Creosote is a natural byproduct of burning wood as moisture and gases released from the wood condense in the flue. It can mix with soot and other gases and compounds produced by the fire to form a tarry material that can coat the inside of the flue. The longer this creosote / tar is left unchecked the more it will build up and this restricts the size of the flue making it more difficult for smoke and emissions to escape. It can also be highly flammable and is a common cause of chimney fires.

It is often said that prevention is better than cure. Wood with a high moisture content produces more creosote so to reduce the amount being produced in the first place, only burn well seasoned wood. Even this contains some moisture so your fire will produce some creosote. The hotter the fire the less creosote is created as more moisture is burned off. A slow burning fire or stove will allow more moisture to escape into the flue. Regular sweeping will stop the build up of creosote, soot and tar but if the chimney is already very tarry, sweeping alone is not enough to get rid of it.

Your chimney sweep should able to identify if your chimney is tarry by sweeping and checking the material that falls and recommend the right treatment to get rid of it. The images below show example of what this creosote tar looks like.