Fly ash is composed of inorganic and organic constituents, either neoformed in the boiler or flue gas from melting minerals of the charring or thermoplastic transitions in coal, primarily vitrinite, or inherited from the coal or auxiliary fuel. Chemical analyses alone are not sufficient to fully describe the complexity of fly ash. For example, the forms of carbons and their relationship to the inorganic fly ash constituents; the burnout of coal and non-coal carbons; and the efficiency of combustion, can all be described by petrographic studies of fly ash. (C) 2012 Elsevier B.V. All rights reserved.