Pure gold is softer than pure silver and its beauty is unmatched by any alloyed gold. However, the extreme softness of pure gold makes it useless for creating durable jewelry.
An alloy is a mixture of different metals. Alloying elements are added to gold to increase the toughness and hardness. While almost any metal can be alloyed with gold, only certain metals will not dramatically change the color or make the metal brittle. As a result, it is important to use the correct metal to achieve the desired composition and color.
Over time, certain percentages of gold have become legally recognized "karats." The karat represents the amount of gold as a percentage of the total. For example, 24 karat is 100 percent gold and 14 karat is 14/24's gold or 58.3 percent gold. Gold standards vary around the world. In the United States, 24k, 22k, 18k, 14k, and 10k gold are the only karats allowed to be sold. Most jewelry is sold in 18k, 14k or 10k gold.
Examples of the gold percentages per karats:
- 24k: 100% Gold
- 22k: 92% Gold
- 18k: 75% Gold
- 14k: 58% Gold
- 10k: 42% Gold
When dealing with karats, there is a known proportion of metals in the non-gold percentage. These metals provide the various colors and hardness of gold alloy. Typical alloying elements and their color are:
Examples of the compositions of different metals used per color:
- Yellow Gold: copper, silver, zinc
- White Gold: copper, nickel (or palladium), zinc
- Rose Gold: copper
- Green Gold: silver
- Copper Reddening
- Silver Greening
- Zinc Bleaching
- Nickel Whitening
- Palladium Whitening
Adjusting the amount of base metals in the gold provides the spectrum of colors available. They also enhance properties such as hardness, corrosion resistance, malleability, strength and ductility. These additions can dramatically change the properties of the gold alloy.
Understanding how the proportions will affect the gold greatly enhances the final product. It is important to have a metal which will elongate or stretch a great deal before fracturing, thus high ductility. The requirement for an earring post would be a high tensile strength (a great deal of force needed to get the material to permanently deform, bend). It is imperative to select the proper gold alloy composition for the desired application.