Gold and Gold Alloys

---

Fundamental Properties of Gold

Gold, designated by the chemical symbol Au, possesses unique atomic and physical characteristics that contribute to its enduring value. With an atomic number of 79 and atomic mass of 196.967, gold exhibits distinctive properties including its characteristic yellow color, which results from specific electron transitions between energy bands. The atomic radius of 0.1442nm, notably smaller than theoretical predictions, influences its interaction with other elements.

Physical Characteristics and Malleability

Pure gold demonstrates remarkable malleability and ductility. A single ounce can be drawn into 80 kilometers of wire merely 5 microns in diameter, making it ideal for electrical contacts and bonding applications. The metal's Young's modulus of 79 GPa, comparable to silver but significantly lower than iron or steel, contributes to its workability while maintaining structural integrity.

Table 1: Chemical composition of gold alloys with color and karat number

Color of Gold	Alloy Composition
Yellow Gold (22 kt)	Gold - 91.67% Silver - 5% Copper - 2% Zinc - 1.33%
Red Gold (18 kt)	Gold - 75% Copper - 25%
Rose Gold (18 kt)	Gold - 75% Copper - 22.25% Silver - 2.75%
Pink Gold (18 kt)	Gold - 75% Copper - 20% Silver - 5%
White Gold (18 kt)	Gold - 75% Platinum or Palladium - 25%
White Gold (18 kt)	Gold - 75% Palladium - 10% Nickel - 10% Zinc - 5%
Gray-White Gold (18 kt)	Gold - 75% Iron - 17% Copper - 8%
Soft Green Gold (18 kt)	Gold - 75% Silver - 25%
Light Green Gold (18 kt)	Gold - 75% Copper - 23% Cadmium - 2%
Green Gold (18 kt)	Gold - 75% Silver - 20% Copper - 5%
Deep Green Gold (18 kt)	Gold - 75% Silver - 15% Copper - 6% Cadmium - 4%
Blue-White or Blue Gold (18 kt)	Gold - 75% Iron - 25%
Purple Gold	Gold - 80% Aluminum - 20%

Understanding Gold Purity and Karat System

The karat system provides a standardized method for expressing gold purity. Pure gold, designated as 24 karat (24K), contains greater than 99.7% gold content. Common jewelry grades include:

• 18K gold: 75% pure gold (18 parts gold, 6 parts other metals)
• 14K gold: 58.3% pure gold (14 parts gold, 10 parts other metals)
• 10K gold: 41.7% pure gold (10 parts gold, 14 parts other metals)

Colored Gold Alloys

The natural yellow color of pure gold can be modified through careful alloying. Copper, the only other naturally colored pure metal, produces redder hues when added to gold. Silver, zinc, and other metals create progressively paler colors. Lower karat alloys permit a broader spectrum of colors due to higher proportions of alloying elements.

White Gold Development

White gold alloys, developed in the 1920s as platinum alternatives, fall into two primary categories: nickel-based and palladium-based compositions.

Table 2: Composition of palladium-containing white gold

	Gold	Palladium	Silver	Copper	Zinc	Nickel
18 kt	75	20	5
	75	15	10
	75	10	15
	75	10	10.5	3.5	0.1	0.9
	75	6.4	9.9	5.1	3.5	1.1
	75	15		3.0		7.0
14 kt	58.3	20	6	3	1
14 kt	58.5	5	32.5	20.5	1.4
10 kt	37.5		52	4.9	4.2	1.4

Modern White Gold Variations

Contemporary white gold formulations often incorporate alternative whitening elements:

• Manganese-based alloys for nickel-free applications
• Chromium and iron additions for enhanced hardness
• Platinum and cobalt for specific property combinations

Mechanical Properties and Alloying Effects

Alloying elements significantly influence gold's mechanical characteristics. Silver, with its slightly larger atomic size, moderately increases strength and hardness. Copper, having a smaller atomic size, produces more pronounced strengthening effects through greater crystal lattice distortion.

Table 3. Mechanical Properties of Typical Gold Alloys

Carat	Composition %, wt.		Condition	Hardness HV	Tensile Strength N/mm²
	Silver	Copper
24	-	-	Annealed	20	45
			Worked	55	200
22	5.5	2.8	Annealed	52	220
			Worked	138	390
	3.2	5.1	Annealed	70	275
			Worked	142	463
21	4.5	8.0	Annealed	100	363
			Worked	190	650
	1.75	10.75	Annealed	123	396
			Worked	197	728
18	12.5	12.5	Annealed	150	520
			Worked	212	810
	4.5	20.5	Annealed	165	550
			Worked	227	880

Strength and Hardness Modifications

The relationship between karat number and mechanical properties follows a distinct pattern. Reducing gold content from 24K to 18K substantially increases strength and hardness. However, further reduction below 18K yields diminishing returns in mechanical property enhancement.

Heat Treatment Considerations

Copper-containing alloys between 8 and 18 karats can achieve additional hardening through heat treatment. These alloys form hard secondary phases below 400°C, necessitating water quenching after annealing to maintain ductility for subsequent working operations.

Modern Applications and Developments

Jewelry Manufacturing

Contemporary jewelry manufacturing employs various gold alloy compositions to balance aesthetic requirements with mechanical properties. The selection of specific alloys depends on: Manufacturing requirements Desired color characteristics Wear resistance needs Cost considerations.

Industrial Applications

Beyond decorative uses, gold alloys serve crucial roles in: Electronic components and connections Dental applications Specialized industrial equipment.

Environmental and Health Considerations

Recent developments in gold alloy composition reflect growing awareness of metal sensitivities and environmental concerns. The European Union's emphasis on nickel-free white gold has spurred development of alternative compositions, particularly those utilizing manganese and palladium-based systems.

Conclusion

Understanding gold alloy compositions and their effects on properties remains crucial for both traditional and emerging applications. Continued development of new alloy systems addresses evolving requirements for performance, aesthetics, and biocompatibility while maintaining gold's fundamental appeal.