Carl Herrmann AHS Capstone Paper 3 12/17/2009

The Chemistry and Metallurgy of

Iron and workers employed a variety of techniques to convert iron ores into metallic iron, all of which utilized the same basic chemical changes. Their goal was first to liberate the metallic iron from the oxygen in iron ores, then to use oxygen to remove other impurities from the molten iron. The different techniques of doing so represented different levels of technological development, and often achieved a similar material through a less labor-intensive process. However, some methods, such as the Bessemer process, produced a different and superior material.

Before the advent of cheap steel, artisanal converted iron ore into two different materials – and . Whether producing the tough, durable wrought iron or brittle cast iron, skilled artisan converted the ore to saleable product with little understanding of the fundamental chemical reactions. While the chemistry behind these reactions changed little between the processes, the workers had little understanding of the chemical nature of the changes they performed, and the day to day work in forges and blast furnaces bore little resemblance to one another.

Smelting iron is the conversion of mineral iron ores into metallic iron. Although their chemical composition varies according to the ores, all iron ores share one characteristic. They all contain iron and oxygen. Some, such as Hematite (Fe2O3) and Magnetite (Fe3O4), contain only these two elements, while others, such as goethite (FeO(OH)) and (FeCO3 ) contain other elements. To remove the oxygen pure is burned, creating CO gas. This strong reducing agent combines with the oxygen in the mineral ore, producing CO2 and leaving behind metallic iron (Gordon 90). All ores also contain other, unwanted minerals, called gangue. These are combined with a flux, often consisting of , to form a . A slag is the term for the non-metallic compounds produced from ironmaking that chemically separate from the iron. The slag performs the useful function or removing impurities dissolved in the iron, while also acting as a chemical barrier, shielding the iron from further chemical reactions. Whether they understand the chemical processes involved, when ironworkers used different techniques to produce iron, they went about achieving these basic chemical processes in different manners, and the final material reflected these differences.

Bloomeries represent the oldest and least capital-intensive method of producing iron – they required just a hearth, bellows and hammer, and were often the first method set up in a new ironworks. In a hearth, an artisan would prepare a burning bed to provide the carbon monoxide gas to reduce the ores. Charcoal, produced by charring wood in an enclosed container, is 90% carbon (Gordon 34). While more expensive than mineral , it is also contains lower amounts of impurities such as and sulfur that would affect the material properties of iron in undesirable ways; it was worth the additional expense. As it reached the proper heat, artisans spread on this bed in a particular pattern. As the ore heated up in the presence of the CO gas from the burning charcoal, chemical reactions began to take place. , a common impurity in the mineral ore, combined with some of the iron to produce Faylite (Fe2SiO4 ), a main portion of the slag (Gordon 100). This slag, common to the bloomery chemistry, contained iron and thus represents a reason why bloomery technique produces lower iron yields than other methods.

As the silicon and other impurities were removed into the slag, carbon monoxide reduced the ores into metallic iron. Initially, these start as small amounts of iron which grow slowly. Although metallically bonded to the other iron atoms, other elements like carbon could diffuse into them. Carbon acted to lower the melting point of iron, reducing it so it could melt in the (relatively) moderate heat of the fire. As the iron congealed and fell through the hearth, a coating of the slag protected them from the oxygen that would convert them back into ores. When enough of these particles had congealed together, forming a ‘loup’ in the hearth, a bloomer brought the loup to a hammer. There, the white-hot metallic particles were welded together under the blows of a hammer, while the slag was squirted out.

The final product of this bloomery process is known as wrought iron. The name, in fact, refers to the process of pounding it to shape it and remove the slag. This material was not a homogenous product. In fact, it was a composite mixture of relatively pure iron interspersed with solidified slag. The purity of the metallic iron made the product both highly ductile and weldable. The ductility of the material made it good for forming into various shapes, as well as strong and tough in its final application. Higher grades of wrought iron containing smaller amounts of slag could be obtained by cutting the resulting bar into strips, heating them, and once more hammering them together again. This additional hammering, often repeated several times, removed successive amounts of slag producing a progressively more pure product.

While produced iron directly from ore, the process required a great deal of highly skilled labor and its output was limited by the size of the batches workers could handle as well as the time it took each batch to finish. While the labor required to produce iron by this method reduced by 86% over the course of the nineteenth century (Gordon 99), in the short term interested in expanding production often invested in different techniques entirely.

Similar to Bloomeries, blast furnaces use burning charcoal to produce heat and the carbon monoxide necessary to reduce iron ores to metallic iron. However, blast furnaces attain a much higher heat than bloomery hearths. While the chemical reactions taking place remain the same, the greater heat allows the products to remain molten through the process. This additional heat greatly increased the total output of the ironworks. Blast furnaces are also much larger than bloomeries, allowing them to handle much more metal (thousands of pounds of metal per batch, as opposed to hundreds). When compared to the bloomery process, they also required much less human labor and could therefore offer the final product at a lower price (Lewis 10).

In contrast to bloomeries, where fine particles of iron and slag combined in a semi-solid mass shielded from further chemical reactions, in blast furnaces the liquid iron pools directly at the bottom of the hearth. The slag floats on top of this mass, preventing its oxidation. However, as the liquid iron passes through the carbon fuel, it absorbs any of the impurities present in it. These can be any from silicon to sulfur. Although usually not regarded as an impurity, the carbon absorbed into the liquid iron alters its physical properties in a significant way. The carbon present in cast iron, usually to the saturation point, results in a very strong iron. This strength, however, reduces its ductility to the point where becomes brittle. While still useful in stove plates or other applications that don’t require toughness, cast iron is useless in structural beams or most tools. It is widely used as an intermediate material in the process of iron, and was quite suitable in this role (Gordon 125) When cast iron intended for further processing left the , it most often found its way to a finery, to be refined into wrought iron. Fineries worked much like bloomeries, creating a loup in a charcoal fire, and hammering the resulting iron under a large to separate the slag from the metallic iron. The final product of this was also known as wrought iron, and was indistinguishable from the bloomery product. The primary difference between the finery and the bloomery lies in the chemical reactions taking place. While the bloomery takes in ore and uses carbon monoxide to remove oxygen from the iron, a finery takes in and uses heat and oxygen to remove carbon and silicon from the cast iron. (Gordon 128). While requiring similarly skilled workers to control the chemical reactions taking place, the chemical reactions in a finery took place more quickly, so finers could produce more iron per day than bloomers. Thus by breaking down the bloomery process into two separate steps, first removing the oxygen from the iron in a blast furnace, then by removing the silicon and carbon in a finery, capitalists and managers could increase their production. However, the great skill required slowed their ability to train workers, and, because the charcoal and iron came into intimate contact, charcoal was still the only available fuel source.

To address these problems, iron masters in the 1830s began using the process. While similar in concept to the fining process, where pig iron is melted and exposed to air to remove excess carbon, it differs in one important aspect – the use of a reverbratory furnace in place of a hearth. A reverbratory furnace burns the fuel separately from the iron, allowing ironmasters to use cheaper mineral coal without contaminating the iron. More importantly, however, it left the molten iron in sight of the worker, making his task much easier, and therefore cheaper, to perform. (Gordon 138).

In a reverbratory furnace, the coal burns on one side of a low wall; the iron sits in the hearth on the other side of the wall. On the other side of the iron sits the chimney, which draws the hot gasses over the hearth, heating the iron. In the process, the puddler inserts pig iron into the hearth. After it melts, the puddler used iron rods to stir the molten iron. On the surface of the iron, the combustion gasses oxidized impurities such as silicon and phosphorus, removing them from the iron to the slag (Gordon 142). To fully expose all parts of the iron to this atmosphere, the puddler stirred this with bars of wrought iron. An improvement on this process involved adding some iron (a product of other iron processes), which would react more vigorously with the carbon, removing it more quickly (Gordon 134). A puddler went through different stages, modifying the temperature and the oxidation of the flame to remove impurities. In the final stage, when removing carbon from the iron, its melting point began to rise and it became less of a liquid and more of a viscous mass. The puddler balled up this iron to prepare it for further processing.

Bloomeries, Fineries, and Puddlers all produced wrought iron. While each improvement in the process made it easier for workers to produce this wrought iron, the batch sizes were still limited by what men could handle on their own, about 200-lbs. Additionally, wrought iron’s physical makup, the non-homogenous mixture of high-purity iron and slag, is due to these methods of production and made it undesirable for railroad rails. Steel, on the hand, is a homogenous mixture of iron and carbon; all steel-making processes pour the slag off separately from the steel ingots, producing a uniform product.

The Bessemer process made use of the “pneumatic principal.” This stems from the observation that when air comes in contact with molten iron, the iron actually heats up. This is due to the same reactions which take place in a blast furnace, where oxygen combines with the carbon dissolved in the iron to become CO2 gas, releasing energy in the process. In the Bessemer converter, molten pig iron is poured into a vessel lined with refectory brick. Air is then blown through this molten iron through the bottom, providing oxygen for reaction. Enough energy is released, in fact, to maintain the iron in a molten state despite its increasing melting temperature (Misa 11). The Bessemer process represented more than a way to increase production of wrought iron – it’s product was in fact better. Since the iron remains molten the entire time, the slag can float to the top, to be poured off separately. This allows the steel produced by the Bessemer process to be slag free. The air also provides agitation and mixing, creating a highly uniform product. Beyond being merely more uniform, Bessemer were in fact stronger. After the carbon was removed from the iron, a small amount of carbon and was reintroduced. This amount could be closely controlled, allowing for a more predictable final product. All of these effects combined to produce a product that was much better suited to rolling into a variety of metal products (Misa 19).

One problem associated with the adaptation of Bessemer converters was their strict chemical requirements. The previous processes used constant human interaction, which allowed a skilled artisan to look at the metal for clues about the process, such as when a puddler watched for a color change from red to slight blue, indicating when he was ready to remove the carbon (Gordon 142). The artisan did not know what exactly he was doing – he simply understood the relationship between these subtle clues and a superior final product. Such attention is impossible in a Bessemer converter. In order to re- assert control over the chemistry of the reactions, steelworks required a better understanding of the fundamental chemistry. Thus steel works began hiring chemists, who could monitor the chemical nature of the inputs to the converters to ensure the proper chemical reactions would take place (Misa 29).

An example of these strict chemical requirements is most easily seen in the way the Bessemer process limited the choice of ores. Initially, Bessemer Converters were lined with silica, a commonly available brick. This brick, when heated to very high temperatures, created an acidic environment. This acidic environment caused made conditions unfavorable for the removal of phosphorus. This limited the ores and fuels available to the blast furnace to those low in phosphorus. Otherwise, they produced a product that was high in phosphorus. This phosphorus caused a steel to be “cold short,” or brittle at room temperature. This was obviously not a desirable feature for iron or steel (Lewis 38).

Thus, early Bessemer converters limited by this chemistry began to understand the nature of their problem, which they addressed in two ways. First, they began hiring university – trained chemists to look into the matter and ensure the correct chemistry took place in these converters. Secondly, they combated this and other problems by restricting the purchasing of their ores to those low in phosphorus (Misa 15).

The greatest benefit of the Bessemer process lies in its ability to convert large quantities of pig iron into steel in a short amount of time. While the size of the puddling and fining processes was limited to around 200 lbs (Gordon 144), Bessemer converters started out at 5 tons and gradually increased in size throughout the course of the century (Temin 132). The vast majority of this Bessemer steel was used in rolling mills to produce rails for the railroad construction booms. These booms required large rail orders to be finished in a short amount of time, and the Bessemer process allowed steel tycoons to supply just that. (Temin 223).

The larger batch sizes also produced higher-quality rails as well. Around the time puddling was adopted, ironworks also adopted the use of rolling to replace hammering. In rolling, iron would be inserted between two rolls which would shape it between them. As workers put red-hot iron through successively smaller gaps between rolls, the iron became successively smaller and longer. Because the batch sizes of puddle, fined, or bloomed wrought iron was smaller than required for a standard railroad rail, several individual bars of iron would be strapped together and rolled together. Ideally, the high heat and pressure would weld these bars into one solid bar – in practice this rarely happened. The most common mode of failure for these rails was delaminating. Steel ingots, on the other hand, were large enough to be rolled into continuous rails with no seams, greatly prolonging their lifespan. This difference, when combined with the superior strength of steel, allowed them to last up to 17 times as long as iron rails (Temin222).

Around the time Bessemer commercialized his pneumatic process for refining molten pig iron, William Seimens developed reverbratory furnaces to the level that they could keep iron liquid even at low carbon contents. In terms of the chemical reactions involved, it differs little from the puddling and fining processes: the oxygen combines with silicon, phosphorus, and carbon in the iron, and floats into the slag. In a Bessemer converter, these heat from these reactions prevent the iron from cooling and solidifying - a costly occurrence. Because the open hearth process does not rely on these reactions to maintain its heat, they can take as long as necessary without worrying about the steel solidifying inside the hearth. Often, workers would sample the steel, testing its chemical composition and adding ingredients as necessary (Gordon 227). Because open hearth steel was as homogenous as Bessemer steel, and its batches just as large, the superior chemical control produced a superior product, often required in demanding structural applications (Misa 82).

The key breakthrough that allowed reverbratory furnaces to attain the temperatures necessary to melt iron came in the methods of preheating the combusting gasses. Early efforts in this area involved cast iron heat tubes, with outgoing gasses on one side and incoming gasses on the other. At very high temperatures, the cast iron would melt. Additionally, the relatively small surface area of the iron pipes impeded the flow of heat from one gas to the other. Although various arrangements were tried, the final breakthrough called for having separate chambers lined with firebrick for the exhaust and incoming gasses to flow through. The exhaust gasses would flow through one brick chamber, heating it up to extreme temperatures. Then, values would re-direct the airflow – incoming gasses would pass through the chamber heated by the exhaust gasses, and exhaust gasses would begin to heat the other chamber. This heated the incoming gasses more efficiently, enabling the hearth to reach temperatures to keep iron molten (Misa 79).

Iron and steel workers employed various techniques to convert iron ore into metallic iron, whether wrought iron or steel, all of which achieved the same basic chemical changes. Ores, chemical compounds of mainly of iron and oxygen, were heated in the presence of carbon to produce carbon dioxide gas and metallic iron. To remove excess carbon, silicon, and other impurities, it was again heated with oxygen and lime, which combined with them to produce excess gasses and slag. Yet, while the chemical reactions were similar, the different techniques produced materials such as wrought iron and steel with significantly different properties.