GB2515403A - Silver alloy compositions and processes - Google Patents

Abstract

A silver alloy suitable for use in jewellery manufacture which comprises (by weight): 50-97 % silver, 0.35-5 % germanium, and the remainder being predominantly copper and unavoidable impurities. Germanium is included in a sufficient amount to impart a significant increase in the whiteness and/or brightness of the alloy as measured using the CIELab colour measurement system, with respect to the a* parameter where the silver content is below 90 % by weight, or with respect to the L* parameter where the silver content is 90 % by weight or greater, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver. The alloy can further comprise 1-40 ppm boron, 0-1 % in total of Au, Pd, Pt and Zn and/or incidental amounts of Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Si, Sn, Ti, V, Y, Yb and Zr.

Description

SILVER ALLOY COMPOSITIONS AND PROCESSES

FIELD OF THE INVENTION

The present invention relates to silver alloy compositions, processes for their preparation and their uses, in particular in the manufacture of jewellery, coins, medals, medallions, trophies, awards, cutlery, silverware and other luxury items.

BACKGROUND OF THE INVENTION AND PRIOR ART

As used throughout this specification, the term "%" means "percent by weight of the total composition", unless specified otherwise. It is also to be understood that all quoted % values are to be considered as intended to be approximate only, unless stated otherwise or the context dictates otherwise.

Alloys of precious metals (gold, palladium, platinum and silver) have a wide variety of uses. Pure silver is considered the brightest and whitest of the precious metals. However when pure it is very soft, so it is generally alloyed with copper to improve its hardness.

At the sterling silver composition (92.5% silver, 7.5% copper) the hardness level is sufficiently high for it to be successfully fabricated into practical jewellery and other luxury precious metal items. However this addition of copper also has the effect of darkening the silver so that it loses its high lustre. This can be rectified by silver plating or by silver plating and subsequent rhodium plating (it should be noted that British Standards exist for the minimum requirements of silver plating flatware and hollowware, B58442-2: 1998).

This "enhancement" of surface whiteness is not restricted to silver alloys. Rhodium plating is a requirement on many white gold alloys to improve the grey colour of the white golds to "white": See C. W. Corti, "What is White Gold? Progress on the issues", in "The Santa Fe Symposium on Jewelry Manufacturing Technology", May 2005, pages 103-119.

As described in Corti, by adding white or grey metals to gold, the gold becomes paler and eventually white.

It is known to use small amounts of germanium in silver-copper alloys to improve firestain and tarnish resistance and to improve casting and hardening characteristics as compared to either traditional silver-copper alloys, or to "deox" silver alloys that are based on silver-copper-zinc-silicon formulations. US 6726877 (A P Eccles) is directed to the hardness and fire resistance (but not colour) of a very wide range of silver alloy compositions, with three worked examples having 92.5% silver and 1.9, 0.04 and 1.0% germanium and 2.35, 3.25 and 3.0% copper.

US 4124380 describes silver-copper-germanium alloys having high oxidation resistant melts and reduced resistance to tarnishing when used in an oral environment (dentistry), with beneficial effects of germanium from 0.1% with a preferred range of 0.5 to 2% by weight for silver alloys. The preferred base alloys are described as light gold in colour, becoming progressively more silver-white as the germanium content increases.

Examples of compositions described as "white gold" include 27.6, 26.3. 24.9, 26.2% copper, and 1.3 or 1.2% germanium, with the remainder being silver or silver and gold or silver and palladium.

us 2255348 (Johns, Metaleurop Recherche) describes alloys having at least 92.5% silver and 4% copper with germanium (0.5-3% and preferably 1.5-3%). The germanium is limited to 3% by weight maximum to maintain the germanium in solid solution.

Most investigations into the colours of precious metal alloys have concentrated on the different colours obtained in gold alloys. From ternary phase diagrams of the gold-silver-copper system (see for example the work detailed in "The Colour of Gold-Silver-Copper Alloys" (Quantitative Mapping of the Ternary system) by German, Guzowski and Wright (Gold Bulletin, Volume 13, Number 3 1980 pages 113-116), it is known that by increasing the copper content, the colour of the "white" silver progressively darkens through whitish/pink to reddish and finally copper-red. This change is gradual and relates to the amount of copper present in the alloy. Similarly the addition of a third element, in this case gold, causes a different colour range to be developed, but again in a manner that can be predicted in proportion to the amount of the element added. When considering white gold alloys, the gold-palladium-silver ternary system illustrates even more clearly how in a three metal component system the relative whiteness of the alloy is linear with respect to the alloy components. A clear illustration of a three metal component system in which the relative whiteness of the alloy is linear with respect to the alloy components can be found in "White Golds, A Review of Commercial Material Characteristics and Alloy Design Alternatives" by Greg Normandeau, Gold Bulletin, 1992, 25 (3), pages 94-103).

In many aspects this diagram can be considered as even more representative of what would be the expected behaviour in the silver-copper-germanium system, as this is also a "white" alloy system.

The colours of the alloy components in white gold are silver (silver), palladium (grey) and gold (yellow). In silver alloys these are silver (silver), germanium (grey) and copper (red).

It is to be expected that when a grey element (germanium) is added to replace a red element (copper) in a silver-copper-germanium alloy the colour change will be similarly uniform as that described for white golds by Normandeau, with the germanium creating a slight whitening effect in proportion to the amount of copper being replaced by germanium.

SUMMARY OF THE INVENTION

However it has now been unexpectedly found that relatively small additions of germanium to certain ranges of silver-copper alloys can result in a surprisingly large enhancement of the whiteness of the alloy as assessed using the ClELab colour measurement system, as compared to replacing copper with silver. In particular, this effect has been found in alloys with relatively high copper contents. It has further been found that germanium additions in the range -0.5-5.0%, and particularly in the range -O.5-3.5%, have a significant whitening effect over and above that which would be expected from a simple "colour-replacement" relationship.

According to a first aspect of the present invention there is now provided a process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, comprising from about 50 to about 97% by weight of silver, from about 0.35 to about 5% by weight germanium, and the remainder being predominantly copper and unavoidable impurities, wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the whiteness and/or brightness of the alloy as measured using the CIELab colour measurement system, with respect to the a* parameter where the silver content is below 90% by weight, or with respect to the L* parameter where the silver content is 90% by weight or greater, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver.

In some embodiments the amount of silver in the alloy may be below 90% by weight, and the amount of germanium included in the alloy may be sufficient to impart an increase in the whiteness of the alloy, as measured using the CIELab colour measurement system with respect to a change (downwards on the numerical scale) in the a* parameter relative to that of the alloy with copper in place of the germanium and with the same proportion of silver, of at least about 10%, possibly at least about 25%, more possibly at least about 50%, of the value of the a* parameter.

In other embodiments the amount of silver in the alloy may be 90% by weight or greater, and the amount of germanium included in the alloy may be sufficient to impart an increase in the brightness of the alloy, as measured using the Cl ELab colour measurement system with respect to a change (upwards on the numerical scale) in the [* parameter relative to that of the alloy with copper in place of the germanium and with the same proportion of silver, of at least about 0.01%, possibly at least about 0.03%, more possibly at least about 0.05%, of the value of the [* parameter.

According to a first species of embodiments of the above first aspect of the invention there is provided a process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, wherein the alloy comprises from about 50 up to about 90% by weight silver, from about 0.5 to about 5% by weight germanium, and the remainder being predominantly copper and unavoidable impurities, and wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the whiteness of the alloy as measured using the ClELab colour measurement system, with respect to the a* parameter, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver. In some of these embodiments the level of increase in whiteness, as measured by the a* parameter, may be as quantitatively defined above.

In some of the above first species embodiments the process may produce an alloy with silver in an amount of from about 50 to about 85% by weight.

In some of the above first species embodiments the process may produce an alloy with germanium in an amount of from about 0.5% to about 3.5% by weight.

In some of the above first species embodiments the process may produce an alloy with germanium in an amount of no more than 2% by weight.

In some of the above first species embodiments the process may produce an alloy with germanium in an amount of from about 0.5 % to about 1.5 %, optionally even from about 0.75% to about 1.25% by weight, even further optionally from about 0.65% to about 1.25% by weight.

In some of the above first species embodiments the process may produce an alloy with with any of the following approximate compositions: 80% silver, 1% germanium, 19% copper; 75% silver, 1% germanium, 24% copper; 65% silver, 1% germanium, 34% copper; and 55% silver, 2% germanium, 43% copper.

In some of the above first species embodiments the process may produce an alloy in which the addition of germanium may be such as to reduce the a* value to less than 50% of the value of the corresponding alloy containing no germanium, but with the same proportion of silver.

According to a second species of embodiments of the above first aspect of the invention there is provided a process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, wherein the alloy comprises from about 90 to about 97% by weight silver, from about 0.35 to about 1.5% by weight germanium, and the remainder being predominantly copper and unavoidable impurities, and wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the brightness of the alloy as measured using the CIELab colour measurement system, with respect to the L* parameter, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver. In some of these embodiments the level of increase in whiteness, as measured by the L* parameter, may be as quantitatively defined above.

In some of the above second species embodiments the amount of silver in the composition may be in the range of from about 92 to about 94% by weight, with the amount of germanium in the composition being in the range of from about 0.35 to about 0.65% by weight.

In some of the above second species embodiments the amount of germanium in the composition may be about 0.5% by weight.

In other of the above second species embodiments the amount of silver in the composition may be in the range of from about 96 to about 97% by weight.

In certain embodiments of any of the above first or second species embodiments of this first aspect of the invention, there may be further included in the alloy one or more additional major alloying ingredients, to replace some copper, in a total amount of up to about 1% by weight, selected from one or more of Au, Pd, Pt and Zn, provided that the effect of germanium in providing the said improvement in the whiteness of the alloy is not unduly adversely affected.

However, in some embodiments of any of the above first or second species embodiments it may generally be preferred that in the alloy any element(s) or compound(s) which are characteristically non-white in colour may be substantially absent, i.e. are not present at all or at a maximum only in trace or impurity amounts.

In certain embodiments of any of the above first or second species embodiments of this first aspect of the invention, there may be further included in the alloy one or more additional alloying ingredients, to replace some copper, in an incidental or trace total amount, selected from one or more of Al, Ba, Be, Cd, Go, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Si, Sn, Ti, V, Y, Yb and Zr, provided that the effect of germanium in providing the said improvement in the whiteness of the alloy is not unduly adversely affected.

However, in some embodiments of any of the above first or second species embodiments it may be preferred that in the alloy nickel (Ni) in particular is substantially absent, i.e. is not present at all or at a maximum only in a trace or impurity amount. This may be particularly desirable for alloys for use in making jewellery, owing to concerns and even legislation concerning dermal sensitivities to nickel.

In some embodiments of any of the above first or second species embodiments it may be preferred that in the alloy any element(s) or compound(s) which lead to hard particles, i.e. particles which lead to hard spots upon polishing, is/are substantially absent, i.e. is/are not present at all or at a maximum only in trace or impurity amount(s). Such substantially absent elements on this criterion may include cobalt (Go) and iron (Fe), which conventionally may be used as grain refining elements. However, because they act as "substitutional" elements and are of sufficient size to be combined in the crystal structure of the primary metals in the alloy (i.e. silver, copper), rather than sitting at the grain boundaries like smaller (and thus the more preferred) interstitial grain refiners (e.g. boron, silicon), the above "substitutional" elements have the potential to act as "hard" particles when the alloy is polished, leading to drag marks in what may often need to, or desirably, be a highly polished surface. Thus, the presence of such "substitutional" grain refining or other elements may be undesirable and thus preferable to avoid as far as possible in alloys according to embodiments of the invention.

In certain embodiments of any of the above first or second species embodiments of this first aspect of the invention, there may be fuither included in the alloy boron, in order to act as a grain refiner. Suitable amounts of boron as a grain refiner may for example be in the range of from about ito about 40 ppm, preferably from about 5 to about 20 ppm.

Such boron may be added for example in the form of a copper-boron master alloy, or alternatively in the form of other boron-containing compounds, e.g. sodium borohydride (NaBH4) or others known in the art. Boron grain-refining additions are described for example in GB 2283934A.

According to a second aspect of the present invention there is provided a silver alloy with enhanced whiteness made by a process according to the first aspect of the invention or any embodiment of any species thereof.

According to a third aspect of the present invention there is provided a silver alloy comprising up to about 5% by weight germanium and having an a* value, as measured by the CIELab colour measurement system, of no more than about 50% of the value for a corresponding alloy containing no gernianium and the same proportion of silver, wherein the alloy has a silver content of up to about 80% by weight, and the remainder of the alloy being predominantly copper and unavoidable impurities.

According to a fourth aspect of the present invention there is provided the use, in the preparation of an alloy of silver for use in jewellery manufacture and comprising from about 50 to about 97% by weight of silver, of an amount of germanium being from about 0.35 to about 5% by weight for enhancing the whiteness of the alloy, the remainder of the alloy being predominantly copper and unavoidable impurities, wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the whiteness and/or brightness of the alloy as measured using the CIELab colour measurement system, with respect to the V parameter where the silver content is below 90% by weight, or with respect to the L* parameter where the silver content is 90% by weight or greater, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver.

According to a fifth aspect of the present invention there is provided an article made from a silver alloy according to the second aspect of the invention or any embodiment thereof, or prepared by a process according to the first aspect of the invention or any embodiment of any species thereof. The article may for example be any of: an item of jewellery, a coin, a medal, a medallion, a trophy, an award, an item of cutlery, an item of silverware, or some other luxury item.

The enhanced whitening effect of germanium additions according to embodiments of the present invention is a new and highly unexpected feature. Previous work on coloured precious metal alloys (specifically the gold-silver-copper and the gold-palladium-silver systems) show that this effect would be expected to be gradual and in proportion to the amount of the element added. This enhanced effect has been found on sterling silver alloys or alloys with a high silver content; we have now found that the replacement of as little of -2.0% of the copper in a -65% silver, -35% copper alloy with germanium can give an alloy with a colour equivalent to a -80% silver, -20% copper alloy; i.e. at this alloy composition, for colour, -2% germanium is the equivalent of -15% silver. For a range of different silver/copper ratios, an even more dramatic effect is achievable with the addition of --1.0% germanium.

A particular advantage of the processes and alloys of embodiments of the present invention is the effect of significantly brightening the alloy so that silver plating and/or rhodium plating may not be required on finished articles. In fact silver-copper alloys with a germanium addition, according to embodiments of the invention, may appear brighter and whiter than silver-plated or rhodium-plated items when the colour of the alloy is measured using the CIELab colour measurement system.

The use of germanium in silver-copper alloys according to embodiments of the present invention enables a wide range of white silver alloys to be produced with exceptional brightness. In addition to this improved whiteness, these alloys may also have much improved hardness properties, owing to the higher copper contents present therein.

These alloys may thus not require silver plating and the bright white finish may remain as the piece is worn, since it is an intrinsic property of the alloy, unlike silver-or rhodium-plated pieces where the plate wears off with time and reveals the colour of the underlying substrate alloy, which may result in returned items and subsequent re-plating costs.

In addition, it is believed that the bright white finish and exceptional lustre of the silver-copper-germanium alloys of the present invention would also provide a good base for the various colour systems (e.g. KLIAR by Legor; COLORIT by Heimerle and Meule) which are in use on silver jewellery. In particular the consistent white, bright colour of these silver-copper-germanium alloys allows these systems to develop a uniform coloration over the areas to which they are applied.

Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention in its various aspects will now be further and more particularly described, by way of non-limiting example only, and with reference to the accompanying drawings, in which: Figure 1 shows graphically the variation of the parameter AE* for a selection of silver/copper alloys; Figure 2 shows graphically the variation of brightness (parameter L*) for the silver copper alloys; Figure 3 shows graphically the variation along the green to red colour axis (parameter a*) for the silver/copper alloys; Figure 4 shows graphically the variation along the blue to yellow colour axis (parameter b*) for the silver/copper alloys; Figure 5 shows graphically the variation of the parameter E* for various silver/copper/germanium alloys; Figure 6 shows graphically the variation along the green to red colour axis (parameter a*) for the silver/copper/germanium alloys; Figure 7 shows graphically the variation along the blue to yellow colour axis (parameter b*) for the silver/copper/germanium alloys; and Figure 8 shows graphically the variation of the brightness parameter [* for silver/copper/germanium alloys with silver contents of at least -90%.

DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS THEREOF

Whiteness Measurement.

There are many different systems that can be used to measure and define the colour and whiteness of different materials, as reviewed by Chris Corti (ibid). Corti states that although the Yellowness Index (ASTM E313-12) is a convenient single value to assess the relative whiteness of a white gold alloy, there is a limiting factor: specifically the Yellowness Index favours white colours with a green tint compared to those with a red tint, so a limit of +3.01-3.5 on the CIELab a* value (green-red axis) is recommended when the Yellowness Index is used to assess white gold alloys.

For this reason the full CIELab system for colour measurement is to be preferred for giving a more complete understanding of alloy colour. The CIELab system for measuring colour (which uses a colour photo-spectrometer that shines a bright white light of defined characteristics on to the sample under test and measures the amount and respective components of the light reflected back) has three coordinates. The coordinate L* measures the degree of brightness or lightness from 0, which is black, to 100, which is bright white; the coordinate a* measures the green-red component of colour; and the coordinate b* measures the blue-yellow colour component. Therefore a perfect pure white would have L* = 100 and a* and b* both = 0. These coordinates give a complete description of any colour as a point within a "colour sphere" described by the CIELab system. A colour sphere also shows how the colour difference between two different colours can be described as the distance between two distinct colour points" in the sphere. In the present case the relative redness of different silver alloys is under consideration, so the differences in the a* parameter for alloys with the same silver content is considered to be the most significant measure of the colour difference for these alloys. However, for silver alloys with a silver content of 90% or above, the brightness parameter L* is a more significant measure, because the level of redness is very low. On the other hand, for silver alloys with a silver content of less than 90%, the whiteness parameter a* is a more significant measure.

If the values of AL*, Aa*, and Ab* are known, then the total difference (or distance) between two colours (Ci and C2) on the CIELab diagram can be stated as a single value, known as AE*, i.e.: aE*(C1C2) = [(zxL2) + (aa2) + (zxb2)]1/2 Sample Production and Preparation.

Samples for testing were produced by melting alloys of the nominal compositions given

in Table 1 below.

Table 1: Sample Designations and Nominal Compositions.

Sample Silver Copper Germanium Designation. Content (%) Content (%) Content (%) A 95.0 5.0 None B 95.0 4.5 0.5 C 95.0 4.0 1.0 D 90.0 10.0 None E 90.0 9.5 0.5 F 90.0 9.0 1.0 G 85.0 15.0 None H 85.0 14.0 1.0 85.0 13.0 2.0 J 80.0 20.0 None K 80.0 19.0 1.0 L 80.0 18.0 2.0 M 75.0 25.0 None N 75.0 23.0 2.0 o 70.0 30.0 None P 70.0 28.0 2.0 o 65.0 35.0 None R 65.0 33.0 2.0 S 65.0 32.0 3.0 T 60.0 40.0 None U 60.0 38.0 2.0 V 60.0 37.0 3.0 W 55.0 45.0 None X 55.0 42.0 3.0 Y 50.0 50.0 None Z 50.0 47.0 3.0 These alloys were produced using an induction melting furnace with an inert gas cover to prevent oxidation of the copper content of the molten alloy. The stirring effect of the induction melting system ensures that the molten alloy is a homogeneous mixture prior to casting. In addition to the major constituents listed above, each melt contained a small amount of boron (in the range -5-20 ppm) as a grain refiner, added as a copper-boron master alloy (the use of boron as a grain refiner in silver-copper-germanium alloys is disclosed for example in GB 2283934A).

These alloys were cast into a graphite mould to give an as-cast ingot size of approximately 50 mm x 20 mm x 5 mm. The ingots were allowed to cool in the mould in air until completely solidified. The cast ingots were not cleaned in acid when removed from the mould as would be common practice when investment casting. This is because acid cleaning has the potential to remove some copper from the surface of the cast pieces and consequently alter the surface composition of the pieces. This would be a particular concern where the copper present on the surface of the cast piece has oxidised to copper oxide on cooling; where sulphuric acid is used as the cleaning acid this would remove the copper oxides from the surface of the piece, leaving a silver-enriched surface layer. This has the potential to be measured as a "brighter" surface finish, which is not an accurate indication of the colour of the alloy.

The surface condition of the cast samples was recorded photographically, and it was observed visually that the silver-copper alloys darken as the proportion of copper increases, while the equivalent silver-copper-germanium alloys have a much whiter appearance.

The samples were then prepared for surface colour measurement by a metallographic polishing process. In order to remove the surface influence from the cast samples and ensure a true measure of the colour of the specific alloy composition, at least 0.75 mm needed to be removed from the surface of each cast sample. This was achieved by grinding back the surface using progressively finer silicon carbide papers, i.e. 60, 120, 240, 600, 1200 grit, followed by final polishing with diamond paste compounds of 5

microns and 1 micron specification.

A metallographic polishing process was chosen over a traditional silversmith's polishing process because of its ability to remove metal at each polishing stage in a controlled manner. Silversmith polishing techniques, using rotating felt wheels and wax impregnated cutting compounds, greatly depend on the skill of the individual polisher to achieve an even and consistent result.

After assessment of the initial results, some additional samples were prepared, to examine in greater detail the properties of high silver-content alloys (-90-97% Ag) with -0.5-1.0% germanium contents; lower silver-content alloys (-50-65% Ag) with -4.0-5.0% germanium; and alloys at around 70% Ag, with 0% germanium, or -1% or -3% germanium. The additional samples were produced in the same way as the initial samples, and were tested in the same way. The nominal compositions of these additional samples, and the associated colour measurement results, are shown in Table 2a below.

Results.

Colour measurement were taken using a Minolta CM 508d Spectrophotometer. Each colour measurement detailed in Tables 2 and 2a below is the average of 5 individual measurements taken at different locations on the polished surface.

Table 2: Colour Measurement Results.

Sample Ag % Cu % Ge% L* 5* b* Designation A 95 5 0 97.93 -0.82 5.55 B 95 4.5 0.5 97.96 -0.59 4.67 C 95 4.0 1.0 96.44 -0.51 5.68 D 90 10 0 97.13 -0A3 7.13 E 90 9.5 0.5 96.95 -0.85 6.39 F 90 9.0 1.0 96.07 -0.63 6.57 G 85 15.0 0 95.37 0.61 8.23 H 85 14 1.0 96.52 -0.57 7.43 13 2.0 95.96 -0.46 6.36 J 80 20 0 96.08 1.64 8.35 K 80 19 1.0 96.39 0.08 7.71 L 80 18 2.0 95.10 -0.41 7.38 M 75 25 0 94.95 2.55 10.64 N 75 23 2.0 94.83 0.10 8.63 0 70 30 0 93.07 2.85 13.02 P 70 28 2.0 94.08 0.70 10.29 Q 65 35 0 93.30 4.24 12.59 R 65 33 2.0 93.15 1.65 11.86 S 65 32 3.0 92.34 1.17 11.91 T 60 40 0 91.29 5.26 14.42 U 60 38 2.0 90.35 2.49 13.57 V 60 37 3.0 88.42 1.94 13.75 W 55 45 0 89.39 5.94 14.41 X 55 42 3.0 88.06 2.92 14.71 Y 50 50 0 90.82 6.92 15.14 Z 50 47 3.0 91.55 2.70 12.77 Table 2a: Additional Composition Data and Colour Measurement Results Sample Ag % Cu % Ge% L* a* b* Designation AA 97.0 3.0 0 97.78 -0.42 3.62 AB 97.0 2.5 0.5 98.36 -0.12 2.54 AC 97.0 2.0 1.0 98.21 -0.02 2.35 AD 96.0 4.0 0 97.60 -0.56 4.65 AE 96.0 3.5 0.5 98.27 -0.08 2.51 AF 96.0 3.0 1.0 97.58 -0.05 2.38 AG 93.5 6.5 0 97.56 -0.24 6.18 AH 93.5 6.0 0.5 98.23 -0.21 2.83 Al 93.5 5.5 1.0 97.41 -0.16 2.47 AJ 92.5 7.5 0 97.23 -0.18 6.24 AK 92.5 7.0 0.5 98.04 -0.18 2.76 AL 92.5 6.5 1.0 96.77 -0.22 2.58 BA 75.0 24.0 1.0 95.89 0.12 7.96 BB 70.0 29.0 1.0 95.63 0.22 8.24 BC 65.0 34.0 1.0 95.02 0.26 8.46 CA 70.0 27.0 3.0 93.14 0.47 10.65 DA 65.0 31.0 4.0 94.05 1.86 11.25 OB 65.0 30.0 5.0 94.22 1.88 12.82 DC 60.0 36.0 4.0 93.56 2.24 12.47 00 60.0 35.0 5.0 93.94 2.17 12.96 OF 55.0 41.0 4.0 93.04 2.56 12.69 OF 55.0 40.0 5.0 93.14 2.48 13.56 OG 50.0 46.0 4.0 92.92 2.73 13.02 OH 50.0 45.0 5.0 92.86 2.61 13.78 EA 75.0 25.0 0 95.05 2.65 10.75 EB 72.0 28.0 0 94.68 2.36 11.84 EC 70.0 30.0 0 93.15 2.91 12.96 Looking at the data in Table 2 above showing the values of the L* and a* parameters for the various alloy samples, the respective percentage changes (zXL* (%) and Aa* (%)) in each parameter for each alloy can be calculated, as shown in Table 2b below. (It will be noted that an upward change on the numerical scale of the value of the L* parameter represents an improvement in the whiteness (as indicated by brightness) of the relevant alloy, whereas a downward change on the numerical scale of the value of the a* parameter represents an improvement in the whiteness (as indicated by actual whiteness) of the relevant alloy.

Table 2b: Additional Colour Measurement Results.

Sample Silver Copper Germanium L* a* b* AL* Aa* Designation _______ _______ ___________ _______ _______ _______ _______ _______ A 95 5 0 97.93 -0.82 5.55 0 n/a B 95 4.5 0.5 97.96 -0.59 4.67 0.03 n/a C 95 4.0 1.0 96.44 -0.51 5.68 -1.52 n/a D 90 10 0 97.13 -0.13 7.13 0 n/a E 90 9.5 0.5 96.95 -0.85 6.39 -0.19 n/a F 90 9.0 1.0 96.07 -0.63 6.57 -1.09 n/a G 85 15.0 0 95.37 0.61 8.23 0 0.0 H 85 14 1.0 96.52 -0.57 7.43 1.21 193 ___________ 85 13 2.0 95.96 -0.46 6.36 0.62 175 J 80 20 0 96.08 1.64 8.35 0 0.0 K 80 19 1.0 96.39 0.08 7.71 0.32 95 L 80 18 2.0 95.10 -0.41 7.38 -1.02 125 M 75 25 0 94.95 2.55 10.64 0 0.0 N 75 23 2.0 94.83 0.10 8.63 -0.13 96 o 70 30 0 93.07 2.85 13.02 0 0.0 P 70 28 2.0 94.08 0.70 10.29 1.09 75 o 65 35 0 93.30 4.24 12.59 0 0.0 R 65 33 2.0 93.15 1.65 11.86 -0.16 61 65 32 3.0 92.34 1.17 11.91 -1.03 72 T 60 40 0 91.29 5.26 14.42 0 0.0 U 60 38 2.0 90.35 2.49 13.57 -1.03 53 V 60 37 3.0 88.42 1.94 13.75 -3.14 63 W 55 45 0 89.39 5.94 14.41 0 0.0 X 55 42 3.0 88.06 2.92 14.71 -1.49 51 V 50 50 0 90.82 6.92 15.14 0 0.0 Z 50 47 3.0 91.55 2.70 12.77 0.80 61 Discussion of Results.

S The expectation is that the colour difference of silver-copper alloys behaves in an incremental, predictable way. This can be shown by calculating the AE* values for the silver-copper alloys in Table 2 relative to the 95% silver, 5% copper alloy; see Table 3 below.

Table 3: Silver-Copper Alloy Colour Difference Measurements (AE*).

Sample No. Silver % Copper % AE* A 95 5 0 D 90 10 1.90 G 85 15 3.97 J 80 20 4.16 M 75 25 6.79 o 70 30 9.64 o 65 35 9.83 T 60 40 12.64 W 55 45 14.04 Y 50 50 14.23 Referring now to Figure 1, this shows graphically the variation of colour difference measurement, E*, with the proportion of silver in the alloy. The solid line links the plotted values and the dashed line shows the best straight line relationship. Within the expected experimental error this shows clearly the proportional relationship between colour change and copper content for silver-copper alloys.

Similar plots for the L*, a* and b* values are shown in Figures 2, 3 and 4, and these all also show the straight line proportional relationship between copper content and the colour of the silver-copper alloy. Of particular interest is the a* graph of Figure 3, which shows the increase in redness with copper content. In all three graphs the solid line shows the data plot and the dashed line the best straight line relationship.

Considering again the graph of Figure 3, it will be noted that the values of a* follow an anomalous trend between about 67 and 75% silver, as the values are significantly below those expected from the straight line graph. This presumably corresponds to the eutectic point being at -72% silver, -28% copper. If the silver content is above -72%, the resulting structure is a silver-rich primary phase with a copper-rich secondary phase; at the eutectic point there will be a single silver-rich phase; and for a silver content below -72% there is a copper-rich primary phase and a silver-rich secondary phase. It can also be expected that the appearance of the specimen in the vicinity of the eutectic point will depend also on the cooling rate of the cast sample, as variations in the casting temperature and in the cooling rate will result in slight changes in the crystallite sizes of the metal phases.

Table 4: Silver-Cu-Ge Alloy -Relative Colour Difference Measurements (AE*).

Sample Designation Ag % Cu % Ge % L* a* b* AA 97 3 0 97.78 -0.42 3.62 0 AB 97 2.5 0.5 98.36 -0.12 2.54 1.26 AC 97 2 1 98.21 -0.02 2.35 1.40 AD 96 4 0 97.6 -0.56 4.65 0 AE 96 3.5 0.5 98.27 -0.08 2.51 2.29 AF 96 3 1 97.58 -0.05 2.38 2.33 A 95 5 0 97.93 -0.82 5.55 0 B 95 4.5 0.5 97.96 -0.59 4.67 0.91 C 95 4.0 1.0 96.44 -0.51 5.68 1.53 AG 93.5 6.5 0 97.56 -0.24 6.18 0 AH 93.5 6.0 0.5 98.23 -0.21 2.83 3.42 Al 93.5 5.5 1.0 97.41 -0.16 2.47 3.71 AJ 92.5 7.5 0 97.23 -0.18 6.24 0 AK 92.5 7.0 0.5 98.04 -0.18 2.76 3.57 AL 92.5 6.5 1.0 96.77 -0.22 2.58 3.69 o 90 10 0 97.13 -0.13 7.13 0 E 90 9.5 0.5 96.95 -0.85 6.39 1.05 F 90 9.0 1.0 96.07 -0.63 6.57 1.30 G 85 15.0 0 95.37 0.61 8.23 0 H 85 14 1.0 96.52 -0.57 7.43 1.83 13 2.0 95.96 -0.46 6.36 2.23 J 80 20 0 96.08 1.64 8.35 0 K 80 19 1.0 96.39 0.08 7.71 1.71 L 80 18 2.0 95.10 -0.41 7.38 2.47 M 75 25 0 94.95 2.55 10.64 0 BA 75 24 1.0 95.89 0.12 7.96 3.74 N 75 23 2.0 94.83 0.10 8.63 3.17 o 70 30 0 93.07 2.85 13.02 0 BB 70 29 1 95.63 0.22 8.24 6.03 P 70 28 2.0 94.08 0.70 10.29 3.62 CA 70 27 3.0 93.14 0.47 10.65 3.36 Q 65 35 0 93.30 4.24 12.59 0 BC 65 34 1 95.02 0.26 8.46 5.99 R 65 33 2.0 93.15 1.65 11.86 2.70 S 65 32 3.0 92.34 1.17 11.91 3.29 DA 65 31 4.0 94.05 1.86 11.25 2.83 DB 65 30 5.0 94.22 1.88 12.82 2.54 1 60 40 0 91.29 5.26 14.42 0 U 60 38 2.0 90.35 2.49 13.57 3.05 V 60 37 3.0 88.42 1.94 13.75 4.44 DC 60 36 4.0 93.56 2.24 12.47 4.25 DO 60 35 5.0 93.94 2.17 12.96 4.32 W 55 45 0 89.39 5.94 14.41 0 X 55 42 3.0 88.06 2.92 14.71 3.31 DE 55 41 4.0 93.04 2.56 12.69 5.26 OF 55 40 5.0 93.14 2.48 13.56 5.17 Y 50 50 0 90.82 6.92 15.14 0 Z 50 47 3.0 91.55 2.70 12.77 4.89 OG 50 46 4 92.92 2.73 13.02 5.14 OH 50 45 5.0 92.86 2.61 13.78 4.96 Considering the same relationships for the silver-copper-germanium alloys, and calculating AE* values for these alloys relative to the respective reference silver-copper alloys, gives the values detailed in Table 4 above. These values are also shown graphically in Figure 5.

When compared to Figure 1 (the silver-copper graph for AE* values) it can be clearly seen that there is a completely different (non-linear) relationship between colour difference and germanium content.

Figure 5 shows a cyclical effect of the germanium addition and clearly show that slightly higher germanium additions will impart additional whitening effects.

Figure 6 shows graphically the variation in the a* parameter (red to green) for all the alloys listed in Table 4. For any one fixed concentration of germanium there is an increase in the a* parameter with the concentration of copper (or a decrease as the concentration of silver increases), at least for a range of values of silver concentration.

This relationship does not hold for high silver concentrations, above about 85% for Ge = -1%, and above about 80% for Ge = -2%; and it also does not appear to hold for values of silver below about 55% for Ge = -3% (where there is only one data point).

Considering the portions of the graphs within which the a* parameter increases with the copper concentration, it is also apparent that the provision of germanium leads to a dramatic reduction in the a* value, that is to say a dramatic change of colour. For example, considering those alloys for which the a* value is 0, this corresponds to an alloy with -89.5% silver (without germanium), or to -80.5% silver (with -1% germanium), or to -76% silver (with -2% germanium). Hence the effect of -1% germanium is equivalent to -9% silvel; while the effect of -2% germanium is equivalent to that of -13.5% silver.

Similarly, considering those alloys for which the a* value is 1.5, this corresponds to an alloy with -81% silver (without germanium)! or to -65.5% silver (with -2% germanium), or to -62.5% silver (with -3% germanium). Hence the effect of -2% germanium is equivalent to that of -15.5% silver, while the effect of --3% germanium is equivalent to that of -18.5% silver.

Figure 7 shows graphically the variation in the b* parameter (blue to yellow) for the alloys listed in Table 4 for the different germanium concentrations. For any one fixed concentration of germanium there is an increase in the b* parameter with the concentration of copper (or a decrease as the concentration of silver increases).

It is also apparent that the provision of germanium leads to a reduction in the b* value, that is to say a change of colour, except for the alloys with more than -90% silver. For example, considering those alloys for which the b* value is 7.5, this corresponds to an alloy with -88% silver (without germanium), or to -84% silver (with -1% germanium), or to -79.5% silver (with -2% germanium). Hence the effect of -1% germanium is equivalent to -5% silver; while the effect of -2% germanium is equivalent to that of -8.5% silver. Similarly, considering those alloys for which the b* value is 11, this corresponds to an alloy with -75.3% silver (without germanium), or to -67.6% silver (with -2% germanium). Hence the effect of -2% germanium is equivalent to that of -7.7% silver.

This relationship can be expressed differently, by considering an alloy with a particular concentration of silver. For example an alloy of silver which has -65% silver (and no germanium) has values a* = 4.3 and b* = 13.7 (taken from the smooth curves). The addition of -2% germanium lowers both values, and the resulting values correspond to an alloy of -80% silver (considering a*) and to an alloy of -73% silver (considering b*).

At least approximately, the resulting alloy is the equivalent of one with -75% silver. As regards the costs of the materials, the cost of -2% germanium is considerably less than that of -10% silver.

Thus from a consideration of the data of both Figures 6 and 7 it is clear that a small proportion of germanium can have an effect on the colour of a silver alloy which is equivalent to that achieved by a much larger change in silver concentration. This effect is most apparent for values of silver concentration less than about 90%, and greater than about 55%.

It will be appreciated that the addition of copper tends to provide a reddish tint, corresponding to a positive value of a*; negative values of a* correspond to an increasingly green appearance. It is desirable to achieve an alloy which is approximately colourless, so proximity to a* = 0 is usually desirable, and negative values of a* are usually not desirable. Another feature which is apparent from the data of both Figures 6 and 7 is that for a wide range of different silver contents, below about 80%, the effect of -1% germanium is markedly greater than that of larger proportions of germanium, and provides an alloy which is closer to a* = 0 than any other germanium proportion.

Thus, for example, considering alloys with a silver content of -65%, in the absence of germanium the a* value is more than 4.0; with germanium loadings of from -2% to -5% the a* value is in the range 1.1 to 1.9; while with a germanium loading of only -1%, the a* value is less than 0.3. That low value of a* corresponds to the value that would be obtained with a silver concentration of about 88% in the absence of germanium. Thus the effect of -1% germanium is the equivalent of an increase of -23% in the silver concentration.

For this same range of silver alloys, below about 80% silver, it is also evident from Figure 7 that about 1% germanium provides a significantly lower value of b* than larger proportions of germanium. Thus, considering the effect on both the colour parameters a* and b* (as shown in Figures 6 and 7), it is apparent that for a range of alloys having from about 50% up to about 80% silver, the optimum whiteness is achieved by providing about 1% germanium. For silver concentrations of from about 80% to about 90%, both -1% and -2% germanium provide a large decrease in both the a* and b* values.

For silver concentrations above about 90% the values of a* are small, and fluctuate significantly with variations in silver or germanium concentration; for this range of silver concentrations the brightness parameter L* is a more useful parameter assessing the whitening effect. Referring now to Figure 8, this shows the variation in brightness, i.e. the parameter L*, for all the alloy specimens with a silver content of about 90% and above

in Tables 2 and 2a.

The non-linear variations with silver concentration provide a further indication that the effect of the germanium addition is not directly related to the amount of copper that is replaced.

It is clear from the graphs of Figure 8 that over this range of silver concentrations the optimum brightness is achieved with a germanium concentration of about 0.5%. In contrast, for much of the range there is a reduction in the brightness with a germanium concentration of about 1%. In the range of silver concentrations of about 92.5% to about 93.5% marked S, which is of particular commercial significance because it corresponds to the requirement for sterling silver (which may be hallmarked in conformity with international regulations where the silver content is above 92.5%), the brightness is increased by adding about 0.5% germanium.

Similarly in the range of silver contents from about 96.0% to about 97.0%, marked B, there is an increase in brightness with germanium contents of both -0.5% and -1%, with the -0.5% addition giving the greatest improvement. This range is also of commercial significance because it corresponds to the requirement for Britannia silver (which may be hallmarked according to United Kingdom regulations where the silver content is above 95.84%).

It is to be understood that the above and below description of embodiments and aspects of the invention has been by way of non-limiting examples only, and various modifications may be made from what has been specifically described and illustrated whilst remaining within the scope of the invention as defined in the appended claims.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Some further embodiments of the present invention may be understood by reference to the following numbered paragraphs: 1. A process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, comprising between 50-90% silver, 0.5-5 % germanium, and the remainder predominantly copper, wherein the amount of germanium is added so as to impart a significant increase in the whiteness of the alloy as measured using the CIELab colour measurement system and as compared to an alloy with silver in place of the germanium, and with the same proportion of copper.

2. A process according to paragraph 1, wherein the alloy comprises between 50 and 85% silver.

3. A process according to paragraph 1 or paragraph 2, wherein the germanium content is between 0.5% and 3.5% by weight.

4. A process according to paragraph 3, wherein the germanium content is no more than 2%.

5. A process according to paragraph 4, wherein the germanium content is between 0.75% and 1.25%.

6. A process according to any one of the preceding paragraphs, wherein the alloy composition is selected from compositions in Table 4.

7. A process according to any one of the preceding paragraphs, wherein the alloy composition is selected from alloys with the compositions: 80% silver, 1% germanium, 19% copper; 75% silver, 1% germanium, 24% copper; 65% silver, 1% germanium, 34% copper; and 55% silver, 2% germanium, 43% copper.

8. A process according to any one of the preceding paragraphs, wherein boron is added as a grain refiner in the range 1 -40 ppm.

9. A process according to any one of the preceding paragraphs, including one or more other major alloying ingredients to replace some copper, in amounts of up to 1 wt%, selected from Au, Pd, Pt and Zn, provided the effect of germanium in providing an improvement in the whiteness is not unduly adversely affected.

10. A process according to any one of the preceding paragraphs, including one or more other alloying ingredients to replace some copper in incidental amounts, selected from Al, Ba, Be, Cd, Go, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Si, Sn, Ti, V, Y, Yb and Zr, provided the effect of germanium in providing an improvement in the whiteness is not unduly adversely affected.

11. A process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, comprising between 90-97% by weight silver and 0.35 - 1.5% by weight germanium, and the remainder predominantly copper, wherein the amount of germanium is added so as to impart a significant increase in the brightness of the alloy as measured using the CIELab colour measurement system and as compared to an alloy with silver in place of the germanium, and with the same proportion of copper.

12. A process according to paragraph 11, wherein the silver proportion is between 92% and 94% and the proportion of germanium is between 0.35 and 0.65%.

13. A process according to paragraph 11 wherein the silver proportion is between 96% and 97%.

14. A silver alloy with enhanced whiteness made by a process of any one of the preceding paragraphs.

15. A silver alloy comprising up to 5% germanium having an a* value no more than 50% of the value for a corresponding alloy containing no germanium and the same proportion of silver, as measured by the CIELab colour measurement system, wherein the alloys have a silver content up to 80% by weight, and the remainder is predominantly copper.

16. An article prepared with an alloy of any one of paragraphs 14 or 15 or prepared by a process according to any one of paragraphs 1 to 13, the article being selected from jewellery, medals, trophies, awards, cutlery, silverware and other luxury items.

Claims (27)

1. CLAIMS1. A process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, comprising from about 50 to about 97% by weight of silver, from about 0.35 to about 5% by weight germanium, and the remainder being predominantly copper and unavoidable impurities, wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the whiteness and/or brightness of the alloy as measured using the CIELab colour measurement system, with respect to the a* parameter where the silver content is below 90% by weight, or with respect to the L* parameter where the silver content is 90% by weight or greater, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver.
2. 2. A process according to Claim 1, wherein the amount of silver in the alloy is below 90% by weight, and the amount of germanium included in the alloy is sufficient to impart an increase in the whiteness of the alloy, as measured using the CIELab colour measurement system with respect to a change (downwards on the numerical scale) in the a* parameter relative to that of the alloy with copper in place of the germanium and with the same proportion of silver, of at least about 10% of the value of the a* parameter.
3. 3. A process according to Claim 2, wherein the amount of germanium included in the alloy is sufficient to impart an increase in the whiteness of the alloy, as measured using the CIELab colour measurement system with respect to a change (downwards on the numerical scale) in the a* parameter relative to that of the alloy with copper in place of the germanium and with the same proportion of silver, of at least about 25%, optionally at least about 50%, of the value of the a* parameter.
4. 4. A process according to Claim 1, wherein the amount of silver in the alloy is 90% by weight or greater, and the amount of germanium included in the alloy is sufficient to impart an increase in the brightness of the alloy, as measured using the CIELab colour measurement system with respect to a change (upwards on the numerical scale) in the L* parameter relative to that of the alloy with copper in place of the germanium and with the same proportion of silver, of at least about 0.01% of the value of the L* parameter.
5. 5. A process according to Claim 4, wherein the amount of germanium included in the alloy is sufficient to impart an increase in the brightness of the alloy, as measured using the CIELab colour measurement system with respect to a change (upwards on the numerical scale) in the V parameter relative to that of the alloy with copper in place of the germanium and with the same proportion of silver, of at least about 0.03%, optionally at least about 0.05%, of the value of the L* parameter.
6. 6. A process according to Claim 1, which is a process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, wherein the alloy comprises from about 50 up to about 90% by weight silver, from about 0.5 to about 5% by weight germanium, and the remainder being predominantly copper and unavoidable impurities, and wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the whiteness of the alloy as measured using the CIELab colour measurement system, with respect to the a* parameter, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver.
7. 7. A process according to Claim 6, wherein the alloy comprises from about 50 to about 85% by weight silver.
8. 8. A process according to Claim 6 or Claim 7, wherein the germanium content is in the range of from about 0.5% to about 3.5% by weight.
9. 9. A process according to Claim 8, wherein the germanium content is no more than 2% by weight.
10. 10. A process according to Claim 9, wherein the germanium content is in the range of from about 0.5% to about 1.5 % by weight, optionally from about 0.75% to about 1.25% by weight, even further optionally from about 0.65% to about 1.25% by weight.
11. 11. A process according to any one of Claims 6 to 9, wherein the alloy composition is selected from alloys with any of the following approximate compositions: 80% silver, 1% germanium, 19% copper; 75% silver, 1% germanium, 24% copper; 65% silver, 1% germanium, 34% copper; and 55% silver, 2% germanium, 43% copper.
12. 12. A process according to Claim 1, which is a process for preparing an alloy of silver with enhanced whiteness suitable for use in jewellery manufacture, wherein the alloy comprises from about 90 to about 97% by weight silver, from about 0.35 to about 1.5% by weight germanium, and the remainder being predominantly copper and unavoidable impurities, and wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the brightness of the alloy as measured using the CIELab colour measurement system, with respect to the L* parameter, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver.
13. 13. A process according to Claim 12, wherein the amount of silver in the composition is in the range of from about 92 to about 94% by weight, and the amount of germanium in the composition is in the range of from about 0.35 to about 0.65% by weight.
14. 14. A process according to Claim 12 or Claim 13, wherein the amount of germanium in the composition is approximately 0.5% by weight.
15. 15. A process according to Claim 12 or Claim 14 (as dependent through Claim 12), wherein the amount of silver in the composition is in the range of from about 96 to about 97% by weight.
16. 16. A process according to Claim 1, wherein the alloy composition is selected from any of the germanium-containing compositions listed in any of Tables 1, 2, 2a, 2b and 4 hereinabove and having the said increased whiteness and/or brightness as measured by the a* or [* parameter, as the case may be.
17. 17. A process according to any one of the preceding claims, wherein boron is added as a grain refiner so as to have a content in the alloy in the range of from about ito about ppm.
18. 18. A process according to any one of the preceding claims, further including in the alloy one or more additional major alloying ingredients, to replace some copper, in a total amount of up to about 1% by weight, selected from one or more of Au, Pd, Pt and Zn, provided that the effect of germanium in providing the said improvement in the whiteness of the alloy is not unduly adversely affected.
19. 19. A process according to any one of the preceding claims, further including in the alloy one or more additional alloying ingredients, to replace some copper, in an incidental or trace total amount, selected from one or more of Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Si, Sn, Ti, V, Y, Yb and Zr, provided that the effect of germanium in providing the said improvement in the whiteness of the alloy is not unduly adversely affected.
20. 20. A process according to any one of the preceding claims, wherein nickel (Ni) is substantially absent from the alloy or is present therein at a maximum only in a trace or impurity amount.
21. 21. A process according to any one of the preceding claims, wherein cobalt (Co) and iron (Fe), as well as any other element or compound which acts as a substitutional grain refiner and can lead to hard particles within the crystal structure of the resulting alloy, is/are substantially absent from the alloy or is/are present therein at a maximum only in trace or impurity amount(s).
22. 22. A process according to any one of the preceding claims, wherein any element(s) or compound(s) which are characteristically non-white in colour are substantially absent from the alloy or are present therein at a maximum only in trace or impurity amounts.
23. 23. A silver alloy with enhanced whiteness made by a process according to any one of claims ito 22.
24. 24. A silver alloy comprising up to about 5% by weight germanium and having an a* value, as measured by the ClELab colour measurement system, of no more than about 50% of the value for a corresponding alloy containing no germanium and the same proportion of silver, wherein the alloy has a silver content of up to about 80% by weight, and the remainder of the alloy being predominantly copper and unavoidable impurities.
25. 25. Use, in the preparation of an alloy of silver for use in jewellery manufacture and comprising from about 50 to about 97% by weight of silver, of an amount of germanium being from about 0.35 to about 5% by weight for enhancing the whiteness of the alloy, the remainder of the alloy being predominantly copper and unavoidable impurities, wherein the amount of germanium included in the alloy is sufficient to impart a significant increase in the whiteness and/or brightness of the alloy as measured using the CIELab colour measurement system, with respect to the a* parameter where the silver content is below 90% by weight, or with respect to the L* parameter where the silver content is 90% by weight or greater, and as compared to an alloy with copper in place of the germanium, and with the same proportion of silver.
26. 26. An article made from a silver alloy according to any one of Claims 23 or 24 or prepared by a process according to any one of Claims 1 to 22.
27. 27. An article according to Claim 26, which is selected from an item of jewellery, a coin, a medal, a medallion, a trophy, an award, an item of cutlery, an item of silverware, or a luxury item.