The powdery black stuff that's sometimes created when fuel burns is called soot. A chimney sweep's job is to clean all the soot and ash from inside a chimney.

Soot is mostly made of carbon, and it forms when matter burns incompletely. Engines, burning coal, and house fires are all sources of soot, and soot is a major contributor to air pollution around the world. It's dangerous for people to breathe too much soot into their lungs. Soot comes from a Germanic root that literally means "what settles."

Definitions of soot
  1. noun
    a black colloidal substance consisting wholly or principally of amorphous carbon and used to make pigments and ink
    synonyms: carbon black, crock, lampblack, smut
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    type of:
    C, atomic number 6, carbon
    an abundant nonmetallic tetravalent element occurring in three allotropic forms: amorphous carbon and graphite and diamond; occurs in all organic compounds
  2. verb
    coat with soot
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    type of:
    coat, surface
    put a coat on; cover the surface of; furnish with a surface
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