What causes soot in chimneys?

The soot that forms in chimneys and stovepipes is a black deposit from wood, coal, oil or other fuel and consists of a combination of partly burned carbon, ash and the finer tarry substance known as lampblack. The proportion of these constituents determines the dryness of the soot, which may vary from a perfectly dry substance to a rather viscous gum, although soot is seldom sufficiently liquid to flow at ordinary temperatures. Particles of carbon, the product of imperfect combustion of the fuel, form the coloring matter of the gases known as smoke. Many of these particles of carbon, while being carried upward in the current of hot air and other gases, adhere to the sides of the pipe and chimney. As a rule the black liquid that exudes from chimneys into houses and soils the walls is a mixture of soot and rain water. A coating of soot in a stovepipe considerably reduces the amount of heat radiated. It has been, estimated that the surface of a stovepipe loses about one-eighth of its heat-transmitting power for each one-eighth of an inch of soot.