Learn about 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

Properties
Alloying additions affect other physical properties as seen in the next table:
Physical Properties of Typical Gold Alloys

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

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.