WO2016084038A2

WO2016084038A2 - Master alloy used for making a white gold alloy

Info

Publication number: WO2016084038A2
Application number: PCT/IB2015/059164
Authority: WO
Inventors: Diego PERETTI; Mauro DI SIRO
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-11-28
Filing date: 2015-11-27
Publication date: 2016-06-02
Anticipated expiration: 2017-05-28
Also published as: WO2016084038A3; CN107208186A

Abstract

A master alloy designed to be mixed with a predetermined weight percentage of gold to obtain a white gold alloy containing a predetermined weight percentage of germanium for gold-whitening purposes, a predetermined weight percentage of at least one plasticizing metal, adapted to impart malleability to the master alloy and a predetermined amount of a binding metal to form a solid solution. The binding metal is selected from the group of elements whose electronegativity value is substantially equal to that of germanium, for homogeneous distribution of the latter within the solid solution. A gold alloy with a Yellow Index of less than 32, comprising a predetermined weight percentage of gold mixed with the master alloy to make a white gold article.

Description

MASTER ALLOY USED FOR MAKING A WHITE GOLD ALLOY

Field of the invention

[0001 ] The present invention generally finds application in the field of metal alloys, and particularly alloys used in the jewelry industry, and relates to a master alloy for making white gold alloys.

[0002] The invention also relates to a gold alloy for making white gold articles.

Background art

[0003] Particular master alloys have been long known, in the jewelry industry, to be melted with gold to make white gold jewels and articles.

[0004] Such master alloys include metals selected from the group comprising silver, copper, zinc and nickel, in weight ratios varying with the karat grade of the desired finished article.

[0005] For example, a 1 8 kt white gold article generally comprises 75% gold, 1 6% copper, 5% nickel and about 4% zinc by weight.

[0006] Yet nickel is known to mainly act on the gold article to change its color in view of obtaining a material of weaker color, arbitrarily identified as white gold.

[0007] Nevertheless, recent studies have shown nickel toxicity to the human body and it was particularly demonstrated that this element may cause contact allergies in sensitive individuals.

[0008] For this reason, the use of this element in the jewelry industry has been strongly restricted as the UNI EN 1 81 1 standard has come into force, setting the maximum amount of nickel release from metal objects in contact with the skin to 0.5 μg/cm²/week.

[0009] In an attempt to obviate this drawback, metal alloys have been developed for making white gold articles, in which nickel was replaced by palladium and/or manganese.

[0010] Nevertheless, palladium-containing alloys do not seem to be free of drawbacks, especially due to their high cost and their less than optimal workability when melted with gold.

[0011 ] Also, manganese-containing alloys are highly toxic to the human body and relatively hard to work, as this element tends to oxidize rapidly and completely in contact with air.

[0012] In response to this additional drawback, master alloys have been developed in which the antioxidant and/or gold whitening agent is germanium.

[0013] US2005/0201887 discloses a germanium-containing metal alloy for making 14 kt white gold articles, whose composition by weight includes 58,34% gold, 35 to 40% silver as a whitening element, 0,5 to 1 ,8% tin and 0,1 to 0,75% germanium.

[0014] This alloy has the drawback of comprising very small amounts of germanium, which have a non-optimal gold whitening effect, and involve the risk that the finished white articles have an excessively yellowish, matte and low-brightness finish.

[0015] Furthermore, the weight ratio of germanium to the other elements cannot be increased in view of achieving an optimal whitening effect, as this would increase the brittleness and complexity of the alloy matrix.

[0016] An additional drawback of this arrangement is that tin decreases the nobility of the alloy, as this element is less noble than the others.

[0017] Furthermore, the use of tin increases the brittleness of the alloy obtained upon melting with gold, and hinders the whitening effect thereon, thereby reducing the action of germanium.

[0018] Also, the use of tin has been recently regulated by the "Dodd-Frank Act", which sets restrictions to the import of this element and makes it less readily available.

[0019] Finally, the last drawback is that the materials used in this arrangement tend to have low chemical compatibility with each other, and this adversely affects the workability of the alloy.

Technical Problem

[0020] In light of the prior art, the technical problem that the present invention intends to solve is to provide a master alloy that has a high whitening effect on gold, optimal workability and a low cost.

Disclosure of the invention [0021 ] The object of the present invention is to overcome the above drawbacks, by providing a master alloy for making a white gold alloy that is highly efficient and relatively cost-effective.

[0022] A particular object of the present invention is to provide a master alloy for making a white gold alloy that has a considerable whitening effect on gold without using nickel, palladium and/or manganese.

[0023] A further object of the present invention is to provide a master alloy for making a white gold alloy that allows the finished article to maintain a bright shiny appearance.

[0024] Another object of the present invention is to provide a master alloy for making a white gold alloy that has such mechanical properties as to afford easy workability throughout the steps of the process of making the white gold alloy.

[0025] Yet another object of the present invention is to provide a master alloy for making a white gold alloy that comprises readily available and relatively low-cost elements.

[0026] A further object of the present invention is to provide a master alloy for making a white gold alloy that is non-toxic to the human body.

[0027] Another object of the present invention is to provide a gold alloy with a Yellow Index of less than 32, that is obtained from the master alloy, and has a particularly bright color.

[0028] These and other objects, as better explained hereafter, are fulfilled by a master alloy for making a white gold alloy as defined in claim 1 .

[0029] According to a further aspect, the invention relates to a gold alloy with a Yellow Index of less than 32, comprising a mixture of gold and the above mentioned master alloy, as defined in claim 14.

[0030] In yet another aspect, the invention relates to the use of the gold alloy for making a white gold article, as defined in claim 1 5.

[0031 ] Advantageous embodiments of the invention are obtained in accordance with the dependent claims.

Detailed description of a preferred embodiment

[0032] The present invention relates to a master alloy for use in the jewelry industry, and adapted to be mixed with a predetermined weight percentage of gold to obtain a white gold alloy.

[0033] Furthermore, the present invention also relates to a white gold alloy obtained using predetermined weight percentages of gold that define precise karat values, and mixed with such master alloy.

[0034] Particularly, the gold alloy has a Yellow Index of less than 32 and is used to make white gold articles.

[0035] In order to make the article, the master alloy shall have to be first mixed with a predetermined weight percentage of gold and will later undergo a melting process as a result of which the gold alloy will be obtained.

[0036] Then, the gold alloy may be formed into 9 kt, 1 0 kt, 14 kt or 1 8 kt white gold articles, in which the weight percentages of gold in use are 37,5%, 41 ,7%, 58,5% and 75% respectively, based on the total weight of the article.

[0037] According to a preferred embodiment of the invention, the master alloy comprises a weight percentage of germanium for gold whitening purposes.

[0038] Conveniently, the weight percentage of germanium may range from 4% to 25% based on the total weight of the master alloy.

[0039] In the periodic table germanium appears with atomic number thirty- two, atomic radius 1 25 pm, and an electronegativity value of 2,01 by Pauling scale.

[0040] This element has a natural silvery white color and has a greater gold-whitening capacity than other elements such as palladium, nickel, manganese and silver.

[0041 ] Furthermore, germanium has anti-oxidant properties, useful to remove any compounds of less noble elements that might form during melting of the master alloy with the weight percentage of gold.

[0042] The master alloy further comprises a predetermined weight percentage of at least one plasticizing metal which is adapted to impart malleability to the master alloy.

[0043] The plasticizing metal may be zinc, and its weight percentage may be less than 35% and preferably range from 5 to 26% based on the total weight of the master alloy.

[0044] Zinc has a high degree of inherent malleability and its addition into the master alloy makes the latter highly malleable. Such malleability remains almost unaltered even upon melting of the master alloy with gold.

[0045] Furthermore, zinc has a low cost and its interaction with gold also has a light whitening effect thereupon, even though its whitening action is much weaker than that of germanium.

[0046] Alternatively, the plasticizing metal may be silver, and its weight percentage may range from 1 0 to 20% based on the total weight of the master alloy.

[0047] Silver further adds nobility to the master alloy and its presence ensures that the color of the white gold article will remain substantially constant with time, while protecting the remaining metals of the master alloy against oxidation caused by oxygen in air.

[0048] The master alloy further comprises a weight percentage of a binding metal other than the plasticizing metal, which is adapted to form a solid solution with germanium.

[0049] The solid solution is a physico-chemical state of the alloys in which the crystal structure of a solvent, here the binding metal, does not change upon mixing with a solute, here germanium.

[0050] In this state the binding metal and germanium form together a single homogeneous phase.

[0051 ] In a peculiar aspect of the invention, the binding metal is selected from the group of elements whose electronegativity value is substantially equal to that of germanium, for homogeneous distribution of the latter within the solid solution.

[0052] Electronegativity is a relative measure of the ability of an atom to attract electrons within a chemical bond and is deemed to be one of the conditions for the formation of a homogeneous solid solutions, in Hume- Rothery rules.

[0053] Particularly, according to one of the rules, when the element that constitutes the solute has an electronegativity value similar to that of the element that constitutes the solvent, it will tend to be incorporated into the crystal lattice and replace the solvent particles without altering the physico- chemical properties, namely hardness.

[0054] Therefore, a binding metal whose electronegativity is similar to that of germanium allows the use of greater weight percentages of the latter, thereby affording a greater gold whitening effect without affecting the physico-chemical properties of the master alloy.

[0055] Furthermore, the article obtained from the master alloy of the invention will exhibit a color that is typically defined as white by the skilled person, as well as a high degree of workability with the known forming processes as used in the jewelry industry to obtain the finished article.

[0056] Preferably, the binding metal may be selected from the group of elements having an atomic radius that is 7% to 8% greater than the atomic radius of germanium.

[0057] This condition fulfills the second of Hume-Rothery rules, according to which the atomic radius of the two elements that form the solid solution must not differ by more than 15%.

[0058] Advantageously, the binding metal is copper, which has an atomic radius of 135 pm and an electronegativity of 1 ,9 by Pauling scale.

[0059] Furthermore, both copper and germanium have a body-centered cubic crystal structure, thereby fulfilling the third Hume-Rothary rule for the formation of a solid solution.

[0060] The weight percentage of copper may range from 50% to 90% based on the total weight of the master alloy.

[0061] According to a preferred embodiment of the invention, the master alloy may comprise 4% to 15% by weight germanium, 50% to 90% by weight copper, and 5% to 26% by weight zinc, based on the total weight of the master alloy.

[0062] If silver is also included as a plasticizing metal, the master alloy may comprise 4% to 15% by weight germanium, 50% to 80% by weight copper, 5% to 26% by weight zinc, and 10% to 20% by weight silver, based on the total weight of the master alloy. [0063] Additional elements may be further provided for imparting chemico- physical properties to the master alloy.

[0064] Particularly, the master alloy may comprise 0,01 % to 0,5% by weight iridium, ruthenium or rhenium and less than 1 0% by weight indium, based on the total weight of the master alloy.

[0065] Advantageously, the first three elements may be used as grain refiners, thereby improving the mechanical strength of the master alloy, whereas indium helps to reduce the surface porosity of the article.

[0066] A few examples of master alloy compositions of the invention will be now given, according to the karat value of gold in the desired article.

Example I - First master alloy composition for a 1 8 kt white gold article

[0067] Here the master alloy comprises 10% by weight germanium, 25% by weight zinc and 65% by weight copper. The weight percentages of the elements are based on the total weight of the master alloy.

[0068] This composition may be described otherwise, by also including the weight percentage of gold corresponding to the 18 kt value, i.e. 75% based on the total weight of the article. Thus, the article will comprise 2,5% by weight germanium, 6,25% by weight zinc and 1 6,25% by weight copper.

Example II - Second master alloy composition for a 1 8 kt white gold article

[0069] Here the master alloy comprises 7% by weight germanium, 25% by weight zinc, 67,95% by weight copper and 0,05% by weight iridium. The weight percentages of the elements are based on the total weight of the master alloy.

[0070] This composition may be also described by including the weight percentage of gold corresponding to the 1 8 kt value, whereby the article will comprise 1 ,75% by weight germanium, 6,25% by weight zinc, 1 6,99% by weight copper and 0,01 2% by weight iridium.

Example III - Third master alloy composition for a 18 kt white gold article

[0071 ] Here the master alloy comprises 10% by weight germanium, 25% by weight zinc, 55% by weight copper, 1 0% by weight silver and 0,05% by weight iridium. The weight percentages of the elements are based on the total weight of the master alloy. [0072] This composition may be also described by including the weight percentage of gold corresponding to the 1 8 kt value, whereby the article will comprise 2,5% by weight germanium, 6,25% by weight zinc, 1 3,75% by weight copper, 2,5% by weight silver and 0,01 25% by weight iridium.

Example IV - Fourth master alloy composition for a 1 8 kt white gold article

[0073] Here the master alloy comprises 10% by weight germanium, 25% by weight zinc, 60% by weight copper, 5% by weight indium and 0,05% by weight iridium. The weight percentages of the elements are based on the total weight of the master alloy.

[0074] This composition may be also described otherwise by including the gold weight percentage corresponding to the 1 8 kt value, whereby the article will comprise 2,5% by weight germanium, 6,25% by weight zinc, 1 5% by weight copper, 1 ,25% by weight indium and 0,01 25% by weight iridium.

[0075] Example V - Fourth master alloy composition for a 1 8 kt white gold article

[0076] Here the master alloy comprises 15% by weight germanium, 1 5% by weight zinc and 70% by weight copper. The weight percentages of the elements are based on the total weight of the master alloy.

[0077] This composition may be also described otherwise by including the gold weight percentage corresponding to the 1 8 kt value, whereby the article will comprise 3,75% by weight germanium, 3,75% by weight zinc and 1 7,5% by weight copper.

Example VI - First master alloy composition for a 14 kt white gold article

[0078] Here the master alloy comprises 6% by weight germanium, 20% by weight zinc, 73,95% by weight copper and 0,05% by weight iridium. The weight percentages of the elements are based on the total weight of the master alloy.

[0079] This composition may be described otherwise, by also including the weight percentage of gold corresponding to the 14 kt value, i.e. 58,5% based on the total weight of the article. Here the master alloy will comprise 2,49% by weight germanium, 8,3% by weight zinc, 30,69% by weight copper and 0,021 % by weight iridium. Example VII - Second master alloy composition for a 14 kt white gold article

[0080] Here the master alloy comprises 10% by weight germanium, 20% by weight zinc, 69.95% by weight copper and 0.05% by weight iridium. The weight percentages of the elements are based on the total weight of the master alloy.

[0081 ] When expressing the composition with the inclusion of the gold weight percentage corresponding to the 14 kt value, the article will comprise 4,1 5% by weight germanium, 8,3% by weight zinc, 29,03% by weight copper and 0,021 % by weight iridium.

Example VIII - First master alloy composition for a 10 kt white gold article

[0082] Here the master alloy comprises 4,3% by weight germanium, 20% by weight zinc, 75,67% by weight copper and 0,03% by weight ruthenium. The weight percentages of the elements are based on the total weight of the master alloy.

[0083] This composition may be described otherwise, by also including the weight percentage of gold corresponding to the 1 0 kt value, i.e. 41 ,7% based on the total weight of the article. Here the master alloy will comprise 2,5% by weight germanium, 1 1 ,66% by weight zinc, 44,1 1 % by weight copper and 0,01 7% by weight ruthenium.

Example IX - First master alloy composition for a 9 kt white gold article

[0084] Here the master alloy comprises 4,3% by weight germanium, 20% by weight zinc, 75,67% by weight copper and 0,03% by weight ruthenium. The weight percentages of the elements are based on the total weight of the master alloy.

[0085] This composition may be described otherwise, by also including the weight percentage of gold corresponding to the 9 kt value, i.e. 37,5% based on the total weight of the article. Here the master alloy will comprise 2,68% by weight germanium, 1 2,5% by weight zinc, 47,29% by weight copper and 0,01 9% by weight ruthenium.

[0086] The master alloy and article of the invention are susceptible to a number of changes or variants, within the inventive concept disclosed in the appended claims. [0087] All the details thereof may be replaced by other technically equivalent parts, and the materials may vary depending on different needs, without departure from the scope of the invention.

Industrial applicability

[0088] The present invention may find application in industry, because it can be produced on an industrial scale in jewelry metalworking factories.

Claims

1 . A master alloy designed to be mixed with a predetermined weight percentage of gold to form a white gold alloy with a Yellow Index of less than 32, wherein said master alloy contains:

- a predetermined weight percentage of germanium for gold- whitening purposes;

a predetermined weight percentage of at least one plasticizing metal, adapted to impart malleability to the master alloy;

a predetermined amount of a binding metal to form a solid solution with germanium,

characterized in that it is free of nickel, manganese or palladium and in that said binding metal is copper, said plasticizing metal being zinc and/or silver, said weight percentage of germanium ranging from 4% to 25% based on the total weight of the master alloy, said weight percentage of zinc and/or silver being greater than the maximum weight percentage of germanium, and said weight percentage of copper being greater than said weight percentage of zinc and/or silver.

2. Master alloy as claimed in claim 1 , characterized in that said weight percentage of germanium ranges from 4% to 1 5%, said weight percentage of copper ranges from 50% to 90%, and said weight percentage of zinc ranges from 5% to 26%, based on the total weight of said master alloy.

3. Master alloy as claimed in claim 1 , characterized in that said weight percentage of germanium ranges from 4% to 1 5%, said weight percentage of copper ranges from 50% to 90%, said weight percentage of zinc ranges from 5% to 26, and said weight percentage of silver ranges from 1 0% to 20%, based on the total weight of said master alloy.

4. Master alloy as claimed in claim 2 or 3, characterized in that it further comprises 0,01 to 0,5% by weight iridium or ruthenium or rhenium, based on the total weight of said master alloy.

5. Master alloy as claimed in claim 4, characterized in that it further comprises less than 1 0% by weight indium, based on the total weight of said master alloy.

6. Master alloy as claimed in claim 2, characterized in that said weight percentage of copper ranges from 40% to 66%, said weight percentage of zinc ranges from 5% to 26%, and said weight percentage of germanium ranges from 5% to 1 1 %, based on the total weight of the master alloy.

7. Master alloy as claimed in claim 4, characterized in that said weight percentage of copper ranges from 40% to 68%, said weight percentage of zinc ranges from 5% to 26%, said weight percentage of germanium ranges from 5% to 8%, and said weight percentage of iridium ranges from 0,05% to 0,1 % based on the total weight of the master alloy.

8. Master alloy as claimed in claim 4, characterized in that said weight percentage of copper ranges from 40% to 56%, said weight percentage of zinc ranges from 5% to 26%, said weight percentage of germanium ranges from 5% to 1 1 %, said weight percentage of silver ranges from 5% to 1 1 %, and said weight percentage of iridium ranges from 0,05% to 0,1 % based on the total weight of the master alloy.

9. Master alloy as claimed in claim 5, characterized in that said weight percentage of copper ranges from 40% to 61 %, said weight percentage of zinc ranges from 5% to 26%, said weight percentage of germanium ranges from 5% to 1 1 %, said weight percentage of indium ranges from 1 % to 6%, and said weight percentage of iridium ranges from 0,05% to 0,1 % based on the total weight of the master alloy.

1 0. Master alloy as claimed in claim 4, characterized in that said weight percentage of copper ranges from 40% to 74%, said weight percentage of zinc ranges from 5% to 21 %, said weight percentage of germanium ranges from 5% to 7%, and said weight percentage of iridium ranges from 0,05% to 0,1 % based on the total weight of the master alloy.

1 1 . A master alloy as claimed in claim 4, characterized in that said weight percentage of copper ranges from 40% to 70%, said weight percentage of zinc ranges from 5% to 21 %, said weight percentage of germanium ranges from 5% to 1 1 %, said weight percentage of iridium ranges from 0,05% to 0,1 % based on the total weight of the master alloy.

1 2. A master alloy as claimed in claim 4, characterized in that said weight percentage of copper ranges from 50% to 76%, said weight percentage of zinc ranges from 5% to 21 %, said weight percentage of germanium ranges from 4% to 5%, and said weight percentage of ruthenium ranges from 0,01 % to 0,05% based on the total weight of the master alloy.

1 3. Master alloy as claimed in claim 2, characterized in that said weight percentage of copper ranges from 40% to 71 %, said weight percentage of zinc ranges from 5% to 1 6%, and said weight percentage of germanium ranges from 5% to 1 6%.

14. A gold alloy with a Yellow Index of less than 32, comprising a mixture of gold and a master alloy as claimed in one or more of the preceding claims.

1 5. Use of the gold alloy as claimed in claim 14 for making a white gold article.