Gold. For thousands of years, gold has been regarded as the finest and most precious metal known to man. In this CDA, I will try to find out why gold is so valuable.

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Gold


Gold

INTRODUCTION

PROPERTIES

Gold is a metal which has had many purposes over the thousands of years. It has been used for jewellery, money, decoration and, more recently, in tooth fillings and electronics. It’s a soft, yellow, metal with useful thermal and electrical properties. Unlike other metals like silver and copper, gold does not tarnish over time and use. Tarnish is a dark layer which forms over some metals such as copper, brass and aluminium. It is very dense metal, at 19.32 g/cm3. To give you an idea of just how heavy gold is, think of it this way: the mass of 1 litre of water is 1 kilogram. The mass of 1 litre of gold is 19.32 kilograms. Imagine carrying a 1-litre water bottle that weighs over 3 stone. The colour of gold is also called gold. It has an atomic number of 79, and a relative atomic mass of 196.97. The symbol for gold is Au, which comes from the Latin word for gold, ‘aurum’. A gold leaf is gold that has been beaten into very thin sheets. Of all elements, gold is the most malleable and ductile. 1 gram of gold can be beaten down to cover an area of 1 square metre. Malleability is the ability of a material to be flattened down into a thin sheet by rolling or hammering. Also, because of its high ductility, solid gold wire can be anything up to around one hundredth of a millimetre thick. Ductility is the extent to which a material can be deformed (made out of shape) to, without fracture. One of the main reasons why gold is so admired by people around the world for thousands of years is because it’s highly unreactive. This means that it can be used over and over again, and for different purposes, without wearing out or losing colour. Gold only has one stable isotope, which is gold-197 (or ). An isotope is a different type of atoms, where the atomic number (number of protons) is the same, but the number of neutrons is different. Gold doesn’t oxidise easily, which means that it doesn’t bond with oxygen, as a result of being highly unreactive. It oxidises with nitric acid to Au3+, although to small amounts.

PURITY

When people buy gold jewellery in shops, the gold isn’t 100% pure gold. It is actually made up of gold and other metals. The purity of gold is measured in carats. It is in parts of 24 carats, so something which has a carat rating of 24 (or simply 24k or 24ct) is made out of pure gold and nothing else. This is very uncommon in the retail market because then it would be very expensive, and wouldn’t be as strong as some of the less-carat gold. It is also measured in ‘fineness’, where the purity of gold is expressed in parts per 1000. Therefore, ‘750 fine’ is ¾ gold (18k) (NOTE: in reality ‘1000 fine’ gold does not exist, since there is no way of being certain that 100% of the material is gold and no other element exists within it. The hallmark would usually say ‘999.9 fine’). An easy way to convert from fineness to the percentage gold is to divide the ‘fine’ number by 10. So, for example, 750 fine = 75% pure gold, since 750 ÷ 10 = 75. Gold is also measured in fineness. They usually say ‘999.9 FINE’, which is saying that they are 99.99% pure gold. Common sense tells us that 24k gold is made up of 100% gold and nothing else. However, in reality, 24k gold is used to refer to gold which is at least 99.7% gold. Fine gold is the name for 24k gold. If the purity exceeds 99.95% gold, then it is called proof gold. These types of nearly-pure gold come as gold bars, and are not used for jewellery. Instead, they are used as gold in gold reserves and trading. This makes it easier because when they are being traded no one has to adjust the price according to fineness. 1 troy ounce (around 31 grams) of proof gold is an international currency, with the symbol XAU. This means that next time you hear on the news about the price of gold, the price refers to one troy ounce of pure gold. TABLE 1 below shows the relationship between different carats of gold with fineness (hallmark rating) and percentage of content which is gold:

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TABLE 2 shows the relationship between the purity of gold and international standards for that purity:

There are several ways in which to test a material of its gold content. Some are destructive (i.e. after the test the material has been damaged in some way, therefore; losing its original value) and some are not. One of the most accurate and well-known ways is by fire assaying. Fire assaying is destructive, but its accuracy is within 2-3 parts per 10,000. The first step in this process is to take a scraping of ...

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