This paper reviews the 20th century advances within the field of the measurement and the prediction of the rheological properties of high-temperature (T > 1000 K) silicate melts at atmospheric pressure with a focus on coal ashes and other melts of compositions relevant to coal ashes. Theoretical considerations are represented firstly by a synopsis of definitions and how-regimes and secondly by an outline of the ruling network theory and the notations used therein. The influence of different cations on viscosity is discussed thoroughly. Experimental points include a listing of the most common apparatus for viscosity measurements on high-temperature liquids and a summary of existing experimental data. Viscosity vs temperature is reported in tables as well as graphically for melts composed of major amounts of one or more of the species: SiO2, Al2O3, FeOx, CaO, MgO, Na2O, K2O, Li2O, MnO, TiO2, B2O3. For each experimental source, relevant information is listed (apparatus, sensor material, atmosphere). With a basis in both theory and experimental data, general equations for liquids, are presented, relating viscosity to temperature and/or concentration of dispersed solids, as are more specific mathematical models, relating the viscosity of silica melts and glasses to temperature and composition. (C) 2001 Elsevier Science Ltd. All rights reserved.