Empirical correlations have been derived by regression analysis to calculate the ash fusion temperatures under reducing conditions and the temperature of critical viscosity from the ash chemical composition. The data-base used in this work includes about 300 ash samples from coals of different sources and biomasses. The standard deviation, a, for these equations are generally about 45-80 degrees C. These fall within the experimental errors of the measurements. These correlations: (I) provide a tool for calculating the thermal properties of the ash from its chemical composition; (2) provide a method to predict the effect of adding minerals (such as CaO) normally used to modify the slag behaviour.