Platinum alloy jewellery wire

This invention relates to fabricating wire and particularly wire at least the surface of which consists of platinum or a platinum-based alloy for use in the jewellery industry. The wire according to the invention comprises a first portion at least the outer surface of which consists essentially of platinum or a platinum-based alloy and a second portion in the form of a coating or layer on the outer surface of the first portion, the second portion consisting essentially of a metal or alloy which is soft relative to the platinum or platinum-based alloy. Preferably, the second portion consists essentially of copper.

This invention relates to an improved form of fabricating wire, in 
particular wire at least the surface of which consists essentially of 
platinum or a platinum-based alloy and used in the jewellery industry, for 
example, for making chain suitable for supporting pendants, necklaces and 
the like. 
Platinum and platinum-based alloys are becoming increasingly popular as 
materials for jewellery manufacture but a well-known disadvantage 
associated with the use of such materials is that they tend to cause an 
abnormally high and unexplained rate of wear on the cutting or other 
working edges or surfaces of fabricating tools such as croppers, mandrels 
and so on. 
Wire for use in the manufacture of platinum or platinum-based jewellery, 
for example, for fabricating chain for use with platinum jewellery, may 
consist essentially of platinum or an alloy of platinum with, say, 5% of 
palladium, copper, cobalt or iridium, but typically consists of a coating 
or outer skin of essentially pure platinum surrounding a solder core. The 
wire is generally fabricated into chain on a machine which firstly forms 
the wire into a helix, secondly crops and holds successive individual open 
coils of the helix, thirdly links two or more, as required for the desired 
pattern of chain, coils together and finally brings together the ends of 
each coil defining the opening therein to form a succession of closed 
links. The chain is then ready for soldering which, in the case of a chain 
formed from solder-cored wire, may readily be effected by passing the 
chain through a furnace maintained at a suitable soldering temperature. 
Whilst machines of the type described give many hours of service when 
fabricating, say, brass or a silver, or a gold-based alloy chain, the 
working and , particularly, cutting edges and surfaces deteriorate 
extremely rapidly when used with wire containing, at least as a surface 
layer, platinum or a platinum-based alloy. 
We have now found that this problem is substantially eliminated if the wire 
contains, as an outer skin or layer, a coating of soft metal, for example 
copper or silver.

According to the invention, therefore, we provide wire suitable for use in 
the manufacture of platinum or platinum-based jewellery and comprising a 
first portion at least the outer surface of which consists essentially of 
platinum or a platinum-base alloy and a second portion in the form of a 
coating or layer on the outer surface of the first portion, the second 
portion consisting essentially of metal or alloy which is soft relative to 
the platinum or platinum-base alloy. 
The coating or layer may be of silver or silver-alloy but preferably, for 
reasons of cost, ease of application and so on, it consists essentially of 
copper. 
The coating or layer preferably should not be less than about 1/3 thou inch 
thick and need not be more than 1 thou inch thick. 
The first portion may, according to the invention, comprise either a solid 
portion consisting essentially of platinum or a platinum-based alloy or it 
may comprise a tubular portion consisting essentially of platinum or a 
platinum-based alloy surrounding and in contact with a core section 
comprising solder. A typical platinum-based alloy consists essentially of 
platinum with, say, 5% by weight of one or more of palladium, copper, 
cobalt and iridium. 
It is a feature of the invention that, after the wire has been fabricated 
into a chain or other article, the copper or other soft metal coating or 
layer is readily removable, for example in an acid medium, desirably one 
formulated not to attack any solder core section in the first portion. 
Platinum or platinum-based wire may most conveniently be coated with a 
layer of soft metal by electroplating or chemical plating. For example, 
and in order to coat a wire with a layer of copper, the wire may be passed 
through an electrolyte comprising a cyanide salt, for example a Rochelle 
salt bath containing "Canning Cupranax 2989" plating salt (80gl.sup.-1), 
sodium potassium tartrate (50gl.sup.-1) and water, at 64.degree. C. The 
wire is initially etched by reversing polarity--that is, by rendering it 
positively charged--and passing a current of 40 amps at 3 volts for 10 
minutes, before restoring the polarity to render the wire cathodic and 
passing a current of 20 amps at 2 volts for 20 minutes. This will 
typically plate a layer of copper onto the wire, the layer having a 
thickness of about 1/2 thou inch or 12-13.mu.. We have found that layers 
thinner than this, say 3-4.mu., tend to flake off. The plated wire is then 
washed leaving the coating of copper in a dull, or matt, state due to 
surface nodules creating roughness. When making fine chain these nodules 
tend to cause clogging in a chain-making machine, such as of the type 
described, so it is desirable to remove them and this may simply and 
conveniently be carried out by drawing the plated wire through a die to 
reduce the overall diameter by about 1/2 thou inch and produce a smooth 
finish. As an alternative to this procedure, the platinum or 
platinum-based wire can be plated from a bright plating bath, thus 
avoiding the need to draw down to a smooth finish. 
The copper coating can readily be removed from the wire after fabrication 
into chain by, for example, immersion in 75% nitric acid for about 20 
seconds. 
We have found, using wire according to the invention, for example, 
fabricating into jewellery chain, that the working and cutting edges and 
surfaces of a chain-making machine of the type described have a useful 
service life comparable with the expected life when using wire other than 
of platinum or a platinum-base alloy. Furthermore, since, after removing 
or stripping the soft metal coating or layer, the material of the chain is 
substantially the same as before the soft metal coating was applied, there 
is no problem in assay tests, hallmarking and the like of the chain itself 
or of jewellery articles associated with the chain.