Microwaves are reputed to enhance the sintering of ceramics, but the uncertainty of temperature in a microwave oven can account for part of the microwave effect. In the two cases presented here (reaction-sintering of alumina-based ceramics and devitrification of LAS glass-ceramics), the microwave effect does not exceed 50 degrees C (at N = 2.45 GHz and T approximate to 1200 degrees C). Theories of microwave effect are still tentative.