Chimney deposit is a term used to describe the accumulation of SOOT and SMUT in a chimney. SOOT is a black or dark grey powdery or flaky substance made up of tiny particles of carbon that is produced when something is burned, while SMUT is a black or dark grey powdery substance made up of tiny particles of ash and other materials that is also produced when something is burned. Both SOOT and SMUT can accumulate in a chimney and form a chimney deposit.