Composition of Gold Alloys
Gold jewelry is not pure gold, it is really an alloy, or mixture of metals. The purity or fineness of gold in the jewelry is indicated by its karat number.
24 karat (24K or 24 kt) gold is as pure as gold for jewelry gets. 24K gold is also called fine gold and it is greater than 99.7% pure gold. Proof gold is even finer, with over 99.95% purity, but it is only used for standardization purposes and is not available for jewelry.
Gold can form alloys with most metals.
The most common alloying metals for jewelry are:
- Silver (Ag)
- Copper (Cu), and
- Zinc (Zn)
However, other metals can be added to make colored gold.
Pure gold is a deep yellow colour (an orange shade of yellow) and is soft and very malleable. The coloured 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 colour shades: green (actually a green shade of yellow), pale yellow, yellow, deep yellow, pink/rose and red. There are also white golds and even unusual coloured golds 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 maximise strength and hardness. There are gold alloys that are optimised for different manufacturing routes such as lost wax (investment) casting and stamping.
Compositions of some common gold alloys:
- Color of Gold & Alloy Composition:
- Yellow Gold (22K): Gold 91.67%, Silver 5%, Copper 2%, Zinc 1.33%
- Red Gold (18K): Gold 75%, Copper 25%
- Rose Gold (18K): Gold 75%, Copper 22.25%, Silver 2.75%
- Pink Gold (18K): Gold 75%, Copper 20%, Silver 5%
- White Gold (18K): Gold 75%, Platinum or Palladium 25%
- White Gold (18K): Gold 75%, Palladium 10%, Nickel 10%, Zinc 5%
- Gray-White Gold (18K): Gold 75%, Iron 17%, Copper 8%
- Soft Green Gold (18K): Gold 75%,Silver 25%
- Light Green Gold (18K): Gold 75%, Copper 23%, Cadmium 2%
- Green Gold (18K): Gold 75%, Silver 20%, Copper 5%
- Deep Green Gold (18K): Gold 75%, Silver 15%, Copper 6%
- Cadmium 4%
- Blue-White or Blue Gold (18K): Gold 75%, Iron 25%
- Purple Gold: Gold 80%, Aluminum 20%
White golds
Apart from copper, all other alloying metals to gold will tend to whiten the colour and so it is possible to make carat golds that are white in colour. White golds for jewellery were developed in the 1920’s as a substitute for platinum. Additions of any white metal to gold will tend to bleach it’s colour. 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 golds:- 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 jewellery purposes. White golds are available up to 21 carat. There is no legal definition of what constitutes a white colour in golds and hence trade description of white gold may not mean ‘detergent white’. Many commercial white golds are not a good white colour.
White gold alloy compositions
Nickel- and palladium-free 14-karat white gold alloy composition can consists of the parts by weight: about 58.34% gold, about 35–40% silver, about 0.5–1.80% tin, and about 0–0.75% germanium.
0 komentar:
Posting Komentar