Properties of Metals.pdf - Geevor Tin Mine

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
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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.
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