Gold Alloys :: Gold Alloy :: Gold Mixtures

Pure gold is soft and very malleable.The carat gold alloys range in gold content from 8 to 22 carats (33.3% - 91.6% gold).

Metallurgists must take into consideration how various precious metals react to one another. Formulating gold alloys is very challenging. Too much of one precious metal can cause the combination to become fragile, hard and will prove difficult to design jewellery. Another combination of precious metal may cause the alloy to become too soft for jewellery fabrication.

The goal is to produce the best combination that provides the most acceptable gold alloy properties: strength, sturdiness, durability and visibly pleasing.

The most common alloying metals for jewelry are:

• Silver (Ag)

• Copper (Cu)

• Zinc (Zn)

However, other metals can be added to influence the properties of gold alloys.

All gold alloy have different mechanical properties such as strength, hardness and malleability (ductility) and some alloys can be heat treated to maximise strength and hardness. There are gold alloys that are optimised for different manufacturing routes such as lost wax (investment) casting and stamping.

As much of the experimentation with gold alloys is now centred around colour, it is difficult to separate an article on gold alloy from information about colour. If it is an article on gold colours that you were looking for then:

Gold Colours :: Gold Colors by Humagaia may be what you are looking for.

Following is a list of the most common compositions of gold alloy that give specific gold alloy colouration:

• Yellow Gold (22K): Gold 91.67wt%, Silver 5wt%, Copper 2wt%, Zinc 1.33wt%
  See Gold :: Yellow Gold by Humagaia for further information on yellow gold.

• Yellow gold (18K): Gold 76wt%, copper 19wt%, and 5wt% aluminum.

  
  

• Red Gold (18K): Gold 75wt%, Copper 25wt%.
  See Gold :: Red Gold by Humagaia for further information on yred gold.

• Rose Gold (18K): Gold 75wt%, Copper 22.25wt%, Silver 2.75wt%.
  See Gold :: Rose Gold by Humagaia for further information on rose gold.

• Pink Gold (18K): Gold 75wt%, Copper 20wt%, Silver 5wt%; and,
  Gold 76wt%, Copper 18wt% Aluminum 6wt%.
  See Gold :: Pink Gold by Humagaia for further information on pink gold.

• White Gold (18K): Gold 75wt%, Platinum or Palladium 25wt%.
  See Gold :: White Gold by Humagaia for further information on white gold.

• White Gold (18K): Gold 75wt%, Palladium 10wt%, Nickel 10wt%, Zinc 5wt%.
• Grey-White Gold (18K): Gold 75wt%, Iron 17wt%, Copper 8wt%.
  See Gold :: Grey Gold by Humagaia for further information on grey gold.

• Blue-White or Blue Gold (18K): Gold 75wt%, Iron 25wt%.
  See Gold :: Blue Gold by Humagaia for further information on blue gold.

• Purple Gold: Gold 80wt%, Aluminum 20wt%.
  See Purple Gold :: Amathyst Gold :: Violet Gold by Humagaia for further information on yellow gold.

  Nickel- and palladium-free 14-karat white gold alloy composition can consists of the parts by weight: approx 58.34wt% gold, approx 35–40wt% silver, approx 0.5–1.80wt% tin, and approx 0–0.75wt% germanium.

For most uses of gold the pure metal is too soft on its own and is therefore hardened with the addition of alloying elements:

• Copper
• Silver
• Nickel
• Palladium
• Zinc.

Gold is, of course, yellow and the various colours seen such as red, white and green are simply alloys of gold. The final colour is dependent on the ratios and type of alloy added.

There are hundreds of gold alloy and gold mixtures possible. General the addition of silver will color gold green. The addition of copper will colour it red. A mix of aboutd 50wt%:50wt% copper : silver will give a range of yellow gold alloys. Those that you are accustomed to seeing at the jewellery store..A small amount (0.2wt%) of zinc will harden the alloy.

Gold is alloyed to produce varying carat (karat) gold. These carat gold grades or purities range from 8k to 22k (24k, pure gold, is rarely used for jewellery manufacture).

A full explanation of carat gold and the various grade / purity / fineness can be found in Gold Grade :: Gold Grades by Humagaia.

So, what are the metals that are alloyed with gold?

Gold will form alloys with most metals, but for jewelry, the most common alloying metals are silver, copper, and zinc. Silver, copper and zinc are the precious metals that are useful for making pure gold harder. They also cause the gold alloy to be less likely to become scratched or dented.

You, the consumer, should concern yourself with certain gold alloy inclusions if you are allergic to certain metals or your body shows acidic properties. Acid can tarnishto  jewellery black and appear of poor quality.

The following are standard gold alloys as related to standard gold fineness, grade or purity.

• 24k Gold - pure gold, or fine gold, is very soft, it is rarely used as jewelry without being alloyed.
• 22k Yellow Gold - 91.8wt% gold alloyed with 8.2wt% copper and silver.
• 18k Yellow Gold - .75wt% gold alloyed with 25wt% copper, silver, zinc and other alloys.
• 18k Nickel White Gold - 75% gold alloyed with 25wt% nickel, silver and other alloys. Nickel can be allergenic.
• 18k Palladium White Gold - 75wt% gold alloyed with 25wt% palladium, silver and other alloys. For those that have an allergy to nickel.
• 14k Yellow Gold - 58.5wt% gold alloyed with 41.5wt% copper, silver, zinc and other alloys. 14k is a very strong, somewhat brittle with a pale gold colour
• 14k Nickel White Gold - 58.5wt% alloyed with 41.5% nickel, silver and other alloy. Nickel can be allergenic.
• 14k Palladium White Gold - 58.5wt% alloyed with 41.5% palladium, silver and other alloy. For those that have an allergy to nickel.

These are just examples or the more commonly available gold alloys.

For further reading on gold fineness see Gold Grade :: Gold Grades by Humagaia.

White gold's properties can vary dramatically depending on the metals and proportions used in the mix. This is done deliberately for different purposes for example,

• Mixing gold with nickel makes it harder. This is perfect for rings and brooch pins.
• Mix gold with a soft metal like palladium, then it becomes pliable. This is good for gemstones settings.
• Mix gold with 10wt% -12wt% palladium to impart a good white colour.
• Mix gold with platinum, chromium, cobalt, zinc, tin and silver to gain a white colour.
• Mix gold with silver for excellent working properties.
• Mix gold and copper to improve the ductility.

There are some more unusual alloys of gold. Where gold is mixed with silver, Iron, cadmium or aluminium one can obtain green, blue and purple gold compounds.

intermetallic compounds are materials with properties very different from the individual metals that constitute them. An intermetallic compound is defined by an intermediate phase in an alloy system, having a narrow range of homogeneity and relatively simple stoichiometric proportions. They are usually brittle, which makes their use in traditional jewellery virtually impossible, but can, however, be faceted and used asgemstones or inlays. The best known among these compounds is AuAl2:

Purple Gold is formed with79wt% gold and 21wt% Aluminium. AuAl₂ has a higher melting point than its constituents, indicating the compound’s thermodynamic stability. The compound is characterized by its purple colour.

he purple gold compound is brittle, it is stable and does not disintegrate with heat treatment. AuAl₂ has a gold content close to18 carat alloy (75wt% gold), The purple colour is preserved until the aluminium content decreases to 15wt%.

See Purple Gold by Humagaia for further information on purple gold.

Blue Gold

Two other intermetallic compounds that are known to produce colours in gold alloys. AuIn₂, with a clear blue colour, and AuGa₂, which has a bluish hue. The gold indium intermetallic compound AuIn₂ forms at 46wt% gold, and AuGa₂ at 58.5wt% gold. Both have a similar crystal structure to the purple gold compound.

See Blue Gold by Humagaia for further information on blue gold.

Another technique, which involves the formation of a coloured oxide layer on the alloy during heat treatment in air is surface oxidation.

As gold does not discolour in air, the metal is alloyed with a base metal characterized by a low oxidation resistance. The oxide layer is only a surface film, and has limited thickness. This could result in the coloured layer being damaged when used in high wear applications.

Surface oxidation is performed as the final treatment after the article has been buffed and polished. Liver of sulfur (potassium sulphide) can be used to colour gold alloys containing a significant quantity of copper (18ct and lower), and a range of colours is produced

• Brown
• Black - black gold generally contains cobalt, which forms a black cobalt oxide layer on the surface with heat treatment. Black gold alloys also contain at least one of the platinum-group metals, silver, or nickel.
• Sapphire Blue - an alloy that turns to a rich sapphire blue with heat treatment at 1800°C. The gold, with content between 20k and 23k is alloyed with ruthenium, rhodium and three other alloying elements.
• Blue oxide layer - on heat treatment an 18k gold alloy, which consists of 24.4wt% iron and a maximum of 0.6wt% nickel, forms a blue oxide layer.
• Blue-green - the oxide layer turns to a blue-green with an increase in gold content to 85wt%.
• Olive-green hue - the addition of chromium results in a thinner oxide layer which consists mainly of Cr₂O₃

Other alloying elements also known to give a blackish layer on oxidation are

• Copper
• Iron
• Titanium.

Certain Au-Cu-Al alloys form a surface texture resulting in a new family of gold alloys known as ‘Spangold’. Two different colours are produced by these intermetallic compounds:

• Yellow - from a 76wt% gold, 19wt% copper and 5wt% aluminium alloy, and a
• Pink - from a 76wt% gold-18wt% copper and 6wt% aluminium alloy.

The decorative spangling effect is the result of a change in the crystal structure which creates many small facets on the previously polished surface.

The spangling heat treatment is normally conducted by heating the item in a bath of hot oil and then cooling it to below room temperature.

Gold jewellery has enchanted people for thousands of years. Hues and colours are only variations on the same artistic theme of beauty and brilliance. Gold will always be distinctive due to its unique yellow colour, but the need for diversity and originality will cause some of the coloured gold alloys to remain in fashion. New designs and ideas will certainly arise, and with today’s knowledge and technologies, new solutions will most likely be found for the challenges posed by some of the coloured gold alloys.

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