a metallic coated substrate capable of reaching a predetermined surface temperature upon being exposed to microwave energy of a known strength including a base , a metal coating on the base , the coating being formed in a plurality of discrete metal areas having predetermined surface resistivity , the size of the areas being below the arcing size for the surface resistivity , and the resisitivity being such that the predetermined surface temperature will be reached when the substrate is exposed to the microwave energy . different areas on the base may contain discrete areas of different surface resistivity so that the different areas reach different temperatures . the spacing of the discrete areas may be varied so that the rate of energy emission from those areas differs .