibacterial, and effects to alleviate dryness, irritation, and flare-ups in sensitive skin. Regulatory oversight ensures safe application; the U.S. Food and Drug Administration (FDA) classifies as safe and effective for over-the-counter topical use in concentrations up to 5% for and related dermatoses, with higher strengths available by prescription. Historically, Victorian-era featured complex mixtures incorporating with tar derivatives for relief and cough suppression, though such combinations have been discontinued due to safety concerns.

Health, Safety, and Environmental Impacts

Toxicity and Health Risks

Exposure to tar, particularly and its derivatives, can cause acute health effects primarily through direct contact and inhalation of fumes. contact with tar often leads to , burns, and , characterized by redness, itching, and potential blistering due to its irritant and phototoxic properties. Inhalation of tar vapors, which may contain and other volatile compounds, can result in respiratory , coughing, sneezing, , , and ; the (OSHA) sets a of 1 ppm for over an 8-hour workday to mitigate these risks. Eye exposure may cause and corneal damage from acute fume inhalation. The primary routes of tar exposure in occupational settings are of fumes and vapors, accounting for a significant portion of cases, and dermal absorption through skin contact with tar or contaminated surfaces. is particularly prevalent during heating or application processes, while dermal exposure occurs via direct handling, often leading to systemic absorption of polycyclic aromatic hydrocarbons (PAHs). is less common but possible through hand-to-mouth transfer. These routes contribute to both immediate and prolonged health hazards in industries like paving and roofing. Chronic exposure to tar poses substantial carcinogenic risks, largely attributed to PAHs such as benzopyrene, which the International Agency for Research on Cancer (IARC) classifies as Group 1 carcinogens; coal tar itself and coal-tar pitch are also designated Group 1 due to sufficient evidence of carcinogenicity in humans. Occupational studies have linked prolonged tar exposure to elevated lung cancer risk, with standardized mortality ratios indicating up to 1.5-fold increases among tar-distillery and coal gas workers, and historical data from the early 20th century suggesting even higher relative risks in exposed cohorts. Skin, bladder, and kidney cancers are also associated with long-term dermal and inhalational exposure. Mitigation of tar-related health risks relies on (PPE) and adherence to standards established by OSHA since the 1970s, including respirators for fume and impervious gloves, clothing, and eye protection to prevent dermal absorption. Regular monitoring and further reduce exposure levels in controlled industrial environments, though tar's use in some medical applications requires similar precautions to minimize risks.

Environmental Concerns and Regulations

Tar, particularly coal tar and petroleum-derived variants, poses significant environmental risks through contamination of soil and water bodies. Spills and leaks from tar storage or application sites lead to persistent pollution, as tar's viscous nature allows it to infiltrate soil pores, reducing permeability and altering microbial activity. In aquatic environments, tar residues introduce polycyclic aromatic hydrocarbons (PAHs), which are hydrophobic and adsorb to sediments, contaminating groundwater and surface water over extended periods. PAHs from tar exhibit high bioaccumulation potential in aquatic organisms, concentrating in fish tissues and magnifying through food chains, which can indirectly affect human health via seafood consumption. Air emissions from tar production, use, and contribute to atmospheric , including and volatile PAHs that participate in formation. During coal tar application or from paved surfaces, PAHs volatilize or become airborne as , reacting with other pollutants to form secondary aerosols that exacerbate urban . These emissions are particularly notable in and settings where tar-based materials like sealants are prevalent. Notable incidents highlight tar's ecological footprint. The 1989 Exxon Valdez oil spill released approximately 11 million gallons of crude oil into , , which weathered into tar balls and residues persisting in sediments for decades, causing ongoing contamination of intertidal zones and affecting marine ecosystems. In urban areas, runoff from sealed pavements has polluted stormwater ponds and nearby water bodies; for instance, studies in the U.S. Midwest found coal tar sealants accounting for up to 67% of PAHs in metro-area sediments, leading to infiltration and broader aquatic toxicity. Regulatory frameworks address these concerns through restrictions on tar and its components. In the , the REACH regulation (Annex XVII, Entry 50) since 2013 limits eight carcinogenic PAHs in consumer s, including those derived from , to 1 mg/kg to prevent environmental release via products like tires and sealants. In the United States, the EPA's land disposal restrictions under 40 CFR Part 268 classify certain tar wastes as hazardous if they exhibit toxicity characteristics, requiring treatment to meet standards before disposal, including wastewater sludges containing tar-derived PAHs. These measures encourage a shift to bio-based alternatives, such as lignin-derived binders from forestry residues, which reduce PAH emissions and comply with mandates in regions banning sealants.