There have been various proposals for making an electric hot plate having a transparent glass top up through which cooking heat is radiated, and which can be incorporated into an electric cooking stove.
In 1971 U.S. Pat. No. 3,612,828 disclosed the idea of a refractory fibrous pad, such as one made of rock wool, glass wool, slag wool, asbestos or the like, having a top surface in which a deep sprial groove is formed and in which a sinuous strand of flat resistance wire is nested to form a heating element with the glass plate spaced above. To prevent excessive heat conduction from this wire to the pad which via the grooves encloses the bottom and sides of the wire, the fibrous material contains a uniform dispersion of finely divided opacifier substance so that the pad's bottom and side walls forming the groove diffusely reflect heat back to the wire which directly contacts these walls.
In 1974 U.S. Pat. No. 3,833,793 proposed that the surface of the pad be formed as a flat planar surface and that the resistance wire be in the form of coiled round wire pinned to the pad by staples, the glass plate being spaced above this round wire element. This is for the purpose of reducing heat conduction from the wire to the pad.
Both of the above patent proposals used metal alloy resistance wire which must be operated at temperatures below 2000.degree. F. and both relied on the wire element being extensive enough to provide a large heat radiating surface area. Therefore, both have the disadvantage of being slow to heat from ambient temperature up to cooking temperature as compared to a gas stove, and both have the disadvantage that excessive heat is lost by conduction to the pad.
In 1975 U.S. Pat. No. 3,912,905 proposed a glass-top hot plate using a layer of molybdenum disilicide resistance wire resting directly on a pad of compacted fibrous refractory material, with a glass plate spaced above. The pad surface is roughened to provide upstanding fibers on which the wire rests. The wire is designed to be heated to temperatures substantially above 2000.degree. F. at which temperature the upstanding fibers fuse, the intent being to in this way position the wire against shifting on the pad if roughly treated as during shipping. Testing showed that the fibers, being ceramic in nature, became fragmented and did not perform their intended function. However, because molybdenum disilicide wire is preferred, the element can be designed so that upon being powered it substantially immediately flashes from ambient to full operating temperatures embracing the temperature of 2350.degree. F. which U.S. Pat. No. 3,612,828 discloses as being the infrared optimum heater element temperature. To this extent, this hot plate might be competitive with the instantaneous heating obtained by a gas stove. However, it has been found that a substantial amount of heat is lost by conduction to the pad on which the molybdenum disilicide wire directly rests after fusion of the fibers provided to anchor the wire against shifting.
In 1976 U.S. Pat. No. 3,987,275 proposed a glass-top unit using metal sheathed electric resistance wire with the sheath flattened and pressed directly against the bottom of the glass by a metal spider, with the pad spaced below and its top surface covered by aluminum foil to act as a reflector. The effect is primarily a heated glass cooking surface relying on conduction rather than heat radiation through the glass to the cooking vessel.
Consequently, it can be seen that the prior art has been presented with the unsolved problem of providing a glass-top hot plate that is competitive with a gas stove as to rate of heat production from ambient temperature, which does not involve a substantial loss of heat to the refractory fibrous pad supporting the resistance element, and which heats a cooking vessel truly by radiation as contrasted to conduction.