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Patent Abstract
Timepiece or jewelry part manufactured in an alloy comprising at
least 75% gold and between 15% and 18% copper or at least 18% copper,
by weight. According to the invention, the alloy also comprises
between 0.5% and 4% platinum with the exclusion of the 0.5% content
when the copper content is equal to 15%. Preferably, the weight
content of copper is between 20% and 22% for a weight content of
platinum between 1.5% and 3%.
Patent Claims
1. A timepiece or jewelry part manufactured in an alloy comprising
at least 75% gold and between 15% and 18% copper or at least 18%
copper by weight, wherein the alloy also comprises between 0.5%
and 4% platinum, with the exclusion of the 0.5% content when the
copper content is equal to 15%.
2. The timepiece or jewelry part as claimed in claim 1, wherein
the alloy comprises between 20% and 22% copper and between 1.5%
and 3% platinum.
3. The timepiece or jewelry part as claimed in claim 2, wherein
the alloy comprises, by weight, at most 0.5% of any one of the elements
chosen from silver, cadmium, chromium, cobalt, iron, indium, manganese,
nickel or zinc.
Patent Description
[0001] The invention relates to a timepiece or jewelry part manufactured
in an alloy comprising at least 75% gold and at least 6% copper
by weight. More particularly, the invention relates to such a timepiece
or jewelry part manufactured in an alloy comprising at least 75%
gold and at least 15% copper by weight.
[0002] The color of such gold alloys depends on their copper and
silver contents. A copper content of greater than 18% and a silver
content of around 4% give them a red color. The color changes toward
pink and then toward yellow if the copper content decreases from
18% to 15% and then from 15% to 6% and if the silver content increases
from 4% to 15%. Color is conventionally defined by a point in the
CIELAB space formed by a red/green x-axis, a yellow/blue y-axis
and an axis representative of the contrast (cf. Standard ISO 7724
drawn up by the Commission Internationale de l'Eclairage [International
Commission on Illumination]). The colors of gold alloys are defined
in the trichromatic space according to Standard ISO 8654.
[0003] The Applicant has observed that watch cases or bracelets
manufactured in these standard gold alloys have a tendency to undergo
a progressive modification in their color through the action of
tap water, sea water, swimming pool water, salt water or even soapy
water.
[0004] Document DE-A-19958800 discloses a timepiece or jewelry
part manufactured in an alloy comprising between 40% and 80% gold,
between 0% and 15% copper, between 1% and 40% silver, between 1%
and 15% iron and between 0% and 15% palladium. Iron is alloyed to
these elements in order to replace nickel (regarded as allergenic),
to limit the content of palladium (regarded as expensive) and to
give the alloy a white gold color. The alloy may contain between
0% and 0.5% of any of the following elements: platinum, ruthenium,
rhodium, iridium, tungsten or tantalum in order to refine the grain
size.
[0005] The specialized literature reports an accelerated tarnishing
study carried out on an alloy intended for the manufacture of items
of jewelry, comprising 75% gold, 12% copper and 12% silver. Tests
were carried out in the gas phase or in the liquid phase. Tarnishing
was determined quantitatively by the difference in color of the
alloy before and after the test. The alloy was exposed to contact
with reactants which essentially comprised pure sulfur or sulfur
compounds. The observed tarnishing was attributed to the formation
of silver sulfide Ag.sub.2S (cf. "Tarnishing of AuAgCu alloys",
43, pp. 48-55, 1992, Werkstoffe und Korrosion).
[0006] Document CH-219 711 discloses an alloy intended for the
manufacture of dental prostheses, which contains between 65% and
75% of a gold/platinum alloy in which the platinum content is between
2% and 5%, between 1% and 6% silver, between 8% and 14% copper,
between 8% and 14% cadmium and between 0.1% and 1% zinc. Platinum
is alloyed to these elements in order to give this yellow gold alloy
good tarnishing resistance and corrosion resistance in the mouth.
[0007] Another study related to an alloy comprising at most 71%
gold, between 12% and 14% copper, between 7.5% and 25% silver, between
0.6% and 4% platinum and between 0.9% and 3.7% palladium in order
to determine the biocompatibility thereof with a view to using it
to manufacture dental prostheses. Corrosion tests were carried out
at room temperature in an aqueous solution containing lactic acid
and sodium chloride, at an acid pH of about 2.3. An increase in
the metal ion concentration showed that copper and silver pass into
solution. The depletion of the two constituents was confirmed by
analysis of the first few atomic layers of the surface of the alloy
carried out by Auger spectroscopy. Under the experimental pH conditions,
the depletion of the copper appeared to be greater the lower the
gold and platinum content. In contrast, the platinum content had
no appreciable effect on the dissolution of the silver (cf. "Biocompatibility
of dental alloys", 3(10), 2001, Advanced engineering materials).
[0008] Document GB-A-2 279 662 discloses an alloy intended for
watchmaking or jewelry, comprising between 33% and 90% gold, between
0.1% and 2.5% iron, between 0.01% and 62.5% silver, between 0.01%
and 62.5% copper and between 0.01% and 62.5% zinc and having a hardness
of between 100 and 280 Hv. Iron was alloyed to the other elements
of the alloy in order to give it a greater hardness and to prevent
grain growth during soldering operations. Moreover, better resistance
to color changes was observed in heat treatments. The alloy may
contain between 0.01% and 25% palladium, nickel or cadmium, between
0.01% and 10% indium, tin, gallium, cobalt, platinum or rhodium
and between 0.01% and 3% iridium, ruthenium, silicon or boron. The
alloys provided by way of example all comprise 37.53% gold, 8.70%
or 9.20% silver, 42.40% copper, 10.87% or 10.67% or 10.57% or 10.37%
zinc and 0.5% or 0.7% or 0.8% or 1% iron.
[0009] Finally, a timepiece or jewelry part manufactured in an
alloy comprising at least 75% gold and between 15% and 23% copper
is known from Japanese Patent Application JP 10245646 published
in 1998. The alloy furthermore comprises between 0.3% and 5% palladium
in order to have a higher crack resistance when casting the part.
[0010] One of the objects of the invention is to improve the resistance
to color change of a timepiece or jewelry part manufactured in a
gold alloy and exposed, during use, to slightly aggressive aqueous
media.
[0011] For this purpose, the subject of the invention is a timepiece
or jewelry part manufactured in an alloy comprising at least 75%
gold and between 15% and 18% copper or at least 18% copper by weight,
wherein the alloy also comprises between 0.5% and 4% platinum, with
the exclusion of the 0.5% content when the copper content is equal
to 15%.
[0012] The platinum content makes it possible to increase the resistance
to color change of the part exposed to the action of tap water,
sea water, swimming pool water, salt water or even soapy water.
[0013] The timepiece or jewelry part may be manufactured in an
alloy further comprising at most 4% palladium in order to increase
the resistance to color change. This is the case, for example, for
an alloy of yellow color comprising between 6% and 15% copper.
[0014] Other advantages will become apparent in the light of the
description of one particular embodiment of the invention, illustrated
by the drawings.
[0015] FIG. 1 shows two experimental discoloration curves obtained
respectively on a red alloy according to the invention--curve (b)--and
on a 5N red alloy according to the prior art--curve (a).
[0016] FIGS. 2a and 2b show two concentration profiles obtained
on the respective two alloys that have undergone the discoloration
test illustrated by FIG. 1.
[0017] Table I gives the discoloration test results obtained on
various alloys according to the invention.
[0018] A 5N control alloy of red color comprising 75% gold, 20.5%
copper and 4.5% silver was subjected to a discoloration test. The
alloy was immersed in a neutral solution saturated with sodium chloride
at a temperature of 40.degree. C. for several tens of days. The
color was measured according to Standard ISO 7724. The rate of discoloration
is illustrated by curve (a) in FIG. 1. Plotted on the x-axis is
the immersion time in days and plotted on the y-axis is the norm
of the vector .DELTA.Elab connecting the representative points of
the color of the alloy in the CIELAB space, at the initial time
and after the various immersion times. Over the time period explored,
the discoloration appears as a continuous monotonic curve with immersion
time.
[0019] An alloy of red color according to the invention, comprising
76% gold, 21% copper and 3% platinum, was tested under the same
conditions as those of the control alloy. The rate of discoloration
is illustrated by curve (b). This shows that the norm of the vector
connecting the representative points of the color of the alloy according
to the invention at the initial time and after the various immersion
times is less than that for the control alloy containing no platinum.
In other words, the presence of platinum has increased the discoloration
resistance of the alloy according to the invention. Quantitatively,
an improvement factor F is defined by the ratio of the color change
of the control alloy to the color change of the alloy according
to the invention, both changes being considered after the same immersion
time. In the present case, the improvement factor is around 3 after
an immersion time of 60 days.
[0020] RBS (Rutherford Backscattering Spectroscopy) analyses were
carried out in order to scan a significant depth 35 of material
relative to the path of the light waves in the two alloys tested
above, that portion of the reflected light waves determining the
color of the alloy.
[0021] FIGS. 2a and 2b show the concentration profiles obtained
on the control alloy 5N and on the alloy according to the invention,
respectively, after 60 days of immersion in the test solution. In
the case of the control alloy 5N, FIG. 2a shows, with respect to
the bulk concentrations of copper and silver, a reduction in the
copper concentration proportional with that of the gold over a depth
of material between the first ten and the first twenty nanometers,
while the silver concentration is maintained over this same depth.
In contrast, in FIG. 2b the copper concentration in proportion to
that of gold decreases less strongly and less deeply in the case
of the alloy according to the invention.
[0022] It is apparent from these analyses that the discoloration
of the control alloy 5N is due to copper dissolving in a deep layer
over a few tens of nanometers. The platinum content makes it possible
to limit the dissolution of copper in the alloy according to the
invention and thus to increase the discoloration resistance of the
latter in the test solution.
[0023] Referring to curve (b) of FIG. 1, the rate of discoloration
of the alloy according to the invention tends toward a limiting
value after about the fifteenth day. The existence of this limiting
value stems from the stable thermodynamic equilibrium that the composition
of the alloy gives the material. Such color stabilization of the
alloy remains a very unexpected result under the conditions of the
discoloration test used. This test may be useful from the industrial
standpoint for the finishing of a timepiece or jewelry part manufactured
in an alloy comprising, by weight, at least 75% gold, between 15%
and 18% copper, or at least 18% copper and between 0.5% and 4% platinum,
with the exclusion of the 0.5% content when the copper content is
equal to 15%, whereby the part is immersed in a saturated saline
solution at neutral pH for a time and at a temperature that are
defined in order to achieve the equilibrium value of the color of
the part. In general, any solution allowing surface dissolution
of copper until the equilibrium color is reached could be used.
It should be pointed out that the limiting discoloration value illustrated
by curve (b) remains within the eye's limit of perception of a color
change of the part.
[0024] Table I gives the results of the discoloration test carried
out on alloys of various compositions numbered from 1 to 20. The
headers of the table indicate the gold, copper, platinum and palladium
contents of the alloy, and also the limiting discoloration value
.DELTA.ELab and the discoloration improvement factor F after a 60
day immersion test. The experimental conditions are the same as
those indicated previously, namely immersion in a saturated sodium
chloride solution at neutral pH and a temperature of 40 degrees
Celsius.
[0025] The alloys of the compositions numbered from 20 to 9 in
table I typically exhibit a discoloration resistance improvement
factor between 1.5 and 4. The alloys denoted by 5N and 4N serve
as controls in calculating the improvement factor of alloys 1 to
18 and of alloys 19 and 20, respectively.
[0026] An alloy comprising 91.7% gold and 8.3% copper has an improvement
factor of less than unity, as indicated by the reference number
8. This result shows that simply seeking an increase in the gold
content has an effect of reducing the discoloration resistance of
the alloy.
[0027] Likewise, the addition of elements such as aluminum, niobium,
tantalum, titanium or silicon, for the purpose of forming an oxide
layer suitable for limiting the dissolution of copper in the saturated
saline solution at neutral pH does not lead to an improvement in
the discoloration resistance of the alloys either. In contrast,
the alloys whose compositions are numbered from 7 to 3 in table
I exhibited an improvement factor of at most 1.
[0028] Finally, the results indicated in table I for reference
2 show that the addition of zinc for the purpose of forming a sacrificial
anode at the surface of the alloy does not lead to an improvement
in the discoloration resistance either.
[0029] The improvement factor depends on the weight content of
copper in the alloys according to the invention. Preferably, this
content is between 20% and 22% for a platinum content of between
1.5% and 3%.
[0030] In addition, a platinum content between 0.5% and 4% gives
timepieces or jewelry parts according to the invention a color that
it was impossible to obtain hitherto. Although copper has a reddening
effect and silver a greening effect, platinum has a blanching effect.
The addition of platinum or palladium with a graying effect thus
makes it possible to pass gradually from warm and lush colors in
the case of the lowest contents through to more specialized, cooler
colors in the case of the highest contents.
[0031] More particularly, a timepiece or jewelry part manufactured
in an alloy comprising, by weight, at least 75% gold, between 20%
and 22% copper, between 1.5% and 3% platinum and at most 0.5% of
any one of the elements chosen from silver, cadmium, chromium, cobalt,
iron, indium, manganese, nickel or zinc possesses a nominal color
having, in the CIELAB space, an abscissa of 7.41 on the red/green
axis, an ordinate of 15.67 on the yellow/blue axis and a contrast
value of 86.75. Depending on the precise composition of the alloy,
these coordinates may vary between 5.71 and 8.51 on the red/green
axis and between 13.67 and 16.67 on the yellow/blue axis for a contrast
value L varying between 76.75 and 96.75.
[0032] The invention applies to any timepiece or jewelry part manufactured
from an alloy using the standard processes, such as machining, stamping
or lost wax casting. |