EP1888797A1 - Silver-copper-germanium alloy manufacturing - Google Patents

Abstract

Silver alloys containing copper and germanium e.g. about 1 wt% Ge and of very low copper content e.g. about 0.8 wt % Cu can be precipitation hardened to 65 HV or above, whereas alloys of similar copper content and not containing germanium remain soft. In an embodiment, a silver alloy comprises 92.5 - 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si5 In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities. The said alloy preferably comprises boron as grain refiner added as a boron hydride, e.g. sodium borohydride. A further group of alloys comprises a ternary alloy of silver, copper and germanium containing from more than 93.5 wt% to 95.5 wt% Ag, from 0.5 to 3 wt% Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride. Silicon-containing casting grain that gives rise to bright as-cast products is also disclosed. In a further embodiment, a zinc-containing silver alloy resistant to tarnish under severe conditions e.g. exposure to human sweat or French dressing comprises 1-5 wt% Zn, 0.7-3 wt% Cu, 0.1-3 wt% Ge, 0-0.3 wt% Mn, 0-0.25 wt% Si, B in an amount effective for grain refinement, up to 0.5 wt% incidental ingredients, the balance being Ag in an amount of 92.5-96 wt%, and impurities. A preferred manufacturing method giving an alloy with favourable physical properties involves melting together the ingredients, and incorporating boron by dispersing into molten silver alloy to form the whole or a precursor part of said alloy a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron- containing metal halides and mixtures thereof. The alloy is particularly suitable for castings which may be hardened in an oven e.g. at about 3000C for 30-45 min.

Description

SILVER-COPPER-GERMANIUM ALLOY MANUFACTURING

FIELD OF THE INVENTION

This invention relates to a method of manufacturing precious metal alloy, and to precious metal products made of the above alloy.

BACKGROUND TO THE INVENTION

Many attempts have been made to produce silver alloys that are resistant to tarnish and/or firestain.

In all but the largest manufacturing companies, most of the annealing and soldering required to assemble finished or semi- finished articles is carried out with the flame of an air-gas blowtorch. The oxidising or reducing nature of the flame and the temperature of the articles are controlled only by the skill of the silversmith. Pure silver allows oxygen to pass easily through it, particularly when the silver is heated to above red heat. Silver does not oxidise in air, but the copper in a silver/copper alloy is oxidised to cuprous or cupric oxide. Pickling of the oxidised surface of the article in hot dilute sulphuric acid removes the superficial but not the deeper seated copper oxide so that the surface consists of fine or unalloyed silver covering a layer of silver/copper oxide mixture. The pure silver is easily permeated during further heating, allowing copper located deeper below the surface to become oxidised. Successive annealing, cold working and pickling produces a surface that exhibits the pure lustre of silver when lightly polished but with heavier polishing reveals dark and disfiguring stains known as 'fire-stain' or 'fire'. Soldering operations are much more productive of deep fire-stain because of the higher temperatures involved. When the depth of the fire-stain exceeds about 0.025mm (0.010 inches) the alloy is additionally prone to cracking and difficult to solder because an oxide surface is not wetted by solder so that a proper metallurgical bond is not formed. US-A-3811876 (Harigaya et a\. K. K. Suwa Seikosha) discloses a tarnish- resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10 wt% Sn, 0.5-12 wt% In, and 0.1-5 wt% Zn, the remainder being silver. The Sn, In and Zn are alleged to be synergistic in their effects. Small quantities of Ti, Zn, Be, Cr, Si, Al, Ge and Sb when used in addition to Sn, In and Zn are alleged to further increase resistance to tarnishing by sulphur-containing materials. The alloy is said not to suffer from fϊrestain because of the absence of copper but this property is not confirmed by torch annealing experiments carried out by the present applicants and furthermore the alloy is soft.

US-A-4973446 (Bernhard et ah, United Precious Metal Refining) discloses a silver alloy composition of the Sn, In, Zn type that also contains copper and boron. It comprises 89-93.5 wt% Ag, 0.01-2 wt% Si, about 0.001-2 wt% B, about 0.5-5 wt% Zn, about 0.5-6 wt% Cu, about 0.25-2 wt% Sn, and about 0.01-1.25 wt% In. Silicon is added as a de-oxidant. Boron is added to reduce the surface tension of the molten alloy, and to allow it to blend homogeneously. Zinc is added to reduce the melting point of the alloy, to add whiteness, to act as a copper substitute, to act as a deoxidant, and to improve fluidity of the alloy. Copper is added as a conventional hardening agent for silver, as well as acting as the main carrying agent for the other materials. Tin is added to improve tarnish resistance, and for its hardening effect. Indium is added as a grain- refining agent, and to improve the wetability of the alloy. Silver must be present in the necessary minimal percentage to qualify as either coin silver or sterling silver. In the experience of the present inventors, although tarnish resistance is exhibited to some extent, together with some fϊrestain reduction on investment casting, firestain resistance on soldering or annealing is not obtained because of the copper content.

US-A-5039479 (Bernhard, United Precious Metals) discloses a silver alloy composition allegedly exhibiting the desirable properties of reduced fire scale, reduced porosity, reduced grain size and reduced oxide formation when heated. It consists essentially of about 89-93.5 wt% Ag, about 0.02-2 wt% Si, about 0.001-2 wt% B, about

0.5-5 wt% Zn, about 0.5-6 wt% Cu, about 0.25-6 wt% Sn, and about 0.01-1.25 wt% In. US-A-5817195 (Davitz, Astrolite, Inc) discloses an alloy alleged to be highly tarnish resistant, corrosion resistant and non-brittle, which comprises 90% to 92.5 wt% Ag, 0.25% to 0.5 wt% Ni, 0% to 0.5 wt% In, metal silicate consisting of 5.75% to 7.5 % zinc by weight of the silver coloured alloy, 0.25% to <1 wt% copper by weight of the silver coloured alloy and 0.1% to 0.25 wt% silicon by weight of the silver coloured alloy.

US-A-5882441 (Davitz) discloses a silver coloured allegedly highly tarnish resistant, corrosion resistant and non-brittle free alloy consisting essentially of 90% to

94 wt% Ag, 3.75% to 7.35 wt% Zn, 1% to 3 wt% Cu and 0.1% to 0.25 wt% Si. A preferred alloy formula is 92.5 wt% Ag, 4.5 wt% Zn, 2.9 wt% Cu and 0.1 wt% silicate

(sic).

US-A-6841012 (Croce) discloses an allegedly tarnish-resistant silver alloy comprising at least 85 wt% Ag, the balance including zinc, copper, indium, tin and iron, see also WO 04/097056 (Croce, Steridyne Laboratories, Inc). Copper contents of up to 1.5 wt% are disclosed, e.g. an allegedly a tarnish resistant silver alloy including at least

90 wt% Ag, 0.5 - 1.5 wt% Cu, 2 - 5 wt% Zn, about 0.1 wt% In and about 0.2 wt% Au.

US 2004/0219055 (Croce) discloses iurther allegedly anti-tarnish silver alloys of the Zn, Cu, In, Sn family, the alloys having at least 85 wt% Ag and the balance also including Fe. Boron is an optional ingredient.

US-A-2005/0186107 (Davitz, Sterilite LLC) discloses a silver-coloured, tarnish- resistant, corrosion-resistant alloy. It includes 92.5-95 wt% silver combined with a master alloy of 24-34 wt% Zn, 60-74 wt% Cu, 0.5-1.8 wt% Si and 0.0-8.0% Sn or 0.0- 1.5 wt% In. The alloy can be used for jewellery items, tableware, dental items or other items that should resist tarnishing or corrosion and which require a non-brittle alloy.

US-A-2005/0211342 (Menon, United Precious Metal Refining, Inc) discloses a manganese sterling silver alloy composition alleged to exhibit the properties of improved hardness and reversible heat treatability, in addition to reduced fire scale formation, reduced porosity, and reduced grain size. It consists essentially of the about 92.5-92.8 wt% Ag, about 2.0-3.0 wt% Cu, about 2.0-3.0 wt% Zn, about 0.03-0.05 wt% In, about 0.01-0.03 wt% Sn, about 0.20-0.50 wt% Cu/B alloy (2.0 wt% B, 98.0 wt% Cu) about 0.50-0.90 wt% Si/Cu alloy (10.0 wt% Si, 90.0 wt% Cu), and 0.01%-0.10 wt% Mn.

Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche) discloses a silver alloy that maintains the properties of hardness and lustre inherent in Ag-Cu alloys while reducing problems resulting from the tendency of the copper content to oxidise. The alloys are ternary Ag-Cu-Ge alloys containing at least 92.5 wt% Ag, 0.5-3 wt% Ge and the balance, apart from impurities, copper. The alloys are stainless in ambient air during conventional production, transformation and finishing operations, are easily deformable when cold, easily brazed and do not give rise to significant shrinkage on casting. They also exhibit superior ductility and tensile strength. Germanium exerts a protective function that is responsible for the advantageous combination of properties exhibited by the new alloys, and was in solid solution in both the silver and the copper phases. The microstructure of the alloy is constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver and copper. The germanium in the copper-rich phase inhibits surface oxidation of that phase by forming a thin GeO and/or GeO₂ protective coating that prevents firestain during brazing and flame annealing. Furthermore the development of tarnish is appreciably delayed by the addition of germanium, the surface turning slightly yellow rather than black and tarnish products being easily removed by ordinary tap water. The alloy is useful inter alia in jewellery and silversmithing.

US-A-6168071 (Johns) describes and claims inter alia a silver/germanium alloy having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper apart from any impurities, which alloy contains boron as a grain refiner at a concentration of up to about 20 parts per million. It further discloses providing the boron content by a master Cu/B alloy having a boron content of about 2 percent by weight. The boron in the copper/boron alloy is elemental boron. Providing the boron content within a Cu/B alloy is stated to overcome the problem of handling pure boron, which typically is a lightweight powder. Such copper/boron master alloys are said to be commonly available, and for example Belmont Metals Inc offers on its website a copper-based master alloy containing 2 wt% boron with any of As, Be, Cd, Cr, Fe, Li, Mg, Ni, P, Si, Te, Ti, Zn or Zr.

US 6726877 (Eccles) discloses an allegedly fire scale resistant, work hardenable jewellery silver alloy composition comprising at least 86 wt% Ag, 0.5 - 7.5 wt% Cu, 0.07 - 6 wt% of a mixture of Zn and Si wherein 0.02 - 2 wt % Si and 0.01 - 2.0 wt% Ge are present. The alloy may also include rheology modifying and other additives to aid in improving the castability and/or wetting performance of the molten alloy. For example, about up to 3.5% by weight of a modifying additive selected from In, B or a mixture thereof may be added to the alloy to provide grain refinement and/or provide greater wettability of the molten alloy. For manufacturing jewellers the copper content of a 925 alloy is recommended to be in the range of 2-3 wt%, the amount of zinc being 2-4 wt%, and silicon being added in proportion to the amount of zinc incorporated and being preferably 0.15-0.2 wt%. The compositions may be formed by the addition of a master alloy to fine silver, the master alloy comprising e.g. 52.5 - 99.85 wt % Cu, 0.1 - 35 wt % Zn and 0.05 - 12.5 wt% Ge. Experiments by the present applicants have not confirmed fire resistance of available embodiments of the alloy, especially during torch annealing.

Silver alloy according to the teaching of GB-B-2255348 and EP-B-0729398 is now commercially available in Europe and in the USA under the trade name Argentium, and the word "Argentium" as used herein refers to these alloys. The 925 grade Argentium alloy comprises 92.5 wt% (minimum) Ag, 1.1-1. 3 wt% Ge, 6 ppm B, the balance being copper and impurities. The alloy shows excellent resistance to tarnishing even under very arduous conditions. A passive layer is formed by the germanium, which significantly slows the formation of silver and copper sulphides, the main cause of tarnish on conventional silver alloys. Even in a hydrogen sulphide atmosphere the degree and depth of tarnish is significantly less compared to a conventional silver alloy or a silver plated item. The same mechanism that creates the tarnish resistance also results in the formation of a passive layer which significantly reduces the depth of fire- staining' or the 'fire layer' that is produced in this alloy when torch annealing in air. Trials have shown that Argentium is substantially free from firestain, which reduces the amount of polishing that the alloy requires and can give rise to considerable cost savings. As previously explained, other commercial alloys develop firestain on simple torch annealing, and it has been found that in practice just over 1% Ge in a silver jewellery or silversmithing alloy is desirable to avoid firestain even in subsequently tested alloys containing relatively low copper levels and even with zinc.

Despite the advantages of existing Argentium alloy grades, in enbodiments of the invention there is a need for further improvement of the alloy with respect to its stability under thermal processing and in particular to its resistance to pitting and/or sagging when heated for the purposes of annealing or joining. There is also a need for alloys that combine these favourable properties with hardness and resistance to tarnishing.

WO2004/106567 discloses the desirability of reducing or avoiding the formation and/or melting of the above mentioned binary copper-germanium eutectic which melts at 554°C. During the production of e.g. 925 Argentium silver alloys, the formation of this phase can be avoided by careful control of the casting conditions since under equilibrium cooling conditions the crystallisation is complete at below 640°C. However, this binary phase can create problems during subsequent thermal treatment of the alloys, e.g. using brazing alloys which typically have melting points in the range 680-750°C and torch annealing which typically involves heating a workpiece to a dull red heat at 700-750°C. On heating the workpiece to or beyond these temperatures incipient melting occurs with a small amount of material corresponding to this binary phase becoming molten while the bulk remains stable. When the workpiece returns to ambient temperature, porosity develops where the alloy has liquefied. This contributes brittleness and e.g. as noted in GB-B-2255348 there is a tendency for the alloy to sag when heated for joining or annealing operations. Although the use of the boron grain refiner of US-A-6168071 and EP-B-0729398 significantly reduces the pitting and sagging consequent on formation and melting of the binary eutectic, the formation and melting of that eutectic is, as previously mentioned, not eliminated and there is still scope for the further development of the ternary alloy to improve its pitting and sagging properties. By increasing the silver content above the level for Sterling but less than that for Britannia (95.84 wt% Ag) it is possible to produce an alloy in which the above binary eutectic either does not form or gives rise to reduced problems in subsequent heat treatment. This provides alloys with a much greater inherent stability under thermal processing. The germanium addition prevents the reduction in hardness that would be seen in a silver-copper alloy of this composition. The alloy also shows resistance to tarnishing, even under very arduous test conditions.

The invention of WO 2003/106567 therefore provides a ternary alloy of silver, copper and germanium containing from more than 93.5 wt% to 95.5 wt% Ag, from 0.5 to 3 wt% Ge and the remainder, apart from incidental ingredients, impurities and grain refiner, copper. A typical alloy that has been found to be suitable contains about 94.5 wt% Ag, about 4.3 wt% Cu and about 1.2 wt% Ge. In the above alloy the weight ratio of Cu to Ge is about 3.6 : 1 whereas in the existing 925 grade Argentium the ratio can be from 5.8 : 1 (1.1 wt% Ge) to 4.8 : 1 (1.3 wt% Ge). The applicants suggested that it is the reduction in the Cu: Ge weight ratio that is responsible for the reduced thermal processing problems, the CuGe eutectic either not forming or forming in a significantly reduced amount during post-melt thermal processing. In particular the ratio is preferably from 4: 1 to 3: 1, more preferably about 3.5 : 1. Above 4: 1 the alloy is more likely to exhibit firestain, whereas below 3: 1 the high germanium content gives rise to formability problems. In the above alloy, preferred Ag contents ranged from about 94.0 to about 95.5 wt%, lower values being preferred for reducing the expense of the silver.

SUMMARY OF THE INVENTION

We have observed a surprising difference in properties between conventional sterling silver alloys and other silver alloys of the Ag-Cu family on the one hand and silver alloys of the Ag-Cu-Ge family on the other hand. Gradual cooling of e.g. the binary Sterling-type alloys results in coarse precipitates and little precipitation hardening, whereas gradual cooling of Ag-Cu-Ge alloys (including those containing the further additives and incidental ingredients set out above) results in fine precipitates and useful precipitation hardening, especially in those embodiments where the silver alloy contains an effective amount of grain refiner e.g. boron.

Experimental evidence has shown that Ag-Cu-Ge alloys of Ag content at least 92.5 wt% become precipitation hardened following cooling from a melting or annealing temperature by baking at e.g. 200°C-400°C even at copper contents below 2 wt %, e.g. below 1.7 wt%, e.g. below 1.5 wt % e.g. below 1 wt% down to e.g. 0.5 wt%, and that baking the alloy can achieve a hardness of 65 or above, preferably 70 HV or above and still more preferably 75 HV or above which is equal to or above the hardness of standard sterling silver used to make jewellery and other silverware. These advantageous properties are believed to be the result of the combination of Cu and Ge in the silver alloy and are independent of the presence and amounts of Zn, In, Sn, Sb, Mn or other incidental alloying ingredients. This behaviour contrasts with that of high silver low copper alloys not containing Ge which do not precipitation harden. Silver alloys of very low copper content can exhibit sweat resistance and can perform well in salt spray tests.

From one aspect, the invention provides a shaped object of Ag-Cu-Ge of a silver alloy including at least 92.5 wt% Ag, 0.5-2 wt% Cu and 0.1-3 wt% Ge together with boron as grain refiner, the object being precipitation hardened to at least 65 HV, e.g. to at least 70 HV and preferably to at least 75 HV.

In another aspect, the invention provides a process for making a finished or semi- finished article of silver alloy, said process comprising the steps of: providing a silver alloy containing silver in an amount of at least 77 wt%, copper, an amount of germanium that is at least 0.5 wt% and is effective to reduce tarnishing and/or firestain and boron incorporated by dispersing throughout said alloy a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof; making or processing the finished or semi- finished article of the alloy by heating at least to an annealing temperature; cooling the article gradually, without an abrupt cooling step, so that cooling to ambient temperature takes more than 10 seconds; and reheating the article to effect precipitation hardening thereof.

Addition of germanium to sterling silver changes the thermal conductivity of the alloy compared to standard sterling silver. The International Annealed Copper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106%, and standard sterling silver 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 56%. The significance is that the Argentium sterling and other germanium-containing silver alloys do not dissipate heat as quickly as standard sterling silver or their non-germanium-containing equivalents, a piece will take longer to cool, and precipitation hardening to a commercially useful level (e.g. to about Vickers hardness 70 or above, preferably to Vickers hardness 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling.

Furthermore, the ability of the alloys of the Ag-Cu-Ge family to precipitation harden to useful values without the need for quenching is retained to copper contents as low as lwt% or even as low as 0.5 wt%, whereas other silver alloys become unacceptably soft at such low copper contents and cannot be hardened sufficiently by heat treatment. The ability of the present silver alloys containing 0.5 wt% Cu or above e.g. 1 wt% copper or above and optionally zinc and/or palladium as well as germanium to precipitation harden makes it practical to reduce the copper content of the alloy. Even though an alloy of lower copper content may be relatively soft as cast, reheating at a low temperature e.g. 150°C or 200°-400°C e.g. 300°C may bring the hardness up to the level of normal sterling silver or better. This is a significant advantage because from the standpoint of corrosion resistance the copper content is the most detrimental part of the alloy, but reduction of copper in a standard Sterling alloy gives rise to unacceptably low hardness. In the present alloys, if the copper content is reduced, the silver content may simply be increased or there may be incorporated zinc e.g. in an amount of 1-2 wt%. Other possibilities include increasing the germanium content or adding further zinc or another alloying element e.g. palladium . Silver alloy of Ag 973 parts per thousand and containing about 1.0 wt% Ge, balance copper, has been successfully precipitation hardened by gradual air cooling from an annealing temperature, and it is believed that Ag-Cu-Ge alloys with silver content above this level are also precipitation hardenable. Significant hardness has been reported for air cooled/quenched Ag-Zn-Cu alloys containing as little as 0.8 wt% Cu.

The benefit of not having to quench to achieve the hardening affect is a major advantage of the present silver alloys. There are very few times in practical production that a silversmith can safely quench a piece of nearly finished work. The risk of distortion and damage to soldered joints when quenching from a high temperature would make the process not commercially viable. In fact standard sterling can also be precipitation hardened but only with quenching from the annealing temperature and this is one reason why precipitation hardening is not used for sterling silver.

In order to distinguish the operations of annealing and precipitation hardening

(which are regarded as distinct by silversmiths) annealing temperatures may be defined to be temperatures above 500°C, whereas precipitation hardening temperatures may be defined to be in the range 150°C - 400°C, the lower value of 150°C permitting embodiments of the alloys of the invention to be precipitation hardened in a domestic oven.

Other alloys of the invention are described below.

Group 1 alloys

It has been realized that a good way to reduce sag in AgCuGe alloys when heated for soldering and annealing is to remove some of the copper. Ideally the copper needs to be below 3%. Increasing the silver content is effective to a degree, but at 95% and above, the alloys were very soft after annealing or soldering (although they could be hardened by precipitation) . Silver alloys of the first group comprise 92.5 - 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner incidental ingredients or impurities.

Alloys of this group also comprise 92.5 - 97 wt% Ag, 1-3 wt% Cu, 1-4 wt%

Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-2 wt% Mn, the balance being boron as grain refiner added as an alkali metal borohydride, incidental ingredients or impurities.

The alloys of this group may be provided as casting grain as aforesaid containing silicon in an amount effective to produce an as-cast silvery appearance and inhibit mould reactions in articles made by investment casting. Such reactions are generally nor detrimental to the properties of finished products, but require processing for their removal and can be disconcerting for those new to the use of the present alloys. The alloys of this group therefore include silver alloy casting grain comprising 92.5 - 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0.05-2 wt% Si, 0 to 0.2 wt% In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.

There is also provided the use in Ag-Cu-Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% and preferably 1-3 wt%, especially 1-1.5 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and/or to increase annealed hardness.

There is also provided the use in Ag-Cu-Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% and preferably 1-3 wt%, especially 1-1.5 wt% of 1-3 wt% Zn e.g. 2-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and up to 0.2 wt% of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy.

There is also provided a ternary alloy of silver, copper and germanium containing from more than 93.5 wt% to 95.5 wt% Ag, from 0.5 to 3 wt% Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride.

It has also been noted that when the silver content of the alloy was above 95.5%, firestain surprisingly reappeared. It has now been found that if the content is raised above 92.5% and about 1-2% Zn is added, sagging can be reduced or eliminated, and surprisingly a better annealed hardness is obtained using higher silver in combination with zinc, rather than replacing the copper with silver or zinc alone. This is significant as high silver or high zinc silver alloys are normally too soft to be used as general-purpose alloys. Higher levels of zinc (above 2%) result in heavy zinc oxide when annealing or soldering in air but it has been observed that a small amount of Sn, In or Si (>0.2) will suppress the zinc oxide formation. The resulting alloy also has superior corrosion resistance to attack by acids. The invention therefore contemplates alloys containing Sn, In or Si in an amount effective to suppress zinc oxide formation during torch annealing. Surprisingly, the presence of zinc has not proved detrimental to the ability of Ag-Cu-Ge based silver alloys to precipitation harden using the methods described below. If desired palladium may be added in partial or total replacement of zinc in an amount of up to 3 wt%.

Boron may be added as described below. Grain refining Ag-Cu-Ge-based silver jewellery or silversmithing alloys using sodium borohydride can also improve hardness by about 10 HV. An optimum combination of Ag, Zn and sodium borohydride can produces a sterling silver alloy with improved annealed hardness and superior mechanical properties. Furthermore, sodium from e.g. sodium borohydride could make the alloy useful as an electrical contact material. Sodium at ppm levels has arc- quenching properties, as also does germanium.

If desired, the germanium or copper content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected. The weight ratio of germanium to incidental ingredient elements may range from 100: 0 to 80: 20, preferably from 100: 0 to 60: 40. The term "incidental ingredients" permits the ingredient to have ancillary functionality within the alloy e.g. to improve colour or as- moulded appearance, and includes the previously mentioned metals or metalloids Si, Zn, Sn or In in amounts appropriate for "deox".

Silicon, in particular, may be added to silver alloys for casting grain e.g. in an amount of up to 0.5 wt % more usually 0.1-0.2 wt%, and is conveniently provided in the form of a copper-silicon master alloy containing e.g. about 10 wt% Si. When incorporated e.g. into casting grain of a silver-copper-germanium ternary alloy it can provide bright investment castings 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.

Group II alloys

A second group of silver alloys resistant to tarnish under severe conditions e.g. exposure to human sweat or French dressing comprises 1-5 wt% Zn, 0.7-3 wt% Cu, 0.1- 3 wt% Ge, 0-0.3 wt% Mn, 0-0.25 wt% Si, B in an amount effective for grain refinement, up to 0.5 wt% incidental ingredients, the balance being Ag in an amount of 92.5-96 wt% and impurities.

The alloys of this second group may be made by melting together the ingredients; and incorporating boron by dispersing into molten silver alloy to form the whole or a precursor part of said alloy a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.

Shaped articles or casting grain of the alloys of this group are also within the invention.

There is also provided a method of making a shaped article, which comprises: providing a molten silver alloy comprising 1-5 wt% Zn, 0.7-3 wt% Cu, 0.1-3 wt% Ge, 0-0.3 wt% Mn, 0-0.25 wt% Si, B in an amount effective for grain refinement, up to 0.5 wt% incidental ingredients, the balance being Ag in an amount of 92.5-96 wt% and impurities; casting an article in a mould with said molten silver alloy; allowing said cast article to cool; and reheating said article to effect hardening thereof.

Silver contents of this group of alloys may be from 92.5-96 wt%, preferably 92.5-95.5 wt%, and most preferably at or closely above 92.5 wt%. For present purposes, it is desirable to maximize the content of zinc which is inexpensive compared to silver.

Copper content is desirably in the range 0.7-3 wt%, preferably about 1-2 wt% and especially about 1.5 wt%. A certain amount of copper is desirable for hardness and ability to precipitation harden, but the proportion of copper is relatively low in order to minimise tarnishing.

Zinc content may be in the range 1-4.5 wt% especially about 2-4 wt%, and should preferably be close to the upper limits of the above ranges. Contrary to the teaching of the prior art, zinc-containing alloys have in the past been relatively soft, and it is the ability of germanium-containing alloys to precipitation harden that has enabled useful hardnesses of the order of 100 HV to be achieved with relatively high zinc content alloys. The very fine grain structure consequent on the addition of sodium borohydride or other decomposable boron compounds also contributes to the achievement of unexpectedly good hardness and/or other physical at high zinc content because of the excellent grain refinement that can be achieved. One unexpected advantage is that the borohydride-treated alloys exhibit relatively high ductility compared to standard material. Items which are liable to breakage on bending e.g. claws for rings which need to bend back and forth when setting stones into the ring can be bent back and forth with relatively few problems using the present alloys. Manganese has unexpectedly been found to improve tarnish resistance and is incorporated in amounts of e.g. 0.02-0.2 wt%, especially about 0.1-0.2 wt%. An additional or alternative advantage of manganese is that it adds hardness in the annealed state after a slow cool.

It is preferable to use silicon to give brighter castings. Silicon is also believed to inhibit the formation of zinc oxide. Amounts of silicon may be 0.04-0.25 wt% e.g. about 0.1 wt%.

The alloy may contain one or more incidental ingredients known per se in the production of silver alloys in amounts that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material. Cadmium may also be added in similar amounts although its use is presently not preferred. Tin may be beneficial, typically in an amount of 0.5 wt%. Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy. Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.

Boron is incorporated into the present silver alloys as a grain refiner and may be incorporated as described below.

Group III alloys

The invention also relates to Ag-Cu-Ge alloys of Ag content at least 92.5 wt% and Ge content 0.1-3 wt%, preferably 0.8-1.5 wt%, more preferably about 0.8-1 wt% that have copper contents below 2 wt %, e.g. below 1.7 wt%, e.g. below 1.5 wt % e.g. below 1 wt% down to e.g. 0.5 wt% (in some embodiments as low as 0.2 wt%), become precipitation hardened following cooling from a melting or annealing temperature by baking at e.g. 200°C-400°C and on baking can achieve a hardness of 65 HV or above e.g. 70 HV or above. The invention also relates to shaped articles of the alloys mentioned above that have been precipitation hardened to 65 HV or above e.g. 70 HV or above.

The alloys of this group in typical embodiments have silver contents of 93-94 wt%. The combined content of Cu and Ge ranges from 0.6 wt% to 5 wt% leaving at least 1 wt% and usually at least 3 wt%, more usually at least 4 wt% of other silver- compatible metals that make up the remainder of the alloy together with incidental ingredients (if any) and impurities. There are various possibilities for making up the balance of the alloy.

One possibility is to avoid other alloying ingredients, except incidental ingredients (if any), and increase the amount of silver e.g. up to 97 wt%. As previously explained, if the silver content rises above 96 wt%, then the problem of firestain reappears, but in some embodiments this disadvantage can be accepted. In other embodiments up to 4.5 wt% Zn e.g. 3-4 wt%, up to 2.5 wt% Sn e.g. about 1 wt% Sn or up to 3 wt% Pd may be incorporated into the alloy.

Manganese has been found to improve tarnish resistance and may be incorporated in amounts of e.g. 0.02-0.2 wt%, especially about 0.1-0.2 wt%. As previously explained, an additional or alternative advantage of manganese is that it adds hardness in the annealed state after a slow cool.

For casting alloys, it is preferable to use silicon to give brighter castings. Silicon is also believed to inhibit the formation of zinc oxide. Amounts of silicon may be 0.04- 0.25 wt% e.g. about 0.1 wt%.

The alloy may contain up to 0.5 wt% of one or more incidental ingredients known per se in the production of silver alloys in amounts that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material. Cadmium may also be added although its use is presently not preferred. Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy. Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.

Boron may be incorporated into the alloy using the procedures indicated below. With the relatively low quantities of copper present in this group of alloys, it is considered particularly advantageous to incorporate the boron as a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.

General methods for adding boron

A boron compound may be introduced into molten silver alloy in the gas phase, advantageously in admixture with a carrier gas which assists in creating a stirring action in the molten alloy and dispersing the boron content of the gas mixture into said alloy. Suitable carrier gases include, for example, hydrogen, nitrogen and argon. The gaseous boron compound and the carrier gas may be introduced from above into a vessel containing molten silver e.g. a crucible in a silver-melting furnace, a casting ladle or a tundish using a metallurgical lance which may be a elongated tubular body of refractory material e.g. graphite or may be a metal tube clad in refractory material and is immersed at its lower end in the molten metal. The lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep into the molten silver alloy. Alternatively the boron-containing gas may be introduced into the molten silver from the side or from below e.g. using a gas-permeable bubbling plug or a submerged injection nozzle. For example, Rautomead International of Dundee, Scotland manufacture horizontal continuous casting machines in the RMK series for the continuous casting of semi- finished products in silver and gold. 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 <5 ppm oxygen and <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 an 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 favourable from the standpoint of avoiding development in the metal or alloy of boron hard spots. Compounds which may be introduced into molten silver or gold or alloys thereof in this way include boron trifluoride, diborane or trimethylboron which are available in pressurised 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. NaBH₄ or NaBF₄ into the fluidized gas stream as a finely divided powder which forms an aerosol.

A 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-«-butylborane and methoxydiethylborane which for safe handling may be dissolved in hexane or THF. The liquid boron compound may be filled and sealed into containers of silver or of copper 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 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 vaporised therein.

Preferably the boron compound is introduced into the molten silver alloy in the solid phase, e.g. using a solid borane e.g. decaborane B₁₀H₁₄ (m.p. 100°C, b.p. 213°C). However, the boron is preferably added in the form of either a boron containing metal hydride or a boron containing metal fluoride. When a boron containing metal hydride is used, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron containing metal fluoride is used, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH₄ which has a molecular weight of 37.85 and contains 28.75% boron.

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, as in a continuous casting process for grain. This facility is not available when using a copper/boron master alloy because adding boron changes the copper content and hence the overall proportions of the various constituents in the alloy.

It has surprisingly been found that when adding a borane or borohydride that more than 20 ppm can be incorporated into a silver alloy without the development of boron hard spots. This is advantageous because boron is rapidly lost from molten silver: according to one experiment the content of boron in molten silver decaying with a half- life of about 2 minutes. The mechanism for this decay is not clear, but it may be an oxidative process. It is therefore desirable to incorporate more than 20 ppm boron into an alloy as first cast i.e. before investment casting or before rolling into strip, and amounts of e.g. up to 50 ppm, typically up to 80 ppm, and in some instances up to 800 or even 1000 ppm may be incorporated. Thus there could be produced according to the present method silver casting grain containing about 40 ppm. boron. Owing to boron loss during subsequent re-melting and investment casting, casting to form strip, rod or wire, strip rolling or other downstream processes, the boron content of finished pieces may be closer to the 1-20 ppm characteristic of the prior art, but the ability to achieve relatively high initial boron concentrations means that improved consistency may be achieved during the manufacturing stages and in the final finished products. Furthermore higher boron content is desirable for master alloys which will be melted with precious metal to make casting grain and then further melted to form rod, wire, or investment casting. Articles made e.g. by casting the present alloy may be hardened by heating in an oven e.g. at about 300°C for about 45 minutes.

Polishing

Finished articles are advantageously polished with a protective agent. As protective agent there may be used a compound containing a long chain alkyl group and a -SH or -S-S- group, e.g. an alkanethiol, dialkyl sulfide or dialkyl disulfides in which the chain is preferably at least 10 carbon atoms long and may be C₁₂-C₂₄. The -SH or - S-S- compounds that many be used include straight chain saturated aliphatic compounds containing 16-24 carbon atoms in the chain, for example cetyl mercaptan (hexadecyl mercaptan) and stearyl mercaptan (octadecyl mercaptan) and cetyl and stearyl thioglyco Hates whose formulae appear below.

Octadecyl mercaptan is a white to pale yellow waxy solid that is insoluble in water and that melts at 30⁰C. Hexadecyl mercaptan is also a white or pale yellow waxy solid that melts at 15-16°C.

How the invention may be put into effect will now be further described with reference to the following Examples: Example 1

A silver-copper-germanium-silicon alloy (Ag = 94.7 wt%, Ge = 1.2 wt%, Cu = 3.9 wt% Si = 0.2 wt%, added as a Cu/Si master alloy), is prepared by melting silver, copper, germanium and master alloy together in a crucible by means of a gas-fired furnace which becomes heated to a pour temperature of about 1093°C (2000°F). The melt is covered with graphite to protect it against atmospheric oxidation and in addition a hydrogen gas protective flame is provided. Stirring is by hand using graphite stirring rods. When the above ingredients have become liquid, pellets of sodium borohydride to give up to 100 ppm boron e.g. 80 ppm are packaged or wrapped in pure silver foil of thickness e.g. about 0.15mm. The foil wrapper holds the pellets of sodium borohydride in a single group and impedes individual pellets becoming separated and floating the surface of the melt. The wrapped pellets are placed into the hollow cupped end of a graphite stirring rod and plunged beneath the surface of the melt which at this stage is covered with a ceramic fibre blanket to quench the resulting flame from decomposition of the borohydride. The hydrogen burns off over a period of about 1-2 minutes with a stirring action being applied, after which evolution of hydrogen ceases and the boron content is substantially incorporated into the melt together with at least some of the sodium which is believed innocuous to properties of the resulting alloy.

After boron addition, the crucible pivots to permits the molten alloy to be poured into a tundish whose bottom is formed with fine holes. The molten silver pours into the tundish and runs through the holes in streams which break into fine pellets which fall into a stirred bath of water and become solidified and cooled. The cast pellets are removed from the bath and dried.

The resulting alloy granules are used in investment casting using traditional methods and using a calcium sulphate bonded investment, and are cast at a temperature of 950-980°C and at a flask temperature of not more than 676°C under a protective atmosphere. The investment material, which is of relatively low thermal conductivity, provides for slow cooling of the cast pieces. Investment casting with air-cooling for 15- 25 minutes followed by quenching of the investment flask in water after 15-25 minutes gives a cast piece having an expected Vickers hardness of about 70, which is approximately the same hardness as sterling silver. The resulting casting has a matt silvery finish when removed from the mold, and an even finer grain structure than when Cu/B master alloy is used, due e.g. to the relatively high boron content permitted by the sodium borohydride and the energetic dispersion of the boron into the molten silver as the borohydride decomposition reaction proceeds. The alloy can be polished easily, is free from boron hard spots, and gives products that exhibit excellent tarnish and firestain resistance. Precipitation hardening to expected hardness values of e.g. about 110 Vickers can be achieved by subsequent torch annealing, quenching and reheating in an oven at about 300°C.

However, a harder cast piece can be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having an expected Vickers hardness of about 110 which is similar to the value that can be achieved by the torch anneal/quench/reheat method. Contrary to experience with Sterling silver, where necessary, the hardness can be increased even further by precipitation hardening e.g. by placing castings or a whole tree in an oven set to about 300°C for 20-45 minutes to give heat-treated castings of an expected hardness approaching 125 Vickers.

Example 2

A ternary silver-copper-germanium alloy (Ag = 94.7 wt%, Ge = 1.2 wt%, Cu = 4.1 wt%) is prepared by melting silver, copper and germanium and master alloy together and adding sodium borohydride as described in Example 1 and is formed into sheet. Pieces of the sheet are brazed together to form shaped articles by passage through a brazing furnace and are simultaneously annealed. Precipitation hardening develops without a quenching step by controlled gradual air-cooling in the downstream cooling region of the furnace. For this purpose, it is desirable that the material should spend at least about 8-30 minutes in the temperature range 200-300°C which is most favourable for precipitation hardening. Articles that have been brazed in a furnace in this way and gradually cooled can achieve hardness of 110-115 Vickers. Exceptionally small grain size and good firestain and tarnish resistance is obtained because of the sodium borohydride addition.

Example 3

Alloys were prepared with the compositions and boron i contents indicated in Table I below using CuB master alloys the source of boron.

Table 1

Precip. Annealed Precip.

Sample B Hardened* hardness

Ag% Ge% Cu% Hardened* ID ppm (air-cooled) (air-cooled) (quenched) HV HV HV

_____

Sterling ^"IT^"

3.1 95.44 1.5 4 3.06 108 115 67

3.2** 96 1.55 Yes 2.45 107 110 64

3.3** 96 2 Yes 2 110 106 63

3.4** 97.30 1 Yes 1.7 93 99 40

3.5** 98.66 1.2 Yes 0.14 28*** 28*** 28***

0 * Precipitation hardening (air cooled) - sample annealed, air cooled, then heated at

300°C for 45 minutes. Precipitation hardening (quenched) - sample annealed, quenched, then heated at 300°C for 45 minutes.

** No final assay results available. Table shows alloy make-up before melting.

*** No precipitation hardening. 5 Further improvements in hardness and greater ease in polishing are obtained by increasing the boron content using sodium borohydride in place of CuB master alloy, melting following the procedure set out in Example 1.

Example 4 0

Zinc containing alloys according to the invention are prepared as set out in Table II below and their hardness is measured. In the above table, boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.

Example 5

Zinc and manganese containing alloys according to the invention are prepared as set out in Table III below and their hardness is measured. In the table III, boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.

It will be noted by comparison of samples 5.4 and 5.8 in table III that the effect of manganese is to increase hardness in the annealed state after a slow cool. Sample 5.4 contains Mn. and has an air cooled hardness of 76 HV. Sample 5.8 does not contain Mn but has an increase in Zinc, otherwise it is the same composition. Its annealed air cooled hardness 64 HV.

Example 6

The alloys of Tables II and III were tested for resistance to human sweat. Samples were polished with metal polish, ultrasonically degreased in an aqueous detergent solution for 2 minutes, rinsed, dried and wiped with acetone. For test purposes, samples were stood upright in a dish containing about 13mm depth of an artificial sweat test solution (Stern-Leach, USA). Droplets of the artificial sweat were splashed onto an upper region of each sample and left to dry.

Alloy such as standard Sterling silver with a high copper content and no germanium or zinc exhibited poor resistance to the artificial sweat. A germanium- containing Silver alloy of high copper content with no added zinc or manganese (Argentium Sterling silver) showed improved resistance both to droplets of the artificial sweat and to immersion in the artificial sweat but were subject to some discoloration. Other germanium-containing silver alloys with no zinc content or with added zinc below about 0.8 wt% showed a similar level of sweat resistance. Increase in zinc content permitted germanium content to be somewhat reduced without significant detriment to sweat resistance. Alloys of high silver content, zinc content of 1 wt% or above and germanium content of lwt% or above showed good sweat resistance properties. The best sweat resistance amongst the alloys tested is obtained with an alloy containing a high level of zinc with low copper and relatively low germanium content. Addition of Mn is believed to significantly improve sweat resistance. Results are indicated in Table IV.

Example 7

It is well-known that sterling silver tarnishes faster than pure silver because of corrosion of the copper phase. French Vinaigrette Dressing (pH level 3.5) test has been used to attack the copper phase in a selection of silver alloys as set out in Table V, containing different quantities of copper. Samples of silver alloys were partially immersed in French Vinaigrette Dressing for 24 hours. A round 'blob' of vinaigrette is placed on the samples approximately lcm above the surface of the liquid. In this test, standard sterling Sterilite B and the ternary alloy all exhibited discoloration, whereas the alloys 5.4 and 5.7 exhibited substantially no discoloration.

Example 8

The casting grain of the first stage of Example 1 made using sodium borohydride as grain refiner is re-melted with about 2 wt% zinc in a continuous caster and cast to give strip of composition about 95.7 wt% Ag, 2 wt% Zn, 1 wt% Ge, balance copper, grain refiner and impurities. The resulting alloy strip can be made into shaped articles by stamping, further hardens by oven treatment at less than 500° e.g. 300°C for 30-45 min to >about 100 HV, and exhibits resistance to tarnish by artificial sweat and by French dressing.

Example 9

An alloy is made comprising Ag 93.20 wt%, Cu 1.456 wt%, Ge 1.00 wt%, Mn 0.20 wt%, Zn 3.00 wt%, Pd 1.00 wt% and CuB 0.144 wt%. It combines the properties of firestain resistance, high Vickers hardness and resistance to corrosion in a salt spray test.

Example 10

An alloy is cast comprising Ag 93.8 wt%, Zn 4.0 wt%, Ge 1.0 wt%, Cu 0.82 wt%, Mn 0.2 wt% and Cu/B 0.18 wt% (Cu 98.2 wt%, B 1.8 wt%). The above alloy has an annealed and air-cooled hardness of about 75 HV, similar to that of standard sterling silver. An alloy developed at Sheffield Hallam University and being sold under the trade name Carrs Lustre Silver by Carrs of Sheffield and which contains 1.6 wt% copper when treated under the same conditions has an HV of 45. For the HV test, the cast samples are air-cooled from annealing temperature for 3 minutes, after which they were water quenched. It gives good performance in sweat resistance and tarnish resistance tests.

Table II

Precip. Hardened* Precip. Hardened* (quenched) Annealed hardness

Zinc alloys Ag% Ge% B ppm Cu% Zn%

(air-cooled) HV _ H_V (air-cooled) HV

4.1** 95 1.5 4-8 2.5 1 Tθ9 ^"74^" 4.2** 93.2 1.3 4-8 4.8 0.7 113 117 56 4.3** 93.2 1.1 0 5.2 0.5 101 115 64 4.4** 92.7 1.3 4-8 4 2 113 117 72

Table III

5.8**

94.2 1 4-8 2.6 2.2 115 64 47

(No Mn)

5.9** 200 Grams 5.7 0.2 68 49

Precipitation hardening (air cooled) - sample annealed, air cooled, then heated at 300°C for 45 minutes. Precipitation hardening (quenched) - sample annealed, quenched, then heated at 300°C for 45 minutes.

10 ** No final assay results available. Table shows alloy make-up before melting.

MT(Tf m D-PPT

TABLE IV

Rank Alloy Constituents Summary of Annealed & air- Sweat test results

(l=best) constituents cooled or as-rolled

1 5.6 Ag - 93.2% Ge - 1% High Zn Mn As-rolled Clean.

Cu/B O.14% Zn - 4% Lower Ge

Mn - 0.2% Cu - 1.46% V. low Copper Standard Ag

5.6 Ag - 93.2% Ge - 1% High Zn Mn Annealed & air-cooled Extremely slight discolouration - barely

Cu/B O.14% Zn - 4% Lower Ge visible. Faint light broken line at top of

Mn - 0.2% Cu - 1.46% V. low Copper immersed section. No discolouration Standard Ag from splashed solution.

4.1 Ag - 95% Ge - 1.5% High Ag Annealed & air-cooled Dark speckles (grey with red tinge) Cu/B O.14% Zn - 1% High Ge covering immersed section. Speckles Cu - 2.36% Medium Zn easily removed with dry cotton wool. No Low Cu discolouration from splashed solution.

4= 4.1 Ag - 95% Ge - 1.5% High Ag As-rolled Dark speckles (grey with red tinge &

Cu/B-0.14% Zn - 1% High Ge slightly darker than annealed sample) Cu - 2.36% Medium Zn covering immersed section. Speckles Low Cu easily removed with dry cotton wool. No discolouration from splashed solution.

4= 5.4 Ag - 94.2% Ge - 1% High Ag As-rolled Dark speckles (grey with red tinge) Cu/B -0.14% Zn - 2% Med. Zn High Mn covering immersed section. Thin dark lini Mn - 0.2% Cu -2.46% Lower Ge across top of immersed section. Speckles Low Cu easily removed with dry cotton wool, darl line not removed. No discolouration from

MTiTf mn.prτ

splashed solution.

(Mn 8) Ag - 94.2% Ge - 1.3% Raised Ag As-rolled Reddish/grey speckles covering immerse

Cu/B - 0.14% Zn - .7% Low Zn High Mn section, removed with dry cotton wool.

Mn - 0.2% Cu - 3.46% Standard Ge No discolouration from splashed solution

Lower Cu

6 5.8 Ag - 94.2% Ge - 1% Raised Ag Annealed & air-cooled Dark speckles covering immersed section CuB-0.14% Zn - 2.2% Higher Zn thin dark line across its top. Speckles Cu - 2.46% No Mn easily removed with dry cotton wool, dar] Low Cu line not removed. No discolouration from splashed solution.

(Br 17) Ag - 94.55% Ge - 1.21% Higher Ag Quenched & air-cooled Dark red speckles covering immersed Cu/B-0.14% Cu - 4.1 % Standard Ge section, removable with dry cotton wool. Discolouration with dried splashed solution not removed by dry cotton wool.

8 Ge/ Ag - 93% Ge - 1.2% Standard Ag, Ge & As-rolled Significantly less attacked than standard Sterling Cu/B-0.14% Cu - 5.66% Cu Sterling (see alloy 9 below). Dark red speckles covering immersed section, removable with dry cotton wool. Discolouration with dried splashed solution not removed by dry cotton wool.

Sterling Ag - 92.5% Cu 7.5% Standard Ag & Cu As-rolled Orange rusty looking tarnish with green No Ge tinge covering immersed section. Thick dark thick line across top of immersed section. Some orange tarnish removed with dry cotton wool, dark line not removed. Discolouration by dried splashed solution not removed by dry cotton wool.

MTiTf mn.prτ

Table V

Alloy Ag% Ge% B ppm Cu% Zn% Mn% Sn% Si%

Standard sterling 92.5 7.5

*Sterilite^® 'B' 92.7 0.42 5.13 0.9 0.8 0.05

AgCuGe ternary alloy 93.2 1.2 4 ppm 5.6

5.4 94.2 1 4-8ppm 2.6 2 0.2

5.7 95.2 1 4-8ppm 1.6 2 0.2

Claims

1. A silver alloy including at least 92.5 wt% Ag, 0.5-4.5 wt% Cu, 0.1 -3 wt% Ge and B in an amount effective for grain refinement, said alloy being precipitation hardened to a hardness of 65VH.

2. The alloy of claim 1, wherein Cu is 0.7-3 wt%.

3. The alloy of claim 1, wherein Cu is 0.7-1.5 wt%.

4. The alloy of claim 1, wherein Cu is 0.8-1 wt%.

5. The alloy of any preceding claim, wherein Ge is 0.8-1.5 wt%.

6. The alloy of any preceding claim, wherein Ge is 0.9-1.2 wt%.

7. The alloy of any preceding claim, wherein B has been added as a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.

8. The alloy of any preceding claim, comprising 0.4-5 wt% Zn.

9. The alloy of any preceding claim, comprising 1 -4 wt% Zn.

10. The alloy of any preceding claim, containing 0.02-0.2 wt% Mn.

11. The alloy of claim 10, containing 0.1 -0.2 wt% Mn.

12. The alloy of any preceding claim, containing about 0.1 wt% Si.

13. A silver alloy comprising 1-5 wt% Zn, 0.7-3 wt% Cu, 0.1-3 wt% Ge, 0-0.3 wt% Mn, 0-0.25 wt% Si, B in an amount effective for grain refinement, up to 0.5 wt%

unvnw.prT incidental ingredients, the balance being Ag in an amount of 92.5-96 wt%, and impurities.

14. The alloy of claim 13, containing any of: (a) 2 - 4.5 wt% Zn;

(b) about 4 wt% Zn;

(c) 1-2 wt% Cu;

(d) about 1.5 wt% Cu;

(e) 0.8-1.3 wt% Ge; (f) 1-1.3 wt% Ge;

(g) 02-0.2 wt% Mn;

(h) 0.1-0.2 wt% Mn;

(i) 0.1 wt% Si.;

G) 1-100 ppra B; (k) 2-25 ppm B; and/or

(1) one or more incidental ingredients selected from the group consisting of Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, and Yb; and/or

(m) boron added as a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.

15. The alloy of claim 13 or 14 in the form of casting grain.

16. A shaped article cast from the alloy of claim 13 or 14.

17. A method for making a silver alloy comprising 1-5 wt% Zn, 0.7-3 wt% Cu, 0.1- 3 wt% Ge, 0-0.3 wt% Mn, 0-0.25 wt% Si, B in an amount effective for grain refinement, up to 0.5 wt% incidental ingredients, the balance being Ag in an amount of 92.5-96 wt% and impurities by melting together the ingredients in a single stage or in two or more stages, and incorporating boron by dispersing into molten silver alloy to form the whole or a precursor part of said alloy a compound selecting from alkyl boron compounds, boron

MDX.030-PCT hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.

18. A silver alloy comprising 92.5 - 97 wt% Ag, 1-4.5 wt% Cu₃ 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.

19. The alloy of claim 18, containing any of: (a) Ag 93.5 - 95.5 wt%; (b) Ag about 94 wt%;

(c) Cu about 2.5 - 3 wt%; (d) Ge l - 1.5 wt%;

(e) Zn 1 - 3 wt%;

(f) boron as grain refiner added as copper boride; and/or (g) boron as grain refiner added as a boron hydride, e.g. sodium borohydride.

20. A ternary alloy of silver, copper and germanium containing from more than 93.5 wt% to 95.5 wt% Ag, from 0.5 to 3 wt% Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride.

21. The alloy of any of claim 20, containing from 1.0 to 1.5 wt% Ge.

22. Silver alloy casting grain comprising 92.5 - 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0.05-2 wt% Si₅ 0 to 0.2 wt% In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.

23. The casting grain of claim 22, wherein (a) Ag is 93.5 - 95.5 wt%; (b) Ag is about 94 wt%;

(c) Cu is about 2.5 - 3 wt%; (d) Ge is l - 1.5 wt%;

XΛΠY nin.prτ (e) Zn is 1 - 3 wt%; and/or

(f) boron as grain refiner added as an alkali metal borohydride.

24. Use in Ag-Cu-Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy.

25. Use in Ag-Cu-Ge silver alloys of silver content 93.5-95.5wt% and having a Ge content of 0.8-3 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and to increase annealed hardness.

26. Use in Ag-Cu-Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and up to 0.2 wt% of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy.

27. Use in Ag-Cu-Ge silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% of 2-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and up to 0.2 wt% of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy.

28. A process for making a finished or semi-finished article of silver alloy, said process comprising the steps of: providing a silver alloy containing silver in an amount of at least 77 wt%, copper, an amount of germanium that is at least 0.5 wt% and is effective to reduce tarnishing and/or fire stain and boron incorporated by dispersing throughout said alloy a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron- containing metal halides and mixtures thereof; making or processing the finished or semi-finished article of the alloy by heating at least to an annealing temperature; cooling the article gradually, without an abrupt cooling step, so that cooling to ambient temperature takes more than 10 seconds; and reheating the article to effect precipitation hardening thereof.

MDX 010- PCT