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Geevor Tin Mine Museum 2009 Properties of Metals Out of the ninety naturally occurring elements around seventy are classified as metals. When we describe something as being metallic it is often because it is hard, heavy, lustrous and strong enough to be made into huge variety of tools, machines and structures. This is true for many metals but there are exceptions to these basic properties. Humans first used metals about 8000 years ago and by 6500 years ago began to extract metals from ores. The most abundant metal on Earth is iron forming around 35% of the Earth’s total mass, most of the iron is concentrated at the earth’s centre where it forms a solid inner and liquid outer core along with some nickel. The crust of the Earth contains about 5% iron. Iron is the metal most widely used by man many millions of tonnes are extracted and smelted each year to make steel etc. Iron is one of the metals essential for life the average human contains enough iron to make a 1inch nail. Some of the rarest metals found on Earth include Francium; the heaviest of the alkali metals, less than an ounce (28gms) is thought to exist in the Earth’s crust. Gold is a comparatively rare metal that has been sought and prized by man for many thousands of years. Gold is often found in the un-combined or native state. The largest single mass of gold discovered was the Holtermann nugget – a slab of slate and gold weighing 213.14 kg. It was found in the Beyers and Holterman Star of Hope Mine, Hill End, NSW Australia in October 1872, it contained 82.11 kg of pure gold. The largest pure nugget was the Welcome Stranger, containing 69.92 kg of pure gold, it was found at Moliagul, Victoria, Australia in 1869 by John Deason, from the Isles of Scilly, and Richard Oats from west Cornwall. Gold is a soft heavy metal (specific gravity 19.3), malleable and the most ductile. One gram of gold can be drawn into a wire 2.4 kilometres in length or one ounce to 43 miles. The metal does not tarnish and has always been used for jewellery and decorative purposes. Much of the world’s extracted gold is held in storage vaults as bullion to support many of the worlds financial systems. Platinum metal like gold is also found in the native state. Platinum is the most important of a group of six closely related rare metals the others being osmium, iridium, palladium, rhodium and ruthenium. Platinum is relatively soft and heavy (specific gravity 21.45) with a high melting point (1773.5°C) and resistance to attack by most chemical reagents. It is used to make jewellery, special scientific apparatus and electrical contacts. Osmium is the densest metal on earth with a specific gravity of 22.48. A 12inch (30 cm) cube of osmium would weigh around 1,345 lbs (610 kgs). A similar cube of lithium, the lightest metal with a specific gravity of 0.585, would weigh only 32 lbs 1 Geevor Tin Mine Museum 2009 (14.4kgs). Osmium when mixed with iridium forms osmiridium a very hard alloy traditionally used for making pen nibs. Tungsten is a very hard metal used to make special steels and carbides for cutting tools, drills, armoured plate and projectiles for military use. It is the metal with the highest melting point (3370°C) and is widely used to make electric light bulb filaments and electrodes for TIG (tungsten inert gas) welding systems. Tungsten is found as the mineral wolframite and was often recovered along with cassiterite in many Cornish mines. The metal with the lowest melting point is mercury or quicksilver. It has a melting point of -38.9°C, thus at normal temperatures it is liquid. Mercury is able to dissolve many other metals, particularly gold, to form alloys (called amalgams). Other uses include the manufacture of batteries, scientific instruments like thermometers, lighting and detonators. Te rare metal gallium has a melting point of 30.15°C thus it would melt if held in the hand. Mercury vapour and many of its compounds are very poisonous but there are several metals that are deadly to man, some in minute quantities. Lead has been used for several thousand years to make water pipes, cisterns and as a component of pewter for tableware and drinking vessels. It has been suggested that a contributing factor to the downfall of the Roman Empire was the poor health suffered by many Roman citizens due to lead poisoning. Other highly toxic metals include antimony, cadmium, barium and thallium. The most toxic of all metals are those that are radioactive. Metals such as uranium, radium, thorium and polonium occur naturally in many parts of the world. When these metals are extracted and concentrated their handling and disposal can be very hazardous and problematic. One of the most deadly of all metals is thorium 228. It has been calculated that exposure to 2.4 x 10 –16 gms of thorium 228 per cubic metre of air would be fatal. 2