Chimney deposits, also known as soots, are the black residue that accumulates in chimneys due to the incomplete combustion of fuel. The soots are composed of a variety of organic and inorganic compounds, including carbon, ash, tar, and other pollutants. These deposits can cause blockages in the chimney, leading to dangerous levels of carbon monoxide in the home. Regular cleaning of the chimney is necessary to prevent the buildup of these deposits.