casing 61 made for example of stainless steel plate with a cylindrical internal opening a side wall 62 and a closed bottom 63 the inside of the casing is lined with one or more layers of a suitable insulating material 64 which can be configured to have an intermediate flange like portion 65 which optionally includes an annular core 66 made of stainless steel welded to the side wall 62 the flange like portion delimits a kiln chamber or compartment 67 and a cooling compartment or chamber 68 the kiln chamber 67 is provided with suitable heating means for example coil shaped electric resistors 69 which are electrically connected by means of a cable 70 to a control unit 71 which is in turn supplied by a suitable source of electric power the cooling chamber 68 is instead surrounded by a metal coil 72 whose ends are connected to a circuit generally designated by the reference numeral 73 for the forced circulation of a heat transferring liquid for example water or diathermic oil which is passed through a heat sink 74 outside the chamber 68 a flange 75 is applied for example welded to the mouth of the metal casing 61 and internally delimits an opening 76 for access to the chambers 68 and 67 the flange protrudes externally with respect to the casing and acts.