Melting point estimates derived from various phase diagrams have to be regarded as yielding hazardous results. The viscosity coefficient K of Bachmann (1980) does not describe actual viscosities within the working range of furnaces and seems to violate basic concepts of phase diagrams. There is no convenient short-cut to either of the parameters -- actual measurements are required. For the archaeometallurgist, the melting points and viscosities of ancient slags are important parameters, which reflect advances in furnace technology. Both parameters can be measured by instrumental methods. However, methods of estimation of these parameters have been presented which, if accurate enough, could be taken instead of actual measurements. -- AATA