Low-density Si3N4 spheroidal beads 3-4 mm in diameter have been made by an alginate route starting from Si3N4 powder. The beads have been characterized by chemical, X-ray diffraction and compression test techniques. The ability of the beads to couple with microwave power has been investigated in nitrogen atmosphere at fixed frequency of 2.45 GHz at power levels to 5 kW, and at variable frequency from 4-6.5 GHz at power levels to 1.4 kW. The beads showed no signs of deterioration in nitrogen at temperatures up to 1800 degrees C, and seem promising as high-temperature microwave furnace insulation.