Creosote is a general term for a family of complex chemical mixtures created through the high-temperature treatment of organic materials, primarily wood or coal. Historically, this substance has been valued for its potent antiseptic and preservative properties, useful in medicine and for protecting wood from decay. The name is derived from the Greek words for “meat” and “preserver,” reflecting its early use in smoking and curing food. Because different types vary significantly in toxicity, application, and regulatory status, understanding their composition is important. This article focuses on the small-scale production of wood-tar oil, the precursor to wood-tar creosote.
Clarifying Creosote: Wood vs. Coal-Tar
The two main types of creosote are wood-tar creosote and coal-tar creosote, which are chemically distinct despite sharing a name. Wood-tar creosote, often derived from the dry distillation of hardwoods like beech or birch, is rich in phenolic compounds, specifically methyl ethers such as guaiacol and creosol. These methyl ethers are less caustic and less poisonous than simple phenols, historically allowing for its use in medicinal applications like cough treatments and as an anti-diarrheal.
Coal-tar creosote is a byproduct of the carbonization of bituminous coal during the process of making coke. This industrial-grade product is a dark, thick, oily liquid containing a high concentration of polycyclic aromatic hydrocarbons (PAHs), including naphthalenes and anthracenes. It is significantly more toxic and is used for industrial wood preservation, such as treating railroad ties and utility poles, due to its superior efficacy against fungi and insects. The United States Environmental Protection Agency (EPA) classifies coal-tar creosote as a Restricted Use Pesticide, meaning it is not available to the general public and can only be applied by certified professionals.
A third common usage of the term refers to the tarry residue that builds up inside chimneys and flues. This chimney creosote is a byproduct of incomplete wood combustion, essentially a form of wood-tar creosote that condenses when wood is burned at low temperatures. This residue is not a usable product but rather a fire hazard that must be regularly removed. The DIY method discussed here focuses on producing the less-toxic wood-tar oil through a controlled process, which is the only practical approach for a small-scale enthusiast.
The DIY Method: Producing Wood Tar Oil
The small-scale production of wood tar oil relies on dry distillation or pyrolysis, which involves heating wood in an environment with little to no oxygen. This thermal decomposition breaks down the wood’s organic matter into three main products: a solid residue (biochar), non-condensable gases (syngas), and a condensable liquid fraction known as pyroligneous acid or wood vinegar, which contains the wood tar oil. The goal is to heat the wood to temperatures between 300°C and 500°C to maximize the liquid yield.
The necessary setup requires a sealed metal container, often called a retort, to hold the wood and prevent oxygen from entering, which would cause the wood to burn instead of pyrolyze. This retort must be connected to a collection system that allows the hot vapors to escape and cool down back into a liquid. A simple collection system involves a pipe leading from the retort into a cooling coil or a water-jacketed condenser, which then drips the resulting liquid into a collection vessel.
The wood feedstock, such as wood chips or small pieces of hardwood like beech, should be dry to ensure an efficient process, as moisture reduces the energy efficiency. As the retort is heated externally, the wood begins to decompose, releasing a mixture of steam, gases, and organic vapors. The vapors travel through the cooling system, where they condense into a liquid that separates into two distinct layers in the collection vessel.
The top layer is a watery, acidic liquid known as pyroligneous acid, or wood vinegar, while the heavier, dark, oily layer settling at the bottom is the crude wood tar oil. This crude wood tar oil is the substance used for small-scale preservation and sealing applications, and it is the precursor to true wood-tar creosote. Achieving a refined wood-tar creosote requires fractional distillation of this crude oil to isolate specific phenolic compounds, a step that is impractical for a home setup.
Safety, Handling, and Regulatory Considerations
Working with creosote and its precursors requires safety protocols due to the health and fire risks involved. All forms of creosote, including the wood-tar oil produced through pyrolysis, contain phenolic compounds that can cause skin irritation, respiratory issues, and are considered potential carcinogens. Personal protective equipment (PPE) is mandatory, including chemical-resistant gloves, long-sleeved clothing, and eye protection to prevent direct skin or eye contact.
The pyrolysis process involves high heat and the production of flammable gases, requiring the operation to be conducted outdoors in a well-ventilated area, far from combustible materials. The syngas produced during the process is highly flammable and must be safely vented or flared off to prevent the buildup of explosive fumes. Inhaling the toxic vapors and smoke released during the heating and condensation phases must be avoided, often requiring the use of a respirator with appropriate organic vapor cartridges.
For storage, the resulting wood tar oil should be kept in clearly labeled, airtight, non-reactive containers away from heat sources, as it remains a flammable substance. Waste disposal is a serious consideration; while the wood vinegar layer is less hazardous, any contaminated materials, such as rags or spent filters, must be treated as hazardous waste and disposed of according to local environmental regulations. Users should always consult local ordinances regarding the use and disposal of wood-tar products to ensure compliance.