The physical and chemical character of fireside ash deposits depend on the processes by which deposits are formed and subsequent reactions within the deposit and with furnace gases. The properties influencing heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer are shown from many measurements to depend on this character. The literature data for the properties are reviewed and are shown to depend principally on the physical ash character and whether the deposits have a particulate character or are sintered or fused. Bounds for the range of properties and expected variations with temperature and deposit porosity and other physical properties are suggested and compared with those for pure oxides and salts. For the radiative properties, theoretical predictions based on the optical properties are shown to predict trends with temperature and particle size for particulate ash and the observed transfer to higher emittance values for slags found at higher temperatures. The influence of the variation of these properties on radiative transfer is then quantified using mathematical models of furnaces.