Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C5/00—Alloys based on noble metals
  • C22C5/06—Alloys based on silver
• • A—HUMAN NECESSITIES
  • A44—HABERDASHERY; JEWELLERY
  • A44C—PERSONAL ADORNMENTS, e.g. JEWELLERY; COINS
  • A44C27/00—Making jewellery or other personal adornments
  • A44C27/001—Materials for manufacturing jewellery
  • A44C27/002—Metallic materials
  • A44C27/003—Metallic alloys

Definitions

• the field of the invention relates to silver alloy compositions, and in particular to sterling silver alloy compositions that have an improved resistance to sulphidation and formation of fire stain.
• Sterling silver alloys are widely used in jewellery and silverware because of their sparkling reflectivity, their artistic heritage, as well as their bactericidal properties.
• Jewellery includes items such as rings, earrings, necklaces, bracelets and so on.
• Silverware includes items such as tableware (cutlery, trays, tea and coffee sets and so on), candlesticks, cigarette lighters, photograph frames, mirrors, coins, musical instruments and the like.
• a requirement of sterling silver is that it must contain at least 92.5% silver in order to be entitled to be termed "sterling silver".
• Conventional sterling silver alloys comprise 92.5% silver and 7.5% copper.
• a problem of sterling silver alloys is that they suffer from sulphidation (also termed tarnishing).
• Silver alloys slowly react with sulphur compounds present in the air to form silver sulphide. Consequences of the silver sulphidation are that the surface of sterling silver products becomes an unattractive yellow colour or the surface darkens as silver-sulphur compounds such as silver sulphide are slowly deposited on the surface. This constitutes a problem for manufacturers of sterling silver articles whose products tend to get darker. Tarnishing is aesthetically unacceptable for most consumers, who must spend time polishing silverware products to remove the tarnish. The most prevalent reaction that causes sulphidation is between silver and H 2 S present in the air, as shown in equation 1 :
• Alloy composition Alloying additions may have a beneficial or 0 deleterious effect on sulphidation of silver.
• copper Cu
• Cu is often added to sterling silver items such as silver forks, knives, salvers and a whole range of silver tableware because of its beneficial effect on mechanical properties and the non-deleterious effect on the colour.
• 92.5% silver - 7.5% copper alloy is consequently very widely used in the silverware sector.
• copper 5 enhances sulphidation. Therefore the addition of copper improves certain properties such as the 'workability' of the silver, but at the expense of increasing the susceptibility of the sterling silver alloy to sulphidation and fire stain (oxidation of alloying elements, especially copper).
• Fig. 1 there is illustrated a table showing the colour change of 92.5% Ag 7.5% Cu sterling silver when subject to a sulphidation test. It can be seen that the reflectivity L decreases with increasing time of exposure to sulphidation, and the colour change ⁇ Lab increases with increasing time of exposure to sulphidation.
• Elements that are commonly alloyed with silver include: Al, Au, Bi, Cu, Ge, In, Mg, Ni, Pb, Pd, Sb, Si, Sn, Te, Tl, Zn.
• the alloys are mostly silver-copper alloys, with 80% or 92.5% of silver.
• Another common addition is germanium (2% -
• the germanium alloys also contain copper that is necessary for appropriate mechanical properties of the alloy and the sulphidation resistance is therefore lowered.
• fire stain is a termed used to describe the discolouration due to oxidation of alloying elements, especially copper. See below).
• the current methods that are widely used to avoid the problem of sulphidation of silver alloys are a) the use of surface treatments or b) the direct manufacture of silver alloys more resistant to sulphidation.
• Extremely thin metal coatings of gold, palladium, platinum or rhodium can be used to reduce the degree of tarnishing of silver. Using these treatments, sulphidation resistance of silver alloys is said to be enhanced up to five times compared to pure silver (see lnder Singh, P. Sabita, V.A. Altekar, 'Silver Tarnishing and its Prevention - A Review', National Metallurgical Laboratory, India). These films are around 100-200 A thick. Rhodium is the most commonly used metallic coating in jewellery.
• Oxides coatings such as BeO, AI 2 O 3 , ZrO 2 , MgO and TiO 2 can be produced by sputtering or by cathodic reduction of solutions containing soluble salts of the metallic ion. Sputtering requires expensive equipment and is complicated, as is cathodic reduction.
• Silver may be anodically passivated in chloride solution or in alkaline solutions by photoelectric polarisation. Oxidation of silver is quite complex and results in the formation of both AgO and Ag 2 O that are resistant to sulphidation.
• oxide coatings are reported to increase the protection of the underneath silver alloy up to five times (see L. GaI-Or, Tarnishing and Corrosion of Silver and Gold Alloys', Institute of Metals, TECHNION - Israel Institute of Technology).
• Varnishes or lacquers based on polymers dissolved in solvents can be used to reduce the degree of tarnishing of silver or silver alloys.
• Varnish or lacquer coatings protect silver by forming a physical barrier between the metal and its environment. They are very resistant against sulphidation.
• Organic coatings also exhibit poor wear resistance. Moreover, another drawback is their poor chemical resistance (against sweat, water, and detergents such as washing-up liquid). Organic coatings are therefore not suitable for silverware protection. Chromatation / Passivation
• Some chemical conversions treatments can be used on silver. They are called passivation or chromatation treatments and consist of a complex chromate layer on the surface. This layer is very sensitive to handling and is destroyed by rubbing. In addition, hexavalent chromium cannot be used for silverware used with food for health and safety reasons.
• Protective organic coatings can be used to increase the resistance to sulphidation, but they often do not last more than few months or years: They can be dissolved by washing products, sweat, or oral contact.
• Chromatation also involves hexavalent chromium that is harmful for 5 health and polluting for the environment.
• Binary Silver Alloy This means an alloy such as Ag / X or [Ag / Cu] / X where 5 'X' is one additional element that should improve sulphidation resistance.
• Germanium is one of the best currently known additional elements that increase sulphidation resistance of silver.
• germanium delays the sulphidation up to 8 times (Ag - 4 % Ge) compared to pure silver. Indeed, germanium fixes oxygen and forms a protective layer of GeO ⁇ . That is why germanium is currently used in very small amounts in dental alloys and high temperature brazing.
• the binary alloy Ag / Ge does not completely prevent tarnishing. Furthermore, germanium is expensive and increases the cost of silver alloy significantly. Germanium is added to a 92.5% silver 7.5% copper alloy and boron is added as a grain refiner. A grain refiner is required to maintain the mechanical properties of the alloy such as the "workability" or ease of processing. However, its "workability” is still not comparable to that of 92.5% silver 7.5% copper alloy, and therefore not suitable for normal processing. Furthermore, the inclusion of substantial amounts of copper reduces the effectiveness of the germanium in terms of its properties of reducing sulphidation. Ag / Si
• silicon is known for its deoxidising properties. As an additional element to silver to reduce tarnishing, silicon gives good results. However, it tends to make the material strongly brittle and prevents cold working.
• this alloy comprises a relatively high amount of copper to improve the mechanical properties of the alloy, and so would be susceptible to "fire stain” discussed below.
• Fire stain is caused by the oxidation of certain alloying elements, and especially copper in the 92.5% silver 7.5% copper alloy. If, for example, it is required to weld two pieces of 92.5% silver 7.5% copper alloy together, fire stain can form leading to an unsightly dark appearance on the surface of the silver object, which is difficult to remove.
• JP 62243725, JP 62010231 and JP 62054046 all disclose a silver alloy composition developed to exhibit sulphidation resistance.
• the principle components included within the alloys include silver, tin, zinc and indium. Whilst these documents do disclose a sulphidation resistant silver alloy, the alloys disclosed are disadvantageous for a number of reasons. In particular, the compositions are not optimised to reduce the amount of indium that is required to be incorporated within the alloy to achieve the desired sulphidation resistance. lncorporation of excessive indium is disadvantageous due to its high cost which is typically four times that of silver.
• the inventors have developed range of sterling silver alloy compositions that have a reduced susceptibility to sulphidation and a reduced susceptibility to fire stain, whilst retaining the properties required for casting and mechanical properties required for various working operations.
• the alloy comprises at least 92.5% silver in order to be entitled to the term "sterling silver".
• the alloy further comprises indium to improve the tarnish resistance and zinc and tin also to improve the tarnish resistance and to improve the mechanical properties of the alloy. Additionally, the alloy further comprises aluminum to provide the required colouration characteristics of the metal and magnesium so as to achieve the desired flowability of the material.
• a sterling 5 silver alloy composition comprising by weight: at least 92.5% silver; 3.0% - 4.0% tin; 1.7% - 2.6% zinc; 0.5% - 1.5% indium.
• the composition further comprises 0.1% - 1.5% aluminium and/or 0.01% - 0.15% magnesium. 0
• the composition further comprises one or more grain refiners, each grain refiner or the total amount of grain refiners representing 0.05% - 0.35% by weight and selected from the list of:
• the composition may comprise copper, manganese and/or iron, each in the range 0.10% - 0.3% or 0.01% - 0.2% by weight.
• the o composition may comprise any one or a combination of the following:
• Nickel, silicon, boron, titanium, iridium or cobalt each within the range 0.05% - 0.2% by weight.
• the composition may comprise 0.01% - 0.1%; 0.05% - 0.35% or no more than 0.05% nickel.
• the composition comprises 0.05% - 0.25% lithium.
• the composition further comprises 0.02% - 0.3% phosphorous.
• the composition may comprise 96.8% silver.
• the composition comprises 92.5% - 93.5% silver.
• a 5 silver alloy composition comprising by weight: at least 92.5% silver > 0% - 5% tin; > 0% - 5% zinc; > 0% - 2% indium; > 0% - 3% aluminum; > 0% - 2% magnesium; wherein any remaining weight % may be silver.
• a silver o alloy composition comprising by weight: at least 92.5% silver; > 0% - 5% tin; > 0% - 5% zinc; > 0% - 2% indium; > 0% - 3% aluminum; > 0% - 2% magnesium; wherein any remaining weight % is silver and/or any one or a combination of the following grain refiners: copper; manganese; iron; nickel; silicon; boron; titanium; iridium; cobalt. 5
• the composition comprises any one of the grain refiners wherein each or the total amount of grain refiners added is within the range 0.05% - 0.35% by weight.
• a sterling silver alloy composition comprising by weight: 92.5% - 93.5% silver; 2.5% - 3.5% tin; 2.0% - 3.0% zinc; 0.3% - 1.1% indium; 0.2% - 0.8% aluminum; 0.03% - 0.1% magnesium.
• Figure 1 illustrates a table showing the colour change of 92.5% Ag 7.5% Cu sterling silver when subject to a sulphidation test.
• FIG. 3 illustrates the processing steps of test samples.
• FIG. 4 illustrates a table showing the Vickers Hardness and colour change after a sulphidation test, and the average grain size of selected ternary silver alloys.
• Figure 5 illustrates a table showing the compositions of a series of sterling 5 silver alloys.
• Figure 6 illustrates a table qualitatively rating the properties of the series of sterling silver alloys.
• FIG. 7 illustrates a table showing the effect of aluminium and germanium additions to sterling silver alloys on the reflectivity of the alloy.
• Figure 8 illustrates a table showing the effect of tin and indium additions to sterling silver alloys on the reflectivity of the alloy. 5
• Figure 9 illustrates a table showing the effect of indium and zinc additions to sterling silver alloys on the reflectivity of the alloy.
• Figure 10 illustrates the variation of hardness with the annealing temperature o and time for standard sterling silver (92.5% Ag, 7.5% Cu) and a sterling silver alloy of the present invention.
• Indium, zinc and tin are alloyed with silver to give an improved sterling silver alloy being less susceptible to tarnishing by sulphidation and formation of fire stain.
• Aluminium may be included within the present composition to achieve the desired colouration of the resulting silver alloy. The weight % of aluminium added may be adjusted to achieve the desired colouration of the resulting silver alloy. Aluminium is considered to be a 'brightener 1 that improves the lustre and reflectivity of the alloy.
• Indium is added to reduce the susceptibility to sulphidation. It is suggested that the mechanism by which this works is that indium in the alloy forms a replenishable oxide layer on the surface of a product manufactured from the alloy. This surface oxide layer is extremely thin, of the order of nanometres. However, it protects the surface of the silver alloy object from sulphur in the atmosphere, or sulphur introduced by handling that would otherwise cause sulphidation and hence tarnishing. If the indium oxide surface layer is removed, for example by polishing or scratching, indium in the silver quickly reacts with oxygen in the atmosphere to form a new replenishable surface oxide layer. It is suggested that the presence of indium protects the silver alloy from tarnishing in this way, as indium has been found to reduce the degree of tarnishing. However, it may be that the indium is protecting the silver alloy from tarnishing by some other mechanism.
• Magnesium, zinc and/or tin may be added for several reasons.
• copper is added to improve the mechanical properties of silver such as hardness, workability and so on.
• Zinc, tin and optionally magnesium have the same effect in the new alloy composition.
• Magneisum, zinc and tin improve the mechanical properties of the alloy without being detrimental to the distinctive colour of sterling silver.
• Magnesium and tin in particular has been found to improve the flowability of the molten alloy in processes such as spinning.
• a grain refiner may also added to refine the grain size of the alloy. Refining the grain size improves the mechanical properties of the alloy in addition to improving the resistance of the surface to tarnishing and the surface appearance of products manufactured from the alloy.
• nickel is preferred to use as the grain refiner, although it may be that other grain refiners can be used.
• Current legislation does not permit the presence of more than 0.05% nickel in silverware that may be used in prolonged contact with the skin, for example jewellery. However, this level of nickel provides adequate grain refinement to the alloy.
• nickel It is difficult to introduce nickel into a cast alloy on its own, as nickel does not readily form a solid solution with the other constituents of the alloy.
• nickel forms a good solid solution with a master alloy of copper-nickel in the ratio of 75/25 copper/nickel. There may therefore be a small quantity of copper added to the alloy, but the low levels of copper are insufficient for it to lead to problems with fire stain.
• nickel it is not essential that nickel is added as a master alloy with copper, under certain conditions nickel can be added without copper.
• Phosphorous can be added in the range 0.02% - 0.3% by weight.
• Lithium may be added in the range 0.05% to 0.2% by weight to improve the castability of the alloy.
• a series of alloys were prepared and cast. Each alloy was cast into a bar test sample. Referring to Figure 3 herein, there are illustrated the processing steps of test samples.
• the bar was milled 301 on its upper and lower surfaces to a thickness of approximately 15.24 mm.
• the bar was then rolled 302 to a thickness of 7.62 mm on the first run (approximately a 50% reduction).
• the bar was then annealed 303 in a retort furnace after the first rolling at 55O 0 C for 20 hours in a 95% nitrogen 5% hydrogen atmosphere.
• a light abrasive clean 304 was then applied to the bar.
• a second rolling 305 to approximately 3.81 mm (a further 50% reduction) was then applied to each bar, followed by a second annealing 306 under the same conditions as the first annealing.
• a third rolling 307 to approximately 1.91 mm (a further 50% reduction) was applied to each bar followed by a third annealing 308.
• the third annealing 308 was performed in a belt furnace to 550 0 C under a 75% hydrogen, 25% nitrogen atmosphere.
• a fourth rolling 309 was then applied to a thickness of approximately 1 mm (approximately 50% reduction) and a fourth annealing 310 was carried out under the same conditions as the third annealing 308.
• a finished sheet of each alloy was then prepared by a performing a fifth rolling 311 to approximately a 50% reduction, such that the final sample thickness was approximately 0.5 mm.
• the Vickers Hardness was measured for a sample of each as-cast alloy, and also measured after the first rolling, the first annealing, the second rolling, the second annealing, the third rolling, the third annealing, and for the finished sheet.
• Vickers Hardness was measured using a Vickers diamond indenter on a polished surface of each alloy sample. A load of 10 kg was applied and the indent size measured optically. The Vickers Hardness was calculated from tabulated figures for Vickers Hardness.
• a colour change test was performed on each finished sheet before and after a sulphidation test.
• the sulphidation test was performed by placing the example in a closed container containing a tarnishing atmosphere.
• the tarnishing atmosphere was obtained using a solution of ammonium sulphide in water. 1 ml of ammonium sulphide 4% was added to 20 ml distilled water (giving a final concentration of ammonium sulphide as 0.2 g/l).
• the ammonium sulphide was prepared in a beaker having a diameter of 50 mm and a height of 70 mm.
• the beaker containing the ammonium sulphide solution was placed in a dessicator having a volume of approximately 5 I. Each sample was placed in the dessicator for a period of 1 hour.
• the colour of each sample was measured before placing in the dessicator and after removal from the dessicator using the Lab system of colour measurement, and the colour change from before the sulphidation test to after the sulphidation test was measured as ⁇ Lab.
• Colour was measured using a spectrocalorimeter using a measurement area of 3 mm.
• a high value of ⁇ Lab indicates a large colour change associated with sulphidation, and a low ⁇ Lab indicates a small or negligible colour change.
• the lower the ⁇ Lab value the lower the degree of sulphidation undergone by the sample and therefore the greater the resistance of the alloy composition to sulphidation.
• a 92.5% silver 7.5% copper prior art sterling silver alloy has a Vickers 5 Hardness for finished sheet of around 133, and a colour change of ⁇ Lab of 48.1 , and an average grain size of 8 ⁇ m.
• An alloy comprising 93% silver, 1% indium, 3.6% tin, 2.2% zinc, 0.1% iron and 0.1% manganese has an as-cast Vickers Hardness of 55, a Vickers Hardness o of 129 after the first rolling, a Vickers Hardness of 44 after the first annealing, a Vickers Hardness of 129 after the second rolling, a Vickers Hardness of 47 after the second annealing, and a Vickers Hardness of 121 for the finished sheet.
• the average grain size is 33 ⁇ m.
• An alloy comprising 93% silver, 1% indium, 3.6% tin, 2.2% copper, 0.1% iron and 0.1% manganese has an as-cast Vickers Hardness of 51 , a Vickers Hardness of 113 after the first rolling, a Vickers Hardness of 48 after the first annealing, a Vickers Hardness of 115 after the second rolling, a Vickers Hardness of 47 after the second annealing, and a Vickers Hardness of 107 for the finished sheet.
• the 0 average grain size is 12 ⁇ m.
• This alloy is provided by way of comparative example.
• 92.5% silver 7.5% copper alloy is as follows: 93.0% silver;
• nickel grain refiner is added.
• the alloy contains 93% silver, and so can still be classified as sterling silver. It has good tarnish resistance and exhibits good resistance to fire stain.
• the inventors provide an alloy composition particularly suitable for lost wax precision casting that also displays suitable properties of sulphidation resistance, fire stain resistance and mechanical properties suitable to allow the alloy to be used as a replacement for 92.5% silver 7.5% copper alloy is as follows:
• Magnesium is added to refine the grain size, and hence the workability of the alloy, and also to improve the castability of the molten alloy. Boron is added as a grain refiner. Aluminium is added to brighten the colour of the alloy and as a grain refiner to improve the workability of the alloy. Nickel is added as a grain refiner. This alloy displays no fire stain and is resistant to sulphidation.
• Figure 5 illustrates a table showing the compositions of a series of sterling silver alloys. These alloys have been produced and the workability through all forming processes, visible grain structure, presence of fire stain, final alloy colour, reflectivity after a final polish and sulphidation resistance have been compared.
• the forming processes mentioned include cold rolling, spinning, deep drawing and tube making.
• the properties of the sterling silver alloys listed in Figure 5 are given in Figure 6. The properties are defined qualitatively as a comparative score with that of 92.5% Ag 7.5% Cu sterling silver alloy.
• Alloy 1 comprises a prior art 92.5% Ag 7.5% Cu sterling silver alloy. It has a small tight grain structure that provides good workability, has a bright reflective colour, but displays fire stain owing to the presence of copper, and has poor sulphidation resistance.
• Alloy 2 comprises 93% Ag, 1% In, 3.6% Sn, 2.235% Zn, 0.115% Cu and
• alloy has over 92.5% Ag, it is entitled to use the legal appellation "sterling silver". It has improved workability compared to 92.5% Ag 7.5% Cu sterling silver alloy, does not exhibit fire stain and has a very good resistance to sulphidation. However, its final colour and reflectivity are not as good as that of 92.5% Ag 7.5% Cu sterling silver alloy.
• Alloy 3 comprises 93% Ag, 1% indium, 2.9% tin, 2.935% zinc, 0.115% copper and 0.05% nickel. Compared to alloy 2, the tin levels of alloy 3 have been reduced and the zinc levels raised. This alloy had similar properties to alloy 2, but was less workable than alloy 2, and also less workable than 92.5% Ag 7.5% Cu
• Alloy 4 comprises 93% silver, 1% indium, 4.5% tin, 1.335% zinc, 0.115% copper and 0.05% nickel.
• This alloy is provided by way of comparative example only. It had poor colour, reflectivity, and workability, although no fire stain was o evident and the sulphidation resistance was good compared to that of 92.5% Ag
• Alloy 5 comprises 93% silver, 1% indium, 3.6% tin, 2.35% copper and 0.05% nickel. This alloy is provided by way of comparative example. Despite the high 5 levels of copper, alloy 5 did not exhibit fire stain. Its castability and workability are comparable to that of 92.5% Ag 7.5% Cu sterling silver alloy, although the colour and reflectivity were inadequate compared to that of 92.5% Ag 7.5% Cu sterling silver alloy. The sulphidation resistance was better than that of 92.5% Ag 7.5% Cu sterling silver alloy, but not so good as alloys 2 to 4. 0
• Alloy 6 comprises 93% silver, 1% indium, 3.6% tin, 1.235% zinc, 0.115% copper, 0.05% nickel and 1% aluminium. Aluminium is added as a "brightener" to improve the appearance of the sterling silver alloy.
• the castability and workability of this alloy are not so good as that of 92.5% Ag 7.5% Cu sterling silver alloy, but 5 the resistance to sulphidation is better than that of 92.5% Ag 7.5% Cu sterling silver alloy, and alloy 6 exhibits no fire stain.
• the final colour and reflectivity of alloy 6 are better than those of alloys 2 to 5.
• Alloy 7 comprises 93% silver, 1% indium, 3% tin, 2.235% zinc, 0.115% o copper, 0.05% nickel and 0.6% aluminium.
• the quantity of tin and aluminium has been lowered and the quantity of zinc has been raised compared to alloy 6.
• the sulphidation resistance was very good, and no fire stain was exhibited.
• the final alloy colour and reflectivity approaches that of 92.5% Ag 7.5% Cu sterling silver alloy.
• the workability is comparable to 92.5% Ag 7.5% Cu sterling silver alloy, and the castability is not quite so good as 92.5% Ag 7.5% Cu sterling silver alloy.
• Alloy 8 comprises 93% silver, 1% indium, 3% tin, 2.235% zinc, 0.05% nickel,
• Alloy 9 has the same composition as alloy 8, with the boron replaced by silicon.
• the properties of alloy 9 are substantially the same as those of alloy 8.
• Alloy 10 comprises 93% silver, 1% indium, 3% tin, 2.035% zinc, 0.05% nickel, 0.6% aluminium, 0.115% boron and 0.2% lithium. Boron is added as a 5 grain refiner to improve workability, lithium is added as a flow aid to reduce the viscosity of the molten alloy and improve castability. It can be seen that the castability and workability of alloy 10 are better than those of 92.5% Ag 7.5% Cu sterling silver alloy. There is no fire stain present and the alloy has a very good resistance to sulphidation. The final colour and reflectivity of the alloy approach 0 those of 92.5% Ag 7.5% Cu sterling silver alloy.
• Alloy 11 comprises 93% silver, 1% indium, 3% tin, 2.035% zinc, 0.05% nickel, 0.6% aluminium, 0.115% boron and 0.2% magnesium. Magnesium is added to reduce the viscosity of the molten alloy, and hence improve its castability. 5 The properties of this alloy are very similar to those of alloy 10.
• FIG. 8 there is illustrated a table showing the effect of tin and indium additions to sterling silver alloys on the reflectivity of the alloy. It can be seen that increasing the amount of tin is detrimental to the appearance of the alloy. It is thought that tin forms an oxide around the surface of the alloy, which 5 reduce the reflectivity and are detrimental to the appearance.
• a feature of alloys 2 to 11 is that they have a high resistance to sulphidation and do not exhibit fire stain under conditions such as soldering, annealing and 5 casting.
• alloy compositions of the present invention are readily prepared using conventional techniques such as casting. Additionally, the alloys may also be prepared using other known techniques including vapour deposition, in particular, o physical vapour deposition (PVD) or chemical vapour deposition (CVD). Exampl ⁇ s of master alloys that can be used to introduce a grain refiner include the following:
• Iron can be added on its own, as it forms an adequate solid solution with silver.
• Other of the elements listed above may also be added on their own without the necessity of using a master alloy. This is useful to keep the levels of copper down in the sterling silver alloy to reduce the risk of fire stain.
• silver content of the alloy it is desirable to keep the silver content of the alloy at least 92.5% by weight in order for the alloy to be entitled to the appellation "sterling silver".
• silver is one of the more expensive elements in the composition, and so it is also commercially desirable to keep the silver content of the alloy to no more than 93% silver. In some applications, the silver content of the alloy can be raised above 93% where necessary.
• alloy 2 of figure 5 of the present invention exhibits similar mechanical properties to standard sterling confirming the suitability of the i o present alloy as a replacement for standard sterling.
• both alloys exhibit similar mechanical properties 2 o with the hardness decreasing with an increase in annealing temperature and time.
• the silver alloy of the present invention may be considered energy saving when compared with standard sterling to achieve a desired hardness given the required annealing temperature and time as illustrated in figure 10.

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