Soot is a fine black to brown powder which is formed through incomplete combustion. The process of partial combustion also makes soot slightly sticky, and it can be found clinging to things like chimneys and exhaust pipes as a result. In addition to being a byproduct of the combustion process, soot can also be useful; it has been utilized for centuries as a source of pigment for inks and dyes, for example. Many consumers are familiar with some form of soot, since combustion is such a ubiquitous process in daily life.

In order to form soot, an object must be burned at a comparatively low temperature with a reduced supply of oxygen. As the fuel burns, it breaks up into small particles; in the right conditions, these particles include soot, which will settle out as a dark powdery deposit. For an example of soot, try holding a heat-safe glass over a lit candle for a moment; a streak of black material will appear where the flame came into contact with the glass.

Soot is also called lampblack or carbon black. Carbon black is used in the vulcanization process to treat rubber, and it is also used in a wide range of pigment, paint, and dye products, ranging from crayons to fine ink for fountain pens. Historically, people also used carbon black in cosmetics, or as part of camouflage paint. Carbon black is also used in toners for laser printers and copiers.

Like some other products of combustion, soot can be harmful. Incomplete combustion promotes the formation of dioxins and other compounds which can be toxic, and particulate matter can be harmful to the lungs. During the Industrial Revolution, soot from factories was linked with serious health problems in the neighboring community as people routinely inhaled soot, causing irritation in their lungs.

In things like chimneys, soot can also clog, causing a range of problems. A clogged chimney will not draw as efficiently, so sooty chimneys can tend to cause smoky fires or inefficient use of fuel. Soot can also be dislodged from a chimney, causing a gust of black, sticky particles to enter a house. As you can imagine, this can cause quite a mess. Soot can also cause chimney fires if it is allowed to build up, which is why it is important to regularly clean chimneys, stove hoods, and other piping which is designed to deal with the byproducts of combustion.