Herefordshire Through Time
Herefordshire Through Time
Iron is a metallic element with the chemical symbol Fe, which comes from the Latin word "ferrum". It has a melting point of around 1150º Centigrade and upwards. Iron is useful in that it has the property of alloying (uniting) with numerous other elements. It is the fourth most densely distributed element in the earth's crust, but it is only found in a limited number of places in a form that can be made use of. Iron does not occur anywhere as an immediately usable metal. It is always found as iron ore, of which there are several types; all of these need to undergo complicated processing techniques before they become a useful metal. All iron ores are a mixture of substances. They are all oxides, which means that they are chemically bonded with oxygen. They also contain small amounts of other elements, as well as materials such as sand and clay.
It is possible to divide processed iron into three major groups: wrought iron, cast iron and steel. The most important and commercially viable of these is steel. Wrought iron is no longer produced on a large scale but cast iron and steel are still manufactured.
Wrought iron is the oldest of the three types of iron, dating back at least 4,000 years, and wrought iron door furniture was commonplace in Roman times. The use of wrought iron as a structural material dates back at least to the Middle Ages. Wrought iron is fibrous in structure and strong in tension, which guaranteed its use in the canal and railway ages.
Cast iron, which was first produced in the 15th century, is crystalline in structure and relatively weak in tension but strong in compression. It is an alloy of iron and carbon, with up to 5% carbon content. It cannot be shaped by hammering, as wrought iron can, but it can be melted and poured into a mould.
Steel is now the most widely used form of iron and also the most versatile. However, bulk steel manufacture only became possible after the invention of the Bessemer process in 1856.
The earliest device for producing iron, the bloomery, was a small furnace built of clay. Charcoal was the fuel, and it was kept alight by manually-operated bellows. Small pieces of iron ore, put into the bloomery on top of the charcoal, became little pieces (or blooms) of wrought iron after several hours. Ironmaking first occurred in the Middle East before spreading across Europe and reaching Britain by about 450BC. As the bloomery was small and needed only clay, iron ore and timber for charcoal, it could be easily set up anywhere these materials could be found.
At the time of their occupation of Britain, from AD43 onwards, the Romans found an established (though still small and scattered) iron industry, and they expanded on it. Iron was useful to the Romans for weapons and tools, as well as for household items. After the Romans left the ironworks suffered from wars and infighting but ironworking never ceased. From the 13th century water power was used in the process, making it possible to use bigger bellows than those operated by hand and so allowing the bloomeries to grow.
In the 15th century a new type of furnace, the blast furnace, was devised in the ironworking district of Liege, Belgium. These made greatly increased outputs of iron possible and introduced another form of iron, cast iron. The process spread to Britain, where the first blast furnace was erected at Newbridge, Sussex in 1496.
Cast iron was made by smelting iron ore with charcoal in the blast furnace, where it melted and accumulated in the bottom before being tapped into sand beds where it solidified and was broken into pieces. There was not much use for cast iron at first, and most of it was converted into wrought iron in a separate furnace called a finery. Some things such as firebacks, graveslabs, cannons and cannonballs were made from cast iron. The first recorded use of a furnace for casting cannons was in 1543 at Buxted in East Sussex.
Furnaces could only be set up where there was the necessary natural combination of iron ore, timber for charcoal and a stream or river for water power. Thus the iron industry settled in a small number of districts. By the 16th century the Weald (an area of south Sussex, Surrey and southern Kent) was Britain's biggest ironmaking area. From 1560 onwards the blast furnace and finery spread to other areas: to Staffordshire in 1561-2; to Shropshire in 1554; and to Derbyshire in 1852. By the end of the 18th century ironmaking was concentrated in the Weald, the Forest of Dean in Gloucestershire/Herefordshire, South Wales, the West Midlands, North Wales, North Staffordshire/Cheshire, Derbyshire/Yorkshire and Furness in Cumbria.
The heat of a blast furnace was maintained by the constant blast of air supplied by a pair of bellows, powered by waterwheels. Due to the intermittent nature of water supply in Britain blast furnaces tended to work continuously for short periods of several months, and then stood idle for months or even years before being worked again.
Another factor limiting output was fuel. Charcoal came from coppiced woodlands, a generally sustainable but ultimately limited source. The use of coal as an alternative was tried but coal contains sulphur, which unites with iron. This was fine if the iron was to be used as cast iron, but was no good for wrought iron as it crumbled at high heat. The answer to this problem was found by Abraham Darby, who in 1709 adapted a blast furnace at Coalbrookdale in Shropshire to use coal in the modified form of coke, which contained no sulphur. Coke was created by burning coal in open heaps to release the sulphur.
However, charcoal was still the fuel for the finery and using raw coal would not have worked. Moreover, the output of the finery was small and there was a need for something with output to match that of the bloomery. The expansion of ironmaking was also inhibited by the lack of water power.
In 1784 Henry Cort of Fontley, Hampshire, applied the coal-fired reverberatory furnace to the conversion of cast iron to wrought iron; he was able to keep the burning coal separate and so prevent sulphur contamination. This process became known as puddling, because the iron was stirred to promote the reaction of air and carbon.
By 1784 the steam engine could provide reciprocating motion for pumping and rotative motion to drive rolling mills, hammers and other machinery. So by the end of the 18th century the iron industry was freed of its restrictions and could be set up anywhere. From a total of 86 in 1788 the number of blast furnaces in Britain had risen to 237 by 1823, with trade dominated by South Wales and Staffordshire.
The coming of cast iron not only made new types of advance possible, but its use also extended into the domestic field for such things as firebacks, graveslabs and later cooking utensils and, later still, architectural work. The products of the ironworks were basically only two: molten cast iron and solid wrought iron blooms. The former could be poured into moulds to make castings of many types, the latter needed hammering or working mechanically before they were of any use.
Bloom had to be worked red-hot; hammering it down was slow and the thinner it got the quicker it cooled. This problem was solved by a machine known as the slitting mill (the first one in Britain was built in 1590). A piece of iron was hammered out to a flat strip, then passed between a pair of flat rollers rotated by water power and so flattened and extended. This strip was then passed between rotating disc cutters which cut (or slit) it into a number of narrow rods.
In 1720 John Hanbury of Pontypool widened the rolls and so made it possible to produce thin iron sheets. In 1783 Henry Cort invented grooved rolls which made it possible to produce grooved sections. The rolling mill could also be used to produce patterns on the iron rods.
The early 19th century saw several important innovations. In 1828 James Neilson of Glasgow patented a method of blowing hot rather than cold air into the blast furnace, decreasing the quantity of fuel required for smelting. Later, methods of harnessing the waste heat of the furnaces to heat the air for the blast furnace were introduced. Modification of the rolling mills enabled wrought iron rails to be mass produced; the British industry was now making a global contribution to the laying of the railways in the mid-19th century.
As industrialisation grew so did the demand for iron. Machines were made of iron, coal and metalliferous mines used cast iron pipes and pumps for drainage and engines. Iron-framed buildings provided another outlet. From the 1820s the railways were a major market for iron, using it for locomotives, carriages, wagons and tracks. In 1821 Aaron Manby built the first iron steamboat, and the advantages of iron were soon acknowledged when the ship went into service and is said to have run aground several times with no serious damage to her hull.
By the mid-19th century iron had become the most important metal of commerce. In 1788 68,300 tonnes of pig iron were produced; in 1852 this number was 2,701,000 tonnes.
By the middle of the 19th century ironmaking was being carried out in several parts of the world. Then came an invention that was to change the whole pattern of industry. This was the invention of the Bessemer process. Henry Bessemer sought an improved method of making wrought iron, but in the process of experimentation created a metal which, whilst chemically similar to wrought iron, was physically different. It proved to be suitable for almost any purpose for which wrought iron was used but was very much quicker and cheaper to produce.
What Bessemer did was to blow cold air through molten cast iron so that the oxygen in the air reacted very rapidly with the carbon in the iron, burning it out and so raising the temperature. This meant that the metal was hotter at the end of the process than it was at the beginning.
Bessemer steel (now called mild steel) had many advantages over wrought iron. It could be converted from cast iron to steel without the use of a fuel. A puddling furnace made about five hundredweight (254kg) in a working cycle of two hours; a Bessemer converter cycle was only about 30 minutes and in this time several tonnes of iron could be converted.
The original Bessemer converter held only seven hundredweight (365kg), but it was soon scaled up to deal with five or six tonnes and later thirty or more, all at the same speed of conversion. This also meant that bigger individual pieces of metal could be made.
Wrought iron began to go into decline, slowly at first and then more quickly. Wrought iron had reached its heyday in the second half of the late 19th century. By 1870 British production was about three million tonnes. By 1900 it was just over one million and by 1930 it had fallen to 113,000 tonnes.
Meanwhile bulk steel production in Britain had risen from zero in 1856 to 4.9 million tonnes in 1900 and 7.33 million in 1930. In 1976 production of wrought iron ceased altogether, although recently more and more British manufacturers are restarting the manufacture of wrought iron for use in gates, garden furniture and much more.
Worldwide production of steel in 1998 was well over 700 million tonnes and is still rising.
[Original author: Miranda Greene, 2005]