EP3775305A1

EP3775305A1 - Tarnishing resistant copper gold alloy, in particular 9k, and method for production thereof

Info

Publication number: EP3775305A1
Application number: EP19726481.5A
Authority: EP
Inventors: Sergio ARNABOLDI; Marta ROSSINI; Marco NAUER
Original assignee: Argor Heraeus SA
Current assignee: Argor Heraeus SA
Priority date: 2018-04-12
Filing date: 2019-04-12
Publication date: 2021-02-17
Anticipated expiration: 2039-04-12
Also published as: CH714880B1; WO2019198042A1; CH714880A1; EP3775305B1

Abstract

The invention refers to an alloy, in particular for jewellery, characterized in that it comprises: Gold in the amount comprised between 330 ‰ and 420 ‰ in weight; Copper in the amount comprised between 500 ‰ and 620 ‰ in weight; Palladium in the amount comprised between 5 ‰ and 35 ‰ in weight. The invention also refers to a method of production of the alloy itself, a method of production of an item for jewellery and an item of jewellery.

Description

TARNISHING RESISTANT COPPER GOLD ALLOY, IN PARTICULAR 9K, AND

METHOD FOR PRODUCTION THEREOF

Field of the invention

The present invention refers to the field of Gold alloys and in particular refers to a Gold alloy with Gold title substantially equal to 9 carats (9K).

The present invention also refers to a method for the production of a Gold alloy.

The Gold alloy and the method for the production of Gold alloys according to the invention are an alloy and a method for the production of Gold alloys for jewellery and watchmaking applications respectively.

Background art

In the field of the jewellery and watchmaking, Gold is not used in pure form, since it is too ductile. For jewellery and watchmaking applications are typically used Gold alloys for jewellery or watchmaking, characterized by a higher hardness with respect to the Gold in pure form and/or with respect to low hardness or high ductility Gold alloys.

It is known that, generally, Gold alloys can undergo over time unwanted color alterations, following interactions with aggressive environments. These interactions bring to the creation of thin layers of reaction products, which staying adherent to the alloy surface, cause an alteration of the color and of the gloss (document “Observations of onset of sulfide tarnish on gold-base alloys”; JPD, 1971 , Vol. 25, issue 6, pag. 629-637).

Environments able to promote color alterations of Gold alloys are various and are linked to their applications.

Colors for Gold alloys can be uniquely measured in the CIELAB 1976 color space, which defines a color on the basis of a first L* parameter, a second a* parameter and a third b* parameter, wherein the first L* parameter identifies the brightness and adopts values comprised between 0 (black) and 100 (white) whereas the second a* parameter and the third b* parameter represent chromaticity parameters. In particular, in CIELAB 1976 color chart, the achromatic scale of greys is detected by points wherein a*=b*=0; positive values for the second parameter a* indicate a color tending the more to red as the higher the value of the second parameter is; negative values for the second parameter a* indicate a color tending the more to green as the value of the second parameter a* is a high absolute value, although negative; positive values for the third parameter b* indicate a color tending the more to yellow as the higher the value of the third parameter is; negative values for the third parameter b* indicate a color tending the more to blue as the value of the third parameter b* is a high absolute value, although negative. Furthermore, it is possible to transform the second a* parameter and the third b* parameter in polar parameters as defined:

The Cab* parameter is defined as“chroma”; the higher the value of Cab* parameter is, the higher is the color saturation; the lower the value of Cab* parameter is, the lower is the color saturation, that will tend to the grey scale. To the knowledge of the Applicant, alloys with a Gold content higher than 750%o, which can be used as such as white or grey Gold alloys and do not require surface rhodium plating, arbitrarily show Cab* values <8. The parameter h_abt identifies on the other hand the tonality of the color.

In particular, the ISO DIS 8654:2017 standard defines seven color designations as for the Gold alloys for jewellery. In particular, these alloys are defined according to the following table, wherein the color is defined on a standard reference specified between ON and 6N.

Table 1

For measuring the color of a Gold alloy, in particular, the ISO DIS 8654 standard specifies that the measuring instrument must comply with the CIE N° 15 publication.

The ISO DIS 8654:2017 standard also shows the nominal values L* a* b* as trichromatic coordinates for alloys of 0N-6N standard color, including the tolerances. Hereinafter is specified an abstract of the standard wherein are defined the chromatic limits of the alloys defined by the ISO DIS 8654:2017 standard as pink/red.

Table 2

In relation to the preceding table, it is then possible to obtain, within the CIELAB 1976 color space, a plurality of areas each of which represents the color spaces within which it is possible to assert that an alloy shows a 0N...6N color and more specifically a 5N-6N color. These areas are represented in details in figure 1.

The ISO DIS 8654:2017 standard also proposes chemical compositions recommended for each of the 0N-6N alloys. In particular, for pink/red alloys, the compositions are the ones specified in the table:

Table 3

The Applicant has noted that the pink/red Gold alloys of known type show a substantial color instability, in particular when exposed to environments wherein there are chlorides or sulphides.

Variations of the color of a Gold alloy according to the color as defined on the CIE 1976 color chart and specified by the E=f (L* a* b*) coordinate, defined:

- L^* ₀ as first parameter in original conditions, at time to=0;

- a₀ ^* as second parameter in original conditions, at time to=0;

- bo as third parameter in original conditions, at time to=0;

are defined in the following equation:

It has also been noted that the human eye of a technician expert in precious materials is able to detect color variations DE (L* a* b*) >1.

In particular, the Applicant has noted that the 5N ISO DIS 8654:2017 Gold alloy in the formulation that uses the minimum reference value as for the content of Silver exposed to vapors of thioacetamide for 150 hours (according to the UNI EN ISO 4538:1998 standard), shows a variation of color DE (L* a* b*) equal to 5.6; when exposed to the action of an aqueous solution 50g/liter of sodium chloride (NaCI) at 35°C for 175 hours, the 5N Gold alloy shows a variation of color DE (L* a* b*) equal to 3.6.

There are also known jewellery alloys, comprising Gold with a title equal to or substantially equal to 9K. These Gold alloys are typically used to make jewels of value lower than those made with 18K or higher Gold alloys. In particular, among the alloys including Gold with a title equal to or substantially equal to 9K, there are known alloys containing Copper, suitable to appear with a pink or red color, according to a growing percentage of Copper in the alloy. In the alloys including Gold with title equal to or substantially equal to 9K, it is known that the presence of Copper has the drawback of being rather prone to vary color especially when the object of jewelry is worn and exposed to human sweating and/or saline environment. The same alloys, including Gold with a title equal to or substantially equal to 9K are known to have significant color variation even just after exposure to air.

For this reason, typically jewellery items made from alloys including Gold with a title equal to or substantially equal to 9K, in particular when including Copper, are covered with a plating with Gold alloys with a title significantly higher than 9K, typically for example 16K or 18K. This plating has some further drawbacks. A first drawback derives from the fact that it is difficult to realize a plating whose color is perfectly superimposable to that of the basic alloy with which the object of jewellery is made.

Furthermore, the plating of a jewellery item with high title Gold alloys is expensive, and significantly contributes to the increase in the production cost of the item not only because of the cost of the high title Gold alloy, but also because of the further necessary processing on the jewellery item, which incidentally for shapes of jewellery items of considerable complexity, may not be insignificant.

Finally, the plating, representing the surface layer of the item, is the portion the most subject to wear or anyhow removal; with the removal of the plating, the item of jewelry may have exposed portions of alloy with Gold title equal to or substantially equal to 9K, which - exposed to chemically aggressive environments - may have different color than the portions that vice versa still carry the plating alloy.

The purpose of the present invention is therefore to describe a Gold alloy, particularly for jewelry and watchmaking, with a Gold title equal to or substantially equal to 9K, which solves the above described drawbacks, in particular being not very prone to turn color when exposed to environments with air, Thioacetamide or NaCI in solution.

The purpose of the present invention is also to describe a method of production of an alloy containing Gold with a title equal to or substantially equal to 9K, which solves the above described drawbacks.

The purpose of the present invention is finally to describe an item of jewelry or part of an item for jewelry, made with the alloy object of the invention, which does not show the above described drawbacks. Summary

These and other purposes are obtained by the alloy and by the method of production thereof described in the following aspects. The following aspects may be combined with each other and/or with portions of the description or claims. Where dependencies are indicated, they are described on a preferred and non-limiting basis.

Forms a first aspect of the invention an alloy, in particular for jewellery, characterized in that it comprises:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight;

Said alloy in particular consists of:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight and, optionally, at least one among: Iron in the amount comprised between 2%o in weight and 25%o in weight, Silver in the amount lower than or equal to 100%o in weight, Zinc in the amount lower than or equal to 100%o in weight, Indium in the amount lower than or equal to 20%o in weight, Tin in the amount lower than or equal to 20%o in weight, Gallium in the amount lower than or equal to 10%o in weight, Iridium in the amount lower than or equal to 0.5%o in weight, or Ruthenium in the amount lower than or equal to 0.5%o in weight, Rhenium in the amount lower than or equal to 0.1 %o in weight. Optionally and not limitedly, the sum of Palladium and Iron is equal to 30%o in weight.

According to the purposes of this invention, as“jewellery alloy” is intended an alloy, in particular a Gold alloy, in which there are no materials toxic to humans and the formulation of which is suitable for, specifically designed for, making jewels, or parts thereof.

According to a 2° non-limiting aspect, the alloy according to the first aspect is a Gold alloy characterized by a dark red color. According to the present invention, as“dark red” is intended a color that, arbitrarily, on the a*, b* color plan according to the CIE 1976 color chart, is not comprised in the spaces defined by the ISO DIS 8654:2017 standard and is enclosed in a polygon at least defined by the following points:

Table 4

According to a 3° non-limiting aspect, the alloy according to a first or second aspect is a tarnishing resistant alloy.

According to a 4° non-limiting aspect, the alloy depending on one or more of the preceding aspects, comprises Palladium in the amount comprised between 8%o, more preferably 10%o in weight, and 32%o, more preferably 30%o in weight.

According to a 5° non-limiting aspect, the alloy according to one or more of the preceding aspects, comprises also Iron in the amount comprised between 2%o in weight and 25%o in weight, more preferably comprised between 5%o in weight and 20%o in weight.

According to a 6° non-limiting aspect, the alloy according to one or more of the preceding aspects is a ternary or quaternary alloy, and the sum of the amounts of Gold, Copper and Palladium is at least equal to 900%o in weight.

According to the present invention, as ternary or quaternary Gold alloy is intended an alloy wherein there are 3 or 4 components respectively, the amount of which is not negligible, and in particular higher than 2%o in weight and more preferably higher than 1 %o in weight. In other words, quaternary or quinary alloys do not comprise components in excess of 2%o in weight and more preferably 1 %o in weight in addition to those explicitly mentioned.

According to a 7° non-limiting aspect, the alloy according to the 6° aspect shows a sum of the amounts of Gold, Copper and Palladium at least equal to 960%o in weight, more preferably 970%o in weight.

According to a 8° non-limiting aspect, the alloy according to the 6° aspect comprises also Silver in the amount lower than 100%o in weight, and/or Zinc in the amount lower than 100%o in weight or Silver and Zinc the sum of respective amounts in weight is lower than 100%o .

According to a 9° non-limiting aspect, the alloy according to one of the aspects from 1 ° to 7°, comprises Palladium in the amount comprised between 15%o in weight and 25%o in weight, more preferably between 18%o in weight and 22%o in weight, more preferably substantially equal to 20%o in weight.

According to a 10° non-limiting aspect, in the alloy according to one of the aspects from 1 ° to 7°, when depending on the 5° aspect, the sum of the amounts of Palladium and Iron is equal to 30%o in weight.

According to a 11 ° non-limiting aspect, in the alloy according to one or more of the preceding aspects, the Gold is present in the amount comprised between 365%o in weight and 385%o in weight and Copper is present between 594%o in weight and 614%o in weight and Palladium is comprised between 15%o and 25%o in weight, more in particular between 18%o and 22%o in weight.

In particular, according to a 12° non-limiting aspect, the alloy according to the

11 ° aspect, is free from Iron.

According to a 13° non-limiting aspect, the alloy subject of the invention is an alloy whose color on the CIELAB1976 color chart shows a coordinate a* > 8.2 and a coordinate b*<13.5.

According to a 14° non-limiting aspect, said Gold alloy for jewellery is an alloy characterized by the absence of Vanadium, and other materials capable to create carbides and oxides, in particular free from Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium, Tantalum, Zirconium, Yttrium, Germanium.

According to a 15° non-limiting aspect, the Gold alloy for jewellery is an alloy free from Nickel, Cobalt, Arsenic and Cadmium. Thanks to this aspect, the alloy is a Gold alloy compatible with being worn or wearable by subjects whose allergic tolerance is significantly low.

According to a 16° independent aspect, it is also object of the invention a method for the production of a Gold alloy; said method is characterized in that it comprises:

a) a step (hereinafter defined as homogenization) wherein all the pure elements constituting the alloy are melted in such a way as to obtain an homogeneous solution or mixture; this mixture comprises in weight:

- Gold, in the amount comprised between 330%o and 420%o in weight; - Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight;

for creating a mixture; and

b) a step of introduction of the mixture in a melting pot, and a subsequent melting through heating until melting.

In particular, and non-limiting to that extent, it can be an aspect of said method, a mixture comprising:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight and, optionally, at least one among: Iron in the amount comprised between 2%o in weight and 25%o in weight, Silver in the amount lower than or equal to 100%o in weight, Zinc in the amount lower than or equal to 1 00%o in weight, Indium in the amount lower than or equal to 20%o in weight, Tin in the amount lower than or equal to 20%o in weight, Gallium in the amount lower than or equal to 10%o in weight, Iridium in the amount lower than or equal to 0.5%o in weight, or Ruthenium in the amount lower than or equal to 0.5%o in weight, Rhenium in the amount lower than or equal to 0.1 %o in weight.

According to a 17° non-limiting aspect, depending on the preceding 16° aspect, said step comprises mixing in particular Palladium in the amount comprised between 8%o, more preferably 10%o, and 32%o, more preferably 30%o in weight.

According to a 18° non-limiting aspect, the alloy according to one or more of the preceding 16°-17° aspects, comprises in particular mixing, in addition to the preceding elements, also Iron in the amount comprised between 2%o in weight and 25%o in weight, more preferably comprised between 5%o in weight and 20%o in weight.

According to a 19° non-limiting aspect, in the alloy according to one or more of the preceding aspects from 16° to 18°, the sum of the amounts of Gold, Copper and Palladium is at least equal to 900%o in weight.

According to a 20° non-limiting aspect, the step comprises the mixing of Gold,

Copper and Palladium in the amount at least equal to 960%o in weight, more preferably 970%o in weight.

According to a 21 ° non-limiting aspect, the alloy according to the 19° aspect comprises the mixing of Silver in the amount lower than 100%o in weight, and/or Zinc in the amount lower than 100%o in weight or Silver and Zinc the sum of respective amounts in weight is lower than 100%o.

According to a 22° non-limiting aspect, according to one of the aspects from 16° to 21 °, said step comprises the mixing of Palladium in the amount comprised between 15%o in weight and 25%o in weight, more preferably between 18%o in weight and 22%o in weight, more preferably substantially equal to 20%o in weight.

According to a 23° non-limiting aspect, when depending on the 18° aspect, the sum of the amounts of Palladium and Iron is equal to 30%o in weight.

According to a 24° non-limiting aspect, depending on one or more of the preceding aspects from 16° to 23°, the Gold is mixed in particular in the amount comprised between 365%o in weight and 385%o in weight and Copper is present between 594%o in weight and 614%o in weight and Palladium is comprised between 15%o and 25%o in weight, more preferably between 18%o and 22%o in weight.

According to a 25° non-limiting aspect, said homogenization is a discontinuous melting, comprising a step of casting wherein the melted material is casted in a refractory mold or refractory or metallic ingot and wherein said melted alloy is an alloy characterized by the absence of Vanadium, and other elements capable to create carbides or oxides, in particular free from Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium, Tantalum, Zirconium, Yttrium, Germanium. The absence of said carbides or oxides makes the Gold alloy suitable for applications of jewelry and watchmaking where polishing or diamond polishing of finished items is required.

According to a 26° non-limiting aspect, during said melting, the melting pot is subject to a gas controlled atmosphere and in particular is subject, at least temporarily, to vacuum condition.

According to a 27° non-limiting aspect, during said casting step, said melting pot is subject to a controlled atmosphere, to pressures lower than the environmental one.

According to a 28° non-limiting aspect, said controlled atmosphere is or comprises an inert gas, preferably argon and/or said pressure is a pressure lower than 800mbar, preferably lower than 700mbar.

According to a 29° non-limiting aspect, said gas is a reducing gas, preferably a hydrogen-nitrogen mixture and/or said pressure is a pressure lower than 800m bar, preferably lower than 700mbar. According to a 30° non-limiting aspect, said melting is a continuous melting, comprising a step of melting and homogenization in a graphite pot and a subsequent melting step wherein the melted alloy is casted in a die realized in graphite and wherein said alloy is an alloy of metals without chemical affinity to graphite and more specifically, in particular at least free from Vanadium, Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium, Tantalum, Zirconium, Yttrium, Germanium.

The absence of elements with chemical affinity to graphite, allows an excellent flow of the melted alloy within the die and facilitates the extraction thereof after solidification. On the contrary, the presence of elements with chemical affinity to graphite, causes a gripping effect of the alloy to the die, preventing the extraction. Furthermore, the absence of carbides and oxides makes the Gold alloy suitable for applications of jewelry and watchmaking where polishing or diamond polishing of finished items is required.

According to a 31 ° non-limiting aspect, after the continuous or discontinuous melting, said alloy is subject to a cooling step followed by one or more hot or cold plastic deformation steps and one or more thermal treatments.

According to a 32° aspect, it is also object of the present invention an item of jewellery, comprising a Gold alloy according to one or more of the preceding aspects concerning said Gold alloy.

According to a 33° aspect, depending on the preceding aspect, said item of jewellery comprises a jewel or a watch or a watch bracelet or a movement or part of a mechanical movement for watches.

According to a 34° aspect, depending on the preceding aspect, said watch or mechanical movement for watches are configured for being respectively worn or installed in wristwatches.

Furthermore, according to another aspect of the invention, it is described an alloy, in particular for jewellery, characterized in that it comprises at least:

- Gold, in the amount comprised between 13.0% and 18.0% in atomic weight;

- Palladium, in the amount comprised between 1.5% and 1.7% in atomic weight;

- Copper, in the amount comprised between 82% and 84.6% in atomic weight; said alloy being characterized in that its color, on the CIELAB1976 color chart, has a coordinate a* > 8.2 and a coordinate b*<13.55.

According to a 35° aspect, that can be combined with one or more of the preceding aspects, the Gold alloy is free from secondary phases, and/or is a homogeneous alloy, and/or is a crystalline alloy, optionally 100% crystalline.

According to the present invention, as“free from secondary phases” or“free from second phases” is intended an alloy free from elements that can generate said second phases, in particular in a proceeding of melting and subsequent solidification without other thermal treatments; second phases that create in the liquid phase and remain downstream of the alloy solidification, are harmful second phases, for example carbides and/or oxides that during the polishing step are visible at naked eye on the surface of the polished item, and that then prevent to obtain items of high surface quality, compatible with the needs required in the high jewellery field. It is possible to expose the alloy to thermal treatment processes, able to give it a hardening, so that due to precipitation subtle precipitates can be present, as results of said thermal treatment; in this case these are precipitates that prevent from the movement of displacement by increasing the mechanical properties in the material, and contrasting the incidence of deformations in the items realized with the present alloys.

Description of drawings

The invention is hereinafter described in preferred and non-limiting embodiments, whose description is associated to the attached figures wherein

- Figure 1 shows a portion of CIELAB 1976 color space according to the coordinates L* a* b* wherein it has been detected an area corresponding to color intervals or tolerances admissible for Gold alloys according to the 5N and 6N ISO DIS 8654:2017 standard and wherein it is shown the typical color position for alloys object of the present invention;

- Figure 2 shows color variation charts according to the time of exposure to

50g/L NaCI solutions for alloys object of the present invention, in relation to the color variation gained by the 5N 18K alloy according to the ISO composition used as reference sample and in relation to the color variation of a reference Gold, Silver and Copper red ternary 9K alloy (LRS 362, defined hereinafter in Tab. 6);

- Figure 3 shows a color variation chart according to the time of exposure to Thioacetamide solutions according to UNI EN ISO 4538 for part of the alloys object of the present invention, in relation to the color variation gained by the

5N 18K alloy according to the ISO composition used as reference sample and in relation to the color variation of a reference Gold, Silver and Copper red ternary 9K alloy (LRS 362, defined hereinafter in Tab. 6);

- Figure 4 shows a curve of variation of the color variation rate of various alloys object of the present invention when exposed for a determined time period to

Thioacetamide according to the content of Palladium;

- Figure 5 shows a curve of variation of the color variation rate of various alloys object of the present invention when exposed for a determined time period to NaCI aqueous solution according to the content of Palladium.

Detailed description

It is an object of the present invention a family of Gold alloys, in particular for jewellery, with Gold title substantially comprised between 8K and 10K, that have tarnishing resistance characteristics.

The alloys that are described in the present invention have been tested in terms of resistance to color variation (tarnishing) in environments comprising Thioacetamide and NaCI solutions (sodium chloride). In this description, any reference to tests carried out in an environment comprising Thioacetamide is made according to the indications of the UNI EN IS04538: 1998 standard. In order to carry out the tests, according to the present invention, the samples are exposed to vapours of Thioacetamide CH₃CSNH₂ in an atmosphere with relative humidity of 75% kept through the presence of a saturated solution of sodium acetate trihydrate CFhCOONa-SFhO in a test chamber with a capacity comprised between 2 and 20 litres and wherein all the materials used for the construction of the chamber itself are resistant to volatile sulphides and do not emit any gas or vapour capable of influencing the results of the test.

With regard to the assessment of the resistance to corrosion and color variation in environments characterized by the presence of Sodium Chloride solutions, the tests have been carried out by immersing the samples of a Gold alloy in a 50g/L NaCI solution at neutral pH, thermostated at 35°C.

In order to obtain tarnishing resistance properties, in particular in environments comprising solutions of Thioacetamide and NaCI according to the above described standards and also to obtain a Gold alloy whose color, measured according to the ISO DIS 8654:2017 standard is contained in the color space defined as“dark red”, the Applicant has conceived a family of Gold alloys comprising:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight;

More in particular, the alloy object of the invention may consist of an alloy, in particular for jewellery, characterised by the fact that it consists of:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight;

- optionally, at least one among: Iron in the amount comprised between 2%o in weight and 25%o in weight, Silver in the amount lower than or equal to 100%o in weight, Zinc in the amount lower than or equal to 100%o in weight, Indium in the amount lower than or equal to 20%o in weight, Tin in the amount lower than or equal to 20%o in weight, Gallium in the amount lower than or equal to 10%o in weight, Iridium in the amount lower than or equal to 0.5%o in weight, Ruthenium in the amount lower than or equal to 0.5%o in weight, Rhenium in the amount lower than or equal to 0.1 %o in weight.

According to the present invention, with“tarnishing” is intended a surface corrosion of the Gold alloy that causes a variation in the alloy color.

The family of Gold alloys object of the invention comprises at least ternary alloys, and more in particular ternary or quaternary alloys. Therefore, the number of elements that are included in a not insignificant amount in the family of Gold alloys object of the invention is at least equal to 3 and, preferably, not higher than 4, although quinary formulations may still be possible and include elements not included in the preceding table.

The Applicant has carried out various experiments for assessing the resistance to tarnishing and the so obtained color of the alloys, and in particular has carried out experiments on the specific embodiments indicated in the following table:

Table 5

In the above table, as in the other tables of the present description, where the boxes are empty, it is intended a zero percentage of the relevant element.

The alloys according to the preceding formulations are preferred and non- limiting examples of Gold alloy for jewellery with a title equal to or substantially equal to 9K. The tarnishing resistance tests carried out by the Applicant and reported below have been carried out with respect to a reference alloy, in particular a reference ternary alloy comprising Gold, Copper and Silver, whose composition is the following.

Table 6

The LRS 362 alloy used as reference test does not have a“dark red” color, but a significantly different color, which lies within the tolerances for alloys whose color is compatible with 6N ISO.

The alloys according to the family generally described as the object of the invention and, consequently, the specific realizations described in table 5 are realizations characterized by the absence of Silver. Silver is known to be an element that in these alloys helps to increase the hardness. As an example, the 362 alloy used as reference test, in the absence of work hardening, has a hardness according to HV5 equal to 139, which becomes equal to 185 with 25% work hardening, equal to 210 with 50% work hardening and equal to 245 with 75% work hardening. In particular, for the alloys listed in Table 5, it has been searched an alternative solution to Silver, which could also help to keep the hardness of the specific formulation of alloy suitable for jewellery applications, such as to make the formulation "for jewellery" as defined in the preceding definition, avoiding the trend to tarnishing that Silver helps to bring for alloys where it is contained.

The following table shows the hardnesses obtained for the specific alloy formulations according to the present invention, as well as for the LRS 362 alloy used as reference test.

Table 7

The Applicant has observed that the specific formulations of Gold alloy described in table 7 have hardness, even under conditions of non-work hardening, compatible with that of alloys for jewellery in accordance with the present invention. In particular, the LRS 496 and LRS 498 formulations are respectively the closest to the behavior in terms of hardness possessed by the LRS362 alloy used as reference test (whose alignment in the table is offset for ease of identification), and even better (LRS 498) with respect to the alloy used as reference test. This shows that in order to obtain hardnesses that are also optimal for the processing of jewels that require considerable resistance in terms of hardness, equal to or substantially equal to the hardness of a ternary Gold-Copper-Silver alloy, that however - due to the presence of a significant amount of Silver - is strongly subject to tarnishing - it is possible to introduce into the alloy a quantity substantially equal to at least 30%o of Palladium and Iron, in which Iron is contained in an amount of at least 10%o. In particular, the Applicant has highlighted that in a 9K quaternary Gold-Copper-Palladium and Iron alloy, having a quantity substantially equal to 40%o of Palladium and Iron, and in particular having Palladium and Iron in amounts respectively comprised, each one, between 1 8%o and 22%o, and even more in particular each one in amounts equal to 20%o allows to obtain alloys whose hardness is substantially higher than the LRS362 reference alloy, at least up to a level of hardening equal to 50%.

The Applicant has observed as it will be better described hereinafter that all the alloys object of the invention and belonging to the above mentioned family, and in particular but non-limiting to the alloys object of table 5 have better performances in Air, Thioacetamide and NaCI solution under the above mentioned conditions, in terms of resistance to color variation, with respect to what can be obtained with the LRS 362 alloy used as reference test. The graphs in Figure 2 and Figure 3 respectively illustrate the evolution of the color of some specific alloy formulations as the number of hours of exposure to NaCI saline solution and Thioacetamide increases, as for the above indicated specifications.

All the alloys according to the general formulation, and in particular all the specific realizations of the alloys according to table 5 have a better behaviour, both in NaCI solution and in Thioacetamide, with respect to the LRS362 alloy used as reference test. This shows that the elimination of Silver, among other things, is beneficial for reducing the trend of the alloy to turn color when exposed to chemically aggressive environment. In fact, although the alloy used as a reference test has a color that is not compatible with that of the alloys object of the invention, the latter have a significantly lower trend to change color.

In particular, the Applicant has observed that the optimization of the alloy behavior in terms of tarnishing is optimized - in the ranges identified in the preceding family, for Palladium values comprised within the following range: [8%o - 32%o] in weight, and even more preferably for Palladium values within the range of [1 0%o - 30%o] in weight.

From the tests carried out in NaCI saline solution as above indicated, the Applicant has surprisingly verified that the optimization of the resistance to tarnishing is obtained, for the family as generally above expressed, for Palladium contents in the amount comprised between 1 5%o in weight and 25%o in weight, more in particular between 1 8%o in weight and 22%o in weight, and in particular for Palladium contents substantially equal to 20%o in weight. In particular, for alloys in which the Gold content is comprised between 365%o and 385%o in weight, and in which Copper content is comprised between 594%o and 61 4%o in weight, and in which Palladium content is comprised between 1 5%o in weight and 25%o in weight, and more in particular between 18%o in weight and 22%o in weight, the Applicant has observed a substantial optimal behavior of performances in NaCI, in particular when Iron content in the alloy is lower than 20%o and in particular when Iron content is lower than or equal to 10%o. In particular, as shown in the graph in Figure 2, alloys according to the LRS 496 and 497 formulation, respectively with an Iron content equal to 10%o in weight and zero, have an optimized behavior. Surprisingly, the Applicant was able to conceive alloys that, with the above mentioned formulations and in particular with Palladium comprised between 18%o and 22%o and Iron in the amount lower than or equal to 10%o, the performances of a 9K alloy are substantially close to that of an alloy according to the 5N ISO standard, which has a significantly higher Gold title, because at 18K. Therefore, it was almost possible to achieve, in terms of tarnishing, the behavior of an 18K alloy with an alloy whose Gold content is significantly lower, being 9K.

The following table describes the color variation characteristics DE (L* a* b*) according to the exposure time in NaCI saline solution.

Table 8

From the above indicated table and from the graph of figure 2 it is possible to observe that the specific LRS 479 and 480 formulations have a worse behavior in NaCI than the LRS362 alloy used as reference test. On the other hand, in Thioacetamide according to the above mentioned conditions, the behaviour of the alloys according to the above mentioned general formulation, and in particular all the alloys according to the above mentioned specific formulations have a better behaviour than the one of the LRS362 alloy used as reference test, even if they cannot match the performances of the 5N ISO alloy in the same environment due to the lower Gold content.

The following table describes the color variation characteristics DE (L* a* b*) according to the exposure time in Thioacetamide.

Table 9

As it can be seen from the reading of Table 9 in conjunction with Table 5 and the graphs in Figures 3, 4 and 5, the Applicant has surprisingly discovered that the behaviour of ternary or quaternary Gold-Copper-Palladium or Gold-Copper- Palladium and Iron 9K Gold alloys is strongly optimized for Palladium values substantially comprised between 18%o and 22%o, with an optimization peak (lower DE values, and minimum of the curve in the graph of Figures 4 and 5) for an amount of Palladium equal to 20%o, both in Thioacetamide (Figure 4) and in NaCI (Figure 5).

In order to reduce the risk that the inclusion of third-party elements in the preceding alloy family may lead to unpredictable behaviors, the Applicant has observed that the alloy must be preferably ternary or quaternary, with the necessary presence of Palladium and with the sum of the amounts of Gold, Copper and Palladium at least equal to 900%o in weight. The remaining 100%o in weight can be of different materials, including Silver or Zinc or combinations of Silver or Zinc in order to vary at least the specific color of the alloy while remaining within the color previously defined as“dark red”.

Table 10

From the preceding Table 10, it is clear that the LRS 362 alloy used as reference test is the only one with a significantly different color with respect to the others, and that it is not“dark red” according to the present invention, since it falls within the tolerance limits defined for alloys whose color complies with the 6N ISO standard.

In particular, all the embodiments specifically identified in Table 5 are specific optimized embodiments taken from a subfamily of alloys in which the sum of the amounts of Gold, Copper and Palladium is at least equal to 960%o in weight and, even more preferably 970%o in weight; alloys with this last characteristic have a substantially assimilable behavior in the complex, observing the color variations in air, in Thioacetamide and in NaCI solution.

Without prejudice to the exclusion of unintended impurities, alloys according to the invention can comprise additional materials in total amount, i.e. in sum, not higher than 2%o and more preferably not higher than 1%_<>; the list of said additional materials comprises Iridium, Ruthenium, Indium and Rhenium. These materials can have, under certain conditions better explained hereinafter, grain refining properties.

In particular, Iridium is preferably used in alloys containing high Copper contents, because it binds in particular with the latter element; preferably, but non- limiting thereto, if present, Iridium is present in an amount equal to or lower than 0.5%o in weight. Rarer is the use of Ruthenium and Rhenium, in a sometimes lower amount, but anyway up to 0.5 %o in weight. Ruthenium and Rhenium are used in Gold alloys containing Palladium or Zinc in the amount lower or equal to 100%o. However, it is noted that the use of Iridium and/or Rhenium and/or Ruthenium is subject to the inclusion of these elements in pre-alloys. In fact, it has been observed that these elements, if not pre-alloyed with the material with affinity thereto, but directly introduced into the pot, do not for alloy, thus contributing to a worsening of the characteristics of the alloy. On the other hand, only if used in pre- alloy with Copper (Iridium) or Palladium (Rhenium and Ruthenium), taking care to make the pre-alloy bind with the rest of the elements composing the alloy itself, it is possible to refine the grain.

With the purpose of reducing the risk of formation of carbides during the processing, the Gold alloy for jewellery is preferably characterized by the absence of Vanadium and other materials capable to create carbides and oxides, in particular free from Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium, Tantalum, Zirconium, Yttrium, Germanium.

This alloy is also an alloy free of Nickel, Cobalt, Arsenic and Cadmium. Thanks to this aspect, the alloy is a Gold alloy compatible with being worn or wearable by subjects whose allergic tolerance is significantly low.

It is also object of the invention a process of production of a Gold alloy.

The Gold alloys object of the invention are made from pure elements, in particular from Gold at 99.99%, Cu at 99.99%, Pd at 99.95%, Fe at 99.99%.

The process of melting of pure elements for the creation of the Gold alloys according to the invention can be in detail a process of discontinuous melting of Gold or a process of continuous melting of Gold. The process of discontinuous melting of Gold is a process in which the mixing is melted and cast into a refractory mold or refractory or metallic ingot mould. In this case the above mentioned elements are melted and cast in a controlled atmosphere. More in particular, the melting operations are carried out only after having preferably conducted at least 3 conditioning cycles of the atmosphere of the melting chamber. This conditioning involves first of all reaching a vacuum level up to pressures lower than 1x1 O ² mbar and a subsequent partial saturation with Argon at 500mbar. During the melting, the Argon pressure is kept at pressure levels between 500mbar and 800mbar. When the complete melting of the pure elements has been reached, a phase of overheating of the mixture takes place, in which the mixture is heated up to a temperature of about 1250°C, and in any case to a temperature above 1200°C, in order to homogenize the chemical composition of the metal bath. During the overheating phase, the pressure value in the melting chamber reaches again a vacuum level lower than 1x1 O ² mbar, useful to eliminate part of the residues produced by the melting of the pure elements.

At this point, in a casting step, the melted material is casted into a mould or ingot mould realized in graphite and the melting chamber is again pressurized with an inert gas, preferably argon, injected at a pressure lower than 800mbar and in particular lower than 700mbar.

After solidification, the bars or casts are extracted from the bracket. When the alloy is solidified, from the mold in graphite are obtained Gold bars or Gold alloy casts which are subjected to quick cooling by means of a step of immersion in water, in order to reduce and possibly avoid solid state phase transformations. In other words, the bars or casts are subjected to a quick cooling phase, preferably but non- limiting in water, in order to avoid phase variations in the solid state.

In a more general embodiment, the production process of the Gold alloy according to the invention comprises, starting from the pure elements according to the above, a mixing step of elements according to the present disclosure, and in particular:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight;

and a step of introduction of the mixture in a melting pot, and a subsequent melting through heating until melting.

The process of continuous melting is a process in which solidification and extraction of the solidified Gold are continuously carried out from one free end of a Gold bar or cast. In particular, a graphite die is used in the continuous melting process. The use of graphite dies is known, since graphite is a solid lubricant, and typically has low friction between its surfaces and those of the solidified metal, permitting to obtain an easy extraction of the element contained therein without fractures and with the minimum amount of defects present on its surface.

When the inclusion of elements such as Iridium, Indium, Ruthenium and

Rhenium is present for grain refinement, the production process comprises a step of realizing of a pre-alloy, in which said pre-alloy comprises:

a) Iridium pre-alloyed to Copper in the already indicated amounts, or alternatively b) Rhenium or Ruthenium pre-alloyed to Palladium in the already indicated amounts.

Subsequently, the bars or casts obtained by discontinuous or continuous melting are subject to a step of cold plastic deformation, preferably but non-limiting to flat rolling.

During the flat rolling and more generally during the cold plastic processing steps, the different compositions synthesized according to the previously described melting procedure are deformed by more than 70% and then subjected to a thermal treatment of solubilization at a temperature higher than 680°C, in order to be subsequently cooled.

Particular embodiments of the previously described method include an initial step in which are mixed in particular Palladium in the amount comprised between 8%o, more preferably 10%o, and 32%o, more preferably 30%o in weight and/or, in addition to the preceding elements, also Iron comprised between 2%o in weight and 25%o in weight, more preferably between 5%o in weight and 20%o in weight.

In a particular embodiment of the method of realization of the Gold alloy, the sum of the amounts of Gold, Copper and Palladium is at least equal to 900%o in weight, and more in particular in the amount at least equal to 960 %o in weight, more preferably 970 %o in weight.

In case the sum of the amounts of Gold, Copper and Palladium is at least equal to 900 %o, it is possible to carry out a step of addition of Silver in the amount lower than 100%o in weight, and/or Zinc in the amount lower than 100%o in weight or Silver and Zinc of which the sum of respective amounts in weight is lower than 100%o .

In a particular embodiment of the method, it is possible to mix Palladium in the amount comprised between 15%o in weight and 25%o in weight, more preferably between 18%o in weight and 22%o in weight, more preferably substantially equal to 20%o in weight, in such a way that, for example, the sum of the amounts of Palladium and Iron is equal to 30%o in weight.

In a particular embodiment of the method of production of the Gold alloy object of the invention, the Gold is mixed in particular in the amount comprised between 365%o in weight and 385%o in weight and Copper is present between 594%o in weight and 614%o in weight and Palladium is comprised between 15%o and 25%o in weight, more preferably between 18%o and 22%o in weight. The advantages offered by the alloy object of the invention are clear in the light of the above obtained description. The alloys are characterized by a low trend to tarnishing for the environments in which an item of jewellery is typically found to be used, consequently it allows to realize items of jewellery or parts of items for jewellery resistant to tarnishing, in substantially red color as above defined, without the need for subsequent plating with high title Gold alloys. Consequently, the item of jewellery thus created is less expensive and less demanding to be processed as well as characterized by a substantially more uniform color even after wear.

The alloys according to the present disclosure are alloys without secondary phases. In detail, the alloys according to the present disclosure are homogeneous Gold alloys, free from second phases, and in particular free from carbides and/or oxides and/or are crystalline alloys, in particular 100% crystalline. This permits to have a high strength and quality and surface uniformity. As“free from secondary phases” or“free from second phases” is intended an alloy free from elements that can generate them, in particular in a process of melting and subsequent solidification without other thermal treatments; second phases that create in the liquid phase and remain downstream of the alloy solidification, are harmful second phases, for example carbides and/or oxides that during the polishing step are visible at naked eye on the surface of the polished item, and that then prevent to obtain items with high surface quality, compatible with the needs required in the high jewellery field.

In the here described process of production of the gold alloy, it is possible to expose the alloy to thermal treatment processes, able to give it a hardening, so that due to precipitation can be present subtle precipitates, results of said thermal treatment; in this case these are precipitates that prevent from the movement of displacement by increasing the mechanical properties in the material, and withstand the incidence of deformations in the items realized with the present alloys.

Finally, it is clear that the object of the present invention may be subject to modifications, additions or variants, which are obvious to an expert in the art, without thereby falling outside of the scope of protection provided by the attached claims.

Claims

1. An alloy, in particular for jewellery, characterized in that it comprises:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight; - Palladium, in the amount comprised between 5%o and 35%o in weight;

2. Alloy according to claim 1 , characterized in that it has a dark red color and/or in that it comprises a color that, on the CIELAB1976 color chart, has a coordinate a* > 8.2 and a coordinate b*<13.55.

3. Alloy according to any of the preceding claims, comprising Palladium in the amount comprised between 8%o, more preferably 10%o in weight, and 32%o, more preferably 30%o in weight.

4. Alloy according to any of the preceding claims, characterized in that it is ternary or quaternary, and wherein the sum of the amounts of Gold, Copper and Palladium is at least equal to 900%o in weight, more preferably at least equal to 960 %o in weight, even more preferably 970 %o in weight.

5. Alloy according to claim 4, comprising also Silver in the amount lower than

100%o in weight, and/or Zinc in the amount lower than 100%o in weight or Silver and Zinc the sum of respective amounts in weight is lower than 100%o.

6. Alloy according to any of the preceding claims, comprising Palladium in the amount comprised between 15%o in weight and 25%o in weight, more preferably between 18%o in weight and 22%o in weight, more preferably in the amount substantially equal to 20%o in weight.

7. Alloy according to any of the preceding claims, comprising also Iron in the amount comprised between 2%o in weight and 25%o in weight, more preferably comprised between 5%o in weight and 20%o in weight.

8. Alloy according claim 7, wherein the sum of the amounts of Palladium and Iron is equal to 30%o in weight.

9. Alloy according to any of the preceding claims, wherein Gold is present in the amount comprised between 365%o in weight and 385%o in weight and Copper is present between 594%o in weight and 614%o in weight and Palladium is comprised between 15%o and 25%o in weight, more in particular between 18%o and 22%o in weight.

10. Alloy according to claim 9 when dependent on claims 1 -6, characterized in that is free from Iron.

11. Alloy according to any of the preceding claims, characterized by the absence of Vanadium, and other metals capable to create carbides and oxides, in particular free from Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium, Tantalum, Zirconium, Yttrium, Germanium, said alloy being also free from Nickel, Cobalt, Arsenic and Cadmium.

12. An alloy, in particular for jewellery, characterized in that it consists of:

- Gold, in the amount comprised between 330%o and 420%o in weight;

- Copper, in the amount comprised between 500%o and 620%o in weight;

- Palladium, in the amount comprised between 5%o and 35%o in weight;

and, optionally, at least one among: Iron in the amount comprised between 2%o in weight and 25%o in weight, Silver in the amount lower than or equal to 100%o in weight, Zinc in the amount lower than or equal to 100%o in weight, Indium in the amount lower than or equal to 20%o in weight, Tin in the amount lower than or equal to 20%o in weight, Gallium in the amount lower than or equal to 10%o in weight, Iridium in the amount lower than or equal to 0.5%o in weight, or Ruthenium in the amount lower than or equal to 0.5%o in weight, Rhenium in the amount lower than or equal to 0.1 %o in weight.

13. A method of production of a Gold alloy for jewellery; said method being characterised in that it comprises:

a) a step of mixing of a homogenization of:

- Gold, in the amount comprised between 330%o and 420%o in weight;

for creating a mixture; and

14. Method according to claim 13, including also a mixing or homogenization step of at least one among: Iron comprised in the amount between 2%o in weight and 25%o in weight, Silver in the amount lower than or equal to 100%o in weight, Zinc in the amount lower than or equal to 100%o in weight, Indium in the amount lower than or equal to 20%o in weight, Tin in the amount lower than or equal to 20%o in weight, Gallium in the amount lower than or equal to 10%o in weight, Iridium in the amount lower than or equal to 0.5%o in weight, or Ruthenium in the amount lower than or equal to 0.5%o in weight, Rhenium in the amount lower than or equal to 0.1 %o in weight for creating said mixture.

15. Method according to claim 13 or claim 14, comprising also a step of mixing of Palladium in the amount comprised between 8%o, more preferably 10%o in weight, and 32%o, more preferably 30%o in weight.

16. Method according to one or more of the preceding claims 14 to 15, comprising also a step of mixing or homogenization of Iron in the amount comprised between 2%o in weight and 25%o in weight, more preferably comprised between 5%o in weight and 20%o in weight for creating said mixture.

17. Method according to one or more of the preceding claims 13 to 16, wherein the sum of the amounts of Gold, Copper and Palladium is at least equal to 900%o in weight.

18. Method according to claim 17, wherein the step of mixing or homogenization comprises the mixing of Gold, Copper and Palladium in the amount at least equal to 960%o in weight, more preferably 970%o in weight.

19. Method according to claim 17 or claim 18, wherein it is present a step of mixing of Silver in the amount lower than 100%o in weight, and/or Zinc in the amount lower than 100%o in weight or Silver and Zinc the sum of respective amounts in weight is lower than 100%o .

20. Method according to one or more of the claims 13-19, wherein said step of mixing or homogenization comprises the mixing of Palladium in the amount comprised between 15%o in weight and 25%o in weight, more preferably between 18%o in weight and 22%o in weight, more preferably substantially equal to 20%o in weight.

21. Method according to one or more of the preceding claims 13-16, when depending on claim 16, wherein the sum of the amounts of Palladium and Iron is equal to 30%o in weight.

22. Method according to one or more of the preceding claims 17-21 , wherein Gold is mixed in particular in the amount comprised between 365%o in weight and 385%o in weight and Copper is present between 594%o in weight and 614%o in weight and Palladium is comprised between 15%o and 25%o in weight, more preferably between 18%o in weight and 22%o in weight.

23. Method according to one or more of the preceding claims 17-22, wherein, said homogenization is a discontinuous melting, comprising a step of casting wherein the melted material is casted in a refractory mold or refractory or metallic ingot and wherein said melted alloy is an alloy characterized by the absence of Vanadium, and other elements capable to create carbides or oxides, in particular free from Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium,

Tantalum, Zirconium, Yttrium, Germanium.

24. Method according to claim 23, wherein the melting pot is subject to a gas controlled atmosphere and in particular is subject, at least temporarily, to vacuum condition.

25. Method according to claim 23 or 24, wherein during said casting step, said melting pot is subject to a controlled atmosphere, to pressures lower than the environmental one and wherein said controlled atmosphere is or comprises an inert gas, preferably argon and/or said pressure is a pressure lower than 800mbar, preferably lower than 700mbar and/or wherein said gas is a reducing gas, preferably a hydrogen-nitrogen mixture and/or said pressure is a pressure lower than 800m bar, preferably lower than 700mbar.

26. Method according to one or more of the claims 17 to 22, wherein said melting is a continuous melting, comprising a step of melting and homogenization in a graphite pot and a subsequent casting step wherein the melted alloy is casted in a mold realized in graphite and wherein said alloy is an alloy of metals chemically not similar to the graphite and more specifically, in particular at least free from Vanadium, Magnesium, Silicon, Titanium, Tungsten, Molybdenum, Niobium, Tantalum, Zirconium, Yttrium, Germanium.

27. Method according to claim 26, wherein after the continuous or discontinuous melting said alloy is subject to a cooling step followed by one or more hot or cold plastic deformation steps and one or more thermal treatments.

28. Item of jewellery, comprising a Gold alloy according to one or more of the preceding claims 1 to 12, wherein said item of jewellery comprises a jewel or watch or bracelet for watch or a movement or part of a mechanical movement for watch.