US3694122A

US3694122A - Apparatus for molding and firing dental porcelain

Info

Publication number: US3694122A
Application number: US70154A
Authority: US
Inventors: Ronald F Macdonald
Original assignee: ALBERT J MACDONALD; JOSEPH L MACDONALD
Current assignee: ALBERT J MACDONALD; JOSEPH L MACDONALD
Priority date: 1970-09-08
Filing date: 1970-09-08
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26

Abstract

Method and apparatus whereby powdered porcelain or plastic material is charged in the opposing cavities of half flasks, which cavities have been formed about a pattern within the investment of each flask half and wherein the flasks are placed in separated condition within the muffle of a furnace and such separation is maintained until furnace heat is achieved slightly in excess of the liquefying temperature of the powder in the flask and then said flask halves are joined to compress the charge to fill all parts of the molded cavity. Refractory spacer means sensitive to temperature maintain separation between the flasks until the critical temperature is reached. The flask halves may thereafter be compressed together either manually or mechanically. The apparatus includes a tiltable furnace and visual observation of the spacer from the furnace exterior.

Description

United States Patent MacDonald, deceased et al.

[54] APPARATUS FOR MOLDING AND FIRING DENTAL PORCELAIN [72] Inventors: Ronald F. MacDonald, deceased, late of Orange, Calif; by Albert J.

MacDonald, executor, Van Nuys, Calif.

[73] Assignee: Joseph L. MacDonald, 'Medfield,

Mass.

22 Filed: Sept. 8, 1970 21 App1.No.: 70,154

[521 US. Cl. ..425/l7l, 425/174 I-IR, 425/405, 425/408 [51] Int. Cl ..B29c 3/00 [58] Field of Search ..18/5.7, 17 D, 47 D, 34.1, 33, 18/32, DIG. 60, DIG. 62, 17 R, DIG. 12;

[ 51 Sept. 26, 1972 3,200,454 8/ 1965 Gramenzi 164/378 X 2,069,746 2/1937 Andrews ..18/33 X 3,411,184 11/1968 McGowan ..18/33 3,109,911 1l/1963 Kremer ..l3/31 X Primary Examiner-4. Spencer Overholser Assistant ExaminerDavid S. Safran Attorney-Wm. Jacquet Gribble 5 7] ABSTRACT Method and apparatus whereby powdered porcelain or plastic material is charged in the opposing cavities of half flasks, which cavities have been formed about a pattern within the investment of each flask half and wherein the flasks are placed in separated condition within the muffle of a furnace and such separation is maintained until furnace heat is achieved slightly in excess of the liquefying temperature of the powder in the flask and then said flask halves are joined to com press the charge to fill all parts of the molded cavity. Refractory spacer means sensitive to temperature maintain separation between the flasks until the critical temperature is reached. The flask halves may thereafter be compressed together either manually or mechanically. The apparatus includes a tiltable furnace and visual observation of the spacer from the furnace exterior,

9 Claims, 9 Drawing Figures PATENTEDsEP2s I972 SHEET 1 BF 3 INVENTOR RONALD F. MccDONALD T TOP/VET PATENTEDSEPZE I912 3.694.122

SHEET 2 OF 3 FIG. 3. 64

49 INVENTOR RONALD F. MOCDONALD T TORNE Y PATENTEflsiPzs I972 SHEET 3 BF 3 INVENTOR RONALD F. Mac DONALD F/QQ.

T TORNE V APPARATUS FOR MOLDING AND FIRING DENTAL PORCELAIN BACKGROUND OF THE INVENTION The invention relates to method and apparatus for processing fired dentures and more particularly to method and apparatus whereby porcelain or plastic powder, which may be tinted, is molded and fired to create quality artificial denture components, such as porcelain mountable on gold, porcelain .bridgework and inlays.

Naturally tinted or colored porcelain or plastic teeth have been previously fabricated by highly skilled dental technicians. The conventional process is laborious and not dependable, relying on the artistry and training of the artisan. Quality denture replacement parts are generally fired porcelain on a gold base. The porcelain must be of a particular quality in order to have the same coefficient of expansion and contraction as the gold and to adhere to the gold, and in order to accept colors which are natural appearing. Gradation of color from one part of the artificial tooth to the other is very important. The method and apparatus of the invention achieve natural-appearing dental prosthetic materials by a method and apparatus which may be implemented by semiskilled craftsmen in a manner which does not require the long experience and keen perceptions of a highly trained craftsman.

While porcelain is used to describe the substances ,of which the dental materials are made, it is to be understood that the term as used covers all of those porcelain-like substances capable of ceramic response to heating and cooling. The substances for dental materials such as bridges, plates, etc. must not only survive a range of temperatures, but must also result in artificial teeth which look natural, remain unchanged after years of exposure to mouth chemicals and foodstuffs. To look natural the material of the teeth must accept coloring agents which can remain gradated through the firing step, dispersed in the material in natural-appearing color differentiation from one area to another.

SUMMARY OF THE INVENTION The process of the invention contemplates the steps of packing an anterior flask half around a conventionally prepared wax pattern with an investment in a plastic state, which hardens about the pattern. The posterior flask half is then packed in like manner with a plastic investment and the flask halves are joined so that each half receives the imprint of the pattern. The flask halves are then separated and the pattern is melted from the flask halves. The anterior flask half cavity is then packed with powdered porcelain previously blended with proper coloring agents, while joined to the posterior half so that the cavity is filled. A parting coating on the investment surface between the flask halves aids separation after they are joined. An overcharge of porcelain powder is then introduced through a vent hole into the flask and the flask is vibrated. The flask halves are then separated slightly and a ceramic spacer rod is placed between the flask halves. The joined flask halves are then mounted on an articulated portion of the furnace so that the flask extends into the muffle of the furnace, posterior half first. The furnace is then fired and the temperature brought to the melting point of the porcelain powder and the spacer rod. The spacer rod melts, indicating critical temperature and the flask halves are joined, compressing the liquefied porcelain powder within the cavity formed about the lost wax pattern.

The invention contemplates unique apparatus for implementation of the inventive method. A vacuum furnace, preferably electric, has a housing, a door on the housing and a muffle, or heating unit within the housing. The housing is pivotally mounted upon a control console. A support beam slidable in the housing door supports removably a separable mold flask which, when closed, supports the flask within the muffle, The flask contains an investment surrounding the mold cavity. The cavity connects with waste vents extending exteriorly of the muffle. The support beam bears against the anterior flask half. A stop pin bears against the posterior flask half. The beam is spring loaded to thrust the halves together, with a spacer rod of ceramic material separating the halves until the rod melts.

In a preferred embodiment of the invention the muffle is movably mounted within the furnace housing by radial thrust springs anchored in longitudinal bearing bar pairs spaced circumferentially about the muffle. Preferably both the muffle and the housing are apertured to provide an observation port through which the spacer rod of the mold flask may be inspected in place.

The inventive furnace includes aflask which is reusable, the flask comprising outer shell halves each of which is impervious to the furnace heat and a hardenable pourable investment which deteriorates in the furnace heat so as to be removable from the shell.

The inventive process and apparatus work together to produce prosthetic dental materials of superbly natural appearance which can be used in any dental restoration device because of their beauty, durability and versatility.

These and other advantages of the invention are apparent from the following detailed description and drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a preferred embodiment of the apparatus invention;

FIG. 2 is a longitudinal section of the embodiment of FIG. 1;

FIG. 3 is a section similar to FIG. 2 with the mold flask in the furnace muffle;

FIG. 4 is a sectional elevation taken along line 4-4 of FIG. 3;

FIG. 5 is a fragmentary sectional view of a mold flask on the support beam of the housing door;

FIG. 6 is a view taken along line 6-6 of FIG. 5;

FIG. 7 is a sectional view taken along line 77 of FIG. 5;

FIG. 8 is a fragmentary sectional view along the axis of the closed furnace showing the flask halves joined; and

FIG. 9 is a side elevation of the furnace of the invention in operating position.

DESCRIPTION OF THE PREFERRED EMBODIMENT An understanding of the entire process the ceramist performs to achieve dental prosthetic materials is helpful in the comprehension of the invention. The general and an investment is packed about the pattern within the flask half. The investment hardens about the pattern and the exposed face of the investment is coated with a lubricant separator, such as Vaselinef. The posterior flask half is also. packed with investment and the flask halves joined such that each half is impressed by the wax pattern. The flask halves are then separated and the wax pattern is melted fromthe flask half. Depending on the type of vpattern material used, the wax maybe lost from the surrounding investment by immersion in boiling water or may be burned out at a temperature of about l,500 F.

The pattern cavity in the flask half is then filled with powdered porcelain which has been blended with color agents. Preferably the porcelain charge is of a type .with a high feldspar content which is self-glazing and contains uranium and nickel cobalt oxides. Coloring agents are preferably mixed with the powdered porcelain prior to its placement in the moldjflask. Depending upon the dental device desired, the charge may be composed of varying amounts of these components so as to'melt at a chosen point between 1,600" and 1,850 F A lubricating separation layer intervenes between flask halves before the halves are joined. The charged flask halves, after joining, are overcharged through the ventholeof the flask while the flask is vibrated. The .vibration tends to solidify the powder so that it remains within the flask when the halves are separated. Preferably the overcharge is equal to 5 to 20 percent of the capacity of the pattern cavity in terms of normal powder density. After the charge is solidified the flask halves are separated and a refractory rod is placed between the mating faces of the flask halves. The halves are then placed on the support beam of the furnace door and the door is closed to position the mold flask within the muffle of the furnace. The charged flask is then subjected to the furnace heat under vacuum, while the flask halves, under joining tension, are separated by the spacer rod.

I The spacer rod is of a refractory or ceramic material compounded such that its melting point is no less than the melting point of the porcelain-charge in the flask. Preferably the rod melts at approximately50 higher temperature than the charge, insuring that the charge has indeed reached its critical fluid condition.

As soon as the melting point of the charge and the rod is reached the tension applied to the flask halves joins them. The charge is thus pressed in the cavity by the joining action of the flask halves at the proper temperature to achieve quality ceramic material. 7 The flask is then removed from the furnace, the mold flask halves separated and the investment removed from about the press-molded porcelain, which can then be subjected to further steps in completing the molded and fired prosthetic device.

Since the porcelain powder for the charge, and the spacer rod, can be manufactured to very close tolerances in terms of melting point and since the desired temperature at which the porcelain charge should be pressed is known, the process achieves exact timing to result in quality porcelain, without the necessity of highly trained ceramists to implement the process.

Further details of the process are set forth in connection with the description of the operation of the apparatusof the invention which follows:

v The furnace, 11 of FIGS. 1-9 may be controlled from a conventional console 12 which has a conventional heat dial 13, heat indicator and thermostat control 14,

vacuum gauge 15 and control switch bank 18. A switch 19 controls the vacuum pump (not shown).

A vacuum hose 21 (FIG. 3) proceeds within a flexible hose conduit 22 from the pump to a cylindrical housing 23 of the furnace 1 l. The housing is suspended above the console on

uprights

25, 26 attached to gimbels like gimbel 27 which have tension nuts 28 so the attitude of the furnace with respect to horizontal may be controlled.

Visual inspection of the interior of the furnace is desirable and therefore the housing is provided with an observation port indicated generally at 31. As can be seen from FIGS. 2 and 3, the port comprises a conventional lens casing 33 within which a quartz lens (not shown) is sealed to close the furnace to both heat and vacuum loss. The casing seats within a stepped aperture 34 of the housing wall 35. A muffle 36 in the housing has an exterior wall 38 and an interior wall 39. The port 31 is in line with

openings

41, 42 ofthe exterior wall 38 and interior wall 39, respectively. The furnace housing 23 comprises the cylindrical wall 35, a back wall 44 and a rod hinged-mounted front door 45. The cylindrical wall 35 is pierced opposite port 31 to receive a sealed vacuum hose coupling 46 to which the inner vacuum hose 21 iscoupled. Flexible shroud 21 also contains

electrical leads

47, 48, 49 which extend from the controls of the console 12 to the muffle 36. While various types of heating elements may be used to practice the invention, the preferred muffle is one like that described in copending application, Ser. No. 771,409 for Muffle Furnace filed Oct. 29, 1968, now US Pat. No. 3,541,293.

The muffle has front and rear

refractory walls

52, 53 in addition to the cylindrical outer case wall 38 and the inner spacer wall 39 between which a plurality of circumferentially spaced heating coils 56 extend within tubes 57 whose nature is best comprehended with respect to the above mentioned copending application. Wall 52 has an opening 59 affording access to the muffle interior. The muffle is removably positioned within the housing by means of a plurality of pairs of spaced bearing bars 61, 62. The bars of each pair are thrust apart by a pair of radial compression springs 63, 64, visible in FIGS. 2 and 3.

As can be seen with respect to FIG. 4, the bearing barpairs are spaced peripherally about the muffle such that the thrust upon the muffle by one pair is counterbalanced by a diametrically opposite bearing bar pair. The compression springs hold the muffle firmly in the desired location and the muffle may be easily removed to be repaired or replaced by overcoming the spring pressure. A stop rod 71 extends through the back wall 53 of the muffle from an adjustment sleeve 72 which intervenes between wall 53 and housing wall 44. Stop 71 is movable within sleeve 72 through the back wall and is fixed in desired position by a set screw 74. The stop is used to locate a divisible mold flask 80 with its posterior flask half 81 and anterior half 83 with respect to the muffle and more particularly with respect to observation port 31 and the

openings

34, 41, 42 in the housing and the muffle walls. The sleeve registers the mufile in the housing so that the muffle openings align with port 31.

The anterior flask half 83 has an extending hollow boss 84 into which a cylindrical support beam 85 fits. The beam passes through and holds a muffle closure 88 and also passes through a threaded gland 89 which is sealed within a threaded aperture'90 of furnace door 45. a

An exterior thread 91 of the gland receives a tension nut 92 within whose cavity 93 a compression spring 94 and a thrust collar 95 reside. The thrust collar bears against a shoulder 96 formed on the beam by the reduction of beam 85 to a small stem 97, which projects outwardly from the outer wall of nut 92. (See FIG. 8) An O-ring 98 seals between the beam and gland 89. A larger compression spring 99 surrounds the beam 85, thrusting muffle closure88 away from the interior surface of door 45. A shield 101 below the opening 59 of the muffle protects the electrical leads to the muffle heating elements and a lead 105 to a heat sensor 106. The shield 101 also aids in guiding closure 88 into the muffle opening 59. All of the electrical leads extend from the muffle through the housing wall 35 into the. flexible conduit 22 and thence to the components of the control console. Conventional heat resistant grommets like the grommet 108 seal the wall passages'of the leads to the conduit.

In FIG. 2 the door 45 of the furnace is shown in open position, supporting mold flask 80 on beam 85 exteriorly of the furnace, with the flask halves 81, 83 separated by a ceramic spacer rod 111. In FIG. 3 the door is closed and the mold flask is positioned against stop 71 within the furnace muffle such that spacer rod 11 1 holds the halves apart along a line coinciding with observation port 31 and the

openings

41, 42 in the muffle. The rod may be of glass, porcelain, a blend of both or other fritted materials.

The door is suspended from a pair of diametrically spaced articulated arms like arm 121 of FIG. 1. Each Each arm is fixed in a radial ear 123 of the door and journalled in a radial tab 125 of the housing. Each arm has a pivot joint 126 of conventional nature spaced an interval from arm head 127 which acts as a stop against extending each arm beyond the housing tabs.

When the door 45 is in the position of FIG. 2 each arm 121 extends forwardly of housing tabs 125 such that the articulated joint 126 is outward of the tab 125. Thus the door may be spaced outwardly from the furnace along the central axis of the furnace housing and then bent downwardly, as shown in FIG. 2, such that the muffle closure 88 and the mold flask may be removed from the muffle through opening 59 and then the door tilted downwardly about the arm joints 126. Conversely, when the flask is to be loaded into the furnace, the door 45 may be placed in the attitude of FIG.

2 with the upper end of beam 85 ready to receive the flask, with the muffle closure 88 held away from the door by the spring 99. Once the flask is in position on the support beam the door may be swung upwardly until joints 126 are aligned with arms 121 and then the door may be closed axially inwardly, introducing the flask into the mufi'le and bringing the closure into the opening 59. The vacuum induced in the furnace holds the door closed. Conventional sealing methods may be used to make the closure tight, such as O-ring 131 in a groove 132 of the inner door face and conventional door latches (not shown) may be used to insure door closure.

Turning now to FIGS. 5-8, which illustrate the preferred mold flask of the invention, the flask is shown in FIGS. 5 and 7 in the position it occupies within the muffle when first introduced into the furnace.

In FIGS. 5 and 7 the posterior flask half 81 is shown to have a partial rear wall 133 which registers against stop 71 when introduced into the muffle. The posterior half has a cylindrical side wall 134 with diametrically opposed

locator tabs

135, 136 extending oppositely from the wall 133. Ninety degrees circumferentially from each

tab

135, 136 is an internally extending boss 137, 138 with a guide aperture 139 therein. Each boss is notched at 141. The flask half, including the notches 141, is filled with an investment 142, now hardened.

The anterior flask half 83 has an anterior wall 145 with an anterior central boss 84 having a stepped recess 146 in which an inner end 147 of the support beam lodges.

A cylindrical cup 149 separable from wall has an outer ring wall 151 with diametrically opposed spools 152 which receive

guide pins

153, 154. The pins are fixed in wall 145 of the anterior flask half and extend posteriorly beyond the anterior half to register in the bosses 137, 138 of the posterior half.

The flask anterior half defined by wall 145 and cup 149 contains an investment 156 which fills the mold flask half about the spools 152 and about a wax-formed cavity 159 now filled with powdered porcelain 161 within the cavity. The porcelain is introduced into the investment cavity 159 through a previously formed conical opening 162 and a sprue gate 163. The gate connects with one of a plurality of openings 164 diametrically aligned in wall 145 of the anterior flask half. Since the various patterns form differently placed cavities in the flasks, one or more of these openings may connect with various sprue gates, depending upon the position within the investment and the attitude of the wax pattern previously therein.

When the investment was shaped, the conical cavity 162 was filled by a conical central protrusion 167 of an investment 171 of the posterior flask half. When the two'halves were united about the wax pattern (which filled the space now occupied by powdered porcelain charge 161) a patterned concave recess 173 resulted in the investment 171 of flask half 81.

In FIGS. 5, 6 and 7 the halves are separated by refractory or ceramic spacer 111. The spacer ends reside in cavities cored in the investment at the time the investment is poured about the wax pattern. Alignment of the spacer rod and the pattern cavities is maintained by the guide pins 153, 154 and by the peripheral tabs opposed pairs 176,

.their decomposition under heat and the difficulty of removing the investment. However, the flask of the invention comprises a shell combined with an investment which deteriorates under the process heat such that it is easily removed. The chrome cobalt flask is filled with an investment which, when mixed to be plastic, is composed of liquid phosphoric acid, ground magnesium oxide and Portland cement. By bulk the proportions of the three principal ingredients are phosphoric acid 1, magnesium oxide 1 and Portland cement 0.5, depending on concentration and fineness.

' This particular investment is degraded by the furnace heat such that it is easily removed from the unaltered flask shell, making the flask shell reusable and reducing the unit cost of ceramic materials made with the process and furnace of the invention. The above described furnace is used to implement the process in the following way:

The flask halves, prepared as previously described, are charged and overcharged with porcelain powder of the high feldspar type. Conventional separators are placed on the facing surfaces of the flask halves. The anterior flask half is placed upon the support beam 85 of the open furnace door while the door 45 is in the attitude of FIG. 2. A spacer rod, such as the rod 111, which is precisely compounded so as to melt at approximately 50 higher than the melting point of the charge, is placed between the anterior flask and the posterior flask. The posterior flask is located upon guide pins 153, 154 as close to the anteriorflask half as the-spacer rod permits. The door is then tilted upwardly from the position shown in FIG. 2 and the arms 121 of the door 45 pushed through the tabs 125 of the housing such that the support beam carries the mold flask through opening 59 of the muffle until it registers against stop 71. During this maneuver closure 88 of the muffle is impelled into the opening under the thrust of compression spring 99.'Tension nut 92 is then adjusted such that compression spring 94 thrusts support beam 85 against anterior flask half 83 sufficiently to join the flask halves when spacer rod 1 l 1 melts. The spring load on the support beam tending to join the halves is about 30 pounds. The controls of the console are activated such that the requisite heat is reached and the furnace interior is exhausted to the proper vacuum point. The normal vacuum operatingrange is from 25 to 29 inches of vacuum. Conventionally firing of dental porcelain is done between l,600 to l,850 F.

As soon as the flask has entered the muffle the furnace housing mama tilted about girnbels 28 from the broken line position 180 of FIG. 9 into the solid line position shown in FIG. 9. This inclined position not only insures against the accidental displacement of the charge from the flask,.but also brings observation port 31 into convenient position for the furnace operator to observe the condition of the mold flask within the furnace and particularly to inspect the spacer rod.

Upon observing the melting of the spacer rod and the spring-induced closure of the flask halves, the tension nut 92 may be turned to increase the torque on the anterior flasks to insure perfect closure between the two flask halves, compressing the charge in the cavity. Alternatively, the operator may choose to observe the melted condition of the spacer rod and apply force to the support beam by adjustment of nut 92 at that time instead of adjusting the out prior to the firing of the furnace.

After the furnace has cooled ambient air is restored and the door may be opened in the manner previously described after the furnace is returned to the generally horizontal position of FIG. 1. The flask may then be removed, cooled and the press-molded ceramic piece removed from the mold. The finishing procedure for most dental prosthetics is then followed, such as staining, additions and glazing.

While the support beam is an element of the embodiment of the furnace as explained above, it may be removed should a different type of flask be desired. Conventional furnace operating techniques may be used with the furnace with or without the support beam. The altered furnace therefore has utility as a conventional furnace as well as being operative to implement the invention process.

Variations in the physical arrangement of the furnace and the support beam will occur to those skilled in the art. The disclosed embodiments are illustrative only and it is desired that the scope of the invention be measured by the appended claims.

What is claimed is:

1. An electrical vacuum furnace having temperature and vacuum controls and comprising a housing, a door sealing the housing, a mold flask adapted to hold a charge, said flask having seperable halves, bias means for urging the separable flask halves, together, destructable spacer means for separating the halves of the flask against the-urging of the bias means during initial heating, said spacer means having a melting point no lower than the melting point of the charge in the flask, stop means for the flask in the housing, means to heat the contents of the furnace to a molding temperature higher than the melting point of said spacer means thereby destroying said spacer means and causing said flask halves to close, and-means for suspending said flask proximate the heating means.

2. Apparatus in accordance with claim 1 wherein the spacer means comprises a ceramic rod, said ceramic being chosen from a class consisting of porcelain, plastic, glass and a fritted combination.

3. Apparatus in accordance with claim 1 wherein the flask support means comprises a beam slidably mounted in the door.

4. Apparatus in accordance with claim 1 .wherein the means urging the flask halves comprises an adjustable compression spring bearing against the flask half and the housing door.

5. Apparatus in accordance with claim 1 wherein the means urging the flask halves together comprises a beam joumalled' in the housing door, said beam bearing against the flask, a thrust shoulder on the beam exterior of the door and a threadably adjustable collar movable with respect to the door to bear against the shoulder and thrust the beam against the flask to alter its spacing with respect to the second flask half.

6. Apparatus in accordance with claim 1 wherein the fumacefurther comprises an observation port in the housing and means for positioning the flask within the housing such that the space between flask halves is optically aligned with the observation port.

7. Apparatus in accordance with claim 1 further comprising swivel means mounting the furnace housing to pivot about a horizontal axis perpendicular to the joining motion of the flask halves. v

8. Apparatus in accordance with claim 1 wherein the heating element comprises a substantially cylindrical muffle, an observation path through the mufile perpendicular to its axis, means for adjustably mounting the mufile within the housing and an adjustable stop rod extending into the mufile to limit motion of the posterior half of the flask.

9. Apparatus in accordance with claim 8 wherein the means mounting the mufile comprises a plurality of bearing bar pairs, each bar pair being spaced from an adjacent bar pair about the exterior of the muffle and compression springs separating the bars of each pair such that one bar bears against the interior of the furnace housing and the other bar of the pair bears against the exterior of the cylindrical muffle.

Claims

2. Apparatus in accordance with claim 1 wherein the spacer means comprises a ceramic rod, said ceramic being chosen from a class consisting of porcelain, plastic, glass and a fritted combination.
3. Apparatus in accordance with claim 1 wherein the flask support means comprises a beam slidably mounted in the door.
4. Apparatus in accordance with claim 1 wherein the means urging the flask halves comprises an adjustable compression spring bearing against the flask half and the housing door.
5. Apparatus in accordance with claim 1 wherein the means urging the flask halves together comprises a beam journalled in the housing door, said beam bearing against the flask, a thrust shoulder on the beam exterior of the door and a threadably adjustable collar movable with respect to the door to bear against the shoulder and thrust the beam against the flask to alter its spacing with respect to the second flask half.
6. Apparatus in accordance with claim 1 wherein the furnace further comprises an observation port in the housing and means for positioning the flask within the housing such that the space between flask halves is optically aligned with the observation port.
7. Apparatus in accordance with claim 1 further comprising swivel means mounting the furnace housing to pivot about a horizontal axis perpendicular to the joining motion of the flask halves.
8. Apparatus in accordance with claim 1 wherein the heating element comprises a substantially cylindrical muffle, an observation path through the muffle perpendicular to its axis, means for adjustably mounting the muffle within the housing and an adjustable stop rod extending into the muffle to limit motion of the posterior half of the flask.
9. Apparatus in accordance with claim 8 wherein the means mounting the muffle comprises a plurality of bearing bar pairs, each bar pair being spaced from an adjacent bar pair about the exterior of the muffle and compression springs separating the bars of each pair such that one bar bears against the interior of the furnace housing and the other bar of the pair bears against the exterior of the cylindrical muffle.