Metals and alloys

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Metals and alloys

Introduction

If we leave an object made of iron out of doors it will start to rust. Iron is an element, and when it is rusting the iron is combining with the oxygen in the air and the water in the rain to form a new substance, a compound of iron with other elements, where the iron atoms are not just mixed with the atoms of other elements but are actually joined with them. If we leave it long enough the iron will rust completely away, so we almost never find iron in the Earth's crust, only compounds of iron. If we want to make iron from compounds containing iron we must use a chemical method.

(The core of the Earth is mostly molten iron, but that is only because there is no water or oxygen in the core.)

Other metals react with water, oxygen and other substances in a similar way, some, like magnesium, more quickly than iron, and some, like copper, more slowly.

Metals of one sort or another make up about a quarter of the rocks and minerals in the Earth's crust, although almost always in the form of compounds with other elements. The rocks or minerals from which we can extract metals are called ores.

Metals can be arranged in a list in an order of reactivity. Here are a few selected metals in their order of reactivity, the most reactive first.

The higher a metal is in the list the more rapidly it reacts with oxygen and water and other substances, and the more difficult it is to extract from its ores.

Aluminium is at the top of this list, and this surprises most people: how come we make window frames out of a metal which “rusts” more quickly than iron? But the answer is at the end of this Page.

The first metal to be produced in a reasonably pure form was gold. Gold is so low in the order of reactivity that it does not often form compounds with other elements. This means that it usually exists in the Earth's crust native, as grains or even nuggets of almost pure gold, and not in the form of compounds, so it only needs to be separated from the surrounding rock by very simple processes. No chemical processes are involved, although the process is very hard work and takes a long time. You may have seen Western films showing gold prospectors panning for gold; today you can pan for gold (actually brass filings) at Theme Parks.

Copper came after gold. Extracting copper involves a chemical process, but only technology based upon a simple wood fire. Tin and lead came next - these need slightly more advanced technology and slightly higher temperatures.

Bronze is an alloy (see next Paragraph) of about 90% copper and 10% tin, and is much harder than copper by itself.

Then came iron - this needs very high temperatures, far higher than can be produced in an ordinary wood fire. These temperatures can only be produced inside a blast furnace, where air is forced at high pressure into the middle of burning charcoal or coke. This why the Bronze Age came before the Iron Age!

Metals above iron in the order of reactivity are very hard to separate from their compounds and few were isolated until the eighteenth century CE.

There is a list of many common and not so common metals on another page of this web site - to link to it click here To Chemical Elements

A mixture of a metal with another metal or another substance is called an alloy. Alloys usually have physical properties such as hardness or melting point quite unlike the metals in them. Many alloys are harder than and have a lower melting point than the metals in them, and many alloys can be cast (see next paragraph) even though the metals in them cannot be. It is these properties which make alloys useful. Some alloys have been known and used for hundreds or thousands of years, and some of these are discussed next. Recently however whole ranges of alloys with totally new properties have been developed. You can read about these at the end of this Page.

Many metals and alloys can be cast: the molten metal is poured into a hollow mould to take its shape as it solidifies. When a substance changes from a liquid into a solid there is often a change in volume: it may get bigger or it may get smaller. Water expands when it freezes, so 10 cm3 of water makes 11 cm3 of ice. If a molten metal or alloy expands when it solidifies, when it is poured into a mould it will be forced into every part of the mould and will take its shape exactly. But if it contracts as it solidifies it will shrink away from the sides of the mould and will not take its shape. Bronze expands when it solidifies so can be cast, but copper and tin both contract when they solidify so cannot be cast.

Gold

Pure gold is very soft indeed and useless for anything except objects which will almost never be used: gold objects to be buried with an ancient Egyptian Pharaoh for example. In Europe and North America it is almost always alloyed with another metal (usually copper) to make it harder.

The purity of gold is measured in carats. Pure gold is twenty four carat. Twenty two carat gold is twenty two parts of gold and two parts of copper (adding up to twenty four), eighteen carat gold is eighteen parts of gold and six parts of copper, and so on.

An alloy of gold and silver is sometimes called electrum. Electrum is much harder than pure gold. In Ancient Egypt there were lots of gold mines but no silver mines. Silver was imported from the Northern Mediterranean, but was very little used. But they did use electrum instead of pure gold for objects to be left out of doors, for example pyramidions (the very tip of pyramids and obelisks), because pure gold would wear away too quickly during sandstorms.

The carat used for describing the purity of gold is not the same as the carat used for weighing precious stones such as diamonds. The carat used for weighing diamonds comes from the Arabic word qirat, meaning little weight. This carat was originally the mass of one of the seeds of the carob plant (which produces very uniform seeds), and is about 200 mg.

Gold is always weighed in Troy ounces. A Troy ounce is about 31 g whereas an ordinary ounce is about 28 g, so an ounce of gold really does weigh more than an ounce of feathers!

Gold is very soft and can easily be made into jewellery and other beautiful things. It has been used in coinage for thousands of years, and it is also used as bullion (blocks of gold just stored in bank vaults for their value).

In electronic equipment such as computers and digital cameras copper connectors tarnish quite quickly (look at the outside of a copper water pipe after it has been in use for a few months) and this affects their ability to conduct electricity. So the connectors are often gold-plated as gold never tarnishes.

In Britain it is illegal to call something gold unless it is hall-marked. So most gold jewellery made in countries such as India cannot legally be sold in Britain as gold even though it may actually be twenty two carat gold.


Copper and bronze

Copper was the first metal to be extracted from the Earth that was hard enough to be used for purposes other than jewellery. It was used for axes, knives, ox shoes, hoes and other early agricultural tools, and also for weapons of hunting and warfare.

The earliest copper object so far found dates from about 9000 BCE. But it was another six thousand years before Man learnt to alloy copper with tin to make bronze - bronze is much harder than copper. Typically bronze is about 90% copper and 10% tin. However most of the stories from Greek mythology are set in the early Bronze Age, and until recently many archaeologists referred to the whole of the period in which copper was used as the bronze age. The problem has been made worse by the fact that the Greeks used the same word khalkos for both copper and bronze. Now that we have a much better understanding of other early civilizations, archaeologists prefer the term chalcolithic (copper/stone) age for the period prior to the introduction of bronze. Although the Ancient Egyptians made some use of bronze, they also continued to use untinned copper, even for weapons, right up to the coming of iron, and used separate words for copper and bronze.

Bronze has a lower melting point than copper and can be cast very easily, that is, you can pour the molten metal into a mould to make objects of different shapes.

Some of the tin needed to make bronze came from Cornwall. There was trade between Cornwall and the countries round the Mediterranean Sea at the time the Pyramids of Egypt were being built - two thousand years before the Romans came to Britain! The oldest Man-made object in the world that has been in continuous use for its original purpose is a section of stone wall built to keep the sheep out of the tin mines.

Pewter and brass

Pewter and brass have been widely used since Roman times. Pewter is an alloy of tin with varying amounts of lead, copper and other substances, and brass is an alloy of copper and zinc. Brass is much harder than copper. (Although zinc was not separated as a metal until the thirteenth century CE brass was being made in India from ores naturally containing both copper and zinc before 1000 BCE, although it was not widely used in Europe and the Near East until Roman times.)

It is quite safe to drink out of pewter tankards and goblets, but you should not store liquids in them. This is because lead is slightly soluble in water, and significantly more soluble in acidic solutions, and drinks which have been stored in pewter vessels may have dangerously high concentrations of lead.

Copper is a very good conductor of electricity so is used for wires and cables, and it is quite soft so the cables are flexible. But we usually use brass connectors inside switches etc because where there are screws or moving parts rubbing against each other copper will wear away very quickly.


Solders

Solders are low melting point alloys used for joining or repairing metals and metal objects. Soft solder is an alloy of tin and lead, and is widely used for joining wires in electrical equipment and for many other purposes. Hard solder is an alloy of copper and zinc and is used in brazing (joining and repairing brass) - the proportions of the copper and zinc in hard solder are different from those in brass and so it has a much lower melting point than brass.



Iron and steel

Iron is extracted from its ores by heating the ore with carbon (initially charcoal but later coke) and limestone in a blast furnace - the limestone combines with many of the impurities, particularly silicates, in the ore to make a slag. The earliest blast furnaces operated at a temperature high enough to melt the slag (which is essential), but not high enough to melt the iron. This meant that you could only make iron in batches, and after you had made a batch of iron you had to let the blast furnace cool and dismantle it to get the lump of iron out. This lump of iron was called a bloom.

The iron produced in this way contained almost no carbon and was pure enough to be immediately suitable for making into tools and other objects by heating the lump of iron to red-hot to soften it (make it malleable) and hammering it into shape, as today a blacksmith makes horseshoes in his forge - iron becomes malleable at a temperature a lot lower than that needed to melt it. The iron produced in this way is called wrought iron. Wrought iron is not very hard, and tools made from wrought iron lose their edge very quickly, although of course they can easily be made sharp again. Also bronze can be cast whereas wrought iron cannot be, so bronze continued to be used for very many purposes a long time after the introduction of iron.

Later blast furnaces operated at a temperature high enough to melt the iron. The molten iron collects at the bottom of the blast furnace and can be run off at intervals without having to stop the furnace. The molten iron ran into a main channel made in sand and then into side channels leading off it, where it solidified. This arrangement looked like a sow (female pig) suckling her babies so the lumps of iron were called pigs, and the type of iron produced pig iron. Each pig was of a shape and weight which allowed it to be carried by two people.

However if the temperature is high enough to melt the iron the molten iron can mix with the carbon, and as a result the material produced in such a blast furnace is actually an iron alloy containing about 4% carbon. This can be cast in a mould to make different objects so it is called cast iron. Cast iron is much harder than wrought iron, but it is also very brittle and cannot be reshaped once it has been cast. Hammering an object made of cast iron, even if it is heated until it is red hot, will shatter it.

Much more useful than either wrought iron (0% carbon) or cast iron (4% carbon) are alloys containing about 1% to 2% of carbon. Changing the amount of carbon in the iron, whether by adding carbon to wrought iron or removing carbon from cast iron, is called steelmaking, and the alloys produced in this way are called steels.

You cannot produce steel in a blast furnace: you can only get either wrought iron or cast iron, depending on the temperature inside the furnace. To make steel you must change the carbon content of the iron after it has come out of the blast furnace and this is not easy. The Hittites (from the area of modern Turkey) were making steel in about 1000 BCE, but it was very expensive and only small quantities were made. Unfortunately most books on the Iron Age do not distinguish between iron objects and steel objects.

Steel-making did not become economically feasible on a large scale until the introduction of the Bessemer process in the 1850s (CE). In this process oxygen is blown through molten cast iron using special hollow tubes. This oxidizes the carbon and the process is continued until the carbon content has been reduced to the right amount. Today steel objects are much more common than iron objects although not everyone understands the difference.

Today other substances such as chromium or nickel are sometimes added to the iron during the steelmaking process. Different steels have different properties which make them suitable for different purposes, but all steels contain at least 70% iron, and we often use the words iron and steel synonymously, as the Greeks used khalkos for both copper and bronze.

The carbon needed to make iron in a blast furnace must be very pure: impurities, particularly phosphorus, in the carbon will make the iron unfit for any purpose. Wood and coal contain phosphorus so they cannot be used, only initially charcoal, and then, much later, coke. Huge numbers of trees were cut down to make charcoal. The Industrial Revolution began, in a small town called Coalbrookdale, on the River Severn, in Shropshire, when iron was first made using not charcoal made from wood but coke made from coal. This brought the cost of iron down to the point at which it could be used for making steam engines and railway lines, and machinery to be used in factories, they even used it to build a bridge across the River Severn. No one had ever made an iron bridge before and the people who built it were carpenters. So all the joints in it are mortice and tenon, just as they would have been in a wooden bridge. Today the place is called Ironbridge.

Until Man learned how to extract iron from its ores by smelting all iron came from meteorites and was more valuable than gold. For more about meteorites please see the page on shooting stars. There was a dagger made from meteoric iron in Tutankhamen's tomb (about 1330 BCE). Meteoric iron contains no carbon so can be wrought; it also contains a small amount of nickel so does not rust!

Man first discovered how to extract iron by smelting about 2000 BCE in Asia, but the technology spread very slowly and did not reach the Mediterranean countries until about 1000 BCE. Countries which knew how to make iron were very careful to try to keep the technology to themselves, as today countries with nuclear weapons try to keep others from having them. The story of David and Goliath is set at the time of the transition from the Bronze Age to the Iron Age and you can find out more about it by clicking here To David and Goliath



Mercury

Mercury is a liquid at normal temperatures, and forms an alloy on contact with any other metal. This is called amalgamating, and an alloy containing mercury is called an amalgam. Amalgams have very different properties from the original metal, and for all practical purposes mercury destroys any metal with which it comes into contact. It is particularly destructive of gold, and very great care should be taken to avoid even the least contact between mercury and gold jewellery. A married woman should always remove or cover up her gold wedding ring before cleaning up the pieces of a broken mercury thermometer - make certain your Mum knows this!

The platinum metals

Platinum is a noble metal like gold and occurs native in the Earth's crust. It is however very much harder than gold and has a very much higher melting point. It is therefore very difficult to purify and make things out of, and although it was known in ancient times very little use was made of it until the end of the nineteenth century. It has almost never been used as bullion or in coinage.

Platinum jewellery was quite fashionable, among both men and women, at the beginning of the twentieth century, but is much less fashionable, particularly among women, today. Women tend not to like it because they think their beautiful platinum brooch might be mistaken for silver, a much cheaper metal.

Because of its chemical inertness, hardness, and other properties, platinum and platinum alloys are widely used in industry. The catalytic converter in the exhaust systems of most modern cars contains platinum.

Platinum is usually found with and is very similar to palladium, rhodium, iridium, osmium and ruthenium, and these are therefore referred to as the platinum metals. Osmium has the highest density of any metal, and an alloy of platinum and ruthenium is, after diamond, one of the hardest materials known to Man, and is used in the finest surgical instruments.


Aluminium

Aluminium is actually a very reactive metal, and it was not extracted from its ores until the middle of the nineteenth century CE, but it seems not to corrode so it is used for aeroplanes and window frames and drinks cans and cooking foil and lots of other things.

The reason that it seems not to corrode is because it is so reactive that as soon as it comes into contact with the air it becomes coated with aluminium oxide. The more reactive the metal is the more stable are its compounds, so aluminium oxide is very stable, and resistant to oxygen and water, and most acids and other chemicals. It is also very hard and difficult to scratch, and sticks like glue to the surface of the aluminium rather than flaking off like rust on iron. So within a few seconds of being made anything made of aluminium is covered with a protective coating of aluminium oxide! Aluminium window frames are often anodised, treated in a special way to give them an extra thick layer of aluminium oxide.

There is more about aluminium and anodising here.

However aluminium oxide does not stick to an aluminium amalgam (see above, under mercury). If you are at school you might ask your science teacher to allow your class to do this very simple and exciting experiment - do not try it at home!

You will need a small piece of aluminium foil (ordinary kitchen foil is fine) and a pair of tongs for each person, and also a paintbrush, a few cm3 of 0.1M mercury chloride solution, and a bucket of water.

Make certain your hands are clean and dry (so that the foil will not stick to them). Hold out one hand with the palm facing downwards and place the piece of foil on the back of it. Your teacher dips the brush into the mercury chloride solution and then just touches the foil with it. It might take up to a minute for the amalgam to form, but then things happen very quickly - turn your hand over so the foil falls onto the floor before you burn yourself! (This is why your hands must be clean and dry and why you put the foil onto the back of your hand not into your palm.) Use the tongs to put the piece of foil into the bucket of water. Wash your hands thoroughly afterwards.

Other alloys

There is more about iron and steel and bronze and a description of lots of other alloys on the Wikipedia Alloys web page, but it does get a bit technical.

© Barry Gray June 2016

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