The main difficulty in producing iron is that it does not exist in nature in the form of a metal but as a part of a chemical compound, iron oxide. The metal is not found on its own because it has a strong affinity with oxygen with the result that a piece of iron in normal atmospheric conditions combines with oxygen from the air to form the compound which has very different properties from both of its components. Metallic iron is a bright metal while oxygen is an invisible gas and yet they combine to produce iron oxide, the most recognisable form of which is rust.
This reddish brown, powdery material is very similar to the ore from which metallic iron is smelted and we can paraphrase the common description of the human life cycle and say that, for iron, it is “Rust to rust”. Iron oxide is taken from the ground and converted into iron, which, if left unprotected, will revert to iron oxide within very few years. Iron fences and gates, for example disintegrate into rust within a few decades when painting is neglected.
Iron oxide occurs very widely in nature providing large quantities of ore from which the metal can be made by smelting. The ore appears in several different varieties of rock or as sand or as a reddish brown, water borne deposit and all have been used in the smelting processes.
Iron was first made as wrought iron, a metal that could be shaped by hammering and bending to make many useful objects and components. It was also capable of being modified by a number of techniques that could make it harder or tougher, both very useful properties for a number of tools and weapons.
The Bloomery Process
The continental Celtic origins of iron making appear to be indicated by the words “smelt” and “bloom”, both of which found their way into Old English from teutonic sources. “Bloom” appears to have had the same origin as the blooming of a flower and although this seems strange today it probably made a great deal of sense initially as the metal appeared from a pile of earth with a little human assistance.
From the Iron Age to medieval times, iron was produced by the bloomery method in which iron ore and charcoal were placed (generally after a preliminary roasting of the ore) within a small furnace that may have been little more than a hollow in the ground, especially in pre-Roman times. Air was supplied by bellows worked by hand or foot to keep the charcoal burning but the air supply was kept low to make sure that the charcoal did not burn fully. It seems that, from Roman times onward, the typical furnace has been around a metre in diameter, or less and typically not much more than a metre high and has been constructed of clay or stone with a clay lining.
Early iron smelters did not have the benefit of modern science to explain what they were achieving but we now know that they were producing carbon monoxide gas by incomplete burning of the charcoal rather than carbon dioxide which would be the result if the carbon was completely burned.
The burning charcoal and hot gases heated the ore within the furnace and although iron has an affinity for oxygen the carbon monoxide has a greater pull and as the gas passed over the iron oxide in the ore it removed the oxygen becoming carbon dioxide in the process and leaving the iron behind. This was one of two essential processes that took place within the furnace.
The iron was parted from the oxygen but it remained within the mass of rock that formed the ore until the rock was melted and then it flowed to the bottom of the furnace and out through a hole to a hollow in the ground where it cooled and set to form slag.
The iron was left behind in the furnace and because it was dispersed throughout the mass of ore it remained as a sponge-like lump (the bloom). It was removed from the furnace but still with some slag trapped in its internal spaces. This slag was removed by reheating the bloom to soften it when it was hammered to consolidate the metal and to force out the slag. The metal could then be forged to any required shape.
The bloomery process was not easily scaled up so individual furnaces remained quite small for centuries and the power for their operation did not need to be greater than that which a man could provide. Since the bloom was also small (about the size of a football) and capable of easy handling, mechanisation was not a priority.
Because the bloomery process was manual it could be carried out in any convenient location although this was usually near the ore source which would also typically have clay and stone available since the geology that provides iron ore often provides these other requirements too. In early times timber for the manufacture of charcoal was also available close to most sites but when this was not the case it was often better to bring timber and make the charcoal on site to prevent this fragile material from being damaged in transit.
There were improvements in production in Roman and medieval times but these were achieved as a result of organisation rather than changes in the smelting process. Numbers of furnaces were operated on a single site to provide large amounts of iron with some level of continuous production. In such situations water powered hammering of the bloom was often provided and iron smelting became increasingly located in areas where water power could be exploited.
In later medieval times it is possible that powered blast and other developments led to a “High Bloomery” method of smelting, a larger scale bloomery process and one that is still the subject of research. Even if this process did exist locally its output was still limited because of the need to terminate and restart the process after each bloom was made. This disadvantage was overcome by the blast furnaces that probably reached Nidderdale by the early 16th century and that resulted in the decline of the bloomery process. There are no bloomeries in commercial production in England today and there appears to have been no iron smelting of any kind in the Nidderdale area for four centuries because of the development of the blast furnace.
The blast furnace brought enormous improvements in the efficiency of iron making generally but it also caused the end of a Nidderdale iron industry and probably permitted the development of Nidderdale as an Area of Outstanding Natural Beauty because the quality and quantity of Nidderdale’s iron ore could not compete with areas such as Leeds, Bradford and Teesside. The decline in iron manufacture locally appears to have begun in the early 17th century when historical records tell us that the terminal decline commenced, a fact that must have been a disaster for those who depended upon it but which has made possible the beautiful landscape that we see today.
The basic principle of the blast furnace was similar to that of the bloomery furnace in that oxygen was removed from the iron oxide by carbon monoxide from burning charcoal (or coke from the 18th century) but in this process the smelting temperature was higher (or is higher, for the process is still in use today) and everything was melted, iron and rock. The slag is the lighter material so it floats on top of the metal and was drained through a hole called the slag notch by knocking out a clay plug. The metal was removed in a similar manner by the removal of a plug in the tap hole at the bottom of the furnace.
The blast furnace is more efficient that the bloomery process because it permits continuous production, the furnace remaining in operation throughout the production period with slag and metal being tapped off as required and more ore and fuel being added as necessary.The process also permits the scaling of the furnaces to almost any size, the larger furnaces giving even greater improvements in efficiency.
Unfortunately, it also brought some disadvantages particularly in the fact that a large amount of carbon is retained within the metal and this imparts hardness and brittleness so that the metal taken from the blast furnace cannot be shaped by hammering or bending.
The direct output from a blast furnace is cast iron which flows well when molten and can be poured into a mould to produce very intricate shapes. This was not very important in the beginning when the main need was for wrought iron but it did lead to a whole new industry, particularly in the 18th and 19th centuries when cast iron became very important for machine components and for domestic items such as fire grates and cookers.
Fortunately wrought iron can be made from cast iron by remelting the metal and blowing air into it to burn the retained carbon and removing it from the metal. When iron was to be used in this way it was tapped from the blast furnace into a channel formed in sand and from this into moulds formed in the sand which, when the metal had cooled, provided “pigs” that were conveniently sized blocks to be carried to the next stage of the process, hence, the raw material from the blast furnace that could be turned into wrought iron was also known as pig iron.
A furnace that was used to remove carbon from cast iron was called a finery and it produced a bloom of wrought iron similar to that which came directly from the bloomery process. Because the bloom from the finery could be quite large, blast furnace sites that were dedicated to the production of wrought iron also included a third process in a building called the chafery, in effect, a forge. Here the bloom was heated and hammered into an appropriate size and shape for its intended use. While “bloom” and “smelt” had teutonic origins “finery” and “chafery” appear to have come from the French words for refining and heating, which is probably the source that would be expected in medieval times with close contact with France nationally and through monastic orders, especially the Cistercians.
Early blast furnaces used charcoal but from 1709 a major change took place when it was discovered that coke could be used for smelting instead of charcoal. This resulted in the concentration of iron production in areas where suitable coal and ore were available in the abundant quantities for blast furnaces that became ever larger as experience was gained and as technology developed.
Iron smelting caused enormous disruption to the landscape because of the pits and adits that were made in order to gain the ore, clay and stone that were required. Buildings and furnaces were constructed together with trackways to access them and water courses were modified to provide the necessary water power. In addition there was significant pollution of the ground and the air caused by charcoal burning, ore roasting, furnace gases and slag.
The passage of 400 years since the end of Nidderdale’s iron industry has meant that nature and human activity have restored the environment to one that can be truly defined as an area of natural beauty despite the disruption of 2000 years of industrial activity.
(From "Nidderdale Iron - A Forgotten Industry", Howber Ltd, 2005)