Creosote comes from three very different sources depending on context: the incomplete burning of wood in a fireplace or stove, the industrial distillation of coal tar, or the natural resin of a desert shrub. Most people searching this question are dealing with the dark, flammable buildup inside a chimney flue, but the word “creosote” also applies to an industrial wood preservative and a plant that shares the name. Here’s what each one actually is and how it forms.
Chimney Creosote: Unburned Wood Gases
The creosote most people encounter is the sticky, dark residue that accumulates inside chimneys and stovepipes. It forms when wood doesn’t burn completely. As wood heats up to around 500°F, it begins to break down chemically, releasing volatile gases that carry 50 to 60 percent of the wood’s total heat energy. If the fire is hot enough (above 1,100°F) and has enough oxygen, those gases ignite and burn off. If it isn’t, they drift upward as smoke.
As that smoke travels through the chimney, it cools. When the temperature of the flue gases drops below roughly 250 to 270°F, the unburned volatiles and water vapor condense onto the cooler chimney walls. Over time, the water evaporates and leaves behind a dark, highly combustible coating. That coating is creosote.
Two conditions accelerate the process: incomplete combustion (a smoldering, oxygen-starved fire) and cool flue temperatures. A slow-burning fire in a large firebox, a poorly insulated chimney, or an oversized flue that lets gases cool too quickly will all produce more creosote. Burning wet or unseasoned wood makes things worse because the fire spends energy evaporating moisture instead of reaching the temperatures needed to burn off volatile compounds.
The Three Stages of Buildup
Chimney creosote doesn’t stay the same consistency. It progresses through three stages, each harder to deal with than the last. In Stage 1, fresh creosote appears as a light, sooty or flaky residue that can be brushed away during a routine chimney cleaning. In Stage 2, it hardens into a tar-like, crusty material that requires heavy scraping to remove. Stage 3 is the most dangerous: a thick, glossy, hardened glaze that is extremely flammable and difficult to remove without professional tools. Stage 3 creosote is the primary fuel in chimney fires.
How to Minimize Chimney Creosote
The most effective step is burning dry wood. Firewood should have a moisture content of 20 percent or less, which typically means splitting and air-drying it for at least six months. You can check with an inexpensive moisture meter. Beyond that, keeping the fire burning hot enough to fully combust those volatile gases matters more than any other variable. Avoid damping down the fire to a smolder for long periods, and make sure the stove or fireplace has adequate air supply.
Maintaining a flue temperature above 250°F at the top of the chimney prevents condensation from forming in the first place. A stovepipe thermometer mounted on the flue can help you monitor this. Proper chimney insulation, correct flue sizing, and annual inspection by a certified chimney sweep all reduce the risk of dangerous buildup.
Coal Tar Creosote: An Industrial Product
Industrial creosote is a completely different substance. It comes from coal tar, which is a byproduct of heating coal to extremely high temperatures in the absence of air (a process called destructive distillation, used in coke production). When the hot gases released during this process are cooled back to ambient temperature, they condense into a thick, dark liquid: coal tar.
That tar is then distilled at atmospheric pressure, with different chemical compounds boiling off at different temperatures up to about 400°C. The resulting distillate is coal tar creosote, an oily liquid ranging from yellowish-green to brown. It’s a complex mixture: roughly 85 percent polycyclic aromatic hydrocarbons (a class of compounds formed when carbon-rich materials burn incompletely) and 2 to 17 percent phenolic compounds. It also contains naphthalene, anthracene, and phenanthrene derivatives, along with sulfur and nitrogen compounds.
Coal tar creosote has been used since the 1800s as a wood preservative, primarily for outdoor applications like railroad ties, utility poles, and marine pilings. It works by making wood toxic to termites, fungi, and other organisms that cause decay. In the United States, creosote-treated wood products are restricted to commercial and industrial use. They are not available to homeowners, though people sometimes encounter recycled creosote-treated railroad ties repurposed as landscaping timbers. The EPA has noted that creosote may pose risks to fish and aquatic invertebrates when treated wood is used near water.
Wood Tar Creosote: The Medicinal Version
There’s a third type that predates the industrial version. Wood tar creosote, first prepared in Germany in 1830, is distilled from the tar of hardwoods like beech. Its chemical profile is distinct from coal tar creosote. It consists mainly of phenol, cresols, guaiacols, and xylenols, without the heavy load of polycyclic aromatic hydrocarbons that makes coal tar creosote toxic.
This version was historically used as medicine. It appeared in the German, American, and Japanese pharmacopoeias as an antibacterial treatment for pulmonary tuberculosis, diarrhea, and wound care. Western countries have since dropped it from their official drug registries, but it remains listed in the Japanese Pharmacopoeia. In Japan, a popular over-the-counter digestive remedy called Seiro-gan combines wood creosote with herbal ingredients and is widely used for food poisoning and diarrhea.
The Creosote Bush: A Plant, Not a Byproduct
The creosote bush is a desert shrub native to the American Southwest and Mexico. It earned its common name because its resin has a tarry smell reminiscent of industrial creosote, but chemically the two have almost nothing in common. The plant’s resin is overwhelmingly phenolic, with that fraction making up 83 to 91 percent of the total resin content. Its signature compound, nordihydroguaiaretic acid, accounts for 5 to 10 percent of the dry weight of the leaves and acts as a potent antioxidant. The resin also contains waxes, alkaloids, and phenolic acids. None of the polycyclic aromatic hydrocarbons found in coal tar creosote are present.
So while the word “creosote” gets applied to all four of these substances, they come from entirely different sources and carry very different risks. The chimney variety is a combustion byproduct you want to prevent. The coal tar version is a regulated industrial chemical. The wood tar version is a mild medicinal product still in use in parts of Asia. And the creosote bush is simply a plant with an unfortunate name.