Soot is a black or dark brown substance found in smoke. Soot consists chiefly of particles of carbon. These particles have a diameter less than 1/50 the width of a human hair. They form when carbon-containing fuels, such as coal, wood, or oil, do not burn completely.
Airborne soot is a form of air pollution. It sticks to any surface it touches. Soot can harm the respiratory system, particularly the lungs. It also can cause widespread damage to property. Smoke blowing through a city leaves soot on buildings, making them look dingy. This soot can eventually damage the surfaces of the buildings by reacting chemically with them.
However, soot is valuable as a pigment (coloring matter). Two kinds of soot used as pigments are bister and lampblack. Bister, a shiny brown powder containing dried tar, is the kind of soot found nearest to wood fires. Lampblack, a black soot, is found farther from such fires. Lampblack also is produced in the incomplete burning of such fuels as oil and natural gas.
