What is creosote?
Creosote is a natural byproduct of burning wood.

It is produced when the wood fuel is not completely burned, allowing soot and oils to be carried up in the smoke. The smoke cools as it rises through your chimney and the oils and soot condense on the interior surface of the flue. The black oily substance left on the interior of the flue is creosote.

Why is creosote a problem?

Over the course of a burning season, the creosote deposits in your flue can grow to be several inches thick. This obviously creates an immediate problem because it makes your flue narrower, making it more difficult for smoke to escape from your chimney. This can lead to smoke seeping back into your home where it can cause discoloration and damage. The second worry is that creosote is highly flammable, which is an obvious danger in an environment that is exposed to large amounts of heat, like a chimney. Creosote chimney fires account for 25% of all residential fires in the United States.

How is creosote removed?

There are several different types of creosote buildup, and some are easier to remove than others. It starts as a flaky, sooty form, which is easy to brush away. The next stage is when it begins to accumulate in hard, brittle deposits, but professional chimney sweeps are used to dealing with these as well. The real problem is 'glazed' creosote, which is recognizable from its shiny, dense appearance. This is the creosote that causes the most serious chimney fires, and it is also the most difficult to remove.