Solution for electroplating a gold-copper-cadmium alloy

A bath and method for electrodepositing gold-copper-cadmium alloys. The electroplating solution contains cadmium ions complexed by a soluble hydroxyalkyl diacetate complexing agent and, in addition, a brightener in the form of a phosphoric ester of a chain of ethylene oxides bonded to an alkyl group, and/or a surfactant in the form of one or a mixture of more than one amidopropyldimethylaminoxides of fatty acids.

This invention relates to the art of electroplating, and more particularly 
to a solution for the electrolytic deposition of a gold-copper-cadmium 
alloy, a method of using this solution, and an article obtained by this 
method. 
The electrolytic deposition of a gold-copper-cadmium alloy has taken on 
great importance because it yields a bright, workable coating, although 
relatively thick, of gold alloy, the color of which may vary from pale 
yellow to pink, while still retaining a gold content of from 12 to 20 
karats. Gold-copper alloys including a white metal to give them a paler, 
more yellowish color have been used for many years. At present, cadmium is 
accepted to have the best electrochemical qualities required for being 
deposited together with gold and copper. 
Numerous patents describe methods relating to the deposition of such 
gold-copper-cadmium alloys, particularly U.S. Pat. No. 4,309,256 and Swiss 
Pat. Nos. 542,934, 556,916, and 621,367, which are representative of the 
state of the art forming the point of departure for the present invention. 
All these methods use cyanided solutions containing a complex gold 
cyanide, a complex copper cyanide, potassium cyanide, a complex organic 
cadmium salt stable enough to exist in a cyanided solution, and an organic 
brightening agent. In these plating solutions, two chemical agents control 
the mechanism of deposition, viz., the cadmium complexing agent and the 
brightener; for although the gold-copper alloy is normally deposited from 
the complex cyanides of these metals, the cadmium must necessarily be 
obtained from an organic complex which makes it possible to stabilize the 
cadmium content of the alloy deposited. If a complex cyanide were also 
used, without chelating agent, for the cadmium, either brittle, white 
deposits would be obtained, or else the cadmium content of the plating 
solution would have to be maintained at very low levels, which would 
present problems as regards the regeneration and useful life of the 
solution. On the other hand, it is economically necessary to obtain a 
bright deposit even for thicknesses of 30 or 40 microns. It is therefore 
important to add brightening agents to the electrolytic solution, for 
otherwise the deposit is dull and requires polishing, always an expensive 
operation. 
The complexing agents used to regulate the amount of cadmium in the 
electrolytically deposited gold-copper-cadmium alloy are most often 
soluble organic products, the molecule of which contains one or more atoms 
of nitrogen and carboxylic acid groups which ensure the solubility. Thus, 
the aforementioned Swiss Pat. Nos. 542,934 and 556,916 relate to the use 
of ethylene diamine tetraacetic acid (EDTA) and nitrilotriacetic acid 
(NTA), U.S. Pat. No. 4,309,256 to the use of NTA alone, and Swiss Pat. No. 
621,367 to the use of compounds containing nitrogen and carboxylated 
propyl groups. 
Experience has shown that NTA and EDTA strongly complex the cadmium. Hence 
the concentrations of cadmium and complexing agent must be kept within 
rather strict limits, which does not allow a very flexible control of the 
solution. If the concentration of complexing agent is too low, the deposit 
becomes too pale, and if it is too high, the deposit becomes pink because 
the cadmium is no longer deposited. The gold-copper-cadmium alloy is then 
easily attacked by nitric acid, which is not the case when the alloy is 
correct. The aforementioned Swiss Pat. No. 621,367 mentions these 
shortcomings and proposed remedying them by using complexing agents having 
a molecule comprising several carboxylated propyl groups. Since the 
stability constant of the cadmium complexes of these products is less than 
that of NTA AND EDTA, such compounds may be present in the solution in a 
relatively broad range of concentrations. However, when the concentration 
is too low, secondary effects cause brittle deposits with poor 
homogeneity, whereas an excess leads to pitted deposits and uneven 
thicknesses. 
It is an object of this invention to provide an improved electroplating 
solution and method of depositing a gold-copper-cadmium alloy whereby the 
solution can be more easily controlled than is possible in the case of the 
prior art. 
To this end, the solution according to the present invention comprises a 
complex gold cyanide, a complex copper cyanide, cadmium ions, free 
potassium cyanide, a wetting agent as a brightener and a complexing agent 
of the general formula 
##STR1## 
wherein R signifies an alkylene group having 1-4 carbon atoms, and M 
signifies sodium, potassium or ammonium ions. 
The proposed electrolytic solution contains complexing agents having a 
stability constant better adapted to the simultaneous deposition of 
cadmium, gold, and copper than that of the prior art complexing agents. 
The present agents are of the hydroxyalkylaminodicarboxylic type of the 
general formula 
##STR2## 
wherein R signifies an alkylene group having 1-4 carbon atoms. The 
stability constant of these compounds is on the order of 9 when R is a 
methyl group, 7.5 when R is an ethyl group, 6.2 for a propyl group, and 
about 5 for a butyl group. The presence of the hydroxy group seems 
particularly important for conferring on these molecules a stability 
constant favorable to the co-deposition of the cadmium with the gold and 
the copper in a cyanided medium. The carboxylic acid group must not 
comprise more than two carbon atoms. With three carbon atoms, the 
stability constant is too low to permit good co-deposition of the cadmium 
in the presence of potassium cyanide. Whereas the complexing agent margin 
for obtaining a correct deposit is only 4-6 g/liter with NTA and 0.8-1.0 
g/liter with EDTA, the complexing agents present in the solution proposed 
by the present invention and, in particular, sodium, potassium, or 
ammonium hydroxyalkylimino diacetate, may be added with a much larger 
margin for maneuver, in quantities up to ten times as great, without 
changing the qualities of the gold-copper-cadmium alloy deposit as regards 
both its color and its resistance to corrosion. 
Besides the cadmium ion complexing agent, solutions for depositing a 
gold-copper-cadmium alloy generally contain a product intended to produce 
a bright deposit, even for very thick coatings. To achieve this, organic 
molecules are used which act at the level of the layer of electrolyte 
closest to the surface to be coated and which influence the diffusion of 
the metal ions through that surface by varying the surface tension of the 
liquid. Hence these products act upon the crystallization of the alloy 
deposited. The patents previously mentioned disclose the use of the 
phosphoric ester of the condensation product of nonyl phenol and a chain 
of ethylene oxides, substances having a molecule comprising both an 
aliphatic chain and a benzene nucleus, or the use of a polyoxyalkylene 
derivative. It has been found, however, that these products cause 
irregularities in the form of pits or craters in the alloy deposited as 
soon as their concentration exceeds a certain maximum, so that they can be 
used only within a narrow margin of concentrations. 
For this reason, the solution proposed by the present invention also 
preferably includes, as brighteners in the solution for electrolytic 
deposit of a gold-copper-cadmium alloy, products having a molecule 
comprising only aliphatic chains bonded to a phosphoric ester of a chain 
of ethylene oxides. In particular, products corresponding to the following 
general formulae have been used successfully: 
##STR3## 
wherein x signifies a number from 8-18 and y signifies a number from 6-15. 
The molecular weight of these various compounds is from 600 to 800 for the 
monoesters and from 800 to 2000 for the diesters. The alkyl chains may be 
straight or branched (iso). 
Such products having an aliphatic chain are much better tolerated by the 
constituents of the solution than those where the molecule contains a 
benzene nucleus, and as a result it is possible to use concentrations from 
five to ten times higher, in the form of sodium, potassium, or ammonium 
salts, which are easily soluble. 
According to the present invention, there is used alternatively a 
surfactant which would act at the same time as a brightening agent; in 
particular, the amidopropyldimethylaminoxide-type derivatives of fatty 
acids corresponding to the following general formula have been used 
successfully: 
##STR4## 
wherein z signifies a number from 11-17. Such amidopropyldimethylaminoxide 
derivatives of fatty acids are much better tolerated by the constituents 
of the solution than the polyoxyalkylene compounds and may therefore be 
used in concentrations from 10 to 20 times higher. 
The electrolytic solutions for depositing a gold-copper-cadmium alloy 
according to the present invention are preferably used at a pH of 9-11, 
with a current density of 0.6 to 1.5 amps/sq. dm., and at a temperature of 
50.degree.-75.degree. C. Agitation or stirring of the parts subjected to 
this process is necessary to ensure uniform distribution of the current 
and, consequently, uniform thickness of the deposit. The concentrations of 
the diverse constituents of these solutions must each be situated within a 
range which has been determined experimentally. Thus, the gold, in the 
form of a complex cyanide, must be present at from 1-20 g/liter, 
preferably 3-5 g/liter; the copper, likewise in the form of a complex 
cyanide, from 6-70 g/liter, preferably 40-65 g/liter; and the cadmium, in 
the form of cyanide, an inorganic salt such as sulfate, for example, or a 
complex organic salt, from 0.3 to 5 g/liter, preferably 0.5 to 2.5 
gram/liter. The concentration of free potassium cyanide may vary from 3-40 
g/liter. The concentration of organic cadmium-complexing agent, especially 
of sodium, potassium, or ammonium hydroxyalkylimino diacetate, may vary 
from 5 to 100 g/liter, preferably 7.5-20 g/liter. Finally, the brightener 
in the form of a phosphoric ester of a chain of ethylene oxides having 
6-15 carbon atoms bonded to an alkyl group having 8-18 carbon atoms, may 
be present within the limits of 5-20 cc/liter, preferably 5-7.5 cc/liter, 
and the surfactant, in the form of an amidopropyldimethylaminoxide-type 
derivative of fatty acids, may be present within the limits of 0.1 to 50 
ml/liter. To increase the conductivity of the solution between the 
electrodes, it is necessary to add alkaline salts such as carbonates, 
borates, or phosphates, in amounts from 10-40 g/liter, which also act as 
pH regulators.

The following examples illustrate various preferred possibilities of 
depositing gold-copper-cadmium alloys of different compositions. 
EXAMPLE 1 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 
1 g/l 
Free potassium cyanide 27 g/l 
Potassium hydroxyethylimino diacetate 
25 g/l 
Wetting agent (phosphoric ester of a chain of 
5 ml/l 
10 molecules of ethylene oxide bonded to an 
alkyl chain having 8 carbon atoms) 
Dipotassium phosphate 20 g/l 
pH 10.5 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 0.75 amps/sq. dm. and a temperature of 60.degree. C. It was coated with 
a bright, uniform deposit, pale pink of color 4 N, having the following 
composition: 
______________________________________ 
gold 55.4% (thus 13.3 kt). 
copper 36.1% 
cadmium 6.9% 
______________________________________ 
Using EDTA or NTA complexing agents, it would not be possible to obtain a 
pale pink 4 N deposit under these conditions. Only a red deposit with a 
little cadmium would be obtained. 
EXAMPLE 2 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 
1.25 g/l 
Potassium hydroxyethylimino diacetate 
25 g/l 
Free potassium cyanide 27 g/l 
Wetting agent (phosphoric ester of a chain of 
5 ml/l 
10 molecules of ethylene oxide bonded to an 
alkyl chain having 10 carbon atoms) 
Dipotassium phosphate 20 g/l 
pH 10.5 
______________________________________ 
A 5.times.2 cm steel plate was likewise plated in this solution at a 
current density of 0.75 amps/sq. dm. and a temperature of 60.degree. C. It 
was coated with a pinkish deposit of color 3-4 N, having the following 
composition: 
______________________________________ 
gold 62.1% (thus 14.9 kt). 
copper 28% 
cadmium 9.5% 
______________________________________ 
EXAMPLE 3 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 1.5 g/l 
Sodium hydroxyethylimino diacetate 
25 g/l 
Free potassium cyanide 27 g/l 
Wetting agent (phosphoric ester of a chain of 
5 ml/l 
10 molecules of ethylene oxide bonded to an 
alkyl chain having 12 carbon atoms) 
pH (to be corrected with a 20% solution of 
10 
phosphoric acid) 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 0.75 amps/sq. dm. and a temperature of 60.degree. C. for 30 minutes. It 
was coated with a uniform, bright yellow deposit of color 2-3 N, having 
the following composition: 
______________________________________ 
gold 66% (thus 16 kt). 
copper 23.5% 
cadmium 10.5% 
______________________________________ 
EXAMPLE 4 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
60 g/l 
Cadmium, in form of cyanide 1 g/l 
Sodium hydroxymethylimino diacetate 
15 g/l 
Wetting agent (phosphoric ester of a chain of 
1 ml/l 
10 molecules of ethylene oxide bonded to an 
alkyl chain having 8 to 10 carbon atoms) 
Free potassium cyanide 30 g/l 
Dipotassium phosphate 20 g/l 
pH 10 
______________________________________ 
A 5.times.2 cm plate was plated in this solution at a current density of 
0.6 amps/sq. dm and a temperature of 55.degree. C. It was coated with a 
bright, very even deposit of color yellow 2N 18, having the following 
composition: 
______________________________________ 
gold 75% (thus 18 kt). 
copper 17% 
cadmium 8% 
______________________________________ 
EXAMPLE 5 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
70 g/l 
Cadmium, in form of cyanide 1 g/l 
Potassium hydroxyethylimino diacetate 
10 g/l 
Free potassium cyanide 25 g/l 
Wetting agent (phosphoric ester of a chain of 
5 ml/l 
10 molecules of ethylene oxide bonded to an 
alkyl chain having 8 carbon atoms) 
Dipotassium phosphate 20 g/l 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 1 amps/sq. dm. and a temperature of 58.degree. C. It was coated with a 
bright, homogeneous, yellow deposit of gold-copper-cadmium alloy at a rate 
of 10 microns per 20 minutes. It had the following composition: 
______________________________________ 
gold 67% (thus 16 kt). 
copper 23% 
cadmium 10% 
______________________________________ 
The film of alloy is particularly ductile. In this connection, it should be 
noted that according to the prior art, it would be necessary to use a 
concentration of 2.5 g/liter of cadmium ions and a current density of 
1.8-3.0 amps/sq. dm. in order to obtain a yellow deposit of 16-kt 
gold-copper-cadmium alloy. Under these conditions, the deposit is yellow 
but brittle, apparently owing to the co-deposition of hydrogen at these 
relatively high current densities. Example 5 shows that with the method 
now proposed, it is possible to obtain a 16-kt yellow deposit which is 
really ductile with a current density of only 1 amp/sq. dm. and a 
cadmium-ion content in the solution of only 1 g/liter. 
EXAMPLE 6 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
50 g/l 
Cadmium, in form of complex cyanide 
0.8 g/l 
Potassium hydroxypropylimino diacetate 
25 g/l 
Free potassium cyanide 25 g/l 
Wetting agent (phosphoric ester of a chain of 
5 ml/l 
10 molecules of ethylene oxide bonded to an 
alkyl chain having 10 carbon atoms) 
pH (to be corrected with a 20% solution of 
10 
phosphoric acid) 
______________________________________ 
The test was carried out in a Hull cell at 0.5 amps and at 60.degree. C. 
for 10 minutes. The plate was coated with a deposit going from yellow for 
the low current densities to pink for the high current densities, 
uniformly bright. 
EXAMPLE 7 
In this example, the same solution was used as in Example 6, but with a 
potassium hydroxypropylimino diacetate concentration of 100 g/liter, thus 
quadrupling the concentration. The test was carried out in a Hull cell at 
0.5 amps and at 60.degree. C. for 10 minutes. The plate cas coated with a 
deposit of colors similar to that obtained in Example 6. 
EXAMPLE 8 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 
1 g/l 
Free potassium cyanide 27 g/l 
Potassium hydroxyethylimino diacetate 
25 g/l 
Wetting agent (amidopropyldimethylaminoxide 
5 g/l 
of saturated fatty acids, between 11 and 
17 carbon atoms) 
Potassium carbonate 20 g/l 
pH 10.5 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 0.75 amps/sq. dm and a temperature of 60.degree. C. It was coated with 
a uniform, bright, pale pink deposit of color 4 N, having the following 
composition: 
______________________________________ 
gold 55.4% (thus 13,3 kt). 
copper 36.1% 
cadmium 6.9% 
______________________________________ 
Using EDTA or NTA complexing agents, it would not be possible to obtain a 
pale pink 4 N deposit under these conditions. Only a red deposit with a 
little cadmium would be obtained. 
EXAMPLE 9 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 
1.25 g/l 
Potassium hydroxyethylimino diacetate 
25 g/l 
Free potassium cyanide 27 g/l 
Wetting agent (amidopropyldimethylaminoxides 
5 ml/l 
of saturated fatty acids, between 11 and 17 
carbon atoms) 
Dipotassium phosphate 20 g/l 
pH 10.5 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 0.75 amps/sq. dm. and a temperature of 60.degree. C. It was coated with 
a pinkish deposit of color 3-4 N, having the following composition: 
______________________________________ 
gold 62.1% (thus 14.9 kt). 
copper 28% 
cadmium 9.5% 
______________________________________ 
EXAMPLE 10 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
60 g/l 
Cadmium, in form of cyanide 
0.6 g/l 
Sodium hydroxyethylimino diacetate 
12 g/l 
Free potassium cyanide 25 g/l 
Wetting agent (amidopropyldimethylaminoxides 
of saturated fatty acides, between 11 and 17 
carbon atoms) 
pH 10.3 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 0.7 amps/sq. dm. and a temperature of 55.degree. C. It was coated with 
a yellow deposit of color 2 N 18, assaying 18 karats. 
EXAMPLE 11 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 
1 g/l 
Sodium hydroxyethylimino diacetate 
12 g/l 
Free potassium cyanide 28 g/l 
Wetting agent (amidopropyldimethylaminoxides 
40 ml/l 
of saturated fatty acides, between 11 and 17 
carbon atoms) 
pH 10.3 
______________________________________ 
This solution was used to plate brass watch cases, working at a current 
density of 1 amp/sq. dm. and at 60.degree. C. A bright, 10-micron deposit 
of a ductile alloy assaying 16 karats of the following composition was 
obtained: 
______________________________________ 
Gold 67% 
Copper 23% 
Cadmium 
10% 
______________________________________ 
EXAMPLE 12 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
60 g/l 
Cadmium, in form of cyanide 
1 g/l 
Sodium hydroxymethylimino diacetate 
15 g/l 
Free potassium cyanide 30 g/l 
Wetting agent (amidopropyldimethylaminoxides 
2 ml/l 
of saturated fatty acides, between 11 and 17 
carbon atoms) 
Potassium borate 20 g/l 
pH 10.0 
______________________________________ 
A 5.times.2 cm steel was plated in this solution at a current density of 
0.6 amps/sq. dm. and a temperature of 55.degree. C. It was coated with a 
very even, bright deposit of color yellow 2 N 18, having the following 
composition: 
______________________________________ 
gold 75% (thus 18 kt). 
copper 17% 
cadmium 8% 
______________________________________ 
EXAMPLE 13 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
65 g/l 
Cadmium, in form of cyanide 1 g/l 
Potassium hydroxyethylimino diacetate 
10 g/l 
Free potassium cyanide 27 g/l 
Wetting agent (amidopropyldimethylaminoxides 
20 ml/l 
of saturated fatty acides, between 11 and 17 
carbon atoms) 
Dipotassium phosphate 20 g/l 
______________________________________ 
A 5.times.2 cm steel plate was plated in this solution at a current density 
of 1 amps/sq. dm. and a temperature of 58.degree. C. It was coated with a 
bright, homogenous yellow deposit of gold-copper-cadmium alloy at the rate 
of 10 microns per 20 minutes. It had the following composition: 
______________________________________ 
gold 67% (thus 16 kt). 
copper 23% 
cadmium 10% 
______________________________________ 
The film of alloy is particularly ductile. In this connection, it should be 
noted that according to the prior art, it would be necessary to use a 
concentration of 2.5 g/liter of cadmium ions and a current density of 
1.8-3.0 amps/sq. dm. in order to obtain a yellow deposit of 16-kt 
gold-copper-cadmium alloy. Under these conditions, the deposit is yellow 
but brittle, apparently owing to the co-deposition of hydrogen at these 
relatively high current densities. Example 13 shows that with the method 
now proposed, it is possible to obtain a 16-kt yellow deposit which is 
really ductile with a current density of only 1 amp/sq. dm. and a 
cadmium-ion content in the solution of only 1 g/liter. 
EXAMPLE 14 
______________________________________ 
Composition of solution: 
______________________________________ 
Gold, in form of complex cyanide 
4 g/l 
Copper, in form of complex cyanide 
50 g/l 
Cadmium, in form of complex cyanide 
0.8 g/l 
Potassium hydroxypropylimino diacetate 
25 g/l 
Free potassium cyanide 25 g/l 
Wetting agent (amidopropyldimethylaminoxides 
0.5 ml/l 
of saturated fatty acides, between 11 and 17 
carbon atoms) 
pH (to be corrected with a 20% solution of 
10.0 
phosphoric acid). 
______________________________________ 
The test was carried out in a Hull cell at 0.5 amps and at 60.degree. C. 
for 10 minutes. The plate was coated with a deposit going from yellow for 
the low current densities to pink for the high current densities, 
uniformly bright. 
EXAMPLE 15 
In this example, the same solution was used as in Example 14 but with a 
potassium hydroxypropylimino diacetate concentration of 100 g/liter, thus 
quadrupling the concentration. The test was carried out in a Hull cell at 
0.5 amps and at 60.degree. C. for 10 minutes. The plate was coated with a 
deposit of colors similar to that obtained in Example 14. 
It will be seen that when the concentration of the complexing agent used in 
this invention is greatly increased, the composition of the alloy 
deposited varies very little; this could not happen with the complexing 
agents used in the prior art. 
The foregoing examples show that the cadmium-complexing agents and the 
brighteners contained in the solution according to the present invention 
make it possible to obtain bright, homogeneous, even deposits or 
gold-copper-cadmium alloys which may range from 12 to 20 karats and more. 
The concentrations of the cadmium-complexing agents and the brighteners 
may vary to a wide extend without affecting the quality of the deposit, 
contrary to what happens in the prior art. 
The article treated in the proposed solution, presented by way of example 
as being a steel plate, may naturally be made of any other conductive 
material or even of an insulating material covered beforehand with a 
conductive coating. 
The electroplating solution according to this invention and the method 
utilizing it yield gold-plated articles of all shapes and sizes; and the 
layer of plating, of 12 to 20-kt gold according to the proportions of the 
constituents of the solution, may be economically thin or else thick 
enough to give the article properties similar to those of solid gold 
articles, particularly jewelry.