Precious metals include gold, silver, platinum and platinum group metals, which are palladium, rhodium, ruthenium, iridium and osmium. Their appearance is glossy and beautiful, and they possess strength, high density and resistance to chemicals and weathering. Precious metals are also ductile and can be soldered with other metals.
This group earned its name "precious metals” because of the materials’ high prices. Their properties also evoke an additional moniker: "noble." Precious metals are rare in nature and, therefore, require a lot of effort to find. These metals play a dual role:
- Precious metals are designed for industrial applications (in machinery, electronics, medical equipment, prosthetics, etc.).
- They are used for investment (i.e., used in the production of coins, jewelry, and in many countries’ foreign exchange treasury reserves).
CHARACTERISTICS OF PRECIOUS METALS
Gold is a precious metal that is a brilliant bright yellow color. In its pure form, gold is resistant to corrosion and has tremendous elasticity. For instance, one gram of pure gold can be elongated into a thread length of three kilometers. Gold dissolves only in aqua regia (a mixture of HN03 and HC1 in a ratio of 1:3). Its density is 19,320 kg/m3 and its melting temperature is 1064°C. Gold seems as if it were created by nature specifically for the minting of coins and manufacturing of jewelry. It is also used by almost all central banks in the world as a part of their foreign exchange reserves.
Silver is a white precious metal that is shiny, very ductile and malleable. It can be rolled into thin sheets up to 0.00025 mm and then drawn into the thinnest of wires. Silver has a very high reflectivity (up to 95%) and is a good conductor of electricity and heat. Its density is 10500 kg/m3 and its melting point is 961.9°C. It has resistance to moisture but interacts with both acids and alkalis. Silver can blacken in the presence of air, water, or hydrogen sulfide, forming a characteristic patina.
Platinum is a precious metal that is a brilliant white color. Its properties include refractory, excellent ductility and chemical resistance. It has a high density of 21450 kg/m3 and a high melting temperature of 1772°C. Platinum is insoluble in acids (except aqua regia). It is harder than both gold and silver and is not oxidized. Platinum is widely used in jewelry.
Palladium is a silver-white precious metal, which is soft and malleable. It is the easiest, lowest-melting, most flexible and ductile of all the platinum metals. It can be easily rolled, stretched into wire, and perfectly polished. Palladium does not tarnish and is resistant to corrosion. The density of palladium is 12020 kg/m3 and its melting temperature is 1552°C. The chemical properties of palladium are slightly lower than those of platinum. It dissolves easily in aqua regia and nitric acid. The jewelry industry uses palladium as a component of gold alloy and in white, high-temperature soldering.
Rhodium is precious metal that is bluish-white in color. It is refractory and possesses high hardness, brittleness and reflectivity. Chemically, rhodium is very passive; it is not oxidized in air or water and does not react with acid mixtures. It is soluble in alkaline cyanide solutions. Its density is 12420 kg/m3 and its melting point is 1960°C. In jewelry, rhodium is used as a decorative and protective coating material, especially in silver and gold (white gold) jewelry. World reserves of this precious metal are estimated at only a few tons, and annual production is measured in hundreds of kilograms. Therefore, rhodium is very expensive and is most often only used when absolutely necessary.
Ruthenium is a silvery-white metal that looks similar to platinum. However, is it harder, and more brittle and refractory. This precious metal is named in honor of Russia. It is used in the manufacturing of wires, contacts, electrodes, laboratory glassware and jewelry. Ruthenium is the rarest metal of the platinum group. It has excellent chemical resistance and can be used as a component of platinum alloys. Its density is 12370 kg/m3 and its melting temperature is 2950°C.
Iridium is a grayish-white precious metal that differs from the others in its hardness, brittleness and high melting point. Iridium was discovered in 1803. Separate use infrequently and often it is used as a ligature. Chemically, iridium is unusually stable. It does not react with alkalis, acids, and mixtures thereof; it is also difficult to use in manufacturing machinery, as pressure can only be applied to it in its incandescent state. The density of iridium is 22420 kg/m3 and its melting point is 2450°C. This metal is used in the chemical industry and in jewelry manufacturing.
Osmium is white with gray-blue shades. This precious metal is very refractory, heavy, hard and brittle. The Greek word “osme” (smell) was given to this precious metal 200 years ago due to its unpleasant, irritating odor, which is similar in smell to a mixture of bleach and garlic. In nature, pure osmium is found in platinum-bearing river sands. Its melting temperature is the highest of the platinum group metals at 3047°C. Its density is 22480kg/m3. As it does not dissolve in acid and aqua regia, it cannot be used in machinery. Osmium is used in alloy metals combined with platinum to impart hardness and elasticity.
Stamps on precious metals
Stamps are made on precious metals to determine the quantity of the pure metal (silver, palladium, platinum, or gold) in ligature alloys. The following stamps in jewelry are used around the world:
- Gold – 375=9K, 585=14K, 750=18K, and 999 =24K;
- Silver - 800, 830, 875, 925, 960 and 999;
- Platinum - 850, 900 and 950;
- Palladium - 500, 850.
CURRENCY PRECIOUS METALS
Of all the precious metals, only four have become commodities and are used for investment.
- Gold – its chemical element symbol is Au. Its stock exchange symbol is XAU.
- Silver – its chemical element symbol is Ag. Its stock exchange symbol is XAG.
- Platinum – its chemical element symbol is Pt. Its stock exchange symbol is XPT.
- Palladium – its chemical element symbol is Pd. Its stock exchange symbol is XPD.
Precious metals are rare in nature, and their production is very laborious. Historically gold and silver, as well as platinum and palladium, have been used to make coins. These noble metals were used as a means of accumulation and status.
Today, gold serves as the main metal used in currency, though silver also used to play a role. Currently, silver is stored in foreign exchange reserves of some central banks, but in sufficiently small quantities.
Precious metals in their pure forms can be relatively soft (particularly gold and silver), brittle (iridium or osmium) or refractory (iridium, ruthenium and osmium). Therefore, in their pure forms, these metals are rarely used for industrial purposes or in the manufacturing of jewelry and other household products. To make these precious metals’ properties ready for industrial use, they are combined in different proportions with other metals. These metals are called alloy metals or ligature. Alloying can be done with precious and non-precious metals, but the resulting alloys are called precious. Each metal alloy has a role in determining the characteristics of the alloy:
Silver as an alloying metal with gold lends softness, malleability, lowers the melting point and changes the color of gold.
Platinum significantly increases the melting point of the alloy, making it firmer and more resilient. The addition of platinum (and platinum group metals) changes the color to white gold alloy.
The addition of copper increases the hardness of gold alloy, while maintaining ductility and malleability. This alloy acquires a reddish tint.
Jewelry usually uses three types of alloys: gold, silver and platinum. The main content of the alloy of the precious metal is its breakdown.