Abstract:
A method to grain refine and deoxidize a precious metal alloy or a master alloy includes the steps of (a) forming a precursor melt consisting essentially of constituents of the precious metal alloy or master alloy and inevitable impurities; (b) dispersing a compound selected from the group consisting of boron containing metal hydrides, boron containing metal fluorides and mixtures thereof throughout the precursor melt; and (c) solidifying the boron containing precious melt alloy or master alloy. One suitable compound is solid sodium borohydride (sodium tetrahydroborate). To minimize evaporation of the boron on contact with the precursor alloy melt, the sodium borohydride may be wrapped in a metal foil formed from constituents of the precious metal alloy or master alloy. The cast precious metal alloy or master alloy has been found to have a reduced number of hard spots and reduced silicon contamination when compared to conventional casting methods.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This patent application claims priority to U.S. provisional patent application Ser. No. 60/672,566 by P. G. Johns and S. A. Davis that was filed on Apr. 19, 2005; United States non-provisional patent application Ser. No. 11/132,621 by S. A. Davis, N. D. Baker, J. J. Riskalla and R. V. Carrano that was filed on May 19, 2005; and U.S. provisional patent application Ser. No. 60/732,784 by P. G. Johns and S. A. Davis that was filed on Nov. 2, 2005. The subject matters of all three U.S. patent application Ser. Nos. 60/672,566; 11/132,621 and 60/732,784 are incorporated by reference herein in their entireties. 
     
    
     U.S. GOVERNMENT RIGHTS  
       [0002]     Not Applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     This invention relates to a process to manufacture boron containing precious metal alloys and master alloys. More particularly a solid compound that is either a boron containing metal hydride, and preferably a solid tetrahydroborate, or a boron containing metal fluoride is dispersed throughout a molten precious metal alloy or master alloy.  
         [0005]     2. Description of the Related Art  
         [0006]     Precious metal jewelry alloys are frequently worked into complex ornamental shapes. To withstand extensive working without fracture, the precious metal alloys require high ductility and high strength. High ductility and high strength are facilitated by an alloy having a low oxygen content and a fine grain structure.  
         [0007]     Boron is known to both deoxidize and refine the grain of precious metal alloys. When boron scavenges oxygen from a melt and other oxides in the melt, it cleanses surfaces of the metal. U.S. Pat. No. 5,384,089 to Diamond discloses the use of boron as a deoxidizer for gold-base alloys. This patent discloses that boron causes hard spots. U.S. Pat. No. 6,168,071 to Johns discloses a diffusion bondable silver-copper-germanium alloy that may contain up to 20 parts per million of boron as a grain refiner. The boron is disclosed as added as a component of a copper-2%, by weight, boron master alloy. Both U.S. Pat. No. 5,384,089 and U.S. Pat. No. 6,168,071 are incorporated by reference herein in their entireties. Throughout this patent application, all percentages are weight percent, unless otherwise specified.  
         [0008]     A conventional method of introducing boron into a precious metal alloy or master alloy is through the use of the 98% copper-2% boron master alloy. However, the use of such a master alloy frequently introduces hard spots into the products. These hard spots are believed to be non-equilibrium phase CuB 22  particles that form in copper saturated with boron when cooled from the liquid phase to the solid phase. Hard spots can also form with other metal-boride compounds such as iron borides (for example Fe 5 B 2  and FeB 2 ). The hard spots are frequently not detected until after the precious metal jewelry alloy is polished and inspected resulting in needless expense for the processing of ultimately unsatisfactory product.  
         [0009]     Copper—2% boron master alloys are frequently contaminated with silicon. The silicon contamination may lead to brittleness, as a result of the formation of brittle intermetallic compounds, oxides and low melting eutectics. A low silicon content is required for fine, white gold and fine silver.  
         [0010]     A further disadvantage with the use of a copper—2% boron master alloy is that the high mass percent of copper may not be desired for the alloy product. Excess copper may cause a silver-base alloy to be subject to tarnish and/or firestain.  
         [0011]     There remains, therefore, a need for a more effective way to introduce boron as a grain refiner and oxygen/oxide scavenger into a precious metal melt.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     In accordance with the invention, there is provided a method to produce a precious metal alloy or master alloy. This method includes the steps of (a) forming a molten precursor alloy of the precious metal alloy or master alloy, (b) disbursing a boron containing compound throughout the molten precursor alloy, and (c) solidifying the boron containing precursor alloy.  
         [0013]     It is feature of the invention that the boron containing compound is either a boron containing metal hydride or a boron containing metal fluoride. When a boron containing metal hydride, the metal may be sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron containing metal fluoride, the metal is sodium. Most preferred as a boron containing compound is sodium boro-hydride.  
         [0014]     It is another feature of the invention that more than 20 ppm of boron can be incorporated into a silver-base, or other precious metal, alloy without the development of hard spots.  
         [0015]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  illustrates in flow chart representation an initial processing sequence for the alloys of the invention.  
         [0017]      FIG. 2  illustrates in flow chart representation subsequent processing of the alloys of the invention in accordance with a first embodiment of the invention.  
         [0018]      FIG. 3  illustrates in flow chart representation subsequent processing of the alloys of the invention in accordance with a second embodiment of the invention.  
         [0019]      FIG. 4  graphically illustrates the rate of boron loss in a batch process of the invention. 
     
    
       [0020]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0021]     The following definitions are used throughout this patent application:  
         [0022]     Master Alloy—Constituents of a precious metal alloy omitting the predominant precious metal. For example, a yellow 10, 14 or 18 karat alloy may contain both silver and gold, only the gold would be omitted in the master alloy. Silver would be present. For a sterling silver alloy, the silver would be omitted and there would be no precious metal constituent present. Master alloys are usually sent to an end user who adds the required amount of precious metal.  
         [0023]     Precious Metal Alloy—An alloy having a desired composition for jewelry applications. The alloy includes required amounts of gold, silver, palladium and/or platinum.  
         [0024]     Precursor Alloy—A composition slightly off specification for a desired master alloy or precious metal alloy. The addition of a metal foil containing boron compound places the composition on specification. If the boron compound is not wrapped in metal foil, for example wrapped in paper or not wrapped, the precursor alloy composition is on specification for the desired master alloy or precious metal alloy.  
         [0025]     The process of the invention is useful to add boron to precious metal alloys and to master alloys with a minimal formation of hard spots. Exemplary of the precious metal alloys are sterling silver alloys and silver alloys containing in excess of 75% silver with the balance being alloying elements, including, but not limited to, copper and zinc, and inevitable impurities. Silver alloys and sterling silver alloys having between 80% and 97% silver are most benefited by the process of the invention.  
         [0026]     The process is also useful for gold jewelry alloys having at least 33%, by weight, of gold (8 karat) with the balance being alloying elements including, but not limited to silver, nickel, copper and zinc as well as inevitable impurities. Most benefited by the process of the invention are those gold alloys having between 37.5% and 77% gold.  
         [0027]      FIG. 1  illustrates in flow chart representation an initial processing sequence of the alloys of the invention. A precursor melt of the precious metal alloy or master alloy is formed by melting 10 appropriate amounts of the precious metal and alloying elements in a suitable crucible. As described below, a boron containing compound may be wrapped in a metallic foil formed from either the precious metal or one of the alloying elements and added to the precursor melt. Accordingly, the additional metal content of the foil is taken into consideration and the composition of the precursor melt is typically slightly different than the composition of the desired end product.  
         [0028]     The alloy is melted 10 in a suitable crucible. For silver alloys, one suitable crucible is formed from clay-graphite and for gold alloys one suitable crucible is ceramic. Other suitable crucibles for silver-base and gold-base alloys include clay-graphite, fused silica, silicon carbide, graphite and zirconia. The metals are heated to a temperature effective to fully liquify and flow the mixture, typically in the range of from 1950° F. to 2300° F., with a nominal temperature on the order of 2150° F. The melting temperature influences the kinetics of boron evaporation which determines the final boron concentration in the cast precious metal alloy or master alloy. The selected temperature should be sufficiently above the liquidous temperature of the alloy to prevent freezing in a die during continuous casting or freezing in a grain box during grain making. While the alloys are readily cast at atmospheric pressures, higher or lower pressures should not affect the benefits of the invention, but will affect the kinetics of boron evaporation.  
         [0029]     To reduce the formation of an oxide slag, the molten precursor alloy should be covered to isolate the metal surface from oxygen. Suitable gas covers include, but are not limited to, a carbon monoxide flame, forming gas flame, argon, nitrogen, hydrogen flame and natural gas flame. Suitable powdered solid covers include, but are not limited to, borax, boric acid, graphite and charcoal.  
         [0030]     Once the precursor melt is at the desired molten temperature, a boron containing compound is added 12 to the precursor melt. Boron is incorporated into precious metal alloys as an oxygen scavenger and, for silver alloys, additionally or alternatively as a grain refiner. It may be added to molten silver by bubbling a gaseous borane, e.g. diborane into the alloy in admixture with a non-reactive gas such as argon, by introducing into the alloy a borane which is solid at ambient temperature, e.g. decaborane B 10 H 14  (melting point=100° C., boiling point=213° C.), or by adding an alkylated borane, e.g. triethylborane or tri-n-butyl borane, although the latter reagents are spontaneously combustible and require care in handling.  
         [0031]     When added to the precious metal in the gas phase, the boron compound is advantageously an admixture with a carrier gas that assists in creating a stirring action in the molten alloy and dispersing the boron content of the gas mixture into the alloy. Suitable carrier gases include, hydrogen, nitrogen and argon. The gaseous boron compound and the carrier gas may be introduced into a vessel containing molten silver or silver-base alloy by using a metallurgical lance that may be an elongated tubular body of refractory material, e.g. graphite, or it may be a metal tube clad in refractory material. The lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep within the molten alloy.  
         [0032]     Alternatively, the boron-containing gas may be introduced into the molten alloy from a side or bottom of the alloy-containing vessel through a gas transport member, such as a gas permeable bubbling plug or a submerged injection nozzle. Rautomead International of Dundee, Scotland, manufactures horizontal continuous casting machines in the RMK series for the continuous casting of semi-finished silver-base and gold-base products. The alloy to be heated is placed in a solid graphite crucible, protected by an inert gas atmosphere which may, for example, be oxygen-free nitrogen containing &lt;5 ppm oxygen and &lt;2 ppm moisture and is heated by electrical resistance heating using graphite blocks. Such furnaces have a built-in facility for bubbling inert gas through the melt. Addition of small quantities of thermally decomposable boron-containing gas to the inert gas being bubbled through the melt readily provides a desired few ppm or few tens of ppm boron content. The introduction of the boron compound into the alloy as a dilute gas stream over a period of time, the carrier gas of the gas stream serving to stir the molten metal or alloy, rather than in one or more relatively large quantities is believed to be favorable from the standpoint of avoiding development in the metal or alloy of boron hard spots.  
         [0033]     Compounds which may be introduced into molten silver or gold or alloys thereof as a gas include boron trifluoride, diborane or trimethylboron which are available in pressurized cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being preferred because apart from the boron, the only other element is introduced into the alloy is hydrogen. A yet further possibility is to bubble carrier gas through the molten silver to effect stirring thereof and to add a solid boron compound e.g. NaBH 4  or NaBF 4  into the fluidized gas stream as a finely divided powder which forms an aerosol.  
         [0034]     The boron compound may also be introduced into the molten silver or gold alloy in the liquid phase, either as such or in an inert organic solvent. Compounds which may be introduced in this way include alkylboranes or alkoxy-alkyl boranes such as triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane which for safe handling may be dissolved in hexane or tetrahydrofuran (THF). The liquid boron compound may be filled and sealed into containers of silver-base or of copper-base foil resembling a capsule or sachet using known liquid/capsule or liquid/sachet filling machinery and using a protective atmosphere to give filled capsules sachets or other small containers typically of capacity 0.5-5 ml, more typically about 1-1.5 ml. As an alternative, especially for gold casting, the capsules or sachets may be of a polymer film, e.g. polyethylene or polypropylene. The filled capsules or sachets in appropriate number may then be plunged individually or as one or more groups into the molten silver or gold or alloy thereof. A yet further possibility is to atomize the liquid boron-containing compound into a stream of carrier gas which is used to stir the molten silver as described above. The droplets may take the form of an aerosol in the carrier gas stream, or they may become vaporized therein.  
         [0035]     Preferably, the boron is added as a metal borohydride, e.g. a borohydride of an alkali metal, a pseudo-alkali metal or an alkaline earth metal, e.g. lithium borohydride. Sodium borohydride is especially preferred because it is widely commercially available and can be obtained in the form of relatively large pellets that are convenient to handle during precious metal melting operations.  
         [0036]     The boron is advantageously solid, e.g a metal borohydride or a higher borane, such as decaborane, and is in the form of pellets or granules which are advantageously wrapped in a layer of foil of precious metal and plunged as a group into the molten metal. Boron can be added to the other molten components both on first melting and at intervals during casting to make up for boron loss if the alloy is held in the molten state for a period of time, such as in a continuous casting process for grain. Addition of boron to a molten copper-base master alloy is not recommended because adding boron changes the copper content and hence the overall proportions of the various constituents in the alloy.  
         [0037]     The boron is added in the form of either a boron containing metal hydride, and preferably as a solid tetrahydroborate, or a boron containing metal fluoride. When a boron containing metal hydride, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron containing metal fluoride, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH 4 , also referred to as sodium tetrahydroborate. Sodium borohydride has a molecular weight of 37.85 and contains 28.75% boron.  
         [0038]     Sufficient boron is added so that an effective amount remains in the cast precious metal alloy or master alloy for effective grain refinement and deoxidation. Between 1 ppm and 1600 ppm boron remaining is effective. Preferably, the boron content is between 100 ppm and 1600 ppm for a master alloy and between 1 ppm and 1000 ppm for a silver- or gold-base precious metal alloy. A nominal boron content in the cast precious metal alloy or master alloy of about 250 ppm is most preferred. Typically, from 0.001% to 0.16% of boron added to the precursor alloy melt is effective.  
         [0039]     Boron reacts to form a gas that evaporates at elevated temperatures and it may be necessary to make sequential additions of boron as described hereinbelow to maintain an adequate concentration for grain refining. To enable better mixing into the precursor alloy, the boron compound may be wrapped in a thin metallic foil. The foil may be any constituent of the master melt or an inert material, such as paper, and is preferably a ductile metal that may be formed into a relatively thin foil. Preferred metals for the foil include silver, copper and gold. The foil has a thickness of from about 0.01 millimeter to about 0.3 millimeter to enable the foil wrapped boron compound to be well submerged in the master melt before the foil melts through releasing the boron compound. Once released, the constituents of the boron compound combine with oxygen in the precursor melt to effectively deoxidize the melt and the boron reacts with some of the elements in the melt to form discrete insoluble particles dispersed throughout the base material which act as nucleation sites promoting the formation of fine grains that are uniform in size and resist growth.  
         [0040]     When sodium borohydride is first added to the master melt, the initial reaction is believed to be decomposition of the boron containing grain refiner. 
 
NaBH 4(s) →Na (g) +B (s) +2H 2(g)   (1) 
 
 When diborane is first added to the molten metal, the decomposition reaction is believed to be: 
 
B 2 H 6 →3B (s) +3H 2(g)   (2) 
 
 The hydrogen is effective to deoxidize the melt. 
 
         [0041]     After decomposition, the sodium, hydrogen and boron are all effective to deoxidize the melt as follows: 
 
Na (g) +0.5O 2(g) →Na 2 O (s)   (3) 
 
H 2(g) +0.5O 2(g) →H 2 O (g)   (4) 
 
B (s) +0.5O 2(g) +0.5H 2(g) →HBO (g)   (5) 
 
         [0042]     To achieve a uniform casting, the boron is dispersed throughout the precursor melt by stirring  14 . Preferably, the boron is stirred  14  for in excess of 1 minute and typically for from 1-5 minutes. Stirring may be by any means which does not contaminate the precursor melt such as with a graphite stirring rod.  
         [0043]     The molten precious metal alloy or master alloy is then cast  16  by a method suitable for forming a desired end product.  
         [0044]     One such useful end product is casting grains. Casting grains are roughly spherical particles which are sold to jewelry manufacturers who then investment cast to form a desired article of jewelry. Subsequent to stirring  14 , molten precious metal alloy is poured into a grain box  18 ,  FIG. 2 . A grain box is a container with openings in the bottom, through which the liquid metal flows to make the desired shape and size of grains. The grain box is made from materials similar to the crucible, such as, but not limited to, graphite, clay/graphite, ceramic and silicon carbide. The molten precious metal alloy is formed into discrete droplets in the grain box as it flows through the openings and is then solidified into roughly spherical particles in grain tank  20 . A grain tank  20  contains water into which the droplets fall and solidify.  
         [0045]     The particles are then removed from the grain tank  20  and dried  22  by centrifugal force and hot air. The roughly spherical grains have a typical diameter of from about 0.1 millimeter to about 5 mm.  
         [0046]     In accordance with a second embodiment of the invention, continuous casting may be used to form wrought mill products such as sheet, tubing and wire that is later made into finished products such as jewelry. The stirred boron containing molten metal alloy is transferred to a die  24 ,  FIG. 3 , and partially solidified in the die such that a cohesive structure may be withdrawn from the die and subjected to secondary cooling to  26  such as by impact with water spray or passing through a chilled coil. The continuous cast structure is then finished  28  such as by passing through rolling mills and shears to achieve a desired cross-sectional shape and surface finish and then coiled  30  for shipment to jewelry manufacturers. Continuous cast sheet has a more consistent boron content than achieved by prior art processes which enables more consistent welding into tubing.  
         [0047]     For a well stirred batch process, as illustrated in the combination of  FIGS. 1 and 2 , the concentration of boron decreases over time in accordance with the equation 
 
 C   B   =C   B,0 exp(− kρt/m )  (5) 
 
 Where C B =the present boron content in ppm. 
 
 C B,0 =the initial boron content in ppm. 
 
 k=a rate constant dependent on alloy composition, temperature, gas cover and melt cover expressed in units of inch 3 /minute. 
 
 ρ=the density of the alloy in troy ounces/inch 3 . 
 
 t=time in minutes. 
 
 m=melt weight in troy ounces. 
 
         [0048]     Equation (5) predicts that the rate of boron evaporation is faster for smaller melt sizes and this was observed in practice.  FIG. 4  illustrates in graphical representation the kinetics of boron loss in the batch melting process in the exemplary condition of a CO gas cover and a graphite powder solid cover.  
         [0049]     For a continuous casting process as illustrated in the combination of  FIGS. 1 and 3 , the material balance must take into account the change in mass in the casting crucible with time. The amount of boron present may be calculated by the equation: 
 
 C   B   =C   B,0 ( m   0 /( m   0   −F   0   t ))exp−( kρ/F   0 )  (6) 
 
 Where C B , C B,0 , t, k and ρ were previously defined. 
 
 m 0 =the initial mass (in troy ounces) of alloy in a crucible at time=0. 
 
 F 0 =the casting rate in troy ounces/minute. 
 
         [0050]     The time, t, is reset to zero and the initial alloy mass m 0 , is recalculated after each incremental addition of boron.  
         [0051]     The above-described process may be used to make alloys of any precious metal, especially fine silver, fine gold and alloys thereof that are required to incorporate boron. In particular, it may be used to make silver/germanium alloys having an Ag content of at least 77%, by weight, a Ge content of between 0.5 and 3%, by weight, an amount of boron effective to enhance ductility and strength and the balance copper along with incidental ingredients and impurities. If desired, the germanium content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, Yb and Zr, provided that the effect of germanium in providing firestain and tarnish resistance is not unduly affected. The weight ratio of germanium to incidental ingredient elements may range from 100:0 to 60:40, and preferably ranges from 100:0 to 80:0. The term “incidental ingredient” permits the ingredient to have ancillary functionality within the alloy, e.g. to improve color or as-molded appearance and includes the metals or metalloids Si, Zn, Sn and In in amounts appropriate for deoxidation. The invention is also applicable for the manufacture of mater alloys, such as Cu/Ge/B and CuB.  
         [0052]     The alloys that may be made according to the process of the invention include coinage grade, 800-grade (80% by weight silver) (including 830 and 850 grades (83% and 85%, by weight, silver, respectively) and the like) and standard Sterling silver and an alloy of silver containing an amount of germanium effective to reduce firestain and/or tarnishing. The ternary Ag—Cu—Ge alloys and quaternary Ag—Cu—Zn—Ge alloys that can suitably be made are those having a minimum silver content of 80%, by weight, and more preferably, a minimum silver content, by weight, of 92.5%. The maximum silver content is 98%, by weight, and preferably, the maximum silver content is 97%, by weight. The germanium content is at least 0.1%, and preferably at least 0.5%, more preferably at least 1.1%, and most preferably at least 1.5%, by weight. The maximum germanium content is preferably 6.5% and more preferably 4.0%, by weight.  
         [0053]     Silicon may be added to silver alloys in an amount of up to 5%, by weight, preferably from 0.5 to 3 weight percent and most preferably in an amount of from 0.1 to 0.2 weight percent. When incorporated into casting grain of an Ag—Cu—Ge ternary alloy, it can provide bright investment casting immediately on removal from the mold. It may be added to casting grain, e.g. before investment casting, or it may be incorporated into the silver at the time of first melting to form an alloy.  
         [0054]     The process of the invention may be used to manufacture gold jewelry alloys having at least 33%, by weight, of gold (8 karat) with the balance being alloying elements including, but not limited to silver, nickel, copper and zinc, as well as inevitable impurities. Particular gold alloys that may be processed in accord with the invention include:  
         [0055]     24K gold—a minimum of 99.7%, by weight, of gold with the balance being grain refiners, hardening additives and impurities.  
         [0056]     22K gold—exemplary is, by weight, 91.67% Au, 5% Ag, 2% Cu and 1.33% Zn.  
         [0057]     18K gold—exemplary are, by weight, 75% Au, 20% Ag and 5% Cu; 75% Au, 15% Ag and 10% Cu; 75% Au, 13% Ag and 12% Cu; 75% Au, 5% Ag and 20% Cu; 75% Au, 2.75% Ag and 22.25% Cu; 75% Au and 25% Cu; 80% Au and 20% Al; 75% Au, 25% Pt, Pd or Ag; 75% Au, 10% Pd, 10% Ni and 5% Zn; 75% Au, 17% Fe and 8% Cu and 75% Au, 23% Cu and 2% Cd.  
         [0058]     14K gold—exemplary are, by weight, 58.33% Au, 24,78% Cu and 0.14% Zn; 58.33% Au, 4.00% Ag, 31.24% Cu, 6.43% Zn, 0.10% Ni, 0.05% Fe; and 0.01% Si and 58.33% Au, 2.08% Ag and 39.59% Cu.  
         [0059]     10K gold—exemplary are, by weight, 41.70% Au, 11.66% Ag, 40.81% Cu, 5.83% Zn, 0.03% Si and 0.02% B; 41.70% Au, 5.50% Ag, 43.80% Cu and 9.00% Zn; and 41.70% Ag, 2.82% Ag and 55.48% Cu.  
         [0060]     The above-described invention is better understood by the examples which follow:  
       EXAMPLES  
     Example 1  
     (Prophetic)—Ag—Cu—Ge—Si Alloy  
       [0061]     A silver alloy is made by melting together 93.2 weight percent fine silver casting grains, 1.3 weight percent germanium in the form of small broken pieces, 0.2 weight percent silicon (added as a Cu/Si master alloy containing 10 weight percent silicon) and the balance being copper granules. Melting is by means of a gas-fired furnace heated to a pour temperature of about 2000° F. The melt is covered with graphite to protect against atmospheric oxidation. In addition, a hydrogen gas protective flame is provided. Stirring is by hand using a graphite stirring rod.  
         [0062]     When the alloy constituents liquefy, 20.2 grams (0.65 troy ounce) of sodium borohydride per 46.7 kilogram (1500 oz.) melt are wrapped in a pure silver foil, about 0.15 mm thick. The foil wrapper holds the sodium borohydride to prevent it from floating to the surface of the melt. The wrapped sodium borohydride is placed into a hollow cup-shaped end of a graphite stirring rod and plunged beneath the surface of the melt. The melt is covered with a ceramic fiber blanket to quench a flame resulting from decomposition of the borohydride. The hydrogen and sodium burns off with a bright yellow flame over a period of 1-2 minutes during which time the melt is continuously stirred. When the evolution of hydrogen ceases, the boron is substantially incorporated into the melt together with at least some sodium.  
         [0063]     After the boron is added, the crucible is pivoted to permit pouring the molten alloy into a tundish having a bottom formed with very fine holes. The molten silver flows into the tundish and through the holes in fine streams that break into fine pellets and fall into a stirred water bath becoming solidified and cooled. The cast pellets are then removed from the bath and dried.  
         [0064]     The pellets are tested by investment casting using a calcium sulphate bonded investment. The resulting casting has a matte silvery finish when removed from the mold, a fine grain structure, and can be easily polished. It is free from boron hard spots and was ductile as exhibited by a capability to make a ring stretch 4-6 sizes whereas a similar material made using copper/boron can only be stretched about two sizes.  
       Example 2  
     (Working)—Manufacture of Sterling Silver Casting Grain  
       [0065]     Two hundred troy ounces of a sterling silver precursor melt were melted in a clay-graphite crucible. The precursor melt had a nominal composition, by weight, of 93% silver, 5.7% copper and 1.3% germanium. The precursor melt constituents were mixed together and heated under a carbon monoxide flame and covered with a one inch thick layer of borax salt. When the precursor melt temperature reached the flow temperature, 0.0125% boron was added as NaBH 4 . The boron compound was wrapped in 0.15 mm silver foil for introduction to the master melt. Sufficient power was provided to maintain the temperature of the molten precious metal alloy at the flow temperature. The molten precious metal alloy was then stirred with a graphite stirring rod for 3.7 minutes and poured into a grain box. The molten precious metal alloy was protected by a reducing atmosphere during pouring at the flow temperature. After about 0.25 minutes, the entire molten precious metal alloy was converted into casting grains.  
         [0066]     The casting grains were assayed and found to have 13.8 ppm boron. The grains were mounted, polished and etched for examination of grain structure and hard spots. The resulting grain structure was fine and contained no boron hard spots. The material was not brittle when reduced 75% by thickness in a rolling mill. Investment cast rings formed from the casting grains contained no fire scale or hard spots. The rings were stretched 3.25 sizes without annealing before failure.  
       Example 3  
     (Working)—Manufacture of Sterling Silver Grains by Batch Process  
       [0067]     Table 1 illustrates that the process of the invention is effective to add boron to a sterling silver precursor alloy and that the melt cover appears to have more of an effect on the boron content in the precious metal alloy than does the pour gas. In no instance were hard spots detected on the cast grains.  
                                                                                     TABLE 1                                               Boron   Time                                   Content   Between   Boron               Amount               in   Boron   Content           Melt   NaBH 4             Pour   Precursor   Addition   in Cast       Melt   Size   Added   Pour   Melt   Temp.   Alloy   and Pour   Grains       Number   (troyoz)   (grams)   Gas   Cover   (° F.)   (ppm)   (minutes)   (ppm)                                0281   200   2.78   N 2 /H 2     Borax   2150   125   3.7   13.8       0322   1500   3.33   N 2 /H 2     Borax   2150   20   4.0   9.5       0326   200   3.04   CO   Borax   2150   137   4.25   23.5       0327   200   3.04   CO   Borax   2150   137   6.5   6.1       0338/3   400   2.02   CO   Borax   2150   45.5   5.0   23.9       0353   1500   5.05   N 2 /H 2     Borax   2150   30.3   5.25   12.2       0339   72   8.09   CO   Borax   2150   1011   3.0   9.0       081006    300   3.03   CO   Borax   2150   91   4.0   11.4       B01   200   17.77   CO   Graphite   2000   800   5.0   0.6       B02   200   8.89   CO   Graphite   2000   400   5.0   0.6       B03   200   5.55   CO   Graphite   2000   250   5.0   1.3       BS4   200   35.53   CO   Graphite   2200   1600   6.4   2.4       BS05   200   8.89   CO   Graphite   2200   400   5.0   0.5       0605   1000   20.2   N 2 /H 2     Borax   2150   182   5   42.9       0610   1000   20.2   N 2 /H 2     Borax   2150   182   5   59.9       0611   1000   20.2   N 2 /H 2     Borax   2150   182   5   85.7                  
 
       Example 4  
     (Working)—Manufacture of Sterling Silver Continuous Cast Products  
       [0068]     4500 troy ounces of a sterling silver precursor melt were melted in a clay-graphite crucible. The master melt had a nominal composition, by weight, of 93% silver, 5.7% copper and 1.3% germanium. The precursor melt constituents were mixed together and heated under a natural gas flame and covered with a layer of charcoal. When the precursor melt temperature reached the flow temperature, 0.0020% boron was added as NaBH 4 . The boron compound was wrapped in 0.15 mm silver foil for introduction to the precursor melt. Sufficient power was provided to maintain the temperature of the molten precious metal alloy at the flow temperature. The NaBH 4  was added incrementally to maintain a good boron concentration by taking into account the change in melt weight and the evaporation of boron with time. A timer was used to add the boron as scheduled below. Each boron addition was placed inside a graphite plunger and mixed into the molten precious metal alloy at the times indicated in Table 2.  
                                         TABLE 2                                   Time From Transfer   Boron Added as NaBH 4 ,           (minutes)   (dry weight grams)                                        0   6.50           6   5.85           12   5.20           18   4.55           24   3.9           30   3.25           36   2.60           42   1.95           48   1.30           54   0.65                      
 
         [0069]     At transfer, to the continuous casting die, the molten precious metal alloy was heated to the transfer temperature and cast into a continuous two inch diameter cylindrical bar at the casting temperature under a natural gas flame and charcoal cover. Samples of the cast precious metal alloy were assayed for boron level. Table 3 summarizes the assay results.  
                                         TABLE 3                                   Time (min.)   Boron Concentration (ppm)                                        8.5   2.9           17   1.8           25.48   2.5           34   1.0           42.5   1.4                      
 
       Example 5  
     (Working)—Manufacture of 18 Karat White Gold Casting Grain  
       [0070]     125 troy ounces of a white gold precious metal alloy were melted in a ceramic crucible. The precious metal alloy melt had a nominal composition of 75% gold and the balance 6% nickel, 14% copper and 5% zinc. The precious metal alloy constituents were melted together and heated under a carbon monoxide flame to the flow temperature at which time 0.08% boron as NaBH was added. The boron compound was wrapped in a paper envelope for introduction to the melt. Sufficient power was provided to maintain the temperature of the molten precious metal alloy at the flow temperature. The molten precious metal alloy was stirred for 1.75 minutes subsequent to the boron addition and then poured into a grain box. The gas cover during pour into the grain box was a reducing atmosphere. After about 0.25 minutes, the entire molten precious metal alloy was converted into casting grain. Analysis of the casting grain showed a clean surface, no hard spots and a fine grain size.  
         [0071]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.