The term ‘precious metals’ is taken to include silver, gold, and the six platinum group of metals-platinum, palladium, ruthenium, rhodium, osmium, and iridium.
Gold and silver have been known and used for many millennia, as has platinum, although to a lesser extent. The others are relatively recent discoveries. These metals and their alloys are used in the electronics, jewelry, dentistry, coinage, textile, automotive, aerospace, ceramics, and chemical industries.
They all have high densities (specific gravities ranging from silver at 10.5 to iridium at 22.6), relatively good corrosion resistance, good electrical conductivity and light reflectivity, and relatively high cost. Despite the latter factor, they are often economically acceptable because of their properties and performance in service. Scrap values are high and recycling is common.
Precious metals are chemically less reactive than most elements. They have a very shiny appearance, which is why they are used for ornaments and jewelry. Its ductile quality means they can be shaped into a desired form without being broken. Gold is both ductile and malleable, which means that it can also be formed into a thin sheet or wire.
They have a higher melting point than normal metals, which means that they require large amounts of heat and electricity to be melted and then reshaped. This contributes to them being very expensive metals. Gold, silver and platinum are the precious metals that are most highly-traded.
The purities of silver and gold are designated in terms of several systems which do not apply to other metals. Fineness is used to designate purity in parts per thousand by weight; hence, 995 fine gold is 99.5% gold, and 925 fine silver (sterling silver) is 92.5% silver. Gold bullion, as traded, is at least 995 fine and silver bullion at least 999 fine. Gold purity for jewelry applications is specified in karats, where 1 karat is 1/24th part of gold; hence, for example, 18 karat gold is 75% gold.
General characteristics of precious metals include:
- A higher melting point than other metals
- Softer in texture – which is also referred to as being ductile
- More lustrous in appearance
- Less reactive than other elements
Gold and silver are the most commonly known precious metals and perhaps the most widely used. The platinum group of precious metals includes iridium, osmium, palladium, rhodium, ruthenium and platinum.
The most popular of the platinum group is platinum itself which also utilized in scientific and medical research in the form of measures, weights and similar items, owing to its great resistance to high temperatures and acids. However the primary use platinum is put to is in the manufacture of up market jewelry. Diamonds and pearls are more often than not placed in settings of platinum these days rather than in any other precious metal.
Palladium was also used as a substitute material for jewelry manufacture during a certain period of history when platinum was not freely available. Used in dental products, a unique characteristic of palladium is its exceptionally high ability to absorb hydrogen.
Rhodium is utilized in the manufacture of components for turbine engines used in airplanes. Owing to its high reflectivity and resistance to corrosion, it is a popular substance for the production of flashlights and mirrors.
Silver was one of the first metals utilized by man. It has the highest thermal and electrical conductivity of all metals. Its chemical symbol is Ag, its atomic number is 47 and it has an atomic mass of 107.9. The metal's physical appearance is white, reflective, and very malleable. Common uses for this metal are jewelry, ornamental, coins, mirrors and electrically conductive paint.
Silver-clad base metals (copper, brass, nickel, iron) are also used, in particular in the electrical and chemical industries. Silver and the other precious metals are described by the UNS system in a single category wit the prefix letter P. Other than for unalloyed silver, the designation given is P07xxy, where xx gives the approximate silver content of the alloy in percent; hence, P07650 contains about 65 % Ag. Refined silver is available at several levels of purity from 99.99% (UNS P07010) to 99.90% (UNS P07020).
Unlike some other precious metals, silver will tarnish when exposed to contaminants such as ozone. Silver has been used by humans since prehistoric times, and the first evidence of a culture separating it from lead dates back to 3000 B.C. Important producers of the world's silver supply include Nevada, Mexico, Peru, Chile and Canada.
Gold has been used by humans for most of human existence. It is yellow in color and has a characteristic reflective shine. In its pure form it will not tarnish. This precious metal is valued for its malleability, sheen and ability to conduct electricity. Common uses for gold include but are not limited to dentistry and medicine, jewelry, art, coins, ingots, scientific and electronic instruments and as an electrolyte in the electro-plating industry. Approximately half of the world's supply of this resource is located in South Africa. Large stores also exist in the United States, Australia, Brazil, Canada, China and Russia. Gold's chemical symbol is Au. It has an atomic number of 79 and an atomic mass of 197.0
Gold has an excellent chemical stability with a high resistance to corrosion and oxidation. However, this is just one of the many properties that gold possesses, which when considered in combination with each other have led to a number of exciting and often unique industrial applications. The purity of gold is measured in Carats. A Carat was originally a unit of mass (weight) based on the Carob seed or bean which was used by ancient merchants in the Middle East. The Carat is still used for the weight of gem stones where 1 carat = 200mg. For gold it is used to measure the purity where pure gold is 24 carats. The following table shows the range from pure gold at 24 Carats to less pure at 9 Carats.
In Europe 18 and 14 carat alloys are most commonly used in jewelry, however 9 carat is popular in Britain.
In many countries the law requires that every item of gold jewelry is clearly stamped with its caratage. Jewelry in many countries is stamped or hallmarked with its caratage. The hallmarking system was developed in London in the 14th century at Goldsmiths’ Hall.
The chemical symbol for the element platinum is Pt. It has an atomic mass of 195.1 and its atomic number is 78. The physical appearance of this precious metal is silver-white to gray. It is known for being one of the heaviest elements. This metal will not oxidize unless it is in the presence of bases. Platinum is one of the rarest of the elements. It is predominantly used as a catalyst for the control of automobile and industrial plant emissions. It is also used a catalyst for making acids, organic compounds and pharmaceuticals. This metal is also used in dental alloys, in capacitors, as conductive or resistive films for electronic circuits, as well as in jewelry. The majority of the world's platinum reserves exist in either South Africa or Russia.
Figure 1: Pouring molten gold into an ingot mold