Smoke from a wood burning fire contains wood particles, gases, and other chemical compounds. As these combustion byproducts cool on their way up the chimney, they condense along the chimney walls. This condensation is what turns into creosote.

How is creosote created?

Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel.

What makes up creosote?

Creosote is a mixture of hundreds of chemicals. A mixture is substances, like water and salt, staying together by physical forces. The major chemicals in coal tar creosote are polycyclic aromatic hydrocarbons (PAHs), phenol, and creosols. Creosote is a thick and oily liquid.