Creosote is a category of oily, complex chemical mixtures historically valued for their preservative and antiseptic properties. The name is derived from the Greek words for “meat” and “preserver,” reflecting its early use in meat preservation. The term creosote does not refer to a single compound, but rather to several chemically distinct products made from different raw materials. This difference in origin causes frequent public confusion regarding its composition and use.
Distinguishing the Commercial Types of Creosote
The commercial market recognizes two primary types of manufactured creosote, separated by their source material. Coal Tar Creosote is derived from fossil fuels, specifically bituminous coal, and is the most common industrial form used globally. Wood Tar Creosote is manufactured from the thermal decomposition of plant material, typically hardwoods. These two products possess distinct chemical profiles that dictate their respective applications. The black, tarry substance accumulating inside chimneys is sometimes called creosote, but this is a misnomer. Chimney creosote is merely a residue of uncombusted organic vapors and soot that condense in a flue when wood is burned at low temperatures.
Manufacturing Coal Tar Creosote
The industrial production of Coal Tar Creosote begins with coal tar, a viscous, black byproduct of manufacturing coke or gas from bituminous coal. Coal tar contains hundreds of different organic compounds, including Polycyclic Aromatic Hydrocarbons (PAHs). The process used to isolate the creosote fraction is called fractional distillation, which separates components based on their boiling points.
The coal tar is heated in a still or tower, causing its chemical components to vaporize sequentially. As these hot vapors rise through the fractional distillation column, they cool and condense back into liquid fractions at different temperature levels. The first fractions to condense are the lighter oils, which contain compounds like carbolic acid and naphthalene. The target creosote oil is collected later, typically condensing between 230 and 355 degrees Celsius.
This fraction is referred to as “heavy oil” or “dead oil” because it is denser than water and is collected after the lighter fractions are removed. The distillation process is controlled to ensure the final product meets industry standards for wood preservation. The residue remaining in the still after the creosote oil is collected is coal tar pitch, a dark solid material. Coal Tar Creosote is characterized by a high concentration of aromatic hydrocarbons, particularly naphthalenes, phenanthrenes, and anthracenes.
Manufacturing Wood Tar Creosote
Wood Tar Creosote is produced through dry distillation, which involves heating wood in a sealed container with a limited or complete absence of oxygen. The preferred raw material is typically beechwood, as it yields a product with a higher concentration of desired medicinal compounds. When hardwoods are heated to temperatures between 400 and 500 degrees Celsius, they undergo thermal decomposition (pyrolysis).
This distillation generates volatile gases and a liquid condensate known as pyroligneous acid, with charcoal remaining as the solid residue. Pyroligneous acid is a mixture of water, acetic acid, methanol, and wood tar. The wood tar separates from the aqueous layer and contains the precursor compounds for wood creosote.
The wood tar must be further refined to isolate the creosote component, which is a mixture of phenolic compounds. This refining step involves treating the wood tar oil with an alkaline solution, such as sodium hydroxide, which selectively dissolves the phenolic components. The alkaline solution is separated from the insoluble neutral oils, then neutralized with an acid (e.g., sulfuric acid) to precipitate the crude wood creosote oil. This process purifies the mixture, yielding a product rich in methyl ethers of phenol, primarily guaiacol and cresol.
Resulting Composition and Practical Applications
The difference in manufacturing results in two products with distinct chemical compositions and uses. Coal Tar Creosote is a dark, oily liquid whose efficacy as a wood preservative stems from its high content of Polycyclic Aromatic Hydrocarbons (PAHs). PAHs are effective biocides, making Coal Tar Creosote the industrial standard for pressure-treating timber used in demanding environments, such as railroad ties, utility poles, and marine pilings. Its application protects wood against fungal decay and insect attack, extending the material’s service life.
Wood Tar Creosote is a lighter, yellowish liquid characterized by its high concentration of phenolic compounds like guaiacol and cresol. Historically, this type of creosote was used in medicinal applications as an antiseptic, expectorant, and treatment for skin conditions. Its phenolic content makes it responsible for the flavor and preservation of smoked meats. A purified form is still used as an active ingredient in some over-the-counter cough and gastrointestinal medicines.