Burning wood causes combustion of the oils in the wood, which are off-gassed as volatiles in the smoke. As the smoke rises through the chimney it cools, causing water, carbon, and volatiles to condense on the interior surfaces of the chimney flue. The black oily residue that builds up is referred to as creosote. Over the course of a season creosote deposits can become several inches thick. Chimney inspections are the only to determine the extent of the damage that the creosote has created.