DESCRIPTION

Technical field of the invention

The invention concerns an alloy for the production of jewels. The invention also concerns a process for preparing the alloy for mechanical machining, its use for precision casting and the items produced therewith.

State of the art

A large number of gold alloys for making jewels is known in the market of jewellery and costume jewellery. Gold is a metal that does not oxidize easily but it is very expensive. The specific weight of gold is relatively high, which limits the reasonable size of jewels, in particular in the case of earrings and chains for necklaces in large sizes. In order to be able to offer jewels also at contained prices, the market of costume jewellery has developed various alloys based on copper and zinc, components of brass, which cost less than gold alloys but pose several drawbacks.

In order to avoid the oxidation of the alloy and thus the alteration of the gilded appearance, it is necessary to coat the item, for example with a golden layer or with galvanic treatments. This layer, however, is subject to wear. Due to the oxidation processes, the alloys that replace the traditional gold alloys do not maintain their gilded appearance over time and need to be frequently coated with alloys having a high gold content. Moreover, often these alloys cannot imitate the external appearance of gold perfectly, meaning its colour and degree of brightness.

The US patent application no. US 2009/0317291 A1 describes gold alloys with various carat ratings, comprising silver, copper, zinc and, in small quantities, silicon and/or germanium. The application, however, admits that some of the alloys with low carat rating present non-negligible oxidations, without specifying, however, what alloys with low carat rating do not suffer from this problem.

Presentation of the invention

The main object of the present invention is to propose an alloy with low gold carat rating that can be used to create jewels having substantially the same brightness and the same colour as 18-carat jewels. It is a further object of the invention to propose alloys with low gold carat rating that have the appearance of traditional gold alloys, in particular yellow gold or rose gold alloys, offer better workability than 18-carat gold and do not oxidize easily.

A further object of the present invention is to propose an alloy whose specific weight is lower than that of traditional gold alloy from 8 carats upwards.

The objects mentioned above and others that are highlighted in greater detail below are achieved by an alloy for making jewels as defined in claim 1.

The variation in the content of copper compared to metals in silver colours, in particular zinc and silver, makes it possible to obtain different colours of the alloy, varying from red or rose through gold yellow until obtaining hues that correspond to those known for white gold.

Surprisingly, a minimum quantity of gold, compared to the quantity of standard gold corresponding to 24 carats, allows items to be produced that may be 40% bigger than 18-carat items, given the lower specific weight, proportionally smaller than with other carat ratings, and makes it possible to obtain an alloy whose colour is like that of 18-carat gold and which, above all, does not oxidize easily.

Moreover, the alloy is very elastic, partly easier to clean, polish and machine than other gold alloys with higher carat rating. The result of lapping after polishing is better than that obtained with 18, 14 and 9- carat gold alloys.

It cannot be excluded that the alloy may contain tin, silocon, germanium, gallium and indium in quantities that are considered traces.

Already with four main components the alloy can be adapted to the most different needs in terms of colour and workability, and this without affecting its resistance to oxidation.

Compared to the alloys described in document US 2009/0317291 A1 , the alloy according to the invention has a higher zinc content and a lower silver content, a choice that cannot be considered a routine solution for the expert when needing to obtain some specific colours, as zinc and silver move in similar directions, or to obtain alloys with low oxidizability, since low oxidizability is more related to silver than to zinc.

The expression "approximately" indicates that the gold percentage corresponds to the weight that in the industry is considered as one carat of gold. This minimum quantity of gold in the composition of the alloy according to the invention is sufficient to guarantee a low oxidizability of the material and makes it possible to define the alloy in terms of gold carats.

Advantageously, the alloy that is the subject of the invention makes it possible to obtain a metallic material that can be used to create jewels having substantially the same brightness and the same colour as 18-ca rat jewels. Preferably, the alloy substantially has the following composition: approximately 4.17% gold by weight, 51.1 to 51.3% copper by weight, 35.4 to 35.6% zinc by weight and 8.9 to 9.1% silver by weight. This composition has the appearance of traditional yellow gold.

Compared to the yellow gold alloys proposed by document US 2009/0317291 A1 , the alloy according to the invention produces a yellow gold colour through an increase in the content of the "silvery" metal zinc and a decrease in the content of copper.

A preferred alloy suited to imitate the appearance of rose gold is the following: approximately 4.17% gold by weight, 69.3 to 69.5% copper by weight, 17.1 to 17.3% zinc by weight and 9.0 to 9.2% silver by weight.

One aspect of the invention concerns the use of the alloy according to the invention for precision casting. After the first melting step the alloy according to the invention can be used for precision casting.

A further aspect of the invention concerns a process that prepares the alloy for mechanical machining.

The process includes the following steps, in the given order: (a) preparation of an alloy according to the invention; (b) repeated casting, preferably double casting, of the alloy, and (c) repeated annealing in the oven at approximately 800°C.

Another important aspect of the invention concerns the items produced with the alloy according to the invention.

According to a preferred variant embodiment of the invention, the item is a jewel. Other possible items are, for example, watches, coins, cutlery, candle holders or other artistic craftwork items.

The invention achieves the object to provide an improved alloy that, in addition to looking like gold and offering mechanical workability and casting workability well above those of 8-carat and 22-carat gold, does not oxidize easily.

Furthermore, the proposed alloy has a specific weight that is lower than that of traditional gold alloys starting from 8 carats.

The invention also achieves the object to provide a process for preparing the alloy for mechanical machining.

Further variants of the invention are described in the dependent claims. In the production stage the alloy, its intended use, the items manufactured therewith and the process that are the subjects of the invention can be subjected to modifications and variants that are not described herein. Said modifications or variants must all be considered protected by the present patent, provided that they fall within the scope of the claims expressed below.

Description of the preferred embodiments of the invention

The production of the alloy follows the methods known in the art for producing gold, silver, brass alloys etc which are not described herein for the sake of brevity.

The following table 1 lists two examples of alloy: one is a yellow gold-coloured alloy, while the other is a rose gold-coloured alloy.

Table 1

The sum of the percentages does not correspond exactly to 100%, due to traces of other elements contained in the raw materials or to possible errors made while determining the percentages or weighing the raw materials.

Concerning the example of the yellow gold-coloured alloy, the table 2 lists in detail also the traces of other elements.

Table 2. More detailed composition of the yellow gold-coloured alloy: