Abstract:
A new nickel-free sterling silver alloy with superior tarnish resistant, yet substantially similar cold working and mechanical properties is disclosed using a specified mixture of zinc, copper, silicon, iridium, and indium with pure silver. A new and improved method of manufacture involving a four-step process whereby the non-silver components are fabricated in an inert gas or reducing atmosphere into a master alloy of pre-determined composition, and in a final step mixed with a predetermined mass of pure silver to produce the new alloy.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims a priority benefit to U.S. Provisional Application No. 61/504,129 entitled “Nickel-Free Sterling Silver Alloy Compositions and Method of Preparation” filed in the United States Patent and Trademark Office on Jul. 1, 2011 by a common Inventor to this instant application, Arthur Taylor. Further the above named Provisional Application in its entirety is hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable 
       REFERENCE TO APPENDIX 
       [0003]    Not Applicable 
       FIELD OF THE INVENTION 
       [0004]    This invention relates to the field of chemical composition for sterling silver metal alloys and the methods for the manufacture of such alloys. 
       BACKGROUND OF THE INVENTION 
       [0005]    “Standard” sterling silver, comprised of about 92.5% silver and about 7.5%, copper has long been used to fabricate jewelry, eating utensils, art objects and coinage. While the addition of copper increases the susceptibility of sterling silver to tarnish more than pure silver, the mechanical properties are much better. Due to the popularity of sterling silver for decorative and ornamental purposes, the properties are widely known. It is common knowledge that a dark film composed primarily of silver sulfide will form on the surface of sterling silver after a comparatively short time. A leading cause of tarnishing is the presence of minute amounts of hydrogen sulfide (H 2 S) in the atmosphere. Certain foods, such as egg yolk and onions, and certain substances, such as vulcanized rubber produce rapid tarnishing due to their sulfur contents. Tarnishing rates on silver alloys are accelerated by increased temperature and moisture. As an example, when sterling silver jewelry is in contact with skin, human perspiration contributes to the discoloration. In most applications, sterling silver tarnishing is an unwanted characteristic and thus it is very desirable to find a means that will provide the desired mechanical and visual properties, yet limit or mitigate the tarnishing process or rate. 
         [0006]    Numerous attempts to prevent or minimize tarnishing have been made by means of alloying. While a number of alloys have been developed that slow the rate of the tarnishing by the substitution of a portion of the copper component with combinations of zinc, cadmium, gallium and indium they all suffer from poor mechanical working properties compared with “standard” sterling comprised of about 92.5% silver and 7.5% copper. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    A new composition for a silver metal alloy is disclosed which is superior to prior art compositions in its ability to resist tarnishing, yet at the same time possesses the prior art desirable mechanical and visual properties. A novel companion method of manufacture for the new alloy comprises a four-step production process using high purity elements, pre-alloys and master alloys. In all of the melting steps that follow, it is preferable to use an inert gas (e.g. argon) as a protective cover over the metal to limit oxygen absorption during melting. Alternatively, incompletely burned methane gas, charcoal, fragments of hardwood or a flux consisting of borax, boric acid and sodium fluoride may be substituted for the inert gas cover. 
       1. Weight/Percentage Calculations of Pre-Alloys and Elements 
       [0008]    In the first step of our method, calculations are performed to adjust the desired sterling silver elemental composition so that the targeted weight percentages of iridium and silicon can be achieved by using a first pre-alloy of 2.5% iridium—97.5% copper, and a second pre-alloy of 10% silicon—90% copper. Since copper is added along with the iridium and silicon pre-alloys, the elemental copper addition to the sterling silver formulation in later steps must be reduced to correct for the weight of copper contained in the pre-alloys. Iridium must be added to the sterling silver formulation via a copper-based pre-alloy to make certain the elemental iridium completely dissolves in the master alloy preparation. Failure of the iridium to completely dissolve in the master alloy will lead to hard particles in the sterling silver alloy and poor crystal structure. Silicon is best added as a 90% copper—10% silicon commercially available pre-alloy to prevent the silicon from oxidizing during the preparation of the master alloy in the third step. A formula is presented below to demonstrate how the pre-alloy and elemental weight/percentages are calculated for a particular formulation of the new and improved sterling silver compound. 
         [0000]    2. Manufacture of the Iridium/Copper Pre-Alloy (2.50 wt/97.50 wt) 
         [0009]    In the second step a pre-alloy of iridium and copper is produced by combining high purity, oxygen-free copper (OFC) with elemental iridium in the weight ratio of 2.5 parts iridium to 97.5 parts copper. Initially only the oxygen-free copper is added to the melting crucible which is made molten. Immediately after the pure copper becomes molten, elemental iridium is slowly added while continuously mixing. The mixing of the now molten copper-iridium mixture continues for about two minutes following the iridium addition in order to completely dissolve the relatively high melting point iridium component. The homogenous molten copper-iridium is then poured into an ingot mold and allowed to freeze. Once cool enough to handle, the copper-iridium bar is chopped into small, random weight pieces of convenient weights for later additions to master alloy melts. 
       3. Manufacture of the Master Alloy 
       [0010]    In the third step, all pre-alloy and elemental materials of the sterling silver formulation except silver are combined into a master alloy. The master alloy is produced by charging a melting crucible with about 50% of the required weight of copper (Cu 31 ) followed by the appropriate weight of the 2.5% Ir—97.5% Cu pre-alloy followed by elemental zinc and then silicon. The silicon addition is made by way of commercially available pre-alloy of 90% copper, 10% silicon. The balance of the elemental copper (Cu 32 ) is added on top of the previous two layers. This mixture is heated until completely molten, mixed briefly with a graphite stirring rod and then the required weight of indium metal is quickly added to the molten copper-based alloy followed by rapid mixing for about 30 seconds. The resulting homogenous molten alloy is quickly poured into tank of cool water to form irregularly shaped granules. The granules or “shot” are then dried in electrically heated centrifugal dryers for about four hours the allowed to cool to room temperature. Alternatively, the molten master alloy can be poured into an ingot mold, cooled then chopped into random pieces for use in sterling silver alloy melt formulations. Master alloys of various compositions may be manufactured so that they will produce different resulting compositions of the improved sterling silver when mixed with pure silver. 
       4. Combine Pure Silver With a Master Alloy 
       [0011]    In the fourth step, 92.5 parts by weight of silver having a purity level of 99.9% or greater is combined with 7.5 parts by weight of the master alloy prepared above. The melting crucible is charged with about ⅔ of the fine silver weight followed by the master alloy followed by the balance of the fine silver. The silver-master alloy mixture is heated until molten then mixed well with a graphite rod for about one minute. The molten metal temperature is raised to about 2050° F. then quickly poured into a tank containing cool water to form sterling silver shot. The resulting sterling silver shot are mechanically separated from the water by sieving then dried in electrically heated, centrifugal dryers and allowed to cool prior to packaging. A sample of the dried shot is customarily sent to a fire assay analysis laboratory to measure the percentage of silver. Some or all of the other elements in the sterling silver formulation may be quantitatively measured via standard instrumental and wet chemical analysis techniques. 
         [0012]    Granulated particles of the new sterling silver may be melted, and then cast into plates for sheet production or rods for wire drawing. The cast plates and rods can then be rolled, drawn, annealed and fabricated into products using well-known, standard procedures. The sterling silver shot may be used to investment cast products using standard investment casting techniques. 
       First Embodiment Formula 
       [0013]    A ratio/percentage formula is disclosed below in Table 2 which used in conjunction with our method will produce a first embodiment of the improved sterling silver with the elemental ratios as shown in Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Element 
                 Percent Wt 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Silver (Ag) 
                 92.500% 
               
               
                   
                 Zinc (Zn) 
                 1.000% 
               
               
                   
                 Copper (Cu) 
                 6.315% 
               
               
                   
                 Silicon (Si) 
                 0.100% 
               
               
                   
                 Iridium (Ir) 
                 0.025% 
               
               
                   
                 Indium (In) 
                 0.060% 
               
               
                   
                   
               
             
          
         
       
     
         [0014]    The formula for calculating the necessary amount of pre-alloys and elemental materials is based on the desired total weight of the final improved sterling silver product T SS . 
         [0015]    The Iridium/Copper pre-alloy is PA IR . 
         [0016]    The Silicon/Copper pre-alloy is PA SI . 
         [0017]    The first copper quantity needed for the Master Alloy in Step 3 is Cu 31 . 
         [0018]    The second copper quantity required for the Master Alloy in Step 3 is Cu 32 . 
         [0019]    The amounts required for a total improved sterling silver output of T SS  are: 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Pre-alloy/Element 
                 Ratio Wt 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 PA IR   
                 0.01 
                 T SS   
               
               
                   
                 PA SI   
                 0.01 
                 T SS   
               
               
                   
                 Cu 31   
                 3.1575 
                 T SS   
               
               
                   
                 Cu 32   
                 1.2825 
                 T SS   
               
               
                   
                 Zn 
                 0.01 
                 T SS   
               
               
                   
                 In 
                 0.06 
                 T SS   
               
               
                   
                   
               
             
          
         
       
     
         [0020]    The above formula shown in Table 2 is a ratio formula, thus whatever the desired units of weight for T SS  are (e.g., grams, ounces, kilograms, pounds), then the pre-alloy and element quantities will be in the same units. It can be converted to a percentage formula by multiplying the constants in Table 2 by 100. 
         [0021]    Using the above ratio formula, the following amounts of pre-alloy and elements are required: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Pre-alloy/Element 
                 Amount % wt 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Pre-alloy Ir/Cu 
                 1.0 
               
               
                   
                 Pre-alloy Si/Cu 
                 1.0 
               
               
                   
                 Zinc 
                 1.0 
               
               
                   
                 Indium 
                 6.0 
               
               
                   
                 Silver 
                 92.5 
               
               
                   
                 Cu 31 
                 3.16 
               
               
                   
                 Cu 32 
                 1.28 
               
               
                   
                 Tss 
                 100 
               
               
                   
                   
               
             
          
         
       
     
       OBJECTS AND ADVANTAGES 
       [0022]    The primary purpose of this invention is to manufacture a new tarnish resistant sterling silver alloy for highly adaptable use in the sterling silver industry. 
         [0023]    Accordingly, several objects and advantages of our invention are: 
         [0024]    (a) calculating the amounts of pre-alloys and elemental materials for each step; 
         [0025]    (b) manufacturing an iridium-bearing pre-alloy in a second step; 
         [0026]    (c) manufacturing a master alloy containing all formulation elements except silver in a third step; 
         [0027]    (b) combining the master alloy compound with pure silver in a fourth step; 
         [0028]    (c) manufacturing the new sterling silver alloy so as to be significantly more tarnish resistant than prior art alloys; 
         [0029]    (d) manufacturing the new sterling silver alloy with mechanical properties so as to be compatible with existing industry fabrication tools and processes; and 
         [0030]    (e) manufacturing a new sterling silver alloy that achieves standard sterling silver hardness values after cold working without the use of nickel as an additive. 
         [0031]    Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [0032]    This invention does not require any drawings as the text of this Specification is fully enabling. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    A purpose of the invention is to provide an alloy formulation that has the same or very similar color and mechanical properties of standard sterling silver but with improved resistance to tarnish from atmospheric hydrogen sulfide (H 2 S) and human perspiration by the substitution of part of the copper in standard sterling silver with alternative elements. 
         [0034]    Another purpose of the invention is to provide a method for combining the various elements in the new sterling silver in such a way that reduces the usual detrimental effects of melting and mixing processes. The result of the new preparation method is an alloy that is homogenous in the solid state with limited oxide and oxygen gas content. 
         [0035]    The new sterling silver alloy consists essentially of silver, zinc, copper, silicon, iridium, and indium. The percentage in weight-weight (w/w) ranges for each of these elements is about: 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Element 
                 Nominal 
                 General 
                 Preferred 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Silver (Ag) 
                 92.5% 
                 92.00-96.00% 
                 92.00-93.00% 
               
               
                   
                 Zinc (Zn) 
                 1.0% 
                 0.10-5.00% 
                 0.10-1.50% 
               
               
                   
                 Copper (Cu) 
                 6.315% 
                 5.30-7.30% 
                 6.30-6.50% 
               
               
                   
                 Silicon (Si) 
                 0.10% 
                 0.01-0.50% 
                 0.01-0.05% 
               
               
                   
                 Iridium (Ir) 
                 0.025% 
                 0.01-0.13% 
                 0.01-0.05% 
               
               
                   
                 Indium (In) 
                 0.06% 
                 0.01-0.30% 
                 0.01-0.12% 
               
               
                   
                   
               
             
          
         
       
     
         [0036]    Copper is added to strengthen the pure silver which alone is generally considered too soft for most practical applications. While copper is sufficient to strengthen pure silver into an alloy that is sufficiently hard and ductile to fabricate jewelry products, copper has the tendency to absorb the oxygen accumulated by silver during melting operations. As the percentage of copper to copper oxide conversion increases the likelihood of copper oxide inclusions increases. Copper oxide inclusions lead to discoloration of the sterling silver (“fire scale”) and increased brittleness. Zinc is added as a deoxidizing element. In relatively low percentages, zinc competes with copper for available oxygen and for most part, leaves the melting system as zinc oxide fume. High levels of zinc tend to make the sterling silver alloy too soft and somewhat porous. Indium is added in trace level percentages to increase tarnish resistance but also has the adverse effect of reducing the alloy hardness and also reduces any increases in the material hardness from cold working. High levels of indium lowers the sterling silver melting point making it difficult to formulate silver solder alloys with melting points sufficiently different so that the sterling silver remains solid while the solder is molten on the surface. Iridium is added in trace level percentages as a grain refiner to reduce the alloy crystal size and increases the alloy hardness in response to cold rolling and wire drawing operations. A smaller crystal size, especially after annealing operations following cold mechanical working, is an important factor in reducing cracking from continued cold mechanical working and from chemical attack (i.e. stress corrosion cracking) in the finished product. Silicon improves the flow of metal into casting molds and also reduces the oxidation of the copper during melting operations. Silicon and silicon oxide diffuse to the surface of investment castings increasing the surface tension. The increased surface tension causes the molten sterling silver to not fill microscopic cavities in the Plaster of Paris type investment mold. The result is a shiny “as cast” surface highly desirable in the casting of jewelry due to the lower amount of labor required for polishing surfaces to a high gloss. 
         [0037]    One advantage of the new sterling silver alloy is the resistance to corrosion in the presence of hydrogen sulfide (H 2 S) gas. Applying test procedure BS EN ISO 4538:1995 Metallic coatings—Thioacetamide corrosion test (TAA test) for three hours, the new sterling silver alloy showed a lower tarnishing rate when compared directly with standard sterling silver. After a long term (10 day) exposure to H 2 S using the BS EN ISO4538:1995 procedure, the new sterling silver alloy showed lower overall corrosion when compared with standard sterling silver. 
         [0038]    Another advantage of the new sterling silver alloy is its improved resistance to corrosion from human perspiration. Following a direct 120 day comparison of exposure to artificial perspiration maintained at 102° F., standard sterling samples showed a non-uniform, dull surface along with patches of grey discoloration and areas of green copper compounds. The new sterling silver alloy samples remained bright and shiny with no noticeable change in color. 
         [0039]    Another advantage of the new sterling silver is that the hardness of standard sterling silver (i.e. 92.5% silver, 7.5% copper) is maintained without the use of nickel as an additive. Nickel dermatitis resulting from skin in contact with jewelry articles that contain leachable nickel is a serious and widespread medical condition. So serious that the EU has banned the sale of jewelry and other decorative metal items that will come into contact with human skin if they fail to conform with BS EN 1811:1999 and/or BS EN 1811:2011. Both BS EN 1811:1999 and BS EN 1811:2011 test for the release of nickel to human skin from prolonged contact with a metal alloy. 
         [0040]    Still another advantage of the new sterling silver alloy is its similarity in working properties to standard sterling silver. Over many years, jewelry manufacturers have built tools and machines and developed processes based on the working characteristics of standard sterling silver. Of particular interest to those skilled in the art is the hardness of the alloy after being reduced in thickness by rolling or drawing. If a new sterling silver alloy showed improved tarnish resistance but responded differently to well established cold working fabrication methods, the new alloy would not be widely accepted due to the high cost of converting machinery and processes to the new characteristics. Therefore, it is very important that a new sterling silver conforms to customary fabrication processes and yield results very similar to standard sterling silver. The new sterling silver alloy disclosed responds to traditional cold working very closely to standard sterling silver. 
         [0041]    The table below illustrates the similarity of the critical hardness values of the two alloys when rolled from an “as cast” (i.e. 0% reduction in thickness) to about 78% reduction in thickness. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Standard 
                 Our New Nickel-free 
               
               
                 % Reduction in 
                 Sterling Silver Alloy: 
                 Sterling Silver Alloy: 
               
               
                 Thickness by Rolling 
                 Rockwell-B Hardness 
                 Rockwell-B Hardness 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.0% 
                 48 
                 44 
               
               
                 19.0% 
                 61 
                 54 
               
               
                 29.0% 
                 63 
                 62 
               
               
                 39.1% 
                 73 
                 71 
               
               
                 44.8% 
                 76 
                 72 
               
               
                 56.2% 
                 78 
                 75 
               
               
                 62.9% 
                 81 
                 80 
               
               
                 71.4% 
                 81 
                 80 
               
               
                 77.6% 
                 81 
                 81 
               
               
                   
               
             
          
         
       
     
         [0042]    While the present invention has been illustrated and described with reference to exemplary embodiments thereof, various modifications will be apparent to and might readily be made by those skilled in the art without departing from the scope and spirit of the present invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but, rather, that the claims be broadly construed.