Every speck of soot hides tiny grains of diamond and graphite
The black grime from a candle flame or a diesel exhaust carries a surprise: soot always contains nanoparticles of graphite and diamond, a quirk chemists call gemmy soot. It forms in just a few milliseconds as fuel burns incompletely, clumping into particles between 6 and 30 nanometres across.
Soot is mostly impure carbon left over when hydrocarbons such as wood, coal or oil fail to burn completely. Definitions shift between fields: atmospheric scientists and toxicologists may mean different things, and some versions include sulfur, traces of metal, or grey-brown carbon instead of pure black. The International Agency for Research on Cancer classes the polycyclic aromatic hydrocarbons in soot as known human carcinogens.
Related terms are often muddled. Carbon black is a nearly pure elemental carbon powder made on purpose in special oil furnaces since the 19th century. Black carbon, a label atmospheric scientists adopted in the late twentieth century, describes unintended light-absorbing particles whose warming effect in the short term is second only to carbon dioxide.
What burns matters. Naphthalenes soot more readily than benzenes, which in turn soot more than simpler chain-shaped hydrocarbons, probably because ring-shaped molecules can build soot directly on their existing structure while chains must first form acetylene. Researchers broadly agree the process runs through building blocks, nucleation of heavy molecules, surface growth, collisions between particles and oxidation, though many details remain disputed. Modelling it is hard; detailed theoretical models can track hundreds of reactions and are slow to run.
Sources range from engines, ship boilers and forest fires to candles, cooking and oil lamps. Diesel exhaust makes up over a quarter of the hazardous pollution in air. Indoors, even faint traces cause ghosting, the dark staining where walls meet ceilings or floors, and thick deposits in chimneys can catch fire. Soot has also been useful: it once went into artists' paints, shoe polish and the blackening of Russia leather for boots, and into printing ink well into the 20th century.
Source: Soot
