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Gold
60000 usd/kg
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)
18 ct white gold is made up of 75% gold mixed with white alloys, such as palladium (Pd) and platinum (Pt). Most commonly used to make jewellery, they are biocompatible and used in dental applications, too. In applications not intended for close contact with the body, nickel (Ni) is used as a hardening alloy.
The name sometimes reflects the proportion, such as AuPd 75/25 or 80/20, for example. White gold is stronger, stiffer and harder than pure gold as a result of the alloys. Metal selection will determine exact properties. In jewellery applications, palladium (a member of the platinum family), is often used, because it is relatively inert and does not produce any allergic reactions.
Platinum mixed with gold (AuPt) gives some impressive mechanical properties. Materials scientists at Sandia National Laboratories produced a platinum-gold alloy (PtAu 90/10) coating that demonstrated ultra-durable properties, better than any other material. Its impressive wear resistance is based on the principal that a material’s resistance to friction is based on is reaction to heat, as opposed to the conventional wisdom that it is based on hardness.
The silvery white colour of finished white gold parts may be enhanced with a coating of rhodium. However, this finish will wear away and so will need to be replenished if the appearance is to be maintained.