Precious Metals Guide

Precious metals are considered precious because they have a good resistance to the attacks by the environment and because of their rarity, considering that they are not only used in jewellery, but also in industry, medicine and as an investment . Gold and silver were, in fact, also used for trading, and many countries still have reserves of gold in their central banks.

Gold and silver have been used since early civilisations, while platinum not until the 18th century and palladium has only recently been used on its own to make jewellery. The platinum group metals (PGMs) are all more recent than gold and silver and they are: platinum, palladium, rhodium, iridium, ruthenium and osmium.

- Mechanical properties of precious metals:

• Hardness: resistance to penetration. It gives an indication of the metal resistance to damage for example by wear or scratching. It is also related to its strength; Harder materials are also stronger.
• Strength: capacity to resist a permanent distortion or fracture when a force is applied, for example when pulling or pressing. It indicates the behaviour while working with it.
• Ductility: ability to deform plastically, therefore it is an indication of the malleability of the metal, that is how easily it can be bent and shaped.

Pure precious metals are too soft top be used on their own and consequently alloys are used, to improve their strength, workability and wearability. In order to be sure of the quantity of precious metal in the jewellery you are buying, the UK Hallmarking Act (1973) requires to hallmark jewellery and objects made of precious metals. The hallmark is an official mark that indicates the finess of the metal applied by the assay office.

- Physical properties of precious metals:

The pure precious metals are chemical elements, meaning they cannot be split up into simpler substances. Their chemical symbols are: Au - gold; Ag - silver; Pt - platinum; Pd - palladium; Rh - rhodium.

• Density: weight per unit volume, a denser metal weigh more.
  The decreasing density order is: platinum, gold, palladium, silver. Therefore, for example, a ring in platinum is heavier than the same ring in gold.

• Melting point:

  Gold	1064°C
  Silver	9761°C
  Platinum	1772°C
  Palladium	1552°C
  Rhodium	1966°C

  An interesting fact showing again their preciousness is that the melting point of gold, silver, platinum and palladium are fixed points on the International Practical Temperature which is used to calibrate all temperature-measuring devices.

• Optical properties: colour, lustre, reflectivity make precious metals beautiful and fascinating.

- Chemical properties of precious metals:

They indicate the way precious metals react with the substances in the environment, like oxygen, water, acids... . For example, the tarnishing reaction that is when thin dullings and discolouring layer of metal oxide or metal sulphide forms on the metal surface.

Pure gold, for example, is totally unaffected by exposure to humidity or oxygen even at high temperatures and will dissolve only in aqua regia and cyanide solutions.

GOLD

Since pure gold is too soft for using in jewellery, alloys are used and to indicate the content of gold in them the word karat is used. It derives from the Arabic word ‘kirat’, the seed of the carob or locust bean tree, which are so similar in size and weight that were used by African and Arab traders to weigh gold, gemstones and other objects. There is often confusion between karat and carat and they are often used with the same meaning: karat is the percentage of gold in the alloy, while carat measures the weight/size of gemstones.

Alloying is also used to change colour to gold, being pure gold yellow, to have rose and white golds. Coloured karat gold are based on the gold-silver-copper (Au-Ag-Cu) alloys. More copper than silver gives a redder colour, while the same amount of copper and silver gives a rich yellow (9k to 18k).

White golds were developed in the 1920s as a less expensive alternative to platinum. The most used alloy for white golds is gold-silver-palladium (Au-Ag-Pd), since nickel causes allergy problem. White golds are rhodium plated to give the the best white colour and lustre. White gold jewellery may require re-plating after some time.

The best karatage to use in jewellery and stone setting is 18 karat, but if a less expensive option is required both 14k and 9k are a good choice.