Creosote is a carbon-based particulate that is carried up the chimney in the smoke given off by a fire. It is left behind when condensation occurs as a result of heated vapors making contact with the cooler brick or metal flue. Their formation occurs in three stages and creosotes are combustible in all of them.
The first stage of creosote formation is a fine powder that can easily be swept out of the chimney with a brush long enough to reach the entire flue. The easiest type of creosote to deal with, stage one creosotes can cause slow-burning chimney fires which often go undetected by the homeowner. Creosote in this stage is soot, made up of carbon particles and ash, so a commercial soot remover may also be a good idea.
In its second stage, creosote takes the form of crunchy rock-like deposits and is more difficult to remove, requiring stiff brushes and scrapers. Creosote is porous in this stage and may at first be flaky, puffy bubbles of unburned carbon that can be broken up and scraped off. A number of commercial products specifically formulated to remove stage two creosote is available, including liquids that can be sprayed onto the buildup and special fireplace logs. Of course, none of these is as effective or safe as calling a professional chimney sweep.
Stage three creosote is a waxy, tar-like accumulation that can both pool in the chimney and drip down it in gooey stalactites. The most difficult stage of creosote formation to deal with, it is also the most dangerous, likely to lead to an explosive chimney fire. Also known as glaze, these dense deposits are fuel for a fire and must be removed, best accomplished with a liquid that is applied to dissolve them.
It is worth remembering that, regardless of the stage in which they exist, creosote is flammable and increases the likelihood of chimney fires. It also produces a strong acrid odor that can move through the house if there is insufficient ventilation or a down draft. In short, removing creosote in the first stage of formation is the easiest and safest way to deal with them and an important precaution for homeowners to take.