A simple model is proposed and tested for simulations of ceramic processing by microwave heating. The model is based on a piecewise constant approximation of the material properties and makes it possible to separate and analyse different effects caused by the sample shape and the dependence of the material properties on temperature. Specifically, the simulation results demonstrate that microwave heating of an alumina sample can be very sensitive to a variation of its dielectric constant with temperature. For different geometries, there is a similarity in the dependences of the thermal state characteristics ( temperature drop across the sample, amount of dissipated power and electric field amplitude at the sample centre) on maximal temperature. It is shown also that a temperature drop between the sample centre and surface can be strongly enhanced in the case of a spherical sample irradiated symmetrically by microwaves.