Gold and Gold Alloys

Properties of Gold

Gold (Au) is renowned for its extreme malleability and ductility. One ounce of gold can be drawn into 80 km of thin gold wire, which is commonly used for electrical contacts and bonding wire. Gold’s Young's modulus is 79 GPa, similar to silver but significantly lower than iron or steel.

Gold's Atomic Structure and Characteristics

Gold has an atomic number of 79, meaning each gold atom has 79 protons in its nucleus. The atomic mass is 196.967, and the atomic radius is 0.1442 nm. Gold's unique yellow color is due to the arrangement of its outer electrons and their transitions between energy bands, which produce the characteristic gold color when they absorb light at specific wavelengths.

Isotopes of Gold

Gold has only one stable non-radioactive isotope in nature, despite the existence of various isotopes due to varying numbers of neutrons.

Gold Purity and Karat System

The purity of gold in jewelry is measured by its karat number:

• 24 Karat (24K): Pure gold, over 99.7% pure, also called fine gold.
• 18 Karat (18K): Contains 18 parts gold and 6 parts other metals, making it 75% gold.
• 14 Karat (14K): Contains 14 parts gold and 10 parts other metals, making it 58.3% gold.
• 12 Karat (12K): Contains 12 parts gold and 12 parts other metals, making it 50% gold.
• 10 Karat (10K): Contains 10 parts gold and 14 parts other metals, making it 41.7% gold, the minimum karat designation for gold in the US.

Colored Gold Alloys

Composition and Colors

Gold alloys can vary in color based on the metals added:

• Yellow and Red: Pure gold and copper.
• White: Alloyed with metals like nickel, palladium, silver, and zinc.

Adding copper to gold makes it redder, while adding silver, zinc, or other metals makes gold paler. Lower karat golds can exhibit a wider range of colors due to more alloying metals.

White Gold

White golds were developed in the 1920s as a platinum substitute. The color of white gold can be achieved by adding nickel, palladium, or platinum, which are strong bleaching agents. Other metals like silver and zinc also contribute to the whitening effect but to a lesser extent.

Palladium White Gold

Palladium increases the melting point, modulus of elasticity, strength, and hardness of gold. Palladium white gold is primarily used in jewelry, dentistry, and spark plug electrodes.

Nickel-Free White Gold

In response to European demand for nickel-free alternatives, new white gold alloys with manganese, chromium, or iron as whiteners are being developed. These alloys may require rhodium plating due to less desirable white color and potential issues with cracking and tarnishing.

Mechanical Properties of Gold Alloys

Alloying gold generally increases its strength and hardness while reducing malleability and ductility. The effects of alloying metals are as follows:

• Silver: Slightly larger than gold atoms, moderately improves strength and hardness.
• Copper: Significantly smaller than gold atoms, strongly strengthens and hardens gold by distorting its crystal lattice.

Impact of Karatage on Properties

Reducing karatage from 24K to 18K increases strength and hardness. Further reduction from 18K to 9K has less impact. Lower karat alloys have a lower melting range and density, and copper-containing golds (8K to 18K) can be further hardened by precipitating hard second phases in the solid state.

Summary of Mechanical Properties

Mechanical properties for typical karat golds vary with composition and are detailed in Table 3.

Table 1: Chemical Composition of Colored Gold Alloys

Table 2: Composition of Palladium Containing White Gold

Table 3: Mechanical Properties of Typical Gold Alloys