Black soot is the product of the incomplete combustion of carbon-containing fuels.
Complete combustion would result in a blue flame, and would produce negligible amounts of soot and carbon monoxide. Until recently, the source for the black soot in homes was unknown. Through interviews and recent experiments, it is now believed that frequent candle burning is one of the sources of black soot. The amount of soot produced can vary greatly from candle to candle. One type of candle can produce as much as 100 times more soot than another type.