Gold

60000 usd/kg
Circularity potential
Very high
Strength
High
Production energy
Extreme
Stiffness
High
Embodied CO2
Extreme
Density
Extreme

Gold is revered for its properties: a combination of colour, corrosion resistance, weight and malleability. While it is biocompatible and suitable for implanting in the body (such as to repair or cap teeth as a dental alloy), and its superior conductivity is used in precision electronics applications, the majority ends up in jewellery and tableware.

Its resistance to corrosion and tarnishing sets it apart from other conductive and malleable metals, like copper and silver. Its high resistance to corrosion means that it maintains it performance over time, giving consistent and reliable results.

The purity of gold is measured in carats (ct or c), also karats (kt or k), and is defined by the % weight of gold in the metal: 24 ct pure gold, 23 ct is 95.8% gold, 22 ct is 91.7% gold, 21k is 87.5% gold, 20 ct is 83.3% gold, 19 ct is 79.2% gold, 18 ct is 75% gold,14 ct is 58.3% gold and 10 ct is 41.1% gold.

For parts that are not required to be solid gold, it can be plated onto the surface of another metal. For example, in applications where it is not practical to use gold, such as for weight or strength reasons, or if it would be too expensive or unnecessary.

The carbon footprint and embodied energy (resource use) are eye-watering – much higher than other engineering materials. They are surpassed only by rhodium (Rh) and platinum (Pt). Much of this is the result of the mining practices and energy consumed during refinement and production. Gold is difficult to extract from its ore.


Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Potential conflict commodity (3TG)


24 ct gold is pure and highly malleable as a result of its crystal structure. Like copper, it is face-centred cubic (FCC), which results in a very closely packed, and dense, structure that allows the crystals to slide past one another relatively easily. It can be hammered, by hand or machine, into wafer (gold leaf) as thin as 0.13 microns. It is very ductile and can be drawn into very thin wire down to 5 microns in diameter.

It is used in many applications beyond cast jewellery, trophies and tableware. Pure gold is non toxic and considered bio-inert. As well as being used as a dental alloy, it is added to food and drink for decoration and opulence. Somewhat wastefully, when consumed it will pass through the digestive system without being absorbed.

It is used as a coating, for parts that do not need to be solid gold, and for technical applications. Principally used in electronics, where gold’s conductivity combined with resistance to corrosion and tarnishing means it is the preferred material for electrical connectors in safety-critical and and high-performance applications that require reliability. Hundreds of tons of gold goes into electronics applications for these reasons.

Spacecraft windows, satellites, telescopes and astronaut helmets visors are electrochemically coated with gold (around 0.1 microns) to protect them from solar heat and radiation. Another rather exceptional example is the Royal Bank Plaza, Toronto, that features 14,000 24 ct gold-coated glass windows.

Gold leaf is used to decorate fabrics, paper and packaging. It is drawn into thin wire or strip, which is used as yarn for embroidering onto fabrics, or as part of the structure of highly prized and decorated lampas, brocade and lace. Less expensive synthetic alternatives to gold have emerged for textile and packaging applications, such as aluminium metallised polyethylene terephthalate (PET), polyester film and fibre.


Design properties
Cost usd/kg
60000
Embodied energy MJ/kg
156000-208000
Carbon footprint kgCO2e/kg
12500-36400
Density kg/m3
19320
Tensile modulus GPa
77.2
Tensile strength MPa
207-221
Shear modulus GPa
27.2
Hardness Mohs
2.5-3
Poissons ratio
0.42
Thermal expansion (µm/m)/ºC
14.4
Melt temperature ºC
1064
Thermal conductivity W/mK
315
Temperature min-max °C
-250 to 400
Thermal
good conductor
Electrical
good insulator
Electrical resistivity µΩ⋅m
0.02