TAR AND CREOSOTE
Burning wood brings with it a unique problem, this being tar build up. As more than 50% of the weight of wood is moisture, even in seasoned wood, it burns at lower temperatures. Even very dry wood has high sap levels, which when burnt are carried in the smoke, and because the temperature of the smoke is already low, the residue from the sap condenses on the sides of the flue, not as a soft soot but a 'treacle' consistency. This is particularly the case when a burner is loaded with wood, then the air supply shut down to maintain a burn overnight. As more fires are lit, producing more condensation, the deposits on the side of the flue turn to a liquid tar (creosote), which you may see leaking down the flue pipe or around the register plate that seals off the chimney.
This wet, tar like substance will then attack the fabric of the chimney, and through capilliary action, work its way out to the plaster finish on the walls, weakening the mortar at the same time. This usually shows up first in bedrooms with a chimney breast, a brown, half moon shape stain appearing, and a similar one on the adjacent ceiling.
Over time the residue will build up, as it goes from a wet state to dry with subsequent fires, and so the flue will become narrower. The solid tar is highly flammable and will readily catch fire. The temperatures reached in the flue during a fire can be well over 2000c, the result being cracks in the stack, pot, flue and plaster or a house fire. If the flue is lined, it will almost certainly destroy a steel liner, and will result in a poured liner crumbling away. Once a fire has occurred, the volatile element of the tar is burnt, leaving a dry crumbly residue. It is therefore essential to sweep woodburning appliances at least twice per year.