Creosote refers to a category of oily liquids derived from the high-temperature processing of organic materials. This single name is confusing because it applies to two chemically distinct substances: coal-tar creosote and wood-tar creosote. Both are byproducts of thermal decomposition processes involving either coal or wood. Although they share a similar physical appearance as dark, viscous liquids, their origins, chemical makeup, and primary applications are fundamentally different.
Coal-Tar Creosote: Chemical Composition and Source Material
Coal-tar creosote is the substance most commonly associated with the name. It is derived from the distillation of crude coal tar, a viscous black residue. Coal tar is a byproduct generated during the high-temperature carbonization of bituminous coal, a process used primarily to produce coke for the steel industry and coal gas. The coal tar is then fractionally distilled to separate the various components, yielding creosote oil.
The resulting creosote oil is a complex mixture containing hundreds of organic compounds. Its character is dominated by Polycyclic Aromatic Hydrocarbons (PAHs), which constitute at least 75% of the mixture. PAHs are molecules characterized by multiple fused aromatic rings. Key PAHs found in coal-tar creosote include naphthalene, phenanthrene, and anthracene.
The exact chemical composition of coal-tar creosote is not uniform. It depends on the temperature used during the initial coal carbonization process, which dictates the yield and type of aromatic compounds in the final distillate. Besides the dominant PAHs, the mixture also contains smaller amounts of phenolic compounds (such as phenols and cresols) and nitrogen- and sulfur-containing heterocyclic compounds.
Coal-tar creosote is an oily liquid with a continuous boiling range starting at approximately 200°C. The substance is heavier than water and possesses a yellowish-dark green to brown color. The high concentration of PAHs gives this material distinct properties, differentiating it from the wood-derived version.
Wood-Tar Creosote: Distillate Origin and Main Components
Wood-tar creosote, sometimes called beechwood creosote, is chemically distinct. It is produced through the destructive distillation of wood, a high-temperature process involving heating specific woods, usually beechwood or birch, in the absence of air. The resulting wood tar is then distilled to isolate the creosote fraction.
The chemical structure of wood-tar creosote is fundamentally different from the coal-derived version because it is predominantly composed of phenolic compounds, rather than PAHs. The primary active components are methoxy derivatives of phenol, meaning they contain a methoxy group attached to the phenol ring. The most significant compounds are guaiacol and creosol (4-methylguaiacol).
Guaiacol is a methyl ether of pyrocatechin, and creosol is a methyl ether of methyl-pyrocatechin. Beechwood is often preferred for production because it yields a higher proportion of these phenolic chemicals in the distillate. The high concentration of these methyl derivatives is a major chemical differentiator from coal-tar creosote.
This wood-derived liquid is colorless or a pale yellowish liquid, possessing a characteristic smoky odor and a burning taste. Due to its chemical composition, wood-tar creosote is less caustic and less poisonous than the coal-tar version. It is also less resistant to water dissolution compared to the PAH-rich coal tar.
How These Different Creosotes Are Used
The distinct chemical makeups of the two creosote types dictate their primary uses. Coal-tar creosote, with its high concentration of toxic Polycyclic Aromatic Hydrocarbons, is used almost exclusively for heavy-duty industrial wood preservation. Its application focuses on protecting wood products exposed to harsh environmental conditions, such as railroad ties, utility poles, and marine pilings. The substance is highly valued for its resistance to rot and insects, and it is applied using pressure methods to ensure deep penetration and long-term protection.
In contrast, wood-tar creosote, containing guaiacol and creosol, finds use in minor applications focused on its distinctive flavor and historical antiseptic properties. It is the active ingredient responsible for the flavor profile in smoked foods, often used as “liquid smoke.” Historically, it was used as an antiseptic, an expectorant, and a digestive aid, though these medical uses have largely been replaced by modern formulations.