Abstract: The present invention relates to thermal reactions performed at rapid transient temperatures, and a furnace (1) able to perform such reactions. The method and the furnace may suitably be applied to perform reactions between reactants where significant losses normally occur at certain transient temperatures or temperature ranges. One practical application of the present invention relates to a carbothermic method for producing Refractory Hard Metal powders, such as borides, nitrides and carbides, and a furnace designed for the performance of the method. In accordance with this method Refractory Hard Metal powders, such as boride powders can be produced with reduced loss of reactants such as C and B2O3. This can be achieved by rapid heating of the mixture containing the reactants in a critical temperature range.