ChicJewelryLive.com The Colors of Gold
Colors Of Gold
Pure (or Fine or 24 carat) gold has a lovely warm rich yellow color that is
highly prized. It is actually an orange shade of yellow. Unlike other precious
metals, gold Jewelry can be produced in various alloys of gold, known as the
carat/karat gold's, in a range of colors from white to yellow and through to
red. In addition, it is possible to achieve other special colors such as blue,
black and purple. So how is it possible to change the intrinsic color of gold?
[An alloy is just a mixture of two or more metals; e.g. brass is an alloy of
copper and zinc.]
The simple answer is that it is like an artist mixing his paints to obtain various hues. Every schoolboy (and girl) knows that mixing yellow and blue produces green and mixing yellow and red produces pink or rose. With metals, we only have the choice of mixing yellow (gold) with red (copper) or white/grey (all other pure metals). So for the carat gold's, we can maintain yellow at the medium and low carats by balancing the amount of copper and silver plus zinc alloyed into the gold. If we add more copper than silver, then we get redder shades and adding more silver or other metals than copper gives us paler colors and even white.
However, we should note that, for a given carats of gold, varying the color also changes other properties, such as hardness and strength. We should also note that we can obtain a wider variation in color as we lower the carats. This is all explained in more detail in the section on gold jewelry alloys. White gold's, in practice, are a little more complex since the whitening - or bleaching - effect of different metals on gold varies: see the section on white gold's.
The special colors such as blue, black and purple are obtained by quite different approaches, either as special metal compounds or by surface treatments to obtain a patina. This is explained in the section on special colors.
White Gold's
What are they? Are they a special form of gold? Answers to the common questions
on white gold's.
Colored Carat Alloys
How color can be varied in carat alloys.
Special Gold Colors
Making Gold: Blue, Black and
Purple!
Pure gold is a deep yellow color and conventional carat gold Jewelry alloys can range from red through yellow to pale yellow/green and even white by varying the alloying metals. But it is possible to make gold Jewelry that exhibits unusual colors such as purple and blue and black. How is this possible? Well, this can be accomplished by one of two techniques: formation of special gold metal compounds (intermetallic compounds) or by a surface coating or patination. Both approaches can yield attractive colors but they do have some disadvantages over normal carat gold alloys.
1. Intermetallic compound colors
a] Purple gold (also known as amethyst or violet gold)
When gold and aluminum are alloyed in a certain fixed ratio, they form a gold intermetallic compound with the chemical formula AuAl2. That is one atom of gold to two atoms of aluminum. This compound has an attractive purple color, as the pendant illustrates. In terms of composition, this compound is about 79% gold by weight and hence is hallmarkable as 18 carat gold.
All intermetallic compounds, and purple gold is no exception, tend to be very brittle. They cannot be easily worked by conventional metal working processes. If one attempted to roll or hammer a piece of purple gold, it would shatter into pieces! It also tends to tarnish easily.
Melting gold and aluminum together to make purple gold is not easy and requires vacuum melting equipment. However, it is possible to melt and cast pieces of purple gold into a mold. The compound has a melting point of about 1060°C, higher than that of both gold and aluminum, which is indicative of the compound's high stability. The purple color can be retained at aluminum contents as low as 15%, but such alloys will be 2 phase, comprising the purple compound and some aluminum-rich solid solution. These non-stoichiometric alloys will tend to be less brittle in their mechanical properties, but the color will be diluted.
Cast pieces can be machined or faceted by grinding or milling to form pseudo 'gem stones' which can be set in conventional gold Jewelry.
An alternative approach to making Jewelry with purple gold decoration is to physically vapor deposit (PVD) the two metals, gold and aluminum, in the correct ratio onto a carat gold substrate. Such processing can be done by a number of PVD techniques such as sputtering. Jewelry made by this approach is commercially available.
A powder metallurgy approach is also possible, with additions of 7-30% cobalt, nickel or palladium powders added to the gold-aluminum powder, which is pressed and sintered. It is claimed that such alloys are of good purple color and have satisfactory workability.
In a new process, ornamental purple gold alloys containing 70-85% gold, the rest aluminum, are claimed which are made by vacuum melting an ingot, atomizing it centrifugally and the powders packed in a mold and electrical discharge sintered. Partial surfaces may be strengthened by diffusion bonding with pure gold, silver or platinum or alloys thereof.
Purple gold wires can be made made by bundling gold-plated aluminum and aluminum-plated gold wires together and drawing them down to produce a composite wire, which is then subjected to a thermal diffusion treatment at 450-700°C in a reducing atmosphere. This way, a wire with a fibrous structure of purple gold (with some gold in a 2 phase structure) is claimed that is tough and flexible. Such a diffusion process can also be used to provide a purple gold effect on gold Jewelry by depositing a layer of aluminum onto the surface and doing a thermal diffusion treatment to form the purple compound. Thermal spraying of gold and aluminum powders onto a substrate can also achieve a purple coating.
b] Blue gold
The intermetallic compound formed between gold and indium, AuIn2, gives rise to a clear blue color and that between gold and gallium, AuGa2, to a bluish hue.
AuIn2 (46% gold) and AuGa2 (58.5% gold) have melting points of 540.7°C and 491.3°C respectively. Off-stoichiometric compositions, like purple gold, will be 2 phase and so can be expected to have some measure of workability and toughness. Manufacturing techniques will also be similar to those for purple gold.
2 colors by surface coatings and patinas
c] Black gold (grey - black & brown)
There are several ways of obtaining a black coloration on carat gold's.
There are several electroplating solutions on the market for the deposition of black coatings, but the most popular are those based on rhodium or ruthenium with special blackening additions. The ruthenium bath gives slightly harder coatings than rhodium. Coatings range in color from grey to 'anthracite' black. The blacker the color, the less wear resistant is the coating. Hardness of the coating ranges from HV 230 to 310 and is inversely proportional to the level of blackening agent. Overall, wear resistance is not high and so rubbing or abrading conditions should be avoided.
PA-CVD has been developed for the watch industry and enables 1 - 1.5 mm thickness coatings of hard, amorphous hydrogenated carbon to be deposited at 200-400°C. The coating has an appearance of Chinese lacquer and can be gloss or matte depending on the substrate surface condition. It is very hard (HV 1800-2000), wears well and is biocompatible.
A black oxide coating or patina can be produced by controlled oxidation of carat gold's containing cobalt, iron or chromium additions. For example, a gold 75% - cobalt 15% - chromium 10% alloy is oxidized in a furnace at 700-950°C (1292-1742°F).
This promotes black oxides of cobalt and chromium, which are wear resistant. However, such alloys are not suited for working and lost wax casting, so are not suitable for mass manufacture of black gold items.
A grey color can be obtained by oxidation of a gold alloy containing 15-20% iron.
Brown to black patinas can also be obtained in copper-containing carat gold alloys at 18 ct or less by treatment with Liver of Sulphur (impure potassium sulphide) or other sulphides to produce a sulphide layer on the surface. They are used dilute and the patina is built up slowly to produce more permanent, denser coatings.
d] Blue gold
A blue patina can be produced on gold alloys by oxidation treatments. In one case a 20 -23 carat gold alloy that turns to a rich sapphire blue is alloyed with ruthenium, rhodium and 3 other metals. It yields a blue surface layer 3 -6 mm thick. In another case an18 ct gold with 24.4% iron and 0.6% maximum nickel forms a blue oxide layer when heated at 450- 600°C. At a higher, 83% gold content, a blue-green color is produced.
Oxidation of gold alloys containing 25% iron or arsenic is also reported in the literature to yield a bluish color.
Note - Many of these coatings will be vulnerable to rubbing or abrasion and
so should be protected
where possible.
Gold Colored Carat Alloys
Gold Jewelry Alloys
Pure (24 carat) gold is a deep yellow color (an orange shade of yellow) and is soft and very malleable. The colored carat gold alloys range in gold content from 8 to 22 carats (33.3% - 91.6% gold) and can be obtained in a range of color shades: green (actually a green shade of yellow), pale yellow, yellow, deep yellow, pink/rose and red. There are also white gold's and even unusual colored gold's such as 'purple gold'. They all have different mechanical properties such as strength, hardness and malleability (ductility) and some alloys can be heat treated to maximize strength and hardness. There are gold alloys that are optimized for different manufacturing routes such as lost wax (investment) casting and stamping.
How can color be varied and why do different gold alloys (an alloy is a mixture of two or more pure metals) have different mechanical and other properties? To answer these questions in depth requires a good technical knowledge of metallurgy. However, it is possible to give some simplified answers.
The Colored Carat Gold's
Almost all conventional, colored carat gold's are based on gold-silver-copper alloys, often with minor alloying additions. All three metals have the same crystal structure (face centered cubic, FCC) and so are compatible with each other over a large range of compositions. Typical minor additions include deoxidizers such as zinc and silicon, grain refiners such as iridium and cobalt and possibly metals such as nickel to strengthen the alloy. Larger zinc additions (about 1-2%) can improve melt fluidity and hence 'cast ability' in lost wax casting, as can silicon, resulting in better filling of the mould and better reproduction of surface detail. Even larger zinc additions (up to 10%) can improve malleability of certain carat gold's, particularly 14 carat and lower, used for making jewelry by stamping from sheet. Additions of low melting point metals such as zinc, tin, cadmium and indium lower melting ranges and hence are used to make carat gold solders.
Color
Gold is yellow and copper is red, the only two colored pure metals. All other metals are white or grey in color. The addition of a red color to yellow, as every school child knows, makes the yellow pinker and eventually red. The addition of a white makes the yellow color paler and eventually white. This principle of mixing colors is the same in carat gold's. Adding copper to gold makes it redder and adding silver, zinc and any other metal makes gold paler. Thus, we can understand that lower carat gold's, because we can add more alloying metals, can have a wider range of colors than the higher carat gold's.
Thus at 22 carat (91.6% gold), we can only add a maximum of 8.4% of alloying
metals and hence can only obtain yellow to pink/rose shades. At 18 carat (75.0%
gold) and lower, we can add 25% or more alloying metals and hence get colors
ranging from green through yellow to red, depending on the copper: silver plus
zinc ratio. Thus at any given caratage we can vary the color by varying the
copper: silver plus zinc ratio. This can be demonstrated in the following table:
Effect of copper: silver ratio on color
|
Type |
Gold % wt |
Silver % |
Copper % |
Color |
|
22 ct
|
91.6 |
8.4 |
- |
Yellow |
|
91.6 |
5.5 |
2.8 |
Yellow | |
|
91.6 |
3.2 |
5.1 |
Deep yellow | |
|
91.6 |
- |
8.4 |
Pink/rose | |
|
18 ct
|
75.0 |
25.0 |
- |
Green-yellow |
|
75.0 |
16.0 |
9.0 |
Pale yellow, 2N | |
|
75.0 |
12.5 |
12.5 |
Yellow, 3N | |
|
75.0 |
9.0 |
16.0 |
Pink, 4N | |
|
75.0 |
4.5 |
20.5 |
Red, 5N | |
|
14 ct
|
58.5 |
41.5 |
- |
Pale green |
|
58.5 |
30.0 |
11.5 |
Yellow | |
|
58.5 |
9.0 |
32.5 |
Red | |
|
9 ct
|
37.5 |
62.5 |
- |
White |
|
37.5 |
55.0 |
7.5 |
Pale yellow | |
|
37.5 |
42.5 |
20.0 |
Yellow | |
|
37.5 |
31.25 |
31.25 |
Rich yellow | |
|
37.5 |
20.0 |
42.5 |
Pink | |
|
37.5 |
7.5 |
55.0 |
Red |
Properties
Alloying additions affect other physical properties as seen in the next table:
Physical Properties of Typical Gold Alloys
|
Carat |
Composition % |
Color |
Density g/cm3 |
Melting Range °C | |
|
Silver |
Copper | ||||
|
24 |
- |
- |
Yellow |
19.32 |
1064 |
|
22 |
5.5 |
2.8 |
Yellow |
17.9 |
995-1020 |
|
3.2 |
5.1 |
Dark yellow |
17.8 |
964-982 | |
|
21 |
4.5 |
8.0 |
Yellow-pink |
16.8 |
940-964 |
|
1.75 |
10.75 |
Pink |
16.8 |
928-952 | |
|
- |
12.5 |
Red |
16.7 |
926-940 | |
|
18 |
16.0 |
9.0 |
Pale yellow |
15.6 |
895-920 |
|
12.5 |
12.5 |
Yellow |
15.45 |
885-895 | |
|
9.0 |
16.0 |
Pink |
15.3 |
880-885 | |
|
4.5 |
20.0 |
Red |
15.15 |
890-895 | |
As carats reduce, the melting range and alloy density are lowered. But at any given carat (gold content), the actual values vary according to the relative silver and copper contents.
As well as affecting physical properties, alloying additions to gold generally increase the strength and hardness, with some reduction in malleability / ductility. The silver atom is slightly larger than that of gold, so alloying gold with silver gives a moderate improvement in strength and hardness. The copper atom is significantly smaller than that of gold and so it has a greater effect on strengthening gold than silver, as it distorts the gold crystal lattice more. Thus reducing carat from 24 carats through 22 ct and 21 ct down to 18 carat gold results in stronger and harder alloys, as can be seen in the following table. Beyond 18 ct down to 10, 9 and 8 carats does not have much further effect.
Mechanical Properties of Typical Gold Alloys
|
Carat
|
Composition %, wt. |
Condition
|
Hardness
HV |
Tensile Strength
N/mm2 | |
|
Silver |
Copper | ||||
|
24 |
-
|
-
|
Annealed |
20 |
45 |
|
Worked |
55 |
200 | |||
|
22 |
5.5
|
2.8
|
Annealed |
52 |
220 |
|
Worked |
138 |
390 | |||
|
3.2
|
5.1
|
Annealed |
70 |
275 | |
|
Worked |
142 |
463 | |||
|
21 |
4.5
|
8.0
|
Annealed |
100 |
363 |
|
Worked |
190 |
650 | |||
|
1.75
|
10.75
|
Annealed |
123 |
396 | |
|
Worked |
197 |
728 | |||
|
18 |
12.5 0
|
12.5
|
Annealed |
150 |
520 |
|
Worked |
212 |
810 | |||
|
4.5
|
20.5
|
Annealed |
165 |
550 | |
|
Worked |
227 |
880 | |||
Mechanical Properties of 18 Carat Gold's
|
Composition, wt% |
Hardness, HV |
Elongation, % | ||||
|
Gold |
Silver |
Copper |
Annealed |
Cold worked |
Annealed |
c.w. |
|
75 |
25 |
- |
36 |
98 |
36.1 |
2.6 |
|
75 |
21.4 |
3.6 |
68 |
144 |
39.3 |
3.0 |
|
75 |
16.7 |
8.3 |
102 |
184 |
42.5 |
3.2 |
|
75 |
12.5 |
12.5 |
110 |
192 |
44.8 |
3.3 |
|
75 |
8.3 |
16.7 |
129 |
206 |
47.0 |
2.6 |
|
75 |
3.6 |
21.4 |
132 |
216 |
42.0 |
1.5 |
|
75 |
- |
25 |
115 |
214 |
41.5 |
1.4 |
However, copper-containing carat gold's in the range of 8-18 carats can be hardened even further because of their metallurgy. Hard second phases can be precipitated out in the solid state as they cool below about 400°C, making the carat gold less ductile. Because of this, such alloys must be quenched in water after annealing to retain the single phase, ductile state if further working is required. This can be seen in the next table.
Effect of Cooling Rate on 18 Carat Gold's after Annealing at 650°C
|
Composition, wt% |
Hardness, HV | |||
|
Gold |
Silver |
Copper |
Slow cooled in air |
Water quenched |
|
75 |
25 |
- |
56 |
56 |
|
75 |
22 |
3 |
90 |
88 |
|
75 |
17 |
8 |
138 |
136 |
|
75 |
12.5 |
12.5 |
160 |
160160 |
|
75 |
8 |
17 |
170 |
165 |
|
75 |
3 |
22 |
196 |
177 |
|
75 |
- |
25 |
242 |
188 |
Apart from copper, all other alloying metals to gold will tend to whiten the color and so it is possible to make carat gold's that are white in color. White gold's for jewelry were developed in the 1920's as a substitute for platinum.
Additions of any white metal to gold will tend to bleach it's color. In practice, nickel and palladium (and platinum) are strong 'bleachers' of gold, silver and zinc are moderate bleachers and all others are moderate to weak in effect.
This has given rise to 2 basic classes of white gold's - the Nickel whites and the Palladium whites. At the 9 carat (37.5% gold) level, a gold-silver alloy is quite white, ductile although soft and is used for jewelry purposes. White gold's are available up to 21 carat.
There is no legal definition of what constitutes a 'white' color in gold's and hence trade description of white gold may not mean 'detergent white'. Many commercial white gold's are not a good white color.
Nickel white gold's
Nickel alloying additions form hard and strong white gold's up to 18 carat. They are difficult to work and suffer from so-called 'fire cracking'. Most commercial alloys are based on gold-nickel-silver-zinc alloys with copper often added to improve malleability. This copper addition, of course, affects color, and so such white gold alloys are not a good white color - more a slight yellow/ brown tint, particularly if nickel content is also low. As a consequence, such white gold jewelry is normally electroplated with rhodium (a platinum metal) which is tarnish resistant and imparts a good white color.
Unfortunately, many people, the female population especially, are allergic to nickel in contact with the skin and this gives rise to a red skin rash or irritation. The European Union countries have enacted legislation valid from January 2000 that limits nickel release from jewelry. Thus, in Europe, nickel white gold's are being phased out and being replaced by palladium white gold's. The USA is taking a more relaxed approach, requiring jewelry to be labeled as nickel-containing, and much jewelry in the West is now advertised as 'non-allergenic' or 'nickel-free'. Some typical nickel white gold compositions are shown in the following table.
Typical Nickel White Gold's
|
|
Gold, |
Copper, % wt |
Nickel, % wt |
Zinc, % wt |
Hardness Hv |
Liquidus °C |
|
18ct |
75 |
2.2 |
17.3 |
5.5 |
220 |
960 |
|
75 |
8.5 |
13.5 |
3.0 |
200 |
955 | |
|
75 |
13.0 |
8.5 |
3.5 |
150 |
950 | |
|
14ct |
58.5 |
22.0 |
12.0 |
7.4 |
150 |
995 |
|
10ct |
41.7 |
32.8 |
17.1 |
8.4 |
145 |
1085 |
|
9ct |
37.5 |
40.0 |
10.5 |
12.0 |
130 |
1040 |
Palladium white gold's
Additions of about 10 -12% palladium to gold impart a good white color. But palladium is an expensive metal, dearer than gold and it is also a heavy metal. Thus jewelry in such palladium white gold's will be more expensive than identical pieces in nickel whites for 2 reasons: firstly, the cost of the palladium and secondly, the impact of density - palladium white gold's are denser and so such jewelry will be heavier and also contain more gold. It is also more difficult to process as the melting temperatures are substantially higher.
Many commercial palladium white gold's only contain about 6-8% palladium plus silver, zinc and copper. Some may even contain some nickel [so a palladium white gold is not necessarily nickel-free]. These may also have less than a good white color and so may also be rhodium plated.
Palladium white gold's tend to be softer and more ductile compared to nickel whites and so will not wear as well. They are available in all caratages up to 21 carat. It is not possible to have a 22 ct white gold, for example. Some typical compositions are given in the following table.
Typical Palladium Alloys
|
|
Gold |
Pd |
Ag |
Cu |
Zn |
Ni |
Hardn Hv |
Liq, °C |
|
18ct |
75 |
20 |
5 |
- |
- |
- |
100 |
1350 |
|
75 |
15 |
10 |
- |
- |
- |
100 |
1300 | |
|
75 |
10 |
15 |
- |
- |
- |
80 |
1250 | |
|
75 |
10 |
10.5 |
3.5 |
0.1 |
0.9 |
95 |
1150 | |
|
75 |
6.4 |
9.9 |
5.1 |
3.5 |
1.1 |
140 |
1040 | |
|
75 |
15 |
- |
3.0 |
- |
7.0 |
180 |
1150 | |
|
14ct |
58.3 |
20 |
6 |
14.5 |
1 |
- |
160 |
1095 |
|
58.5 |
5 |
32.5 |
3 |
1 |
- |
100 |
1100 | |
|
10ct |
41.7 |
28 |
8.4 |
20.5 |
1.4 |
- |
160 |
1095 |
|
9ct |
37.5 |
- |
52 |
4.9 |
4.2 |
1.4 |
85 |
940 |
Alternative white gold's
In Europe especially, there is a demand for cheaper alternatives to white gold's than the palladium whites which are nickel-free. Many new alloys are coming to market, most of which rely on manganese additions as the main whitener. Some are palladium-free and others are low palladium alloys. Chromium and iron are also be used as whiteners. They tend to be hard and more difficult to process. Many of these alloys are not a good white color, requiring rhodium plating, and many suffer cracking problems and tarnishing.