US2309136A - Process for preparing an alloy for cast dentures - Google Patents

Description

R. NEIMAN Jan. 26, 1.943.

PROCESS FOR PREPARING AN ALLOY FOR CAST DENTURES Filed May 4, 1940 REDUCING CONE BRUSH FLA ME INVENTOR. ROBERT NEIMAN BY Q G? ATTORNEY.

atented Jan. 2, l i' r rtoonss son amma AN poaoas'r pnn'rmns Robert Neiman, Louise, assignor to Ed- I mund A. Steinbeck, a

Application my 4,1940, Serial No. seam '1 Claim- (01. ta -m) This invention relates to a'nonferrous alloy or alloys combining high resistance tocorrosion and strength, of which, the hardness and resilience can be simply and easily controlled in the melting and casting process thereof.

While this alloy is well suited for forming a wide variety of commercial articles, it is most eminently suited for producing articles-by the casting process, especially dentures. Its use and advantages for the exacting nature and process of producing dentures will be described hereinafter in detail.

In producing a cast alloy denture the. following steps are the usual'procedure:

1. An impression of the portion of the mouth containing the area to be reconstructed is taken 4 with plaster, plastic materials, or colloidal maheat between the melting point and casting temperature of the alloy 50 as to minimize oxidation or nitrification in the air as well as interaction with the refractory surfaces of the mold. Furthermore, the metal should not form a surface scum or slag which prevents the operator from easily seeing when the nuggets or pieces of metal are fully molten. A shiny surface on the fluid molten metal, not unlike that shown by precious metals or alloys, gives the operator an excellent chance of noticing when the metal has reached the right condition for casting.

The metal should have the property of forming castingsthat are homogeneous, dense, and free from pits or blowholes.

The metal should not change greatly in strength or hardness upon cooling of the mold,

' whether it be by quenching in water or air im- 7. The entire mold is heated in a suitable furnace to such' temperature as will eliminate the ,wax, and expand the mold sufiiciently to counteract the casting shrinkage of the metal, and also to permit the alloy to cast successfully.

s. The mold is placed in a'casting machine, the metal is melted in a crucible, usually by means of an oxyacetylene flame, and-the molten alloy is forced into the cavity in the mold under pressure.

9. The moldis cooled, preferably by quenching in water, the casting is removed, cleaned and polished for use.

To produce the best possible denture with a minimum of effort or cost, the alloy, also commonly referred to'as metal in the art, should have many qualities of which the following are paramountly necessary or desirable.

The metal must have good castability, thatis, it must form a liquid of low viscosity when mediately or at any time after casting. Its casting shrinkage should be sumciently small to enable extreme accuracy'of fit of casting without resort to unusually high expanding investments, the latter being diflicult to attain. The cast denture should have a surface as smooth and free of oxide and other contamination as possible, in order to reduce the time necessary for cleaning. and polishing. This is most necessary on-the surface in contact" with the model where remova -of any appreciable portion of the surface woul obviously prevent accurate fit.

- The metal should have. a low melting point for ease in melting and a low casting shrinkage. A low casting temperature and low reactivity ,at thistemperature are most important advantages as they enable the use of investments with calcium sulfate binders. The latter investments are the easiest to manipulate, stronger and least troublesome to use.

Corrosion resistance to a high degree is absolutely necessary. The metal should not be acted on or visibly discolored by food acid, lactic acid,

heated to casting temperature soas to flow into I or other acid, or'alkalies present in foods, dentifrices, or medicinal agents. Furthermore, it should be tissue tolerant in its lack of metallic taste or other undesirable eflects on the tissues, such as discomforts due to electrolysis effects with other metals present in the mouth. It should becapableof receiving a high polish and must maintain this lustre in use.

The denture must have a highstrength and resilience to withstandsuccessfully the continued subjection to stresses and strains of mastication without losing its initial shape or configuration. In addition it should have the strength and resilience necessary to permit the use of extremely thin and narrow portions or sections. There must be an accompanying toughness and suflicient ductility to enable slight adjustments, especially of clasps, as well as to withstand overloads and occasional undue impacts. This means that the alloy should possess a high elastic limit, ultimate tensile strength, modulus of elasticity, and yet have a sufficient elongation or ductility. High values in impact and fatigue tests are also desirable. This multitude of desirable mechanical properties must not be accompanied by any appreciable brittleness nor should the hardness as measured by the Brinell test be too high. The Brinell number should be preferably within the limits of 200-300. A high Brinell number or the presenceof a hard constituent would mean injury to the teeth when placing and removing the denture. This latter condition means that the alloy should be a homogeneous and preferably a single phase solid solution.

The term denture includes a variety of appliances such as partial dentures, complete dentures, inlays, crowns, and similar articles. In the construction of these varied appliances there is a necessity for using alloys of varying degrees of hardness as is known in the art. Furthermore, some dentists desire a slightly softer alloy than other in order to give them greater freedom in making adjustments of clasps, etc. It would therefore be especially desirable that the technician be able to vary the hardness of the alloy to suit these varying needs without detracting appreciably from the other desired mechanical properties. This process should preferably be performed before the denture is fitted and polished so as to preclude the necessity of double work in polishing and chance of distortion when subjected to such process as heat treatment.

The ability of the alloy to permit melting in an open crucible by such means as an oxy-acetylene flame means less cost and trouble than one requiring inert or reducing atmospheres, or special melting furnaces. The alloy should also permit soldering or welding without complicated or costly apparatus or technics.

A low specific gravity, low cost of metal and casting equipment, a warm platinum like color rather than a harsh bluish tinge of chromium, and good heat conductivity are further desirable properties.

To meet this extremely difficult host of requirements the art has found gold and alloys thereof to meet most but not all of the requirements. A number of non-ferrous, non-precious metal alloys are available that also meet most of the requirements but are usually of high melting point, require costly casting equipment, or fail to give the operator a control of the hardness characteristics.

Alloys consisting of chromium and cobalt or of these two metals in conjunction with tungsten or molybdenum, or both, have the advantage over precious alloys in their comparatively 'low cost and greater modulus of elasticity but their melting point is so high as to preclude the very convenient use of calcium sulfate investments. Even when alloyed with nickel, silicon, manganese, titanium or combinations therefrom the melting point is still too high. Also the hardness is generally too high and cannot be easily controlled by the operator. Ternary alloys of chromium, cobalt, and nickel also offer similar disadvantages.

The addition of beryllium or boron to the latter ternary alloys serve to reduce the melting point but are generally somewhat brittle and not readily susceptible to hardness control by the operator.

Many other commercial alloys are available that resistcorrosion well but lack one or more properties that makes them as suitable as desired. Stainless steel, such as the 18% chromium-8% nickel variety, is fairly corrosion resistant to a number of media but the high melting point precludes its castability in moulds having calcium sulfate as the binder. Another type of alloy-"Nichromecontaining nickel and 20% chromium is also too high melting and too reactive at the casting temperature, moreover, this alloy is too soft. The substitution of cobalt for all or part of the nickel increases the hardness but does not lower the melting point or reactivity at casting temperature sufficiently for use in conjunction with calicum sulfate investments.

It is, therefore, the principal object of this invention to provide an alloy or alloys that possess a comparative high degree of excellence in every one of the following desirable properties and a combination of all of them in such degree as to render the alloys admirably suited for dentures. Obviously, alloys possessing these desirable attributes are also advantages in industry for a multitude of uses.

1. Good castability.

2. Low melting point and casting temperature readily discernible.

3. Form smooth, homogeneous, dense, and blow-hole free castings.

4. Strength not greatly affected by rate of 0 quenching.

5. A low casting shrinkage.

6. Low reactivity at casting temperature.

7. Castable in a calcium sulfate investment.

8. High corrosion resistance.

9. High elastic limit.

10. High ultimate tensile strength.

11. High modulus of elasticity.

1 High modulus of resilience.

13. Sufficient ductility to obviate brittleness.

14. High impact strength.

15. High fatigue value.

16. Desirable and controllable Brinell hardness.

17. Permit soldering and welding readily.

18. Low specific gravity.

19. Low cost of metal.

20. Low cost of melting and casting equipment.

21. Attain a high lustre readily.

22. Possess a warm platinum-like color.

23. Good heat conductivity.

24. Contain little or no high abrasive constituents.

25. Form a homogeneous and preferably a single phase solid solution.

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

In the drawing, the figure illustrates a flame that is employed to obtain the results of this invention.

As above mentioned, cobalt-chromium-molybdenum alloys have many desirable features but are primarily of a high melting point and form solid solutions within a limited field of the ternary diagram.

Binary alloys of nickel and coppermay be made in any proportions as these two metals are mutually soluble. Binary alloys of these two metals especially with high nickel content are fairly corrosion resistant but not as highly as the above ternary alloys. n the other hand, the binary alloys are not as brittle and, consequently, more ductile. I

While no study has ever been made of the complete alloy system containing all five metals, we can surmise from known metallurgic principles certain facts which explain the peculiar desirable results of mixing these five metals in certain limited proportions,

When metals are at least partially soluble in each other, the addition of one metal to another results in an alloy possessing a lower melting point than that of either metal alone. The addition of two new metals to a ternary alloy will thus reduce its melting point considerably. Furthermore, the more corrosion resistant metals will impart this same property to the newer alloy as a whole. Also, the addition of the binary alloys to theabove ternary system tends to form a more homogeneous metal in perhaps a greater range of compositions since there are now present more combinations of metals with mutual affinities for each other. The added nickel-copper also reduces the carbide forming tendency of the alloys as a whole since neither of these two metals form carbides readily, if at all. The

binary alloys alone have a soft and pleasing color and impart this to the completed alloy. The nickel-copper addition also imparts its ductility without deteriorating the toughness of the alloys appreciably.

Of the metals mentioned above, chromium is the basis and makes this type of alloy possible. Whereas precious alloys attain their corrosion resistance through the.noble metals present, alloys containing appreciable amounts of chromium attain theirs through the passivity of the surface due to the chromium present, chromium alloys become passive because the surface film is oxidized in the air or generally, even in aqueous solutions though they be acid, alkaline, or neutral. between 1 and 40 molecules thick. This oxide film is so dense that it is impervious to the further difiusion of oxygen or other corrosive agents.

This film is very tenacious and the prime reason' for its continued protection of the metals is the fact that the film even when broken is immediately self-repairing. This is true wherever there is possible access of oxygen. Furthermore, this film has approximately the same coefiicient of expansion as the metal as a quite abrupt change in temperature will not cause it to flake off. It is this passive oxide film that is corrosion resistant, rather than the metal itself.

Chromium is also most important from the mechanical standpoint as it. imparts strength and hardness to the alloy as a whole. When usedin the greater proportion within the limits specified in the table below, it will tend to harden the metal and unless there are softening metals present, the alloy as a whole while becoming more corrosion resistant will, nevertheless, lose in ductility. Chromium contents within the, preferred proportions given below render the alloy tough and extremely resistant to such acids as nitric, in

fact, the alloy-willresist concentrated nitric acid for unlimited periods. Nitric acid is very useful in forming the passive surface referred to above because of its oxidizing action.

The oxide film formed-is invisible and is Cobalt is agra i h metalwith a slight tint of red. It imparts this reddish tone to its alloys which adds warmth to their color. Cobalt also forms a protection passive oxide film. It is a fairly hard metal and imparts strength to the alloy and when not used in too great proportions, does not cause brittleness.-

. Molybdenum, when'used, within the limits set forth imparts strength and acid resistance, especially to hydrochloric acid -.,-The acid resisting property of molybdenumis. believed bysome to be due tov the factthat it isla noble metal since it is below-hydrogenin theele'ctromotive series. In spite of its high melting'point it seems to lower the melting pointof-the all0'y-.-

Nickel is a strongand tenacious forms a protective oxide It adds toughness and ductility to the finished alloy. U

Nickel may be interchanged'with cobalt within the limit specified and when thus. replacing cobalt, it tends to add ductility to the alloys, whereas, cobalt adds greater resilience. These two metals seem to combine their respective desirable properties when present in approximately equal proportions as given in the preferred alloys below.

Copper has a great afiinity for, and solubility in nickel and renders the alloys more homogeneous and reduces the melting point. While it is not extremely acid resistant alone, it does not seem to reduce the acid resistance of the alloy since it is in solid solution and, furthermore, it also forms a protective oxide film. When using this metal, the higher proportions should preferably be used when the nickel content is also high as, otherwise, it may'tend to border on the range of a two phase system.

Iron is to be kept as low as possible in order to maintain a high corrosion resistance along with other good properties possessed by these alloys. Since iron is-an appreciable impurity in practically all of the constituent elements as commercially available, itwill occur in the finished alloy in amounts generally between 1.0 and 2%. Itcan hardly be'kept'belo'w 1% and in rare cases may be as high as 5%, In any case, the iron is not to be considered asJa necessary alloying element, but merely as anundesirable impurity that must of necessity be tolerated.

To this combination-of five elements, making up from 97% to over 99% of the alloy, it is desirable to add small amounts of certain elements to remove undesirable impurities, to prevent contamination in the melting process, and to modify and control the physical characteristics. I

silicon, in amounts of'approximately .2 of 1% increases the castability byimp'roving the fluidity. It also makes it easier;for the operator to discern when the nietalihifiitzached its casting temperature by reducing ;-th e obscuring, nonmetallic surface film. This small amount is more than likely entirely ,dissolvedin' the alloy and, thus, 'does not form hard intergranular constituents. It also seemsto be eifective in controlling and limiting the amountof carbon dissolved in the alloy during the melting operation and thus acts as a fbrake in preventing carbide formation. :5 f r The manganese serves to combine with the sulphur present as impurities in the other elements by forming globularized manganese sulfide which generally enters. the $1 18 or, if retained in the alloy, has no appreciable;deleterious effect. If

the sulphur is not thus-removed, it tends to alloy with the nickel, especially." These two elements form a very low melting eutectic that coats the metal and also surface of the grains of the alloy and renders the latter extremely Weak and brittle.

Carbon, perhaps of all the elements, has the unusual property of not only hardening these a1- loys but of giving them a springiness or resilience combined with toughness, when present in small amounts. This is very likely true so long as the carbon is dissolved in the alloy. Once an excess of carbon is present, it begins to form carbides, which cause embrittlement of the alloy. Furthermore, carbides are undesirable as they are very abrasive and would injure the teeth when the denture is removed and replaced. The alloys herein given, especially the preferred ones, have been found to absorb the desired amount of carbon when melted by the method hereinafter described. Furthermore, when melted by different operators there seems to be but a negligible variation in carbon absorption provided a given flame.

is used.

The carbon in the original alloy should be lowpreferably under 0.20% and may be added to in the melting process herein described up to approximately 1.0%. Alloys with less than 0.2% carbon may be referred to as practically carbon free alloys.

The melting process in which the carbon is imparted to the alloy in controlled amounts is performed as follows:

The equipment consists of a tank of oxygen, a tank of acetylene, each equipped with regulating gauges and connected by a suitable tubing to the torch. A torch made by the Imperial Brass Manufacturing Co., of Chicago, 111., and using Tip #8 has been found very successful. Substantially, similar torches may be secured from many manufacturers, especially those dealing in the two gases mentioned above.

To adjust the flame, the following procedure is used:

1. Close both valves on torch.

2. Open valves on oxygen and acetylene tanks fully.

3. Set regulator gauge valve for acetylene at lbs. flow pressure, by opening the acetylene torch valve when adjusting the gauge.

4. Set regulator gauge valve for oxygen at 10 lbs. flow pressure, by opening the oxygen torch valve when adjusting the gauge.

5. Light torch.

6. With the acetylene torch valve fully opened, giving 5 lbs. flow pressure, adjust the oxygen flow by means of its torch valve until the flame is as shown in Figure 1. The inner oxidizing cone 10 will be inch to l'inch long the reducing cone II will be approximately 2 inches long. It is important that the fla e, that is, the sum of the oxidizing and reducing c nes,, should measure 3 inches long from the e go of the tip.

'7. The nuggets of metal are melted by impinging this flame in such manner that the reducing cone only is in contact with the metal. It is preferable to have the center of this cone, that is, the portion of the flame between 2 and 2 inches from th edge of the tip, in contact with the metal.

The above flame is known as a 3 inch flame and is the one generally preferred for use with the preferred alloy.

To harden the alloy, the flame may be increased in length to 4 inches, for example. To soften the alloy the flame is decreased to 3 inches or even less. Increasing the flame length imparts more carbon to the alloy, thus hardening it, whereas decreasing the flame length imparts less carbon and thus the alloy remains more ductile.

As a concrete example-an alloy made about midway between the preferred alloy limits shown in the table, contained but 0.15% carbon. It showed a Brinell hardness (BHN) of (500 kg. load) and a Rockwell C (Rc) hardness of 20-22.

When 1% oz. were molten and cast with a 2" flame, the carbon was increased to 0.25% and the BHN was 109, R0 was 12-14. Thus, even though the carbon was increased, the hardness was reduced. This is explained by the fact that the size, shape, heat, and mechanical treatment of the test pieces were different.

Using a 3%" flame the carbon was increased to about 0.5%, the BHN was 130, R0 21 to 23.

The use of a still longer and more reducing flame, one of 4%,", the carbon was increased to about 0.6% and the BHN was 143, R0 21 to 22.

The test pieces for the tests, other than the original alloy were cast under practical conditions and show that the carbon and BHN are increased as the flame is made longer and more reducing. The R0 is also increased but not in exact ratio.

It is interesting to note that it is apparently impossible to exactly correlate the BHN and the Ho. This is very likely associated with the Work hardening of these alloys. It is further interesting to note that whereas the R0 hardness indicates that the alloys are soft enough to machine, it is, nevertheless, practically impossible to mill or file specimens without damaging the milling cutter and the file. For this reason, and also due to the fact that specimens when cast in small and thin pieces-comparable to the size of dentures, it was undesirable to use the 3,000 kg. load Brinell hardness test. The 3,000 kg. BHN would be about twice the BHN 500 kg. load) and, therefore, range between 200 and 300 as is most desirable for the intended uses.

The castings on which tests were made were cast in a mold having a calcium sulfate hinder,

the investment mold having been raised to 1600 A F. in about 1 hrs. and maintained at this temperature for 1 hour. The properties of the alloys are variable somewhat depending on the mold temperature as this determines the rate of solidification. In general a smoother and perhaps a little harder casting will result when cast at lower temperatures and a slightly rougher and more ductile casting will result at higher mold temperatures but greater detail is possible. The more inert and the less possible contaminating agents present in the mold at casting temperature the smoother and more resilient the casting.

The ability to work harden is further desirable in cases where clasps are to be adjusted. The metal as cast is readily adjustable and then, in use is hardened further and thus given a greater resilience. The ability to work harden further makes possible the drawing of the alloy into wires, etc. Thus, clasps may be made of wires and attached to the denture by soldering, welding, or casting the denture to the embedded clasp. Other commercial uses wherein work hardening is necessary or desirable is also possible.

Within the minimum and maximum ranges of the metals forming the alloy, it is possible to increase the carbon absorbing elements (chromium and molybdenum) ,or the cobalt and give the alloy sufficient inherent hardness to permit the use of even a neutral flame, that is, a flame without the center reducing cone. In this case, however, it will be found that in melting the alloy, an undesirable opaque fllm will cover the molten metal and obscure the operators view in adiudging the readiness of the metal for casting by its fluidity or viscosity.

In this case, or even when flames up to approximately 3 inches in length are used, it is highly beneficial to apply a finishing" flame. To do this, the metal is heated by the desired'flame until the nuggets have slumped and the alloy appears to be completely molten, the oxygen torch valve is then closed slightly, without moving the torch, to provide about a 3 inch to 4 inch flame. With this reducing flame, the metal is given a suflicient super-heat, from just above the melting point to the casting temperature, to enable successful casting. By using this finishing reducing flame, the opaque film of oxides is reduced and the surface appears shiny and readily indicates the fluidity or viscosity of the metal by the mobility and spinning action produced under the force of the impinging flame gases. This "finishing flame length cannot be accurately adjusted since it is undesirable to remove the flame from the metal even momentarily, as this will permit .the rapid oxidation and possibly solidification of the alloy. The inaccuracy of the finish flame length is not deleterious as its action lasts for perhaps only seconds and during this short interval very little carbon is impartedto the alloy, since a great portion of it is used up in reducing the oxide film. Bythis means, the alloys provided herein may be easily molten and yet retained almost carbon-free.

The use of the finishing flame is believed to be an entirely new and novel process.

If the more ductile elementsnickel or copper, or both are used in maximum quantities herein set forth, it will be advisable to use a more reducing flame, such as, a 4" or even slightly longer, to provide the alloy with suflicient strength and resilience.

It is thus possible for a manufacturer to provide a single substantially carbon-free alloy and the user in turn can impart to this alloy a desired resilience and hardness within wide limits. Obviously, the manufacturer may add sufiicient carbon to the alloy in the manufacturing process to give a desired hardness when used with a Following is shown a table giving the elements that go to make up the alloys:

Element or metal Minimum Maximum Preferred Per cent Per cent While the above list gives minimum and maximum percentages by weight of each metal it is, of course, recommended to steer clear of extremes. The extreme on one metal is not necessarily undesirable, but the extreme on two or more metals is generally undesirable.

tices; for example, the total of the minimums on cobalt plus chromium, plus molybdenum is 40% and the total of maximums on nickel plus copper is 50%, the flve elements adding up to only 90%; obviously the lower limit of the ternary combination must add up to at least 50%.

To get the best possible results, it is desirable to take a middle course on all elements and this is given in the column entitled, Preferred; even in this column, the middle course is a further improvement. I The preferred alloys are thus best suited and the best results attained from use with a cargiven flame length: the user still can vary the properties, should he desire, by changing the flame length. For those who desire a hard metal not requiring appreciable carbon the manufacturer mayincrease the chromium,- cobalt, or

molybdenum, or any combination of theseand' namely, cobalt, chromium, molybdenum, nickel,

and copper; to this combinationit is most desirable to add some amounts of silicon, manganese, and carbon. While the latter three elements are highly desirable,,it is, nevertheless, possible to produce satisfactory alloys by omitting one or more of them. Instead of manganese it is possible .to use an alloy of manganese and titanium or possibly titanium. alone. ,It is possible to omit the silicon entirely, likewise, it is possible to adjust the five metals in such combination as .to obviate the necessity for carbon. The elements silicon, manganese, and carbon are not positively essential but highly desirable if full advantage of the benefits provided by the alloys and processes given herein are to be utilized to the best possible advantage as herein provided.

their functions. Chromium plus molybdenum are the hardening elements and also most active in resisting corrosion. Such combination of the two as to make up approximately-30% of the alloy seems to give the best combination of high strength and moderate hardness which, in reality, is a definition of toughness. Nickel and cobalt form solid solutions in all proportions and it is 1 possible to interchange these to a great extent,

cobalt lending hardness and nickel lending ductil ity-a superior combination is attained with approximately equal quantities of each. Nickel plus copper are also mutually soluble in all proportions and seem to give the best results when present in the ratio of nickel to copper of between 7 and 10 to 1. Chromium and molybdenum are quite soluble in nickel although they do not form solid solutions in all proportions. It is thus seen that each of the elements has a good solubility and forms solid solutions with nickel. It is thus apparent that this summation of inter-related affinities conveys to the completed alloy an extreme homogeneity that combines the good characteristics of each constituent to the best possible advantage. Furthermore, the above also explains why the five metals mentioned form solid solutions within Wide ranges of composition.

In the matter of minor elements, namely, silicon, manganese and carbon a medium course in composition is also of advantage. In the preferred alloy all three elements make up approxi- Insofar as possible, the above list obviates such pracmately 1.5% of the total. In manufacturing these alloys it is desirable that they be melted in an induction furnace and preferably also in a carbon-free crucible. The alloy should be melted under a glass slag to prevent undue reaction with gases in the air, especially oxygen. This slag is desirable although not absolutely necessary.

Once the alloy is molten it should be cast or poured, into a refractory, or preferably a metal mold, into the form of rods which can then be cut or broken into small pieces suitable for convenient use. If the pieces are appreciably contaminated on the-surface, they should be cleaned by sandblasting or tumbling.

The original alloy made for use should be as carbon-free as possible since even small amounts may be deleterious, not necessarily alone, but

when additional carbon is absorbed in the melting process just prior to casting. Wherever it is desirable to make the original alloy with a given amount of carbon so that a shorter flame may be used in melting, it is possible to use a furnace or crucible containing carbon or graphite. This has been found generally to be inferior to the use of an induction furnace.

The use of small amounts of various elements which function as fluxes and remove impurities in the original metals is possible. For example, calcium is a very powerful de-oxidizer and may be used in small amounts. Similar metals, such as zirconium or titanium, the latter also a powerful dentrifier, may also be used as fluxes, so long as they do not appear in the finished alloy in amounts suflicient to act deleteriously.

The use of the oxy-acetylene flame has been herein described in detail and is generally the preferred form for melting the alloy just prior to casting. It is, however, possible to melt the metal with other gas flame combinations and, also, to melt it by using electricity as the source of heat. In the latter cases it is desirable to adjust the carbon in the original alloy so that the final casting will contain the desired amount of carbon and be within the recommended limits of .1 to 1%. Carbon may also be added to the metal by melting it in contact with carbon.

What is claimed is:

The process of preparing an alloy to make a cast denture in a mold containing a calcium sulfate binder, and said alloy consisting of cobalt 18 to 40%, chromium 18 to 28%, molybdenum 4 to 12%, nickel 19 to 40%, copper 1 to 10%, silicon .2 to 1%, manganese .1 to 1%, and carbon .1 to 1%, and including the steps of melting the alloy with a substantially neutral flame, in which the reducing cone is short or entirely absait, and following with a finishing flame, in which the reducing cone is increased in length to provide a reducing condition, whereby the surface oxide film of the alloy is substantially removed, thus improving the visible condition of the molten a1- loy, and which finishing flame is not used for a sufficient length of time to impart any appreciable amount of carbon to the molten alloy.

ROBERT NEIMAN.

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