2. A furnace according to claim 1, wherein the well in the block of refractory material is so shaped as to form, around the heat source, a reflector capable of redirecting, towards the hearth plate, at least a portion of any heat emanating from the heat source and radiated downwardly and towards the periphery of the well.

3. A furnace according to claim 1, wherein the heat source is a gas burner, the hearth plate is supported across the top opening of the well a plurality of feet engaging with the insulation block in such a manner as to form, between these feet, apertures through which hot gases rising in the well and diverging under the hearth plate can escape upwardly into the cover which has in its top region, an opening for the escape of combustion products,

4. A furnace according to claim 1, wherein the block of refractory material is a felt of slightly compressed mineral fibers

5. A furnace according to claim 1, wherein the hearth plate is formed of refractory stainless steel.

6. A furnace according to claim 3, wherein the opening in the cover for the escape of combustion products is bent upwardly and outwardly to form an external peripheral throat in which a wire loop engages, said loop being extended to form arms which are pivotally mounted on a base of the furnace, whereby the cover may be raised and lowered about said pivotal mounting.

7. A furnace according to claim 3, wherein the burner is formed by a hollow cylinder having its upper end covered by a wire gauze sheet and being connected at its lower end to a housing forming a mixing chamber for the gAs and air.

8. A furnace according to claim 7, wherein said housing is secured to a base of the furnace by a preferred clip which engages round a gas nozzle, and by means of a lug opposed to said clip and which is secured to the base by at least one screw.

9. A furnace according to claim 6, wherein the base is formed by means of two shells secured together to form a hollow housing in the manner of a box and its lid, the arms carrying the cover being pivoted on the sidewalls of the base and extending through slots formed in an upper surface of the base.

10. A furnace according to claim 9, wherein the shells forming the base are secured together by intermediary ribs, the upper end of each rib being secured to the upper shell by means of a screw, while the lower end of each rib is secured to the lower shell by means of a threaded stem connected to the rib and which cooperates with a threaded bore provided in a foot arranged below the lower shell.

11. A furnace according to claim 9, wherein the shells forming the base are secured together by intermediary ribs, the upper end of each rib being secured to the upper shell by means of a screw, while the lower end of each rib is secured to the lower shell by means of a threaded stem connected to a foot disposed below the lower shell and which cooperates with a threaded bore provided in the rib.