EP2943596B1

EP2943596B1 - Master alloy composition for producing gold alloys and gold alloy

Info

Publication number: EP2943596B1
Application number: EP14704895.3A
Authority: EP
Inventors: Massimo Poliero; Andrea Basso
Original assignee: Legor Group SpA
Current assignee: Legor Group SpA
Priority date: 2013-01-11
Filing date: 2014-01-09
Publication date: 2016-10-05
Anticipated expiration: 2034-01-09
Also published as: ITPD20130003A1; US20150354029A1; EP2943596A1; WO2014108849A1; CN104903477A

Description

Technical field

The present invention concerns a master alloy composition for producing gold alloys with innovative refiners system.
Furthermore, the current invention concerns a gold alloy comprising the aforesaid master alloy composition and used to produce precious jewelry, goldsmith, costume jewelry items (or jewels), coins and/or medals, both through a micro-casting process and through a mechanical working.

Prior art

Usefulness of grain refiners as improving chemical elements of the performances of gold alloys as a whole has been long known in producing goldsmith and/or jewelry alloys.
In particular, main benefits observed by reducing the size of the crystalline grain (refining) in gold alloys which follows the introduction of these refiners may be summarized as follows:

improvement of physico-mechanical properties in general, in particular deformability and elongation (physical properties which can be summarized in the term "ductility" or ability to withstand plastic deformation);
increase of the resistance to corrosion understood, for example, as decrease in the release of nickel in white gold alloys or as resistance to acidic emptying solutions in the processes of production of hollow objects (crimped process, hollow tube, etc.).

Among the better known, used and most effective grain refiners for gold alloys, iridium (Ir) and ruthenium (Ru) are included.
The same Applicant has described, in European Patent application published with N. EP1266974 A1 , master alloys compositions to obtain gold alloys comprising at least (percentages by weight): iridium ≤ 0.4%, germanium ≤ 4% and copper for the difference to 100%. Said compositions may also include silver ≤ 72%, nickel ≤ 41%, zinc ≤ 25%, ruthenium ≤ 0.96%. These compositions, due to the germanium that is present in particular as fluidifying element, possess excellent fluidity in the molten state and reduced tendency to oxidation even in absence of silicon and also, thanks to the small size of the crystalline grain, are normally able to not require the use of other refining elements.
EP1266974 discloses gold alloys and master alloys for obtaining them wherein the master alloy for obtaining gold alloys characterised in that it comprises, by weight, at least: Iridium: Ir≤0.4 %; Germanium: Ge≤4 %; Copper: Cu sufficient to reach 100%, Ag≤72%, Ni≤41%, Zn≤25%, Ru≤0.96%.
As it is well known, iridium (Ir) is a transition metallic element belonging to the so-called platinum group metals (PGMs, otherwise also known as platinoids): it has atomic number equal to 77, molecular weight equal to 192 and melts at 2,466°C.
Being a precious element, the cost of this grain refiner is particularly high (about 26÷28 €/g, at the time of filing of the present patent application).
Even ruthenium (Ru) is a transition metallic element of the group of platinoids: it possesses atomic number equal to 44, molecular weight equal to 101 and melts at 2,334°C. Its cost, however, is considerably lower than that one of iridium (about 2.5÷3 €/g at the time of filing of the present patent application).
Since both of these metallic elements have a high melting point and have limited solubility in gold, it is conceivable that they are already present as precipitates (separated compounds, because little soluble in a given liquid solution) upon reaching the liquidus temperature of the gold alloy (including fine gold and master alloy), acting as "impurities" or nucleation (or crystallization) centres for the formation and growth of the crystalline grains.
Their addition in the gold alloys or master alloys for producing the first ones is carried out in small amounts, generally not exceeding a few hundred of parts per million (ppm) on the final gold alloy.
Furthermore, the established practice commonly followed and the current state of the art in the field of production of gold alloys provides that these crystalline grain refiners are added individually, never in combination each other, therefore addition of iridium (Ir) is entirely alternative to addition of ruthenium (Ru).
The addition of only iridium (Ir) or, alternatively, of only ruthenium (Ru) in a master alloy (or in a gold alloy using such a master alloy) involves a significant but still even presumably improvable reduction in the sizes of the crystalline grain compared to those ones of a gold alloy missing these refiners, although the reduction which can be obtained with the addition of iridium (Ir) is greater than that one achievable with the addition of ruthenium (Ru).
Such a situation, according to which the iridium (Ir) is preferable to the ruthenium (Ru) as grain refiner in terms of the improvement of the mechanical properties of the alloy, however, must be weighted at the light of the significantly higher commercial cost of the first one than the second one, which however makes not always convenient its use.
It follows that, in the field concerned, the need to use or add crystalline grain refiners of master alloys or related gold alloys is now strongly felt which are able to further reduce the sizes of the crystalline grain of the gold alloy, thus resulting in a marked improvement of the physical-mechanical properties (in particular, ductility) of the alloy compared to equivalent alloys of known type provided with refiners.
At the same time, at the state of the art the need to use refiners which have not only high performances in terms of the mechanical features but also able to affect the production costs of the gold alloy to a lesser extent than the refiners used in the prior art is felt.
In short, at the current state of the technique of the field of the for jewelry and/ or goldsmith alloys a need to produce a master alloy composition for the production of gold alloys remains which exploits a refiners system alternative to that one constituted by the single use of the most widespread refiners, that is iridium (Ir) and ruthenium (Ru), while trying to exploit or enhancing the merits of both the latter.

Purposes and brief description of the invention

Starting, therefore, from the knowledge of the drawbacks mentioned above of the current state of the art, the present invention seeks to give concretely solution to them.
In particular, primary purpose of the present invention is to provide a master alloy composition for the production of gold alloys which, by using a special and innovative system of refiners, allows to get sizes of the crystalline grain of the gold alloys themselves better than those ones of equivalent gold alloys of known type, including refiners.
It is a second purpose of the invention to improve the physical-mechanical properties, in particular ductility, of a gold alloy compared to similar gold alloys of the known technique. It is another purpose of the present invention to devise a master alloy that provides a refiners system cheaper than that one of similar known master alloys, degree of crystalline grain refining reached being equal.
It is a last but not least purpose of the invention to make available a gold alloy comprising refiners which, generally, while achieving the above purposes in terms of mechanical performance, presents a cost lower than that one of prior art gold alloys equivalent to it. During several laboratory tests, the Applicant has surprisingly found that a system of crystalline grain refiners for gold alloys where iridium (Ir), ruthenium (Ru) and rhodium (Rh) are added simultaneously in the master alloy, then combined by fusion with fine gold to form the gold alloy, is quite beneficial and effective and able to reach the purposes set above. The invention, indeed, highlights how the combined use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) causes a synergic effect of the grain refining properties, and related properties, by these refining metals, in respect to the separated and alternative use of only iridium (Ir) and ruthenium (Ru) in a given master alloy composition, as it typically happens in prior art. Even more: such a synergic effect is associated with an overall reduction of the cost of the refiners used in the master alloy, because the refining system composed of iridium (Ir), ruthenium (Ru) and rhodium (Rh) allows to get a qualitative level of the physico-mechanical properties listed above even better than that one of the known gold alloys but without increasing the content of iridium (Ir), as said metal significantly and notoriously more expensive than ruthenium (Ru).
On the other hand, the combination of refiners according to the technical concept expressed by the current invention surprisingly allows to exploit the rhodium (Rh) as innovative and new grain refiner (never used in the prior art for similar purposes), without causing significant increases of cost since it is added in small quantities (less than 1%).
As known, rhodium (Rh) is a transition metallic element of the group of platinoids: it has atomic number equal to 45, molecular weight equal 102.9 and melts at 1,964°C. Its cost, very high in the past, has drastically declined in recent times (about 30 €/g at the time of filing of the present patent application).
In detail, the combination of a date, however small, amount by weight of ruthenium (Ru) and rhodium (Rh) to iridium (Ir) - without changing the quantity of the latter - surprisingly and advantageously involves a not negligible reduction of the crystalline grain size of the gold alloy and an equally noticeable increase in the ductility of the same.
Said purposes are achieved by a master alloy composition for the production of gold alloys with innovative refiners system and a gold alloy using such a master alloy composition, respectively according to the appended claims 1 and 12, as hereinafter referred for the sake of exhibition brevity.
Integral part of the invention is also the use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as refiners in the production of a master alloy composition for the production of gold alloys, according to claim 13 attached, as hereinafter referred for the sake of exposure brevity.
Object of the present invention is also the use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as synergic refiners of the crystalline grain of gold alloys according to claim 14 attached, as still hereinafter referred for the sake of exhibition brevity.
It is, therefore, a first objective of the present invention, possibly independent and usable autonomously with respect to the other aspects of the invention, a master alloy composition for the production of gold alloys with innovative refiners system, represented particularly by the simultaneous presence of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as refiners of the crystalline grain size of the final gold alloy obtained through such a master alloy composition.
It is a second purpose of the invention a master alloy composition for the production of gold alloys, comprising simultaneously iridium (Ir), ruthenium (Ru) and rhodium (Rh), able to improve, compared to the known technique, the mechanical properties, and in particular the ductility, of the final gold alloy obtained through such a master alloy composition.
It is, therefore, another purpose of the present invention, possibly independent and usable autonomously with respect to the other aspects of the invention, a master alloy composition for the production of gold alloys, comprising simultaneously iridium (Ir), ruthenium (Ru) and rhodium (Rh), able to improve, compared to the prior art, the properties of resistance to corrosion - for example by reducing the release of nickel in white gold alloys - of the gold alloy obtained through such a master alloy composition.
It is, therefore, another purpose of the invention, possibly independent and usable autonomously in relation to the other aspects of the invention, the combined and simultaneous use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as refiners in the production of a master alloy composition for the production of gold alloys.
It is, then, a further purpose of the invention, possibly independent and usable autonomously in relation to the other aspects of the invention, the combined and simultaneous use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as synergic crystalline grain refiners of gold alloys.
Other technical features of detail of the master alloy composition for the production of gold alloys with innovative refiners system, and the corresponding gold alloy, as well as the simultaneous use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) for the production of a master alloy composition, and related gold alloy, according to the present invention, are described in the corresponding dependent claims.
The claims, hereinafter specifically and concretely defined, are considered an integral part of the present description.

Description of the figures

Further features and specificities of the present invention will appear to a greater extent from the detailed description which follows, relating to preferred embodiments of the master alloy composition, gold alloy and uses claimed herein by exclusive, given by indicative and illustrative, but not limitative, way with reference to the enclosed drawing tables, also provided only by way of example, in which the figures 1-3 are three distinct images of the section of a master alloy composition of the invention, captured using a backscattered electrons beam (QBSD) of an electronic scanning microscope equipped with a microprobe (SEM/EDX), at three different levels of enlargement (680x, 2080x and 5670x, respectively).

Detailed description of the invention

While the invention is susceptible to various alternative modifications and implementations, some embodiments thereof will be described below in detail, in particular by means of some illustrative examples.
It should be understood, however, that there is no intention to limit the present invention to the specific embodiments described but, on the contrary, the invention itself intends to cover all the further alternative and equivalents modifications and implementations that fall within the scope of protection as defined in the appended claims.
In the following description, therefore, use of "for example", "etc." and "or" indicates a non-exclusive alternatives without any limitation, unless otherwise stated.
Use of the term "also" means "among which, but not limiting to," unless otherwise indicated. In accordance with the invention, the master alloy composition for the production of gold alloys with innovative refiners system includes:

iridium (Ir): 0.01÷0.8 % by weight;
ruthenium (Ru): 0.002÷0.9 % by weight;
rhodium (Rh): 0.002÷0.8 % by weight;
copper (Cu): 20÷99.965 % by weight,

In particular, the percentage by weight of iridium (Ir) is preferably in the range between 0.03% and 0.17%.
Still more in particular, the percentage by weight of iridium (Ir) is in the range between 0.03% and 0.06% for yellow gold alloys, while it is in the range between 0.1% and 0.16% for white gold alloys.
Preferably, the weight percentage of ruthenium (Ru) is in the range between 0.003% and 0.09%.
In further preferred but not exclusive manner, the percentage by weight of ruthenium (Ru) is in the range between 0.0045% and 0.04% for yellow gold alloys, while it is in the range between 0.01% and 0.08% for white gold alloys.
Preferably, also, the percentage by weight of rhodium (Rh) is in the range between 0.05% and 0.15% and for example it is equal to 0.1% by weight.
Preferably but not necessarily, the master alloy composition concerned further comprises zinc (Zn): 1÷25% by weight.
In a preferred but not binding way, the percentage by weight of zinc (Zn) is in the range between 1% and 18%.
Even more in detail, the percentage by weight of zinc (Zn) is in the range between 1% and 12% for yellow gold alloys and in the range between 7% and 14% in case of white gold alloys; this depending on the title (carat) of the final gold alloy to be obtained.
As mentioned above, it is also possible to provide master alloy compositions deprived of zinc (so-called "zinc free"), as it will be shown in the examples below; the absence of this element, while not compromising the mechanical features and resistance to corrosion nor the performances in terms of color and workability, especially in case of yellow or red alloys, allows to stabilize the weight of the casting and consequently the title of the alloy.
In case of yellow gold alloys, preferably, the master alloy composition of the invention further includes silver (Ag): 20÷64% by weight.
If, instead, the final alloy to be produced provides white gold, preferably the master alloy composition further comprises nickel (Ni): 5÷40% by weight.
As already mentioned, another integral aspect of the invention concerns a gold alloy with innovative refiners system which includes:

gold (Au): 333,33‰ (8k)÷958.33‰ (23k) by weight;
a master alloy composition as just identified and described, for the remaining part by weight.

According to a preferred embodiment of the gold alloy of the invention, the percentage by weight of gold (Au) is equal to 750‰ (18k) and the percentage by weight of the master alloy composition is equal to 250‰.
In accordance with another preferred and alternative embodiment of the gold alloy of the invention, the percentage by weight of gold (Au) is equal to 585 ‰ (14k) and the percentage by weight of the master alloy composition is equal to 415‰.
By way of preferred but not exclusive example, the above gold alloy of the invention provides that the metals of the master alloy composition are bonded together in order to form an intermediate, separated and distinct, compound, subsequently combined to the fine gold (Au) to obtain precisely the final gold alloy.
This does not exclude, however, that in other embodiments of the gold alloy of the invention, the metals of the master alloy composition are bonded directly to gold (Au) during the production phase of the final gold alloy, i.e. without creating an intermediate compound. Another aspect of the present invention relates to the use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as refiners in the production of a master alloy composition for the production of gold alloys, wherein iridium (Ir), ruthenium (Ru) and rhodium (Rh) are used simultaneously and in combination each other.
A further aspect of the present invention refers to the use of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as synergic refiners of the crystalline grain of gold alloys, wherein, again, iridium (Ir), ruthenium (Ru) and rhodium (Rh) are used simultaneously and in combination each other.
Some preferred but not limiting examples of the master alloy composition, and the resulting gold alloy with innovative refiners system, object of the invention, are described below.

EXAMPLE 1 - 750‰ yellow gold alloy.

Table I below compares four master alloy compositions with different refining systems of the crystalline grain, three of these master alloy compositions are of known and traditional type (indicated with the reference "A", "C" and "D"), while the remaining master alloy composition is designed according to the teachings of the invention (reference "B" in bold).
It is also stated precisely that, in the following Table I, the term "bal" for copper (Cu) stands for "balanced" and indicates that this element is present in an amount by weight necessary to complete the master alloy composition to 100%.
In the lower part of Table I the main physico-mechanical properties related to the corresponding gold alloy, having in this case a title of 750‰ (18k), comprising the directly above master alloy composition reported in Table I, are reported.
Master alloy compositions "A" and "C" of the prior art provide different types of refiners inserted individually, while the master alloy composition "B" of the invention provides three different types of refiners inserted simultaneously.
The master alloy composition indicated by the reference "D" is, instead, a traditional version without any kind of grain refiner.
It can be clearly seen that the composition of the master alloy "B" according to the invention, in which the triple refiner system (Ir + Ru + Rh) is inserted holds crystalline grain having sizes about 35% lower than the refined alloy only with Iridium (Ir).
It is also stressed how the triple refiner system (Ir + Ru + Rh) produce a significant improvement in the ductility of the alloy (indicated with A%), amounting to 22%. Table I

Composition (% by weight on master alloy) A B C D

Zn 9 9 9 9

Ag 45 45 45 45

Ir 0,04 0,04 --- ---

Ru --- 0,0045 0,0045 ---

Rh --- 0,1 --- ---

Cu bal bal bal bal

Measured properties (750% - 18k gold alloy)

Grain (µm) 120 70 85 700

A% 38 47 39 38
In figures 1, 2 and 3 attached, referring to the sample of master alloy composition "B" of the present invention shown in Table I, it can be observed some surveys carried out by means of backscattered electrons beam (QBSD) with electronic scanning microscope with microprobe (SEM/EDX), at three different levels of enlargement (Figure 1 = 680x; figure 2 = 2080x; figure 3 = 5670x).
From the images of these figures 1-3 at the origin of the dendrites, the presence of a nucleus consisting of elements of high molecular weight (light color) appears clearly visible.

EXAMPLE 2 - 750‰ yellow gold alloy

The second favourite, although not binding, example of master alloy composition (and corresponding gold alloy) of the invention presented herein is derived from Table II below, which compares three master alloys compositions having different grain refining systems. In particular, Table II shows two master alloy compositions of the known and traditional type, (indicated respectively with the reference "E" and "G", each having a single refiner different from that one of the other), and a master alloy composition designed according to the concept of the invention (indicated with the reference "F" in bold), i.e. with triple refiner. "Bal" term for copper (Cu) used in Table II means again "balanced" and indicates that this element is present in an amount by weight necessary to complete the master alloy composition to 100%.
Moreover, at the bottom of Table II the main physico-mechanical properties related to the corresponding 750‰ gold alloy are shown. Table II

Composition (% by weight on master alloy) E F G

Zn 9 9 9

Ag 47 47 47

Ir 0,04 0,04 ---

Ru --- 0,04 0,04

Rh --- 0,04 ---

Cu bal bal bal

Measured properties (750% - 18k gold alloy)

A% 29 34 30
Also in this case it can be observed how by including in the master alloy composition the triple refiner system according to the invention (iridium (Ir) + ruthenium (Ru) + rhodium (Rh)) produces an increase of ductility (parameter A%) more than proportional to the addition of the individual refiners in a given master alloy composition.

EXAMPLE 3 -750‰ white gold alloy.

Another important novelty introduced by the invention is the ability of the crystalline grain refining system - that involves the simultaneous addition of iridium (Ir), ruthenium (Ru) and rhodium (Rh) - to increase the properties of resistance to corrosion in white gold alloys.
It follows that in white gold alloys which provide the simultaneous presence of iridium (Ir), ruthenium (Ru) and rhodium (Rh) as refiners it has been observed a further reduction of the values of nickel release compared to what has been observed in alloys of equal composition in which the grain refiners, such as typically iridium (Ir) and ruthenium (Ru), are included as individual elements, not in combination.
Such a property is particularly important when it is considered that from the next April 2013 will come into force the new revision EN1811:2011 of the European standard on the nickel release, which provides the adoption of limits of nickel release by goldsmith, jewelry and costume jewelry articles much more restrictive than the current ones.
As known, nickel is a low cost metal, used in jewelry as whitening element of gold.
The problem of allergies (contact dermatitis) caused by the release of nickel on the skin of people who are particularly subject has been well known for many years. The public has shown an increasing level of concern about the issue of the release of nickel, to the point of pushing the European Union to adopt a Community regulatory directive on the use of such a metal, formed in particular by the legislation on the nickel EN1811 whose first publication dates back to 1994.
With the entry into force of the new revision of the regulation EN1811:2011, nickel release values actually determined shall not exceed the values of 0.28 g/cm²/week - for objects in long contact with the skin - and 0.11 g/cm²/week - for objects to be inserted into pierced parts of the body -.
Compared to the previous version of the legislation, the new standard EN1811:2011 de facto determines the reduction of the maximum release limit of more than 18 times with respect to the release limit allowed by the previous version of the legislation concerned.
The entry into force of the new standard will result, therefore, in heavy restrictions with respect to the availability of nickel-based alloys which can be still used by manufacturers. The metallurgical implications that will follow the introduction of the new standard are extremely important.
That being stated, the present invention demonstrates that through the use of the triple refiner system iridium (Ir), ruthenium (Ru) and rhodium (Rh) release values can be further reduced, leading them to a safe range, well below the maximum limit allowed by the new regulations EN1811:2011, as shown by table III below.
In such a Table III, comparative tests among master alloy compositions with different types of crystalline grain refining systems are shown.
More in detail, master alloy compositions indicated with "H" and "L" are of traditional type and each provide a single refiner (either iridium (Ir) or ruthenium (Ru), alternative each other), while the master alloy composition indicated with "I" (highlighted in bold), is designed according to the invention, using a triple refiner system (Ir + Ru + Rh).
The term "bal" for copper (Cu) always stands to shorten the word "balanced" and provides that this element is present in an amount by weight necessary to complete the master alloy composition to 100%.
The lower part of Table III shows the main physico-mechanical properties related to the corresponding white gold alloy, having in the specific case a title equal to 750‰ (18k).
The example of Table III compares the crystalline grain sizes, and the related nickel release values, of white gold alloys having a master alloy composition with single grain refiner (only iridium (Ir) for the master alloy composition "H" and only ruthenium (Ru) for the master alloy composition "L"), with the relevant parameters of white gold alloys having a master alloy composition with triple refiner system, comprising, in particular, iridium (Ir), ruthenium (Ru) and rhodium (Rh) (master alloy composition "I").
It can be clearly appreciated the synergic effect determined by the simultaneous addition of the three refiners iridium (Ir), ruthenium (Ru) and rhodium (Rh) [cf. master alloy composition "I"] not only in refining the crystalline grain but also in reducing the nickel release value, which is conveniently much lower than the limit prescribed by the new standard EN1811:2011 (0.28 g/cm²/week), with the obvious advantages that this entails. Table III

Composition (% by weight on master alloy) H I L

Zn 16 16 16

Ni 17 17 17

Ir 0,14 0,14 ---

Ru --- 0,04 0,04

Rh --- 0,1 ---

Cu bal bal bal

Measured properties (750% - 18k gold alloy)

Grain (µm) 185±15 64±17 95±20

Nichel release (µg/cm²/week 0.203 0.065 0.103

EXAMPLE 4 - 750‰ white gold alloy.

Even Table IV below shows comparative tests, carried out by the Applicant, including alloys compositions with different grain refining systems, intended to form white gold alloys.
More specifically, master alloy compositions indicated with "M" and "O" are of conventional and well known type, each providing a single refiner (either iridium (Ir) or ruthenium (Ru), alternative to each other), while master alloy composition indicated with "N" (highlighted in bold) is designed according to the invention, using simultaneously iridium (Ir), ruthenium (Ru) and rhodium (Rh) as refining elements of the crystalline grain (triple refiner system). As usual in this paper, the term "bal" for copper (Cu) abbreviates the word "balanced" and indicates that this element is present in an amount by weight necessary to complete the master alloy composition to 100%.
The lower part of the table shows the main physical and mechanical properties (grain size and ductility) referred to the corresponding 750‰ (18k) gold alloy.
In the example of Table IV the sizes of crystalline grain, and their nickel release values, of white gold alloys whose corresponding master alloy includes a single grain refiner (only iridium (Ir), master alloy composition "M", and only ruthenium (Ru), master alloy composition "O"), are compared with the same parameters of white gold alloys whose related master alloy composition "N" includes three grain refiners, in this case iridium (Ir), ruthenium (Ru) and rhodium (Rh).
It should be clearly observed the synergic effect of the triple refiner system, particularly iridium (Ir), ruthenium (Ru) and rhodium (Rh), [master alloy composition indicated with "N"] in refining the crystalline grain and reducing the nickel release value that is, advantageously, well below the limit prescribed by the new standard EN1811:2011 (0.28 g/cm²/week). Table IV

Composition (% by weight on master alloy) M N O

Zn 17 17 17

Ni 18,5 18,5 18,5

Ir 0,16 0,16 ---

Ru --- 0,08 0,08

Rh --- 0,08 ---

Cu bal bal bal

Measured properties (750% - 18k gold alloy)

Grain (µm) 170±19 60±21 102±20

Nichel release (µg/cm²/week 0.215 0.055 0.120

EXAMPLE 5 - 750‰ white gold alloy.

It is, finally, considered useful to provide, in Table V below, some additional comparative data between two traditional master alloy compositions (respectively indicated with "P" and "R") and a master alloy composition according to the present invention, indicated with "Q". As usual, the term "bal" for copper (Cu) abbreviates the word "balanced" and indicates that it is present in an amount by weight useful to complete to 100% the master alloy composition. Table V

Composition (% by weight on master alloy) P Q R

Ir

0,15 0,15 0,15

Ru --- 0,1 0,1

Rh --- 0,1 ---

Cu bal bal bal

Measured properties (750% - 18k gold alloy)

Grain (µm) 140 70 115
As it can be easily noted, the composition according to the present invention allows to get a crystalline grain having a size reduced up to about the half compared to that one of the traditional compositions of the prior art, this resulting from the simultaneous and combined use of iridium, ruthenium and rhodium.
On the basis of the description just given, it is understood, therefore, that the master alloy composition for the production of gold alloys with innovative refiners system, the gold alloy using such a master alloy composition and the uses of the present invention reach the purposes and achieve the advantages previously mentioned.

Claims

Master alloy composition for producing gold alloys with innovative refiners system consisting of
- iridium (Ir): 0.01÷0.8% by weight;

- ruthenium (Ru): 0.002÷0.9% by weight;

- rhodium (Rh): 0.002÷0.8% by weight;

- copper (Cu):44.88-99.965% by weight.
optionally zinc (Zn) 1-25% by weight, silver (Ag) 20-64% by weight, nickel (Ni) 5-40% by weight.
Master alloy composition according to claim 1, wherein the iridium (Ir) weight percentage is in the range between 0.03% and 0.17%.
Master alloy composition according to claim 2, wherein the iridium (Ir) weight percentage is in the range between 0.03% and 0.06% for yellow gold alloys and in the range between 0.1% and 0.16% for white gold alloys.
Master alloy composition according to claim any of the previous claims, wherein the ruthenium (Ru) weight percentage is in the range between 0.03% and 0.09%.
Master alloy composition according to claim 4, wherein ruthenium (Ru) weight percentage is in the range between 0.0045% and 0.04% for yellow gold alloys and in the range between 0.01% and 0.08% for white gold alloys.
Master alloy composition according to any of the previous claims, wherein the rhodium (Rh) weight percentage is in the range between 0.05% and 0.15%.
Master alloy composition according to claim 6 wherein the zinc (Zn) weight
percentage is in the range between 1% and 18%.
Master alloy composition according to claim 7 wherein the zinc (Zn) weight
percentage is in the range between 1% and 12% for yellow gold and in the range between 7% and 14% white gold alloys.
Gold alloy with innovative refiners system comprising:
- gold (Au): 333,33‰ (8k) ÷ 958,33‰ (23k) by weight;

- master alloy composition according to any of the claims from 1 to 11, for the remaining part by weight.
Use of iridium (Ir), rhutenium (Ru) and rhodium (Rh) as refiners in the production of a master alloy composition for producing gold alloys according to any of the claims from 1 to 11, wherein said iridium (Ir), rhutenium (Ru) and rhodium (Rh) are used simultaneously and in combination each other.
Use of iridium (Ir), rhutenium (Ru) and rhodium (Rh) as synergistic refiners of the crystalline grain of gold alloys according to claim 12, wherein said iridium (Ir), rhutenium (Ru) and rhodium (Rh) are used simultaneously and in combination each other.