US3025380A - Electric furnaces - Google Patents

Description

March 1962 E. A. STEINBOCK, JR 3, 2

ELECTRIC FURNACES Filed Feb. 4, 1959 2 Sheets-Sheet 1 INVENTOR.

EbMu/vo A. STEM/Bock, J BY Inc-awe] March 13, 1962 STEINBQCK, JR 3,025,380

ELECTRIC FURNACES Filed Feb. 4, 1959 2 Sheets-Sheet 2 IN V EN TOR. fflMUND A. STEINBOCKI JR.

H Home United States Patent Office 3,025,380 Patented Mar. 13, 1962 3,025,380 ELECTREC FURNAE Edmund A. Steinbeck, 31"., 7300 'Greenlawn Road, Louisville, Ky. Filed Feb. 4, 1959, Ser. No. 791,095 7 Claims. (Cl. 219-35) This invention relates to improvements in electric furnaces and particularly to burn-out furnaces as used in preparing cementitious refractory molds prior to the casting of metal therein.

Cementitious refractory molds are used in a number of arts, particularly in the making or casting of dental restorations. As is customary, a pattern of the casting is first made with said pattern formed of wax or other dissipable material with said pattern subsequently encased in a cementitious material generally composed of a binder, a filler and a thermal expanding agent to augment the inherent expansion of the binder. The mold after hardening is then placed in a furnace to dissipate or burn out the pattern and thereby provide the mold cavity. At the same time the mold is heated to substantially the pouring temperature of the metal to be poured therein which effects a thermal expansion of the mold to compensate for metal shrinkage from the molten to the solid state. The heating of the mold also eliminates all water and moisture from the mold prior to the pouring therein of the molten metal.

Some molds are to be heated to a temperature of around 1200 F. while others are heated to a temperature of around 900 F. In order to control such temperature in a given furnace it would be necessary to provide a current metering mechanism which would result in the pricing of a furnace for accomplishing this purpose beyond the reach of dentists and the like.

It is. therefore, the principle object of the present invention to provide a furnace so designed and arranged as to automatically give mold temperatures of l200 F. and 900 F. so that, if desired, molds may be simultaneously burned out at these temperatures.

Another object of this invention is the provision of a small burn-out furnace of the radiant heat type that is extremely simple of construction yet cflicient in operation.

A further object of this invention is the provision of a furnace to accomplish the above objects that has its construction reduced to a minimum of parts and which may be readily disassembled to repair or replace worn out parts, such as heating element or the support therefor.

Other objects and advantages of the present invention should be readily apparent by reference to the following specification considered in conjunction with the accompanying drawings forming a part thereof and it is to be understood that any modifications may be made in the exact structural details there shown and described, within the scope of the appended claims, without departing from or exceeding the spirit of the invention.

in the drawings:

FIG. 1 is a top plan view of an electric furnace embodying the principles of the present invention.

HO. 2 is a front elevational view of the furnace of .FTG. l, as seen from the lower side of said FIG. 1.

FIG. 3 is an enlarged vertical sectional view through the furnace as seen from line 3-3 on FIG. 1.

FIG. 4 is a staggered horizontal sectional view through the furnace as seen from line 4-4 on FIG. 3.

FIG. 5 is a horizontal sectional view taken on a plane partly above and partly below the plane of FIG. 4 as seen from line 5-5 on FIG. 3.

FIG. 6 is a perspective view of a means for supporting a mold in the furnace to burn out same at a given temperature below that normally attained in the furnace.

Throughout the several views of the drawings similar reference character are employed to denote the same or similar parts.

Specifically, the burn-out or radiant heat furnace of the present invention comprises a base or stand 10 having a plan view such as disclosed in FIGS. 4 and 5 and including a front 11 rearwardly of which extends sides 12 and 13 joined by a substantially semi-circular back wall 14. The stand 10 has considerable height to elevate the heating element upwardly of the supporting and which is generally a work table or bench, as will presently be made clear. The stand 10 is hollow for substantially its entire height and includes an inwardly projecting flange 15 at the upper ends of its front, back and side walls and which construction results in said stand having an opening through its top for the major portion of its area.

The base or stand top flange 15 has inwardly projecting therefrom a plurality of lugs 16, there being four such lugs illustrated in FIG. 5, and which lugs are utilized to have secured thereto the furnace, per se, as will presently be made clear. The furnace, per se, indicated in its entirety by the reference numeral 17 comprises a base or bottom member 13 in the form of a shallow pan comprising a body portion 19 with its periphery upturned as a flange 20. The bottom or furnace base member 18 is circular in plan and supports a housing member 21 in the form of a cylinder having an outer diameter substantially equal to the inner diameter of the flange 2d and against the inher surface of which flange 20 the lower end of the housing cylinder 21 engages. Concentric with said outer cylinder housing 21 and disposed on the furnace base body portion 19 to upstand therefrom is an inner cylindrical housing 22 of a diameter substantially less than the inner diameter of the outer housing 21 for thereby providing an air space 23 between said housings. The housings 21 and 22 are maintained in spaced relation to one another by a closing cap ring 24. The cap ring 24 comprises a ring like body portion 25 from the inner periphery of which depends a flange 26 while a similar flange 2'7 depends from the outer periphery of said ring body portion 25. The said cap ring 24, therefore, has, in effect, a channel upwardly of its lower end and with said channel having a diameter within the inner face of its outer depending flange 27 substantially equal to the outer diameter of the outer cylindrical housing 21 and with said cap ring inner flange 26 having an outer diameter substantially equal to the inner diameter of the furnace inner cylindrical housing :22. By this construction the said cap ring inner depending flange substantially snugly enters the inner diameter of the furnace inner cylindrical housing 22 While the said cap ring outer flange 27 rather snugly receives the upper end of the furnace outer cylindrical housing 22 and thereby substantially rigidly spaces said housings from one another in establishing the air space 23 between said cylindrical housings 21 and 22.

The furnace cylindrical housings are secured upwardly of the base or stand 10 by a plurality of heat insulators in the form of collars indicated by the reference numeral 28. The said collars are arranged in groups of three collars and with a group supported by each of the base or stand inwardly projecting lugs 16. Disposed between said supporting collars 28 are baflle plates 29 each circular in plan and of an area slightly in excess of the area of the furnace bottom member or base 18.

Said supporting columns of collars 28, respectively, have their central passageways in alignment with one another as well as respectively in alignment with apertures through the baffle plates 29 and the said base or supporting frame lugs 16 are respectively provided with an aperture 39 in alignment with the said collars 28 passageways. The furnace base body member 19 as well as the cap ring body portion are further each provided with apertures, respectively, in alignment with a supporting column collars central passageway and it will be obvious that the said apertures in the furnace base body portions 19 are located within the area of the space 22 between the furnace cylindrical housings 2i and 22.

Disposed within the said space 23 between the furnace cylindrical housings 21 and 22 are tie-rods 31, there being as many tie-rods as there are supporting columns of collars 27, and with said tie-rods respectively passing through a column of said collars 2S and with each of said rods passing through the aligned furnace base body portion aperture and supporting base lug aperture to project below its said base supporting lug l6. Said projecting portion of each tie-rod 31 has its lower end threaded, as at 32, to receive clamp nut 33 and with a spacing refractory material collar 34 between its nut 33 and the under surface of its supporting lug 16. The upper end of each tie-rod 31 is provided with a tapped socket 35 to receive a screw 3-6 extending through an aligned aperture in the cap ring body 25 and which aperture is counter-sunk to receive the screw head.

From the foregoing it is believed obvious that the furnace cylindrical housings 2i and 22 along with the cap ring 24 and the batlle plates 29 are all secured in operative positions with respect to one another and to the supporting base ill by means of the four tie-rods and which can be readily removed to permit disassembling of the parts particularly for the purpose of permitting replacement of or repairs to the radiant heat heating mechanism presently to be described.

The radiant heat heating mechanism comprises a block or brick base 37 substantially circular in plan and of suitable refractory material having formed downwardly of its upper surface 33 a groove 39 which, as can be seen from FIG. 4, is continuous although having the general appearance of a plurality of concentric grooves. Disposed in said groove 39 is the heating element 40 which takes the usual form of an electric heating element in that it is a wire of high electrical resistance and coiled on itself in the manner of coiled spring. The heating element 4% has its one end connected to a binding post or screw 41 which extends downwardly through the heating mechanism base 37 to a point to be substantially within the hollow interior of the supporting base 10. The other end of the heating element 40 is connected to a binding post or screw 42 which, similar to the binding post or screw 41 extends downwardly through the heating mechanism base 37 to terminate again within the hollow interior of the supporting base it Said terminal screws or binding post at and 42 pass through suitable apertures in the furnace base body portion 19 and in the battle plates 29 and to prevent short circuiting contacts of said terminal screws or posts with said members as well as with one another they are respectively encircled by insulating sleeves 43 and 44 and which sleeves may conveniently be formed of refractory material, as is the usual practice.

The heating mechanism refractory base 37 is supported by a plate 45 having depending legs 46 disposed on the upper surface of the furnace base body portion 1%. 1nterposed between the heating mechanism base plate 45 and heating mechanism brick or block 37 is a relatively soft liner 47 conveniently taking the form of sheet asbestos which not only somewhat resiliently supports the heating element base 37 but at the same time acts as a heating insulator to prevent or limit downward heat radiation and encourages upward heat radiation.

The heating mechanism is secured in operative position by means of a bolt having at one end an enlarged head 49 disposed in a counter-bore 5S substantially centrally of the heating element base 37 and with said bolt projecting downwardly through aligned apertures in said CII heating element base 37, asbestos pad 47, heating mechanism base plate 45, furnace housing base body portion 19 and bathe plates 29 to project below the said baffle plates and receive a securing member 51 and whereby said heating mechanism is secured in operative position on the furnace housing base body portion 19.

The supporting base 10 is provided through its back Wall 14 with an aperture for an electric wire insulator 52 through which passes the service wires 53 and 54 carrying at their outer ends although not shown in the rawings, the usual pronged plug for insertion in a commercial electric current source outlet. The said service wires 53 and S i, therefore, are disposed within the hollow interior of the base 16 and with one of said wires, wire 53, for example, terminating in a toggle switch 55 mounted and carried by the base front wall 11. The said switch 55 is a standard commercial product and includes the switch actuating member 56 which when in one direction effects the closing of the switch and when actuated to a second position effects the opening of the switch, all as is well known. Extending from said switch 55 is a wire 57 which terminates in the screw or binding post and therefore one end of the heating element, per se, or heating coil 40. The screw or binding post 4-2, at the other end of said heating element or coil 40, has connected therewith one end of a wire 58 that terminates at a binding screw 59 carried by an insulating block 60, which for convenience is secured to one of the tie-rods 311 being clamped thereto between its nut 33 and the spacing insulator collar 34 thereof. This same binding screw 59 has connected therewith the service wire 54 and thereby completes the electric circuit to the heating element, per se, or coil 40.

The supporting base 10, in addition, has mounted on its front wall ll a pilot light which includes a socket 61 projecting inwardly of the said supporting base front wall 11 to carry a light bulb exteriorly of the said front wall 11 and which light bulb is enclosed by a guard 62 secured in a well known manner to the socket 61. A second wire 63 extends from the switch S5 to the said electric light socket 61 with a second wire 64 extending from the said socket 61 to the binding post 59, and whereby an electric circuit to the said light socket 61 is complete, as is believed obvious.

It is on the upper surface 38 of the heating element base 37 that materials to be treated are disposed. The said material on said heating element base are subjected to a maximum temperature of 1200 Fahrenheit and which is the maximum temperature within the furnace. The said temperature of l200 Fahrenheit is attained approximately thirty minutes after the furnace is turned on and the said temperature is prevented from going higher due to circulation through the furnace and which circulation is effected by providing the furnace inner cylindrical housing with a plurality of apertures 65 that communicate with the space 23 between the furnace cylindrical housings and which space, in turn, is connected through apertures 66, see FIG. 4, through the furnace housing bottom body portion 19.

The upper end of the furnace housing is closed by a hinged cover 67 which has secured to and projecting from it a hinge member 68 hingedly connected through hinge pin 69 with a second hinge member 70 secured to and rearwardly projecting from the furnace outer cylindrical housing member 21 all as clearly illustrated in FIGS 1 and 3. in order to actuate said cover it has secured to its front end an operating handle 71 which laterally projects therefrom as illustrated in FIGS. 1 and 2. To further maintain the temperature within the furnace housing the cover 67 is provided therethrough with a plurality of apertures 72 and which aperture allow escape of excessive heat radiation.

Interiorly of the furnace is a removable shelf or support comprising a supporting shelf or plate 73 having a plurality of apertures 74 therein, see FIGS. 3, 4 and 6.

Beneath the supporting shelf 73 is a baffle plate 75 of the same area as the shelf 73 but imperforate which acts as a baflle between the heating element, per se, heating coil 40 and the shelf 73. The shelf 73 and baflle plate 75 have provided between them an air space 76 afforded by a plurality of shouldered studs 77 which have their ends projecting through the shelf 73 and plate-75 to be upset as rivet heads 78 and 79. The portion of the studs 77 which projects below the baflle plate 75 are employed for a dual purpose, namely, that of attaching to the removable shelf or support, legs St). As illustrated in the drawings use is made of three legs each of which has its upper end upset and inwardly projecting, as at 81, to provide means for attaching the legs to the shelf or support. As seen in FIGS. 3 and 5, the legs 80 have considerable width whereby they may span the channels 39 in the brick 37 and at the same time have their lower ends inturned, as at 32, in order to firmly support the removable shelf.

The base has inwardly projecting, from a plurality of points on its front and side walls, thickened bosses 83 to the lower end of each of which is secured a supporting foot 84; said securing of each foot being by way of a threaded stud 85 integral with and upwardly projecting therefrom. As shown in FIG. 3 the said thickened bosses have a lower surface in alignment with one another and with said surfaces upwardly of the lower edges of the base front, sides and rear walls and with said thickened bosses aligned lower surfaces having clamped thereagainst a perforated guard plate 86; said clamping of the guard plate being conveniently effected by the supporting feet 84, as clearly shown in FIG. 3.

What is claimed is:

1. In a radiant heat furnace the combination of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing, concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including, a cap-ring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from one another, an outer peripheral flange from said spacing ring body member embracing the upper end of the outer cylindrical housing, and an inner depending flange from the inner periphery of the cap-ring body member disposed within the upper end of the inner cylindrical housing, said cap-ring body portion having a plurality of spaced countersunk apertures therethrough, a headed screw passing through each aperture with its head in the countersink thereof to be flush with the plane upper surface of the cap-ring body portion, heat insulating spacing means between the supporting base and furnace bottom for spacing same from one another, tie-rods upwardly extending from said supporting base through the space between the cylindrical housings and in axial alignment with the cap-ring apertures with each tie-rod having a threaded socket at its upper end receiving a headed screw clamping the cylindrical housings between the cap-ring and furnace bottom, and a cover hingedly secured to the upper end of the outer housing operable to a position on said cap-ring body portion plane upper surface for covering the furnace heating chamber.

2. In a radiant heat furnace the combination of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of said cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including, a cap-ring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from one another, an outer peripheral flange from said spacing ring body member embracing the upper end of the outer cylindrical housing, and an inner depending flange from the inner periphery of the cap-ring body member disposed within the upper end of the inner cylindrical housing, tie-rods upstanding within the space between the inner and outer cylindrical housings between the capring body portion and the supporting base, means passing through the cap-ring body portion securing the tie-rods to the said cap-ring body portion but with said securing means below the cap-ring body portion plane upper surface, heat insulating spacing means between the furnace bottom and supporting base, means on said tie-rods securing the cap-ring, furnace bottom and supporting base to one another and said housings between said furnace bottom and cap-ring in their spaced apart positions and said supporting base and furnace bottom having the heat insulating spacing means therebetween, and a cover hingedly secured to the upper end of the outer housing operable to a position on said cap-ring body portion plane upper surface for covering the furnace heating chamber.

3. In a radiant heat furnace the combination of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of said cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including, a cap-ring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from one another, an outer peripheral flange from said spacing ring body member embracing the upper end of the outer cylindrical housing, and an inner depending flange from the inner periphery of the cap-ring body member disposed within the upper end of the inner cylindrical housing, tie-rods upstanding within the space between the inner and outer cylindrical housings between the cap-ring body portion and the supporting base, means passing through the cap-ring body portion securing the tie-rods to the said cap-ring body portion but with said securing means below the cap-ring body portion plane upper surface, heat insulating spacing means between the furnace bottom and supporting base, means on said tie-rods securing the cap-ring, furnace bottom and supporting base to one another and said housings between said furnace bottom and cap-ring in their spaced apart positions and said supporting base and furnace bottom having the heat insulating spacing means therebetween, baffles carried by said tie-rods between the furnace bottom and the supporting base and with said bafiies outwardly projecting from said furnace outer cylindrical housing member, and a cover hingedly secured to the upper end of the outer housing operable to a position on said cap-ring body portion plane upper surface for covering the furnace heating chamber.

4. In a radiant heat furnace the combination of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of said cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including a cap-ring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from one another, an outer peripheral llange from said spacing ring body member embracing the upper end of the outer cylindrical housing, and an inner depending flange from the inner periphery of the cap-ring body member disposed within the upper end of the inner cylindrical housing, said cap-ring body portion having a plurality of spaced countersunk apertures therethrough, a headed screw passing through each aperture with its head in the countersink thereof to be flush with the plane upper surface of the cap-ring body portion, heat insulating spacing means between the supporting base and fur nace bottom for spacing same from one another, tie-rods upwardly extending from said supporting base through the space between the cylindrical housings and in axial alignment with the cap-ring apertures with each tie-rod having a threaded socket at its upper end receiving a headed screw clamping the cylindrical housings between the cap-ring and furnace bottom, a cover hingedly secured to the upper end of the outer housing operable to a position on said cap-ring body portion plane upper surface for covering the furnace heating chamber, said heat insulating spacing means being in the form of similar length collars on each tie-rod between the supporting base and furnace bottom, and a bafiie between the furnace bottom and supporting base with a heat insulating collar on each side thereof and which bafile extends outwardly of the furnace outer housing.

5. In a radiant heat furnace the combination. of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings, in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of said cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including a capring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from one another, an outer eripheral flange from said spacing ring body member embracing the u per end of the outer cylindrical housing, and an inner depending ilange from the inner periphery of the capring body member disposed within the upper end of the inner cylindrical housing, said cap-ring body portion having a plurality of spaced countersunk apertures therethrough, a headed screw passing through each aperture with its head in the countersink thereof to be flush with the plane upper surface of the cap-ring body portion, heat insulating spacing means between the supporting base and furnace bottom for spacing same from one another, tic-rods upwardly extending from said supporting base through the space between the cylindrical housings and in axial alignment with the cap-ring apertures with each tie-rod having a threaded socket at its upper end receiving a headed screw clamping the cylindrical housings between the cap-ring and furnace bottom, said heat insulating spacing means being in the form of similar length collars on each tie-rod between the supporting base and furnace bottom, and a bathe between the furnace ott m and supporting base with a heat insulating collar on each side thereof and which extends outwardly of the furnace outer housing.

6. In a radiant heat furnace the combination of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of said cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including a cap-ring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from One another, an outer peripheral flange from said spacing ring body member embracing the upper end of the outer cylindrical housing, and an inner depending flange from the inner periphery of the capring body member disposed Within the upper end of the inner cylindrical housing, said cap-ring body portion having plurality of spaced countersunk. apertures therethrough, a headed screw passing through each aperture with its head in the countersink thereof to be flush with the plane upper surface of the cap-ring body portion, heat insulating spacing means between the supporting base and furnace bottom for spacing same from one another. tie-rods upwardly extending from said supporting base through the space between the cylindrical housings and in axial alignment with the cap-ring apertures with each tie-rod having a threaded socket at its upper end receiving a headed screw clamping the cylindrical housings between the cap-ring and furnace bottom, a heating element on said furnace bottom upwardly thereof and including a carrying block having its upper surface upwardly of the heating element and which surface i adapted to support material within the furnace heating chamber, and additional means removably carried by said heating element block supporting surface adapted to support material upwardly of the said heating element block supporting surface.

7. In a radiant heat furnace the combination of a supporting base, a furnace comprising an outer cylindrical housing, an inner cylindrical housing concentric with and inwardly of the outer cylindrical housing forming inwardly thereof the furnace heating chamber, a furnace bottom supporting said housings in spaced apart positions and with a space between them, comprising, a body portion of an area substantially equal to the area of the outer cylindrical housing and a peripheral flange engaging the lower end of said outer cylindrical housing, means at the upper end of said cylindrical housings closing the space therebetween and securing the said cylindrical housings upper ends in spaced apart relation, including a cap-ring having a body portion with a plane upper surface and of a width equal to the distance of said spacing of the housings from one another, an outer peripheral flange from said spacing ring body member embracing the upper end of the outer cylindrical housing, and an inner depending flange from the inner periphery of the cap-ring body member disposed within the upper end of the inner cylindrical housing, said cap-ring body portion having a plurality of spaced countersunk apertures therethrough, a headed screw passing through each aperture with its head in the countersink thereof to be flush with the plane upper surface of the capring body portion, heat insulating spacing means between the supporting base and furnace bottom for spacing same from one another, tie-rods upwardly extending from said sup porting base through the space between the cylindrical housings and in axial alignment with the cap-ring apertures with each tie-rod having a threaded socket at its upper end reciving a headed screw clamping the cylindrical housings between the cap-ring and furnace bottom, a heating element on said furnace bottom upwardly thereof and including a carrying block having its upper surface upwardly of the heating element and which surface is adapted to support material within the furnace heating chamber, additional means removably carried by said heating element block supporting surface adapted to support material upwardly 0f the said heating element block supporting surface, and a cover hingedly secured to the upper end of the outer housing operable to a position on said housings for covering the furnace heating chamber.

References Cited in the file of this patent UNITED STATES PATENTS Pollard Feb. 17, 1925 Goldsmith June 10, 1930 Manternach Apr. 7, 1931 Harris July 21, 1931 Barnsteiner Feb. 18, 1947 Steinbock July 24, 1951 Ratchford May 27, 1958

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