Soot is a general term that refers to the black, impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres, charred wood, petroleum coke, etc. that may become airborne during pyrolysis and which are more properly identified as cokes or chars. The gas-phase soots contain polycyclic aromatic hydrocarbons (PAHs). The PAHs in soot are known mutagens and probable human carcinogens. They are classified as a "known human carcinogen" by the International Agency for Research on Cancer (IARC).
Some soots are produced commercially to be used as pigments, such as lampblack and carbon black. These products have been used for many years as common pigments used in paints and inks, and remain in use today in toners for xerography and laser printers. The black color of rubber tires is due to the use of lampblack or carbon black as an ingredient in their vulcanisation; this use accounts for around 85% of the market use of these products. Bone black, another black pigment and decolorizing agent, is the product of charring bones and is not a soot.
In India lampblack is used for a different purpose. The closest definition found is as follows. "Collyrium or lampblack [or Katuka as called in Telugu], a paste made of lampblack and oil and applied to the eyes to increase their brilliancy. It is also supposed to assist in conjuring and giving second sight.
Soot, as an airborne contaminant in the environment has many different sources but they are all the result of some form of pyrolysis. They include soot from internal combustion engines, diesel engines, power plant boilers, hog-fuel boilers, ship boilers, central steam heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, furnaces, etc. These exterior sources also contribute to the indoor environment sources such as smoking of plant matter, cooking, oil lamps, candles, quartz/halogen bulbs with settled dust, fireplaces, defective furnaces, etc. Soot in very low concentrations is capable of darkening surfaces or making particle agglomerates, such as those from ventilation systems, appear black. Soot is the primary cause of “ghosting”, the discoloration of walls and ceilings or walls and flooring where they meet. It is generally responsible for the discoloration of the walls above baseboard electric heating units.
The production of soot in a flame is a complex process consisting of several chemical reactions taking place in series. In the fuel-pyrolysis zone of the flame, typically clear or blue, the fuel molecules are broken down into various fragments, including carbon-ring structures, acetylene (C2H2), the radical C3H3 (and higher order), as well as monatomic and diatomic hydrogen. As the combustion process continues the radicals quickly combine into new structures, giving off heat. These precursors polymerize into larger "pre-soot" chains then gather into formations of hydrogen-rich spheres in the soot-inception zone. In the soot-growth zone these spheres give up their hydrogen gas through diffusion, resulting in solids consisting of several of the formerly liquid spheres stuck together into larger chains. It is this portion of the flame that has the bright yellow color. Hydrogen-rich examples then further oxidize, releasing more heat. In perfect combustion the soot would break down into almost pure CO2 and H2O; it is only in incomplete combustion that the soot is able to form and escape the flame.
Soot normally forms at about 1400 °C, forming an excellent blackbody radiator of colors in the yellow to red spectrum. The typical yellow color of a candle flame or wood fire is produced primarily by the hot soot forming inside.
The energy being radiated from the soot is an important contributor to the ongoing combustion process, cooling the flame above the soot-growth zone and feeding energy back into the fuel-pyrolysis zone. In "pool fires" of open liquid fuel this process can feed as much as 50% of the flame's energy back into the liquid fuel below, which vaporizes it and keeps the reaction going; it would otherwise burn much more slowly. The same release of energy is responsible for quickly cooling the flame above the soot-growth region, limiting its further combustion into lighter molecules, and explaining why these fires release so much soot. A canonical example is the 2005 Hertfordshire Oil Storage Terminal fire, which released massive amounts of soot and covered the skies over a large portion of the London area.
The separation of flame into zones of different chemical reactions is due to convection forcing the hot reactants upward. In microgravity or zero gravity convection no longer occurs, and such flames tend to become more blue and more efficient, producing much less soot. Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized than in conditions on Earth, because of a series of mechanisms that differ from those in normal gravity conditions.
Lampblack has been used since prehistoric times as the source for carbon black, collected by holding a cold surface over a cool flame. Candles or lamps using animal fats or waxes generate considerable amounts of soot that can be collected and then mixed with a lubricant to produce ink. This process can be easily duplicated today by passing some noncombustible surface, such as a tin can lid or glass, closely through a candle flame. Lampblack produced in this way is among the darkest and least reflective substances known.
Lampblack is also used to coat aluminium foil that has been previously attached to a recording drum for use in a recording barograph or other instrument. The surface is scratched clear by a pointed stylus. In this case, the sooty smoke is produced by burning a small amount of camphor. After recording the image is fixed by spraying the surface with a clear lacquer. Similar coatings were used in direct recording pendulum seismometers. While not a sensitive instrument, these were capable of directly recording the direction of significant horizontal shocks upon a smoked glass plate.
Soot is in the general category of airborne particulate matter, and as such is considered hazardous to the lungs and general health when the particles are less than five micrometres in diameter, as such particles are not filtered out by the upper respiratory tract. Smoke from diesel engines, while composed mostly of carbon soot, is considered especially dangerous owing to both its particulate size and the many other chemical compounds present.
Soot can stain clothing and can possibly cause illness if inhaled. Breathing common urban air pollution (containing soot) is much deadlier than previously thought, according to a major study and an editorial published in New England Journal of Medicine on February 1, 2007.