METHOD FOR MAKING A BLACK SELF-HEALING PART HAVING A PRECIOUS METAL SUBSTRATE, AND PART THUS OBTAINED

Abstract

Disclosed is a method for making a part including a substrate made of a precious metal, including: a step of providing the substrate made of an alloy including between 94% and 99.5% of copper (Cu), between 0.5% and 6% of gold (Au), and between 0% and 4% of at least one added element, the substrate having a surface, called upper surface, and having a thickness of at least 100 μm starting from the upper surface; and a step of preparing a patina bath from a mixture including at least: between 3 g and 10 g of anhydrous or hydrated copper acetate (Cu(CH3COO)2), between 3 g and 20 g of anhydrous or hydrated copper sulphate (CuSO4), and between 0.5 g and 2 g of salt (NaCl); then a step of applying the patina bath over the upper surface of the substrate. A part obtained by such a method.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to FR Patent Application No. 2207267 filed Jul. 13, 2022, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a method for making a black and self-healing trim part having a substrate made of a precious metal.

It also relates to a part obtained with such a method.

Description of the Related Art

Alloys with a black appearance have appeared in about 2000 BC.

Black bronzes with gold date back to the time of the ancient Egyptians (1000 BC) then to the time of Romans where there have been Corinth bronze (although this designation refer to different alloys), or bronzes with gold, which has the particular advantage of being both black, thanks to a patina, and semi-previous by the presence of gold.

The term “black bronzes” is then found in other civilisations (Roman, Saxon, Chinese (Wu-tong), as well as in Japan, in the Middle Ages.

An alloy of this type is still present nowadays in a Japanese alloy called “Shakudo”.

To form a black surface over this alloy, there is a technique for so-called “Niiro” coatings.

Nonetheless, the “Niiro” coatings have a “purple-like” black. Moreover, they are conventionally made using a Japanese radish (for example daikon) to prepare the surface and with an addition of Japanese traditional plum vinegar (“plum vinegar”) into the patina solution which is hardly compatible with a production on an industrial scale.

It is also possible to form a black coating, for example by a dry deposition process such as PVD (“physical vapour deposition”) or DLC (“diamond like carbon”), or by application of a lacquer.

However, such a coating generally generates an adherence problem, in particular on a relatively ductile substrate, which results in a scaling, and/or a delamination of the coating, in particular in the event of an impact.

Furthermore, in case of scratching, it is very difficult, and even impossible, to repair the damaged surface, especially after application of a lacquer.

SUMMARY OF THE INVENTION

An objective is to provide an alloy with a black appearance, at least at the surface, and possibly a deep black, overcoming at least part of the aforementioned drawbacks.

To this end, a method is provided for making a part including a substrate made of a precious metal, the method including:

A step of providing the substrate made of an alloy including (the percentages should be understood in weight percent, also denoted wt. %):

• • a. Between 94% and 99.5% of copper (Cu),
  • b. Between 0.5% and 6% of gold (Au),
  • c. Between 0% and 4% of at least one added element,
  • i. The total amounting to 100%;
  • ii. The substrate having a surface, called upper surface, and having a thickness of at least 100 μm starting from the upper surface;

A step of preparing a patina bath from a mixture including, in proportions, at least:

• • a. Between 3 g and 10 g of anhydrous or hydrated copper acetate (Cu(CH₃COO)₂),
  • b. Between 3 g and 20 g of anhydrous or hydrated copper sulphate (CuSO₄), and
  • c. Between 0.5 g and 2 g of salt (NaCl); then

A step of applying the patina bath over the upper surface of the substrate.

A precious metal herein refers to an alloy including at least one amongst gold, silver or platinum. In this case, it consists more particularly of a gold-based alloy.

Thus, such a method allows making a black trim part having self-healing properties.

At the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterised by colourimetry in the L*a*b* CIE 1976 colour space with a “L” value comprised between 10 and 50, more particularly between 30 and 50, an “a” value comprised between −1.5 and 1; and a “b” value comprised between −8 and 1.

It has been found that the colours rendered by a patina range from black “brown” (when a gold concentration in the substrate tends towards about 0.5 wt. % (Au)) tends to black “blue/black” (when a gold concentration of the substrate tends towards about 5-6 wt. % (Au)).

However, the higher the gold concentration, the “deeper” the black will appears, yet, it has been found that the higher the gold concentration, for example higher gold concentrations, for example 7.5%, the more the patina is difficult to obtain and loses density.

The concentrations may be measured by any method.

According to a preferred example, the gold concentration is measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), for example after dissolving in aqua regia (i.e. a nitric acid and hydrochloric acid mixture, for example respectively in a 1:3 proportion), or by gravimetry (i.e. by attack of an alloy with nitric acid and weighting the insoluble substances which are recovered on a filter which is weighted).

Depending on the measurement method that is used, the measured concentration values may vary up to 10% for example, and even sometimes more.

According to one example of implementation, the method includes a step of forming the alloy of the substrate.

For example, the step of forming the alloy of the substrate includes a step of mixing, in proportions, at least 45 g to 55 g, for example 50 g, of gold (Au) with between 800 g and 1,200 g, for example 1,000 g, of pure copper (Cu).

For a particular example of implementation, the mixing step is configured to produce an alloy including between 4.5% and 4.8%, for example 4.76%, of gold (by weight). For example, the obtained gold concentration is 4.57 wt. % when it is measured by ICP-AES and 4.66% when it is measured by gravimetry. These differences between the targeted theoretical value (i.e. 4.76%) and the measured values are due to measurement uncertainties related to each measurement method, as indicated hereinabove.

According to one example of implementation, the method includes a step of forming the substrate over a base.

For example, the base is made of stainless steel, aluminium, bronze or brass or other, with any desired geometry or dimension.

According to one example of implementation, the step of forming the substrate includes a step of galvanic deposition or projection of the alloy on the base, in particular by techniques like Cold Spray or Arc Spray.

For example, this step is configured to form a thickness of the substrate of at least 100 μm.

For example, the proportions given for the step of preparing a patina bath are for one litter of water, in particular distilled or permuted water.

According to one example of implementation, the step of preparing a patina bath includes, in proportions:

• • A step of heating a litter of distilled or permuted water up to a temperature comprised between 80° C. and 100° C.;
  • A step of adding 3 g to 10 g of anhydrous or hydrated copper acetate, for example 4.82 g of anhydrous copper acetate (Cu(CH₃COO)₂) or 5.3 g of monohydrated copper acetate (Cu(CH₃COO)₂, H₂O), and
  • 3 g to 10 g of anhydrous or hydrated copper sulphate, for example 3.19 g of anhydrous copper sulphate (CuSO₄) or 5 g of pentahydrated copper sulphate (CuSO₄, 5H₂O), in the heated water, producing a first mixture;
  • A step of stirring for a time period comprised between 30 minutes and 90 minutes of the first mixture; then
  • A step of adding 0.5 g to 2 g, for example 1 g, of salt (NaCl), in the first mixture;
  • A step of adding 3 g to 10 g of anhydrous or hydrated copper sulphate, for example 3.19 g of anhydrous copper sulphate (CuSO₄) or 5 g of pentahydrated copper sulphate (CuSO₄, 5H₂O) in the first mixture, producing a second mixture; then
  • A step of stirring the second mixture for a time period comprised between 30 minutes and 90 minutes, producing the patina bath.

According to one example of implementation, the method includes, prior to the step of applying the patina bath, a step of pickling, also called stripping, the upper surface of the substrate.

For example, the pickling step is configured to pull off a possible oxide layer at the surface of the substrate.

For example, the pickling step is carried out with a sulphuric acid solution (H₂SO₄) at a concentration comprised between 0.5% and 10% diluted in water (H₂O).

Before the pickling step, the method may optionally include a degreasing step, for example with a solvent, or with ultrasounds, for example in the presence of a surfactant.

According to one example of implementation, the method includes a step of rinsing the substrate, for example rinsing with water; this step is for example implemented after the pickling step.

According to one example of implementation, the step of applying the patina bath includes a step of immersing the substrate into the patina bath.

For example, the immersion step has a duration comprised between 45 minutes and 120 minutes, for example 60 minutes.

For example, the patina bath has a temperature comprised between 80° C. and 100° C.

Once the patina is obtained, the method may include a drying step, for example in the open air, and/or in the oven.

Once the patina is dried, the method may possibly include a step of polishing the patina.

According to an interesting option, the method may also include a finishing step.

For example, the finishing step may include a step of applying a non-volatile organic substance, for example a protection, for example an organic material, like for example acacia gum, a wax, a resin or other.

By impregnating the patina, this non-volatile organic substance allows switching from a patina with a generally anthracite appearance into a deeper black. Thus, the finish contributes to conferring a deep black appearance on the part.

This deeper black obtained after finishing may be characterised by

colourimetry in the L*a*b* CIE 1976 colour space with a “L” value comprised between 10 and 40, an “a” value comprised between −1.5 and 1; and a “b” value comprised between −4 and 1.

For example, the finishing step is also configured to protect the patina and/or confer a shiny, satin or matt finish thereon.

For example, the step of applying the finish may be carried out by soaking, spraying, buffering, application with a cloth or with a brush.

According to another aspect of the invention, a part obtained by a method as described before is also provided

According to one embodiment, the part includes:

At least one substrate made of an alloy including at least:

• • a. Between 94% and 99.5% of copper (Cu),
  • b. Between 0.5% and 6% of gold (Au),
  • c. Between 0% and 4% of at least one added element,
  • i. The total amounting to 100%;
  • ii. The substrate having a surface, called upper surface, and having a thickness of at least 100 μm starting from the upper surface; and
  • And a patina applied over the upper surface of the substrate, the patina having a thickness of at least 5 μm, for example comprised between 5 and 30 μm, and the patina including at least:
  • d. a copper oxide, for example in the form of cuprite (Cu₂O), and
  • e. gold (Au).

According to one embodiment, the added element of the substrate has a concentration comprised between 0% and 1%, and possibly between 0% and 0.01%.

According to one embodiment, the added element of the substrate

includes at least one amongst: silver (Ag); arsenic (As), zinc (Zn), lead (Pb), iron (Fe), or tin (Sn).

Where appropriate, the silver concentration is for example lower than 1%, for example comprised between 0% and 1%.

According to one embodiment, the alloy of the substrate includes 95.24% of copper (Cu) and 4.76% of gold (Au), in particular when the gold concentration is measured by ICP-AES. Such an alloy is then free of any added element (which corresponds to a concentration of 0%).

According to one embodiment, the patina further includes at least one element in the form of traces, for example at a concentration comprised between 0% and 0.01%.

For example, the element in the form of traces includes at least one amongst: carbon, oxygen, chlorine, potassium, calcium, or iron.

According to one embodiment, the patina includes a gold concentration comprised between 5% and 15%, for example 13%.

For example, the gold concentration in the patina is measured by EDX.

Thus, the part according to an example of implementation features a “deep black” appearance.

At the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterised by colourimetry in the L*a*b* CIE 1976 colour space with a “L” value comprised between 10 and 50, more particularly between 30 and 50, an “a” value comprised between −1.5 and 1; and a “b” value comprised between −8 and 1.

If the part includes a finish, this black may be characterised in the L*a*b* CIE 1976 colour space with a “L” value comprised more particularly between about 10 and 40, an “a” value comprised between about −1.5 and 1; and a “b” value comprised between about −4 and 1.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, according to one embodiment, will be better understood and its advantages will appear better upon reading the following detailed description, given for an indicative and non-limiting purpose, with reference to the appended drawings wherein:

FIG. 1 shows a substrate according to an embodiment of the invention;

FIG. 2 illustrates a step of immersing the substrate of FIG. 1 into a patina bath, according to an example of implementation of the invention;

FIG. 3 shows a part according to an embodiment of the invention;

FIG. 4 illustrates a self-healing property of the part of FIG. 3;

FIG. 5 shows an example of a spectrum made by SEM-EDX on the patina of the part of FIG. 3; and

FIG. 6 shows an example of a Raman spectrum made on the patina of the part of FIG. 3.

DETAILED DESCRIPTION

To make a part 30 made of a precious alloy, such as a piece of jewellery, with a black appearance, and having a self-healing property (also called “self-healing capability”), a method for making such a part 30, according to an example implementation of the invention, includes on the one hand a step of providing a substrate 10 made of an alloy including:

• • Between 94% and 99.5% of copper (Cu),
  • Between 0.5% and 6% of gold (Au), and
  • Between 0% and 4% of at least one added element,

The total amounts to 100%.

The percentages herein refer to weight percentages, also denoted “wt. %”.

The gold concentration may be measured by any method; preferably, it is measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), for example after dissolving in aqua regia (i.e. a nitric acid and hydrochloric acid mixture, for example respectively in a 1:3 proportion), or by gravimetry (i.e. by attack of an alloy with nitric acid and weighting the insoluble substances which are recovered on a filter which is weighted; in this case, only gold is insoluble).

Depending on the measurement method that is used, the measured concentration values may vary up to 10% for example, and even sometimes more.

For example, to obtain such an alloy, the method may include a step of forming the alloy of the substrate including a step of mixing at least 45 g to 55 g, for example 50 g, of gold (Au) with between 800 g and 1,200 g, for example 1,000 g, of pure copper (Cu).

For a particular example of implementation, the mixing step is for example configured to produce an alloy including between 4.5% and 4.8%, for example 4.76%, of gold (by weight). For example, the obtained gold concentration is 4.57 wt. % when it is measured by ICP-AES and 4.66% when it is measured by gravimetry. These differences between the targeted theoretical value (i.e. 4.76%) and the measured values are due to measurement uncertainties related to each measurement method, as indicated hereinabove.

In the context of the description of a particular example of implementation of the invention, the substrate 10 is formed by an alloy including 95.24% of copper (Cu) and 4.76% of gold (Au).

In this particular example, the alloy is free of any added element; but it could nevertheless contain at least one, preferably according to a concentration comprised between 0% and 4%.

For example, such a substrate 10 is shown in FIG. 1. For example, it consists of a substrate configured to form a ring.

The substrate having a surface 11, called upper surface 11, over which a patina 20, described hereinafter, is subsequently applied.

Preferably, the substrate has a thickness of at least 100 μm starting from the upper surface 11.

A thickness of at least 100 μm of the substrate allows best guaranteeing enough depth for anchorage of the patina 20.

The substrate 10 is herein formed by a curved framework, for example a curbed wire with a circular or truncated circular section, herein having a diameter 12 of about 3 mm.

The substrate is herein a solid element.

Nonetheless, the substrate could be a coating layer (made of the aforementioned copper-gold alloy), at least 100 μm thick, applied over a base made of another material, for example made of stainless steel, aluminium, bronze or brass or other, with any desired geometry or dimension.

For example, the method may include a step of forming the substrate including a step of galvanic deposition or projection of the alloy on the base, in particular by techniques like Cold Spray or Arc Spray, of the alloy as described before, this step being configured to form a substrate thickness of at least 100 μm.

To make the part 30 in a precious alloy, the method according to an example of implementation of the invention also includes a step of preparing a patina bath 21.

For example, the patina bath 21 is obtained from a mixture including:

• • Between 3 g and 10 g of anhydrous or hydrated copper acetate (Cu(CH₃COO)₂),
  • Between 3 g and 20 g of anhydrous or hydrated copper sulphate (CuSO₄), and
  • Between 0.5 g and 2 g of salt (NaCl).

For this purpose, the step of preparing a patina bath 21 includes for example:

• • A step of heating a litter of distilled or permuted water up to a temperature comprised between 80° C. and 100° C.;
  • A step of adding 3 g to 10 g of anhydrous or hydrated copper acetate, for example 4.82 g of anhydrous copper acetate (Cu(CH₃COO)₂) or 5.3 g of monohydrated copper acetate (Cu(CH₃COO)₂, H₂O), and
  • 3 g to 10 g of anhydrous or hydrated copper sulphate, for example 3.19 g of anhydrous copper sulphate (CuSO₄) or 5 g of pentahydrated copper sulphate (CuSO₄, 5H₂O), in the heated water, producing a first mixture;
  • A step of stirring for a time period comprised between 30 minutes and 90 minutes of the first mixture; then
  • A step of adding 0.5 g to 2 g, for example 1 g, of salt (NaCl), in the first mixture;
  • A step of adding 3 g to 10 g of anhydrous or hydrated copper sulphate, for example 3.19 g of anhydrous copper sulphate (CuSO₄) or 5 g of pentahydrated copper sulphate (CuSO₄, 5H₂O) in the first mixture, producing a second mixture; then
  • A step of stirring the second mixture for a time period comprised between 30 minutes and 90 minutes, producing the patina bath 21.

For the present example of implementation, the patina bath 21 includes more particularly:

• • 10 g of pentahydrated copper sulphate,
  • 5.3 g of monohydrated copper acetate,
  • 1 g of salt (NaCl), and
  • 1 L of distilled or permuted water.

For example, the distilled or permuted water is heated up to 90° C. (+/−10° C.).

Then, the copper acetate and half the copper sulphate (namely 5 g) are added into the distilled or permuted water.

This first mixture is stirred for one hour, then one gram (1 g) of salt is added as well as the rest of copper sulphate (namely 5 g).

Afterwards, this second mixture is stirred for about one hour too.

Afterwards, the method includes a step of applying the patina bath 21 over the upper surface 11 of the substrate 10.

For this purpose, the method includes, for example, a step of pickling, also called stripping, the upper surface 11 of the substrate.

For example, the pickling step is configured to remove a possible oxide layer at the surface 11 of the substrate.

For example, the pickling step is carried out with a sulphuric acid solution (H₂SO₄) at a concentration comprised between 0.5% and 10% diluted in water (H₂O).

Before the pickling step, the method may optionally include a degreasing step, for example with a solvent, or with ultrasounds, for example in the presence of a surfactant.

After the pickling step, the method may optionally include a step of rinsing the substrate, for example rinsing with water.

Afterwards, the substrate 10 is for example immersed into the patina bath 21, as illustrated in FIG. 2.

For this purpose, the patina bath 21 is for example brought to a temperature comprised between 80° C. and 100° C.

For example, the substrate 10 is immersed into the patina bath 21 for a time period comprised between 45 minutes and 120 minutes, for example 60 minutes.

Once the patina is obtained, the method may include a drying step, for example in the open air, and/or in the oven.

Once the patina is dry, the method may possibly include a polishing step.

According to another interesting option, the method may also include a finishing step. For example, the finishing step may include a step of applying a protection, for example an organic material, like for example acacia gum, a wax, a resin or other.

This organic, non-volatile, substance possibly allows intensifying the black of the patina.

At the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterised by colourimetry in the L*a*b* CIE 1976 colour space with a “L” value comprised between 10 and 50, more particularly between 30 and 50, an “a” value comprised between −1.5 and 1; and a “b” value comprised between −8 and 1.

If the part includes a finish, this black may be characterised in the L*a*b* CIE 1976 colour space with a “L” value comprised more particularly between about 10 and 40, an “a” value comprised between about −1.5 and 1; and a “b” value comprised between about −4 and 1.

In a particular example, L=28.16; a=−0.91; b=0.34.

Thus, the finish contributes to conferring a deep black appearance on the part.

For example, the finishing step is configured to protect the patina and/or to confer a shiny, satin or matt finish thereon.

For example, the step of applying the finish may be carried out by soaking, spraying, buffering, application with a cloth or with a brush.

As described hereinbelow, the obtained patina 20, with a deep black appearance, over such a substrate 10, which is therefore a bronze with gold, primarily consists of cuprite (Cu₂O).

The cuprite is naturally red, but it seems that such a patina results in a black tint thanks to the presence of gold (and/or silver) originating from the substrate.

More specifically, the part 30 obtained in the present embodiment herein includes:

• • The substrate 10 made of an alloy including, according to the aforementioned theoretical dosing, about 95.24% of copper (Cu), and about 4.76% of gold (Au), herein without any added element; and
  • the patina 20, applied over the upper surface 11 of the substrate 10, having a thickness of at least 5 μm, for example comprised between 5 μm and 30 μm, and including a copper oxide, in particular herein in the form of cuprite (Cu₂O), as explained hereinafter with reference to FIG. 6, and gold (Au), for example according to a concentration comprised between 5% and 15%.

Thus, such a part 30 has a “deep black” appearance.

At the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterised by colourimetry in the L*a*b* CIE 1976 colour space with a “L” value comprised between 10 and 50, more particularly between 30 and 50, an “a” value comprised between −1.5 and 1; and a “b” value comprised between −8 and 1.

If the part includes a finish, this black may be characterised in the L*a*b* CIE 1976 colour space with a “L” value comprised more particularly between about 10 and 40, an “a” value comprised between about −1.5 and 1; and a “b” value comprised between about −4 and 1.

In a particular example, L=28.16; a=−0.91; b=0.34.

Such a part 30 further has a self-healing property.

This property is schematically illustrated in FIG. 4.

In FIG. 4A), the part 30 has been scratched to damage the apparent surface of the part 30 and form a “cut” 31.

After exposure to open air and/or light, natural or artificial, for example for a few days and possibly a few weeks, the cut 31 is attenuated and forms a trace 32, illustrated in FIG. 4 B).

It has also been observed that following a contact with the skin of a person wearing the object, for example as a piece of jewellery, the healing effect was quicker (for example a few days to obtain the same attenuation).

This evolution is explained, a priori, by a tarnishing of the surface of the substrate uncovered by the cut 31, and/or a growth of the patina including at least partially the cut 31.

In general, the patina 20 may possibly include at least one amongst carbon, oxygen, chlorine, potassium, calcium, or iron.

For example, the at least one of these elements is present between 0% and 0.01% of the patina, i.e. at most in the form of traces, but ideally, the patina is free of it.

More generally, if the substrate includes an added element, the added element has a concentration comprised between 0% and 1%, and possibly between 0% and 0.01%. For example, the added element includes at least one amongst: silver (Ag), arsenic (As), zinc (Zn), lead (Pb), iron (Fe), or tin (Sn).

FIG. 5 shows a spectrum made with SEM-EDX on the patina 20.

The spectrum includes peaks which primarily show copper and gold, as well as carbon and oxygen.

For example, an elementary analysis gives: 67% of copper (Cu), 13% of gold (Au), 10% of oxygen (O), 5% of carbon (C), and impurities: chlorine (CI), nitrogen (N), potassium (K).

Hence, this spectrum indicates a presence of copper oxide in the patina 20, as well as a presence of gold.

Hence, the patina 20 is interestingly gold-rich.

The relatively low presence of carbon in the present example could for example be explained by a use of acacia gum for a step of finishing the part 30, and/or by pollution.

In addition, as illustrated in FIG. 6, Raman analyses have been

performed on the patina 20 using a Senterra spectrometer from Bruker with a laser source at 532 nm, at a power of 20 mW.

The spectral resolution is within a range comprised between 9 cm⁻¹ and cm⁻¹.

The spectrum shows the presence of two bands at 219 cm⁻¹ and 623 cm⁻¹ which are characteristic of a copper I oxide: Cu₂O or cuprite. One could notice an absence of a band at 297 cm⁻¹ characteristic of a copper II oxide.

Hence, this analysis confirms that the patina 20 primarily consists of cuprite.

Thus, the invention allows providing a part made of a precious metal with a black appearance and self-healing.

Claims

1. A method for making a part including a substrate made of a precious metal, the method including:

A step of providing the substrate made of an alloy including: Between 94 wt. % and 99.5 wt. % of copper (Cu), Between 0.5 wt. % and 6 wt. % of gold (Au), Between 0 wt. % and 4 wt. % of at least one added element, The total amounting to 100%; The substrate having a surface, called upper surface, and having a thickness of at least 100 μm starting from the upper surface;

A step of preparing a patina bath from a mixture including at least: Between 3 g and 10 g of anhydrous or hydrated copper acetate (Cu(CH3COO)2), Between 3 g and 20 g of anhydrous or hydrated copper sulphate (CuSO4), and Between 0.5 g and 2 g of salt (NaCl); then

A step of applying the patina bath over the upper surface of the substrate.

2. The method according to claim 1, wherein the step of preparing a patina bath includes:

A step of heating a litter of distilled or permuted water up to a temperature comprised between 80° C. and 100° C.;

A step of adding 3 g to 10 g of anhydrous or hydrated copper acetate, and

3 g to 10 g of anhydrous or hydrated copper sulphate in the heated water, producing a first mixture;

A step of stirring for a time period comprised between 30 minutes and 90 minutes of the first mixture; then

A step of adding 0.5 g to 2 g of salt (NaCl), in the first mixture;

A step of adding 3 g to 10 g of anhydrous or hydrated copper sulphate in the first mixture, producing a second mixture; then

A step of stirring the second mixture for a time period comprised between 30 minutes and 90 minutes, producing the patina bath.

3. The method according to claim 1, including a step of forming the alloy of the substrate including:

a step of mixing, in proportions, at least 45 g to 55 g of gold (Au) with between 800 g and 1,200 g of pure copper (Cu).

4. The method according to claim 1, including a step of forming the substrate over a base.

5. The method according to claim 4, wherein the step of forming the substrate includes:

a step of galvanic deposition or projection of the alloy on the base, configured to form a thickness of the substrate of at least 100 μm.

6. The method according to claim 1, including prior to the step of applying patina:

A step of pickling the upper surface of the substrate; then

A step of rinsing the substrate.

7. The method according to claim 1, wherein the step of applying the patina bath includes a step of immersing the substrate into the patina bath, the immersion step having a duration comprised between 45 minutes and 90 minutes, and the patina bath having a temperature comprised between 80° C. and 100° C.

8. A part obtained by a method according to claim 1, the part including:

At least one substrate made of an alloy including at least: Between 94% and 99.5% of copper (Cu), Between 0.5% and 6% of gold (Au), Between 0% and 4% of at least one added element, The total amounting to 100%; The substrate having a surface, called upper surface, and having a thickness of at least 100 μm starting from the upper surface; and

And a patina applied over the upper surface of the substrate, the patina having a thickness of at least 5 μm, and the patina including at least: a copper oxide, and gold (Au).

9. The part according to claim 8, wherein the patina includes between 0% and of at least one amongst: carbon, oxygen, chlorine, potassium, calcium, or iron.

10. The part according to claim 8, wherein the added element has a concentration comprised between 0% and 1%.

11. The part according to claim 8, wherein the added element includes at least one amongst: silver (Ag); arsenic (As), zinc (Zn), lead (Pb), iron (Fe), or tin (Sn).

12. The part according to claim 8, wherein the patina includes a gold concentration comprised between 5% and 15%.

13. The part according to claim 8, wherein the alloy of the substrate includes 95.24% copper (Cu) and 4.76% gold (Au).

14. The part according to claim 8, having a black surface characterised by colorimetry in the L*a*b* CIE 1976 colour space with a “L” value comprised between 10 and 50, an “a” value comprised between −1.5 and 1; and a “b” value comprised between −8 and 1.

15. The method according to claim 2, including a step of forming the alloy of the substrate including:

a step of mixing, in proportions, at least 45 g to 55 g of gold (Au) with between 800 g and 1,200 g of pure copper (Cu).

16. The method according to claim 2, including a step of forming the substrate over a base.

17. The method according to claim 3, including a step of forming the substrate over a base.

18. The method according to claim 2, including prior to the step of applying patina:

A step of pickling the upper surface of the substrate; then

A step of rinsing the substrate.

19. The method according to claim 3, including prior to the step of applying patina:

A step of pickling the upper surface of the substrate; then

A step of rinsing the substrate.

20. The method according to claim 4, including prior to the step of applying patina:

A step of pickling the upper surface of the substrate; then

A step of rinsing the substrate.