How creosote forms
Wood smoke carries unburned particles and gases up the flue. As the smoke cools, those materials stick to the inside of the chimney and form creosote.
Burning damp wood or keeping a fire smouldering with low air can make buildup worse, because the smoke is cooler and heavier.
What it looks like
Creosote can be flaky and sooty, sticky and tar-like, or hard and glossy. The hard, glazed kind is the most difficult to remove.
Why it matters
Creosote is flammable, and heavy buildup can feed a chimney fire. It also narrows the flue, which can make a fire harder to start and let smoke back into the room.
How to keep it under control
Burn dry, seasoned wood and give the fire enough air. Have the chimney checked regularly and swept when buildup is found. How often you need a sweep depends on how much you burn and what you burn.