Ash residues that arise during combustion of solid fuels form deposits on heat exchanger surfaces which hinder the heat transfer to the working fluid steam. The thermal conductivity and - especially within the combustion chamber - the optical properties of the deposits determine the transferred amount of heat. Thus, knowledge of the optical properties, here represented by the normal emittance, are crucial for the design and operation of a steam generator. Since the emittance is dependent on both the physical structure of the surface as well as the mineralogical composition, both parameters are subject to the current investigation. In the work presented the spectral normal emittances of mineral samples as well as their dependence on both the chemical composition and physical structure were investigated experimentally. In the course of the research the degree of complexity was increased gradually. Starting with pure quartz sand the influence of the surface structure on emittance has been investigated. By mixing quartz and hematite powder the influence of chemical enrichment with this typical ash component has been investigated. Measurements with real coal ashes complete the experimental program. Additionally, the latter samples were undergone thermal treatment to investigate the effect of particle agglomeration and melting. (C) 2014 Elsevier Ltd. All rights reserved.