Nothing ever burns completely. Wood smoke is a combination of unburned gases and fog of unburned tar like liquids. When these gases come in contact with a cool surface, they will condense and form a nasty dark brown or black substance which has an unpleasant acrid odor. This is creosote.
Creosote starts as a liquid which results from condensation of the flue gases. Creosote comes in a range of forms: sooty, ash like deposits: dry, flaky deposits; sticky, tacky deposits resembling tar; or hard shinny deposits. Creosote collects inside the flue passage, in offsets and in termination parts of your chimney. These deposits reduce the flow of gases through the chimney system which may result in a weak draft or smoke spillage into the room.
Creosote is highly flammable. When it’s allowed to build up, the result could be a chimney fire. No matter what kind of chimney you have, such overheating is dangerous to the chimney structure and the surrounding building. Veteran wood burners know the importance of keeping their chimneys clean. However, many newcomers to heating with wood may be unaware of the potential harm and hazard of creosote buildup.
Three Factors That Influence Creosote Deposits
Smoke density: High smoke density increases the rate of creosote formation. Smoke density can be reduced by increasing the flow of air, and by using smaller pieces of wood or adding less wood more often. Hotter fires will also lessen the smoke density by causing more complete combustion of the wood and gases.
Temperature of the condensing surface: The cooler the surface, the more creosote will condense. One can relate this to water vapor condensing on the outside of a glass of cold water on a humid day, except its in reverse. Condensation occurs on the inside of a chimney, especially when the outside cold air makes the surface of the inner chimney relatively cool. Keeping stack temperatures high will reduce this problem.
Residence time: The longer the smoke stays in your chimney, the more likely it is to condense on the surface.