Method for producing a black-appearing, self-healing part having a precious metal substrate, and part obtained

Abstract

Method for producing a part comprising a precious metal substrate, comprising: a step of providing the substrate in an alloy comprising 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 addition element, the substrate having a surface, called the upper surface, and having a thickness of at least 100 µm from the upper surface; and a step of preparing a patina bath from a mixture comprising at least: between 3 g and 10 g of copper acetate (Cu(CH3COO)2), anhydrous or hydrated, between 3 g and 20 g of copper sulfate (CuSO4), anhydrous or hydrated, and between 0.5 g and 2 g of salt (NaCl); then a step of applying the patina bath to the upper surface of the substrate. Part obtained by such a method. Figure for abstract: Fig. 4

Description

Title of the invention: Method for producing a black-appearing, self-healing part having a precious metal substrate, and part obtained

[0001] The invention relates to a method of producing a black, self-healing part having a precious metal substrate.

[0002] It also relates to a part obtained by such a process.

[0003] Black-looking alloys appear around 2000 BC.

[0004] Black bronzes with gold date back to the Egyptian period (1st millennium BC) then to the Roman period where we speak of Corinthian bronzes (although this name covers different alloys), or gold bronzes, which have the particular advantage of being both black, via a patina, and semi-precious due to the presence of gold.

[0005] The term "black bronzes" is then found in other civilizations (Roman, Saxon, Chinese (Wu-tong), as well as in Japan, in the Middle Ages.

[0006] An alloy of this type is still present today in a Japanese alloy called "Shakudo".

[0007] To form a black surface on this alloy, there is a coating technique called “Niiro”.

[0008] However, the “Niiro” coatings have a “purple” black. Furthermore, they are traditionally made using a Japanese radish (for example daikon) to prepare the surface and with the addition of traditional Japanese plum vinegar in the patination solution, which is difficult to reconcile with industrial production.

[0009] It is also possible to form a black coating, for example by dry deposition such as PVD (physical vapor deposition) or DLC (diamond like carbon), or by applying a lacquer.

[0010] However, such a coating generally causes an adhesion problem, in particular on a relatively ductile substrate, which leads to flaking and / or delamination of the coating, in particular in the event of impact.

[0011] Furthermore, in the event of a scratch, it is very difficult, if not impossible, to repair the damaged surface, especially following the application of lacquer.

[0012] One objective is to provide an alloy with a black appearance, at least on the surface, or even if possible a deep black, overcoming at least in part the aforementioned drawbacks.

[0013] To this end, a method is proposed for producing a part comprising a precious metal substrate, the method comprising: - A step of providing the substrate in an alloy comprising (the percentages are understood as mass percentages, also noted wt.%): - Between 94% and 99.5% of copper (Cu), - Between 0.5% and 6% of gold (Au), - Between 0% and 4% of at least one addition element,

[0014] The total being 100%;

[0015] The substrate having a surface, called the upper surface, and having a thickness of at least 100 μm from the upper surface; - A step of preparing a patina bath from a mixture comprising, in proportions, at least: - Between 3 g and 10 g of copper acetate (Cu(CH3COO)2), anhydrous or hydrated, - Between 3 g and 20 g of copper sulfate (CuSO4), anhydrous or hydrated, and - Between 0.5 g and 2 g of salt (NaCl); then - A step of applying the patina bath on the upper surface of the substrate.

[0016] A precious metal here means an alloy comprising at least one of gold, silver or platinum. Here, it is more particularly a gold-based alloy.

[0017] Such a method thus makes it possible to produce a part with a black appearance and self-repairing properties.

[0018] On the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterized by colorimetry in the CIE 1976 L*a*b* color space with a value of “L” between 10 and 50, more particularly between 30 and 50, a value of “a” between -1.5 and 1; and a value of “b” between -8 and 1.

[0019] It has appeared that the colors rendered by a patina range from black "brown" (when a gold content of the substrate tends towards approximately 0.5 wt.% (Au)) to black "blue / black" (when a gold content of the substrate tends towards approximately 5-6 wt.% (Au)).

[0020] However, the higher the gold content, the deeper the black appears, but on the other hand, it has appeared that the higher the gold content, for example at higher gold contents, for example 7.5%, the more difficult the patina is to obtain and loses density.

[0021] The contents can be measured by any method.

[0022] According to a preferred example, the gold content is measured by ICP-AES (inductively coupled plasma-atomic emission spectrometry), for example after dissolution in aqua regia (i.e. a mixture of nitric acid and hydrochloric acid, for example respectively in a proportion of 1:3), or by gravimetry (i.e. by attacking an alloy with nitric acid and weighing the insolubles which are recovered on a filter which is weighed).

[0023] Depending on the measurement method used, the measured content values ​​can vary by up to 10%, for example, and sometimes even more.

[0024] According to an exemplary implementation, the method comprises a step of forming the alloy of the substrate.

[0025] For example, the step of forming the alloy of the substrate comprises 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 1200 g, for example 1000 g, of pure copper (Cu).

[0026] For a particular implementation example, the mixing step is configured to produce an alloy comprising between 4.5% and 4.8%, for example 4.76%, of gold (by mass). The gold content obtained is for example 4.57 wt.% when measured by ICP-AES and 4.66% when measured by gravimetry. These differences between the theoretical value targeted (i.e. 4.76%) and the measured values ​​are due to the measurement uncertainties associated with each measurement method, as indicated above.

[0027] According to an exemplary implementation, the method comprises a step of forming the substrate on a base.

[0028] For example, the base is made of stainless steel, aluminum, bronze or brass or other, of any desired geometry or dimension.

[0029] According to an exemplary implementation, the step of forming the substrate comprises a step of galvanic deposition or by projection of the alloy on the base, in particular by techniques such as Cold Spray (cold projection) or Arc Spray (arc projection).

[0030] This step is for example configured to form a thickness of the substrate of at least 100 μm.

[0031] According to an example of implementation, the step of preparing a patina bath comprises, in proportions: - A step of heating one liter of distilled or deionized water to a temperature between 80°C and 100°C; - A step of adding 3 g to 10 g of copper acetate, anhydrous or hydrated, for example 4.82 g of anhydrous copper acetate (Cu(CH3COO)2) or 5.3 g of copper acetate monohydrate (Cu(CH3COO)2, H2O), and - from 3 g to 10 g of copper sulfate, anhydrous or hydrated, for example 3.19 g of anhydrous copper sulfate (CuSO4) or 5 g of copper sulfate pentahydrate (CuSO4, 5H2O), in heated water, producing a first mixture; - A stirring step for a duration of 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 copper sulfate, anhydrous or hydrated, for example 3.19 g of anhydrous copper sulfate (CuSO4) or 5 g of copper sulfate pentahydrate (CuSO4, 5H2O) to the first mixture, producing a second mixture; then - A step of stirring the second mixture for a period of between 30 minutes and 90 minutes, producing the patina bath.

[0032] According to an exemplary implementation, the method comprises, prior to the step of applying the patina bath, a step of stripping, also called etching, the upper surface of the substrate.

[0033] The stripping step is for example configured to remove a possible oxide layer on the surface of the substrate.

[0034] The pickling step is for example carried out with a sulfuric acid solution (H 2SO4) at a content of between 0.5% and 10% diluted in water (H2O).

[0035] Before the stripping step, the process may optionally include a degreasing step, for example using a solvent, or using ultrasound, for example in the presence of a surfactant.

[0036] According to an exemplary implementation, the method comprises a step of rinsing the substrate, for example rinsing with water; this step is for example implemented after the stripping step.

[0037] According to an exemplary implementation, the step of applying the patina bath comprises a step of immersing the substrate in the patina bath.

[0038] For example, the immersion step has a duration of between 45 minutes and 120 minutes, for example 60 minutes.

[0039] For example, the patina bath has a temperature between 80°C and 100°C.

[0040] Once the patina is obtained, the process may include a drying step, for example example in the open air, and / or in an oven.

[0041] Once the patina is dry, the process may optionally include a step of polishing the patina.

[0042] According to an interesting option, the method can also include a finishing step.

[0043] 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, such as for example gum arabic, a wax, a resin or the like.

[0044] This non-volatile organic substance, by impregnating the patina, allows it to go from a patina with a generally anthracite appearance to a deeper black. The finish thus contributes to giving the piece a deep black appearance.

[0045] This deeper black obtained after finishing can be characterized by colorimetry in the CIE 1976 L*a*b* color space with a value of “L” between 10 and 40, a value of “a” between -1.5 and 1; and a value of “b” between -4 and 1.

[0046] The finishing step is for example also configured to protect the patina and / or give it a shiny, satin or matte finish.

[0047] The step of applying the finish can for example be carried out by dipping, spraying, dabbing, application with a cloth or brush.

[0048] Also provided, according to another aspect of the invention, is a part obtained by a method as described above.

[0049] According to an exemplary embodiment, the part comprises: - At least one substrate made of an alloy comprising at least: - Between 94% and 99.5% copper (Cu), - Between 0.5% and 6% gold (Au), - Between 0% and 4% of at least one additive,

[0050] The total being 100%;

[0051] The substrate having a surface, called the upper surface, and having a thickness of at least 100 μm from the upper surface; and - And a patina applied to the upper surface of the substrate, the patina having a thickness of at least 5 μm, for example between 5 and 30 μm, and the patina comprising at least: - a copper oxide, for example in the form of cuprite (Cu2O), and - gold (Au).

[0052] According to an exemplary embodiment, the addition element of the substrate has a content of between 0% and 1%, or even between 0% and 0.01%.

[0053] According to an exemplary embodiment, the addition element of the substrate comprises at least one of: silver (Ag); arsenic (As), zinc (Zn), lead (Pb), iron (Fe), or tin (Sn).

[0054] Where appropriate, the silver content is for example less than 1%, for example between 0% and 1%.

[0055] According to an exemplary embodiment, the alloy of the substrate comprises 95.24% copper (Cu) and 4.76% gold (Au), in particular when the gold content is measured by ICP-AES. Such an alloy is then devoid of any addition element (which corresponds to a content of 0%).

[0056] According to an exemplary embodiment, the patina further comprises at least one element in trace form, for example at a content of between 0% and 0.01%.

[0057] The element in trace form comprises, for example, at least one of: carbon, oxygen, chlorine, potassium, calcium, or iron.

[0058] According to an exemplary embodiment, the patina has a gold content of between 5% and 15%, for example 13%.

[0059] The gold content in the patina is for example measured by EDX.

[0060] The part according to an example of implementation thus presents a “deep black”.

[0061] On the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterized by colorimetry in the CIE 1976 L*a*b* color space with a value of “L” between 10 and 50, more particularly between 30 and 50, a value of “a” between -1.5 and 1; and a value of “b” between -8 and 1.

[0062] If the part has a finish, this black can be characterized in the CIE 1976 L*a*b* color space with a value of “L” more particularly between approximately 10 and 40, a value of “a” between approximately -1.5 and 1; and a value of “b” between approximately -4 and 1.

[0063] The invention, according to an exemplary embodiment, will be well understood and its advantages will appear better on reading the detailed description which follows, given for information purposes and in no way limiting, with reference to the appended drawings in which:

[0064] [Fig.l] shows a substrate according to an exemplary embodiment of the invention;

[0065] [Fig.2] illustrates a step of immersing the substrate of [Fig.l] in a bath of skate, according to an exemplary implementation of the invention;

[0066] [Fig.3] shows a part according to an exemplary embodiment of the invention;

[0067] [Fig.4] illustrates a self-repairing property of the part of [Fig.3];

[0068] [Fig.5] presents an example of a spectrum produced in SEM-EDX on the patina of the part of [Fig.3]; and

[0069] [Fig.6] shows an example of a Raman spectrum produced on the patina of the part in [Fig.3].

[0070] To produce a part 30 made of precious alloy, such as a piece of jewelry, with a black appearance, and having a self-healing property (also called “self-repairing capacity”), a method for producing such a part 30, according to an exemplary implementation of the invention, comprises on the one hand a step of providing a substrate 10 made of an alloy comprising:

[0071] - Between 94% and 99.5% copper (Cu),

[0072] - Between 0.5% and 6% gold (Au), and

[0073] - Between 0% and 4% of at least one addition element,

[0074] The total being 100%.

[0075] The percentages here designate mass percentages, also noted “wt.%”.

[0076] The gold content can be measured by any method; preferably, it is measured by ICP-AES (inductively coupled plasma-atomic emission spectrometry), for example after dissolution in aqua regia (i.e. a mixture of nitric acid and hydrochloric acid, for example respectively in a proportion of 1:3), or by gra vimetry (i.e. by attacking an alloy with nitric acid and weighing the insolubles which are recovered on a filter which is weighed; here, only gold is insoluble).

[0077] Depending on the measurement method used, the measured content values ​​can vary by up to 10% for example, sometimes even more.

[0078] For example, to obtain such an alloy, the method may comprise a step of forming the alloy of the substrate comprising a step of mixing at least 45 g to 55 g, for example 50 g, of gold (Au) with between 800 g and 1200 g, for example 1000 g, of pure copper (Cu).

[0079] For a particular implementation example, the mixing step is for example configured to produce an alloy comprising between 4.5% and 4.8%, for example 4.76%, of gold (by mass). The gold content obtained is for example 4.57 wt.% when measured by ICP-AES and 4.66% when measured by gravimetry. These differences between the theoretical value targeted (i.e. 4.76%) and the measured values ​​are due to the measurement uncertainties linked to each measurement method, as indicated above.

[0080] In the context of the description of a particular example of implementation of the invention, the substrate 10 is formed from an alloy comprising 95.24% copper (Cu) and 4.76% gold (Au).

[0081] In this particular example, the alloy is devoid of any addition element; but it could nevertheless contain at least one, preferably in a content between 0% and 4%.

[0082] Such a substrate 10 is for example shown [Fig.l]. It is for example a substrate configured to form a ring.

[0083] The substrate having a surface 11, called upper surface 11, on which a patina 20 is subsequently applied, described below.

[0084] The substrate preferably has a thickness of at least 100 μm from the upper surface 11.

[0085] A thickness of at least 100 μm of the substrate makes it possible to better guarantee a sufficient depth for anchoring the patina 20.

[0086] The substrate 10 is here formed from a curved armature, for example a curved wire with a circular or truncated circular section, here having a diameter 12 of approximately 3 mm.

[0087] The substrate here is a solid element.

[0088] However, the substrate could be a coating layer (of the aforementioned copper-gold alloy), at least 100 μm thick, applied on a base of another material, for example stainless steel, aluminum, bronze or brass or other, of any desired geometry or dimension.

[0089] The method may for example comprise a step of forming the substrate comprising a step of galvanic deposition or by projection on the base, in particular by techniques such as Cold Spray or Arc Spray, of the alloy as described previously, this step being configured to form a substrate thickness of at least 100 qm.

[0090] To produce the part 30 from precious alloy, the method according to an exemplary implementation of the invention also comprises a step of preparing a patina bath 21.

[0091] The patina bath 21 is for example obtained from a mixture comprising: Between 3 g and 10 g of copper acetate (Cu(CH3COO)2), anhydrous or hydrated, Between 3 g and 20 g of copper sulfate (CuSO4), anhydrous or hydrated, and Between 0.5 g and 2 g of salt (NaCl).

[0092] For this, the step of preparing a patina bath 21 comprises for example: A step of heating one liter of distilled or deionized water to a temperature between 80°C and 100°C; A step of adding 3 g to 10 g of copper acetate, anhydrous or hydrated, for example 4.82 g of anhydrous copper acetate (Cu(CH3COO)2) or 5.3 g of copper acetate monohydrate (Cu(CH3COO)2, H2O), and 3 g to 10 g of copper sulfate, anhydrous or hydrated, for example 3.19 g of anhydrous copper sulfate (CuSO4) or 5 g of copper sulfate pentahydrate (CuSO4, 5H2O), into the heated water, producing a first mixture; A stirring step for a period of 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), into the first mixture; A step of adding 3 g to 10 g of copper sulfate, anhydrous or hydrated, for example 3.19 g of anhydrous copper sulfate (CuSO4) or 5 g of copper sulfate pentahydrate (CuSO4, 5H2O) to the first mixture, producing a second mixture; then A step of stirring the second mixture for a period of between 30 minutes and 90 minutes, producing the patina bath 21.

[0093] For the present example of implementation, the patina bath 21 more particularly comprises: 10 g of copper sulfate pentahydrate, 5.3 g of copper acetate monohydrate, 1 g of salt (NaCl), and 1 L of distilled or deionized water.

[0094] For example, distilled or deionized water is heated to 90°C (+ / -10°C).

[0095] Then the copper acetate and half of the copper sulfate (i.e. 5 g) are added to the distilled or deionized water.

[0096] This first mixture is stirred for one hour, then one gram (1 g) of salt is added as well as the rest of the copper sulfate (i.e. 5 g).

[0097] This second mixture is then stirred for about an hour as well.

[0098] The method then comprises a step of applying the patina bath 21 to the upper surface 11 of the substrate 10.

[0099] For this, the method comprises for example a stripping step, also called etching, of the upper surface 11 of the substrate.

[0100] The stripping step is for example configured to remove a possible oxide layer on the surface 11 of the substrate.

[0101] The pickling step is for example carried out with a sulfuric acid solution (H 2SO4) at a content of between 0.5% and 10% diluted in water (H2O).

[0102] Before the stripping step, the process may optionally include a degreasing step, for example using a solvent, or using ultrasound, for example in the presence of a surfactant.

[0103] After the stripping step, the method may optionally include a step of rinsing the substrate, for example rinsing with water.

[0104] Then, the substrate 10 is for example immersed in the patina bath 21, as illustrated [Fig.2].

[0105] For this, the patina bath 21 is for example brought to a temperature between 80°C and 100°C.

[0106] For example, the substrate 10 is immersed in the patina bath 21 for a duration of between 45 minutes and 120 minutes, for example 60 minutes.

[0107] Once the patina has been obtained, the process may include a drying step, for example in the open air, and / or in an oven.

[0108] Once the patina is dry, the process may optionally include a polishing step.

[0109] 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, such as for example gum arabic, a wax, a resin or other.

[0110] This organic, non-volatile substance can possibly intensify the black of the patina.

[0111] On the surface of the part, the black of the patina obtained after application of such a patina bath is for example characterized by colorimetry in the CIE 1976 L*a*b* color space with a value of “L” between 10 and 50, more particularly between 30 and 50, a value of “a” between -1.5 and 1; and a value of “b” between -8 and 1.

[0112] If the part has a finish, this black can be characterized in the space of CIE 1976 L*a*b* color with an “L” value more particularly between approximately 10 and 40, an “a” value between approximately -1.5 and 1; and a “b” value between approximately -4 and 1.

[0113] In a particular example, L=28.16; a=-0.91; b=0.34.

[0114] The finish thus contributes to giving a deep black appearance to the part.

[0115] The finishing step is for example configured to protect the patina and / or give it a shiny, satin or matte finish.

[0116] The step of applying the finish can for example be carried out by dipping, spraying, dabbing, application with a cloth or brush.

[0117] As described below, the patina 20 obtained, with a deep black appearance, on such a substrate 10, which is therefore a gold bronze, is mainly made up of cuprite (Cu2O).

[0118] Cuprite is naturally red, but it appears that such a patina leads to a black tint thanks to the presence of gold (and / or silver) coming from the substrate.

[0119] More precisely, the part 30 obtained in the present embodiment example comprises here: - The substrate 10 in an alloy comprising, according to the aforementioned theoretical dosage, approximately 95.24% copper (Cu), and approximately 4.76% gold (Au), here without any additional element; and - the patina 20, applied to the upper surface 11 of the substrate 10, having a thickness of at least 5 pm, for example between 5 pm and 30 pm, and comprising a copper oxide, in particular here in the form of cuprite (Cu2O), as explained below in connection with [Fig.6], and gold (Au), for example according to a content of between 5% and 15%.

[0120] Such a part 30 thus presents a “deep black”.

[0121] On the surface of the part, the black of the patina obtained after application of such a bath of patina is for example characterized by colorimetry in the CIE 1976 L*a*b* color space with a value of “L” between 10 and 50, more particularly between 30 and 50, a value of “a” between -1.5 and 1; and a value of “b” between -8 and 1.

[0122] If the part has a finish, this black can be characterized in the CIE 1976 L*a*b* color space with a value of “L” more particularly between approximately 10 and 40, a value of “a” between approximately -1.5 and 1; and a value of “b” between approximately -4 and 1.

[0123] In a particular example, L=28.16; a=-0.91; b=0.34.

[0124] Such a part 30 further has a self-repairing property.

[0125] This property is illustrated schematically [Fig.4].

[0126] In [Fig.4] A), part 30 has been scratched to damage the exposed surface of part 30 and form a “cut” 31.

[0127] After exposure to open air and / or light, natural or artificial, for example for a few days or even a few weeks, the cut 31 has attenuated and forms a trace 32, illustrated [Fig.4] B).

[0128] It has also been observed that following contact with the skin of a person wearing the object, for example as jewelry, the healing effect was faster (for example, a few days to obtain the same attenuation).

[0129] This development 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 at least partially closing the cut 31.

[0130] Generally, the patina 20 may optionally comprise at least one of: carbon, oxygen, chlorine, potassium, calcium, or even iron.

[0131] At least one of these elements is for example present between 0% and 0.01% of the patina, i.e. at most in trace form, but ideally, the patina is free of it.

[0132] More generally, if the substrate comprises an addition element, the addition element has a content of between 0% and 1%, or even between 0% and 0.01%. The addition element comprises, for example, at least one of: silver (Ag), arsenic (As), zinc (Zn), lead (Pb), iron (Fe), or tin (Sn).

[0133] [Fig.5] shows a spectrum produced in SEM-EDX on patina 20.

[0134] The spectrum has peaks that represent mainly copper and gold, as well as carbon and oxygen.

[0135] An elemental analysis gives for example: 67% copper (Cu), 13% gold (Au), 10% oxygen (O), 5% carbon (C), and impurities: chlorine (Cl), nitrogen (N), potassium (K).

[0136] This spectrum therefore indicates a presence of copper oxide in patina 20, as well as a presence of gold.

[0137] Interestingly, patina 20 is therefore enriched with gold.

[0138] The relatively low presence of carbon in the present example can be explained for example by the use of gum arabic for a finishing step of the part 30, and / or by pollution.

[0139] Additionally, as illustrated in [Fig.6], Raman analyses were performed on patina 20 using a Bruker Senterra spectrometer with a 532 nm laser source, at a power of 20 mW.

[0140] The spectral resolution is in a range between 9 cm 1 and 15 cm1.

[0141] The spectrum shows the presence of two bands at 219 cm 1 and 623 cm1 which are characterized characteristics of a copper I oxide: Cu2O or cuprite. We can note an absence of a band at 297 cm1 characteristic of copper IL oxide

[0142] This analysis therefore confirms that patina 20 is mainly made up of cuprite.

[0143] The invention thus makes it possible to provide a precious metal part with a black, self-healing appearance.

Claims

Claims

1. Method for producing a part comprising a precious metal substrate, the method comprising: - A step of supplying the substrate in an alloy comprising: - Between 94 wt.% and 99.5 wt.% of copper (Cu), - Between 0.5 wt.% and 6 wt.% gold (Au), - Between 0 wt.% and 4 wt.% of at least one additive, The total being 100%; The substrate having a surface, called the upper surface, and having a thickness of at least 100 μm from the upper surface; - A step of preparing a patina bath from a mixture comprising at least: - Between 3 g and 10 g of copper acetate (Cu(CH3COO)2), anhydrous or hydrated, - Between 3 g and 20 g of copper sulfate (CuSO4), anhydrous or hydrated, and - Between 0.5 g and 2 g of salt (NaCl); then - A step of applying the patina bath on the upper surface of the substrate.

2. The method of claim 1, wherein the step of preparing a patina bath comprises: - A step of heating one liter of distilled or deionized water to a temperature between 80°C and 100°C; - A step of adding 3 g to 10 g of copper acetate, anhydrous or hydrated, for example 4.82 g of anhydrous copper acetate (Cu(CH3COO)2) or 5.3 g of copper acetate monohydrate (Cu(CH3COO)2, H2O), and - 3g to 10g of copper sulfate, anhydrous or hydrated, for example 3.19 g of anhydrous copper sulfate (CuSO4) or 5 g of copper sulfate pentahydrate (CuSO4, 5H2O), in heated water, producing a first mixture; - A stirring step for a duration of 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), to the first mixture; - A step of adding 3 g to 10 g of copper sulfate, anhydrous or hydrated, for example 3.19 g of anhydrous copper sulfate (CuSO4) or 5 g of copper sulfate pentahydrate (CuSO4, 5H2 0) to the first mixture, producing a second mixture; then - A step of stirring the second mixture for a period of between 30 minutes and 90 minutes, producing the patina bath.

3. A method according to any one of claims 1 or 2, comprising a step of forming the alloy of the substrate comprising: - a step of mixing at least, in proportions, from 45 g to 55 g, for example 50 g, of gold (Au) with between 800 g and 1200 g, for example 1000 g, of pure copper (Cu).

4. A method according to any one of claims 1 to 3, comprising a step of forming the substrate on a base, the base being for example made of stainless steel, aluminum, bronze or brass.

5. Method according to claim 4, in which the step of forming the substrate comprises: - a step of galvanic or projection deposition of the alloy on the base, configured to form a thickness of the substrate of at least 100 μm.

6. Method according to any one of claims 1 to 5, comprising, prior to the step of applying the patina: - A step of stripping, for example with a sulfuric acid solution (H2SO4), the upper surface of the substrate; then - A step of rinsing the substrate, for example rinsing with water.

7. A method according to any one of claims 1 to 6, wherein the step of applying the patina bath comprises an immersion step of the substrate in the patina bath, the immersion step having a duration of between 45 minutes and 90 minutes, for example 60 minutes, and the patina bath having a temperature of between 80°C and 100°C.

8. Part obtained by a method according to any one of claims 1 to 7, the part comprising: - At least one substrate made of an alloy comprising 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 addition element, the total being 100%; The substrate having a surface, called the upper surface, and having a thickness of at least 100 pm from the upper surface; and - And a patina applied to the upper surface of the substrate, the patina having a thickness of at least 5 pm, for example between 5 and 30 pm, and the patina comprising at least: - a copper oxide, for example in the form of cuprite (Cu2O), and - gold (Au).

9. A part according to claim 8, wherein the patina comprises between 0% and 0.01% of at least one of: carbon, oxygen, chlorine, potassium, calcium, or iron.

10. Part according to any one of claims 8 or 9, in which the addition element has a content of between 0% and 1%, or even between 0% and 0.01%.

11. A part according to any one of claims 8 to 10, wherein the addition element comprises at least one of: silver (Ag); arsenic (As), zinc (Zn), lead (Pb), iron (Fe), or tin (Sn).

12. A part according to any one of claims 8 to 11, wherein the patina has a gold content of between 5% and 15%, for example 13%.

13. A part according to any one of claims 8 to 12, wherein the alloy of the substrate comprises 95.24% copper (Cu) and 4.76% gold (Au).

14. Part according to any one of claims 8 to 13, having a black surface characterized by colorimetry in the CIE 1976 L*a*b* color space with a value of “L” between 10 and 50, a value of “a” between -1.5 and 1; and a value of “b” between -8 and 1.

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