During the Middle Ages the materials which armourers worked with most often were iron and steel. Iron is an element, its ore is common and plentiful in the Earth’s crust and its ease of extraction and malleability after smelting has made it one of the defining materials of civilisation, highly prized for its utility as well as for its working properties. It has been used for weapons and armour, as an architectural element, and for tools for all types of craftsmen.156
Unlike copper, iron is almost never found in nature in its metallic state and is instead found as an ore, the element chemically bonded with oxygen.157 These ores are composed of varying percentages of iron and impurities, with magnetite ores having the highest amount of iron at around 65% and hematite ores with slightly less at 50-60%. There are other types of ores with even less which are more difficult to process into usable iron.158 These include bog ores, a type of limonite that has the appearance of growing quickly by collecting in nodules.159 This may have given rise to the idea of regenerative ores recorded by Biringuccio in reference to iron from Elba, where so much ore had been mined over time that ‘not only the mountains but even two islands like that one should have been levelled’, and that some people believed that the ore ‘regenerated anew in that soil which has already been mined’.160
156
W. K. V. Gale, Iron and Steel (London: Longmans, 1969), p. 1. 157
Native, or telluric, iron is very rare, but is found in some places, for example in the American state of Connecticut and most significantly on Disko Island, off Greenland. While the Disko Island iron is workable, these deposits are not pure iron and had no effect on medieval Europe. See Rostoker and Bronson, Pre-Industrial Iron, p 41, and Paul T. Craddock, Early Metal Mining and Production (Edinburgh: Edinburgh University Press, 1995), pp. 101-03.
158
Gale, Iron and Steel, p. 2. 159
Rostoker and Bronson, Pre-Industrial Iron, p. 42. 160
Biringuccio, Pirotechnia, pp. 61-62. Black lead in Britain is said to regenerate by Pliny the Elder, and Strabo states that stone regenerates on Elba and Rhodes. See J. F. Healy, ‘Pliny on Mineralogy and Metals’, in Roger French and Frank Greenaway, eds., Science in the Early Roman Empire: Pliny the Elder, his Sources and Influence (London: Croom Helm, 1986), pp. 111-46 (p. 116).
Like any metal, iron has certain working properties which a smith must take into account and these dictate the stresses which a piece can withstand, both during forming and in use after fabrication. One of the most important properties of iron is its ability to be worked at high temperatures, the high ductility of the material allowing it to be formed into many shapes. Iron can also be worked cold to a lesser degree, and can be easily forge welded to join pieces together. Due to its grain structure and the tendency for slag inclusions from the smelting process it is also prone to delaminating along the grain boundaries if forged at too low a temperature. Brittleness and toughness are two other factors which affect how the material is worked, brittleness indicating that the material breaks easily, and toughness that the material has the ability to deform without damage.161
When carbon is added to the pure iron the element becomes the alloy known as steel. Steel and iron behave differently under working conditions and have different properties concerning their strength and their ability to be heat treated. While iron cannot be hardened appreciably, an exception being high-manganese ‘steely iron’, the carbon content of steel allows it to be hardened and tempered in a process known as heat treatment.162 This may either be a collective, two stage process where hot steel is hardened by rapid cooling then tempered by slow reheating, or it may be single-stage ‘slack quenching’, where the hot steel is immersed repeatedly in a quenchant, allowing the residual heat to temper while at the same time hardening.
Heat treatment makes the steel hard, useful for keeping an edge and resisting impact, but also greatly increases its toughness. Under the hammer the high carbon content of steel also makes it brittle in comparison to wrought iron and susceptible to cracking while being forged if not hammered at the correct temperature. Poorly
161
Rostoker and Bronson, Pre-Industrial Iron, p. 2. 162
tempered steel is also prone to cracking during and after cooling, known as cold shunting, either spontaneously or through repeated bending.
Before these metals can be worked they must first be smelted, the process of reduction which removes the oxygen from the ore. Oxygen and iron react easily, most commonly seen as rust, and in smelting the oxygen is removed by chemically bonding with carbon, which is also the fuel for the fire in the form of charcoal. In the furnace, ‘the carbon unites with the oxygen and goes off in the form of a gas, leaving the iron behind’.163
By the beginning of the fourteenth century ferrous metals had been known and worked for thousands of years, but in that time little had changed in mining and smelting technology.164 Small furnaces were still used to reduce the ore into a mass which was relatively free of impurities, the sponge or bloom, which could then be worked into useable iron.165 What was known of the smelting process was gained only by experimentation and observation in what has been called a ‘triumph of
empiricism’.166
During the early part of the Middle Ages and before, small hearths were used in conjunction with manually driven bellows to provide the environment necessary for reduction. Temperatures created by this system never rose high enough to melt the iron, resulting in a lump of spongy iron which was removed from the furnace and hammered into a piece of wrought iron. This type of smelting is known as the direct process, a one-stage operation with a resultant product that can be heated and forged, or solid state reduction, since the ore does not melt in the furnace.167
163
Gale, Iron and Steel, p. 3. 164
Aitchison, A History of Metals, I, 111. 165
Aitchison, A History of Metals, I, 100. 166
Rostoker and Bronson, Pre-Industrial Iron, p. ix. 167
In the fourteenth century the combination of water power with larger smelting furnaces created the blast furnace, which differs from the earlier types of furnaces by melting the ore inside the reducing chamber.168 Water power had first been harnessed for use in ironworking during the thirteenth century, for example in 1273 at S.
Salvatore, in Siena, Italy.169 Water-driven bellows are larger and can deliver a more powerful blast of air for as long as the wheel is driven, a great advantage over human- powered bellows.
Agricola’s instruction in De Re Metallica on iron produced by the blast furnace begins with a description of the shape and size of the hearth, and how it was to be charged by the master who ‘first throws charcoal into the crucible, and sprinkles over it an iron shovel-ful of crushed iron ore and unslaked lime’. The furnace was thus filled in layers, and the process of smelting could take eight to twelve hours.170 The master was responsible for the level of ore and fuel, for tapping slag, and for controlling the flow of water which powered the bellows of the furnace, the result of his labours being that ‘iron is melted out and a mass weighing two or three centumpondia may be made, providing the iron ore was rich’.171
As the iron travels down through the chamber it absorbs a great deal of carbon from the charcoal fuel, more than earlier furnaces due to the blast furnace’s larger incandescent zone. This reduces the melting point of the iron from 1500°C to 1150°C. The molten iron is tapped from the furnace bottom and cast into ingots.172 This cast
168
D. W. Crossley, ‘Medieval Iron Smelting’, in Medieval Industry, ed. by D. W. Crossley, CBA Research Report, 40 (London: Council for British Archaeology, 1981), 29-41 (p. 29). 169
Maria Elena Cortese, ‘Medieval Ironworking on Mount Amiata (Siena, Italy): Economy, Society, Technology’, in Prehistoric and Medieval Direct Iron Smelting in Scandinavia and Europe: Aspects of Technology and Society, ed. by Lars Christian Nørbach (Aarhus: Aarhus University Press, 2003), pp. 55-59 (p. 56). 170 Agricola, De Re Metallica, pp. 420-21. 171 Agricola, De Re Metallica, p. 421. 172
iron can then be re-melted to burn off the carbon and create wrought iron, or combined with already produced wrought iron for making steel.173 This two stage method of iron production is known as the indirect process owing to the intermediate cast iron stage which cannot be forged by a smith due to the high carbon content and requires further treatment to create a useable product.
Whether smelted in a one or two stage process, the resulting bloom of iron must be extracted from the furnace or finery and beaten with hammers.174 According to Agricola it is first put on the floor where it is worked with hand hammers before being moved, still hot, to the trip hammer, a large hammer whose shaft is raised by cams and then allowed to fall onto the anvil below.175 Beginning in the thirteenth century water powered tilt hammers were increasingly being used for this purpose. As a result of the water driven bellows and hammers used in the medieval iron industry it was important to locate suitable sites for the mill.176
Working the bloom with hand sledges and the heavy trip hammer serves several functions. First, at high temperatures the bloom is welded into a single unit and can be welded with other blooms to create larger pieces of iron. Second, during the welding process residual slags are forced out of the iron. Third, the grain structure of the iron is formed and elongated, with the remaining slags being extruded between the grain boundaries producing a more homogenous structure, though still quite heterogeneous by today’s standards.
After processing the iron by hammering it is cut, again using the trip hammer and a set chisel. After cutting, ‘These pieces, after they have been re-heated in the
173
Crossley, ‘Medieval Iron Smelting’, p. 39, and R. F. Tylecote, A History of Metallurgy (London: The Metals Society, 1976), p. 65.
174
It is from this hammering that the term ‘wrought iron’ is derived. 175
Agricola, De Re Metallica, pp. 421-23. This hammer is also sometimes called a tilt or helve hammer.
176
blacksmith’s forge and again placed on the anvil, are shaped by the smith into square bars or into ploughshares or tyres, but mainly into bars’.177 The iron in bar form would have been easier to transport than larger masses of iron and could be shaped to standard sizes for the market.
Iron produced using the indirect method of smelting is made using, for the first stage, either a high bloomery, a 1444 reference to fining pig iron providing the earliest reference in Germany, or a blast furnace for the production of cast iron, and for the second stage a fining hearth which decarburized the cast iron through ‘exposing it to hot oxidizing conditions in a charcoal fired hearth’.178 The capability of melting the ore is one of the main distinctions of the blast furnace which differentiates it from earlier smelting furnaces which could only create solid blooms, though the high bloomery, as an intermediate development, was able to produce both solid blooms and cast iron.179
As the cast iron melts in the furnace it collects near the bottom of the hearth. The resulting mass, called a ‘loup’, was stirred and re-melted in the furnace until it could ‘no longer be melted under the tuyère blast which indicates that the carbon has been entirely removed’. The loup was therefore essentially the same as the bloom from a
bloomery.180 Working the loup would then continue in the same way as working the bloom in the direct process.
Iron may be alloyed with carbon to create steel and cast iron. Steel is much stronger than iron and may be heat treated, while cast iron during the Middle Ages was a step in the process of iron and steel production resulting from the use of the blast furnace. Carbon is added to the iron either during or after smelting, less than 2% being
177
Agricola, De Re Metallica, p. 423. 178
Starley, ‘Medieval Iron and Steel Production’, pp. 31-32 and 35. 179
Starley, ‘Medieval Iron and Steel Production’, p. 33 and H. F. Cleere, ‘The Classification of Early Iron-Smelting Furnaces’, Antiquaries Journal, 52 (1972), 8-23 (pp. 8-9).
180
required for steel.181 According to Paul Craddock, ‘To produce steel which regularly contains a controlled amount of carbon has been one of the principal aims of the smith for over three thousand years’.182 Experimentation led to advances in steelmaking techniques, which were the product of empirical observation since the role, indeed the existence, of carbon would not be understood until the late eighteenth century.183
Biringuccio’s description of the nature of steel includes an accurate account of the visible changes in the crystalline structure of steel which has been heated and
rapidly cooled. To Biringuccio, after iron had been changed into steel it seemed ‘almost to have been removed from its original nature’, though he understood that they were still in the same group of metals and therefore treated them together.184 Agricola’s instructions for steel production are much like those in Pirotechnia.185
David Starley identifies four methods of creating steel during the Middle Ages and early Renaissance:
1. Primary carburisation of iron within the bloomery furnace. 2. Secondary carburisation of iron, from either bloomery or finery. 3. Partial fining of cast iron.
4. The Brescian process.186
Primary carburisation involves the production of steel within the bloomery, as part of the larger mass of iron. The amount of the bloom with enough carbon to have become steel must be removed from the rest and forged similarly to iron.187
181
Craddock, Early Metal Mining and Production, p. 236. 182
Craddock, Early Metal Mining and Production, p. 252. 183
Cyril Stanley Smith, ‘The Discovery of Carbon in Steel’, Technology and Culture, 5 (1964), 149-75 (pp. 167-73).
184
Biringuccio, Pirotechnia, p. 67. 185
Cyril Stanley Smith, in Biringuccio, Pirotechnia, p. 68, n. 1. 186
Starley, ‘Medieval Iron and Steel Production’, p. 41. 187
Secondary carburisation involves the case carburisation, or case hardening, of a piece of iron.188 In this process carbon diffuses into the iron’s surface, creating a layer of steel over an iron core. This may be a final treatment, or several pieces of case hardened iron may be forge welded together, allowing further diffusion of carbon and creating a more homogenous steel.
In his description of files needed by the craftsman, Theophilus makes a
distinction between solid steel files and files that are made ‘so that they are stronger in the middle, of soft iron inside but outside covered with steel’.189 He describes case hardening these soft iron files,
When they have been incised with the hammer, or chisel, or with a knife, smear them with old hog’s lard, bind them round with strips cut from goat- skin, and tie them up with flaxen thread. Afterwards cover each one separately with kneaded clay leaving the handles bare. When they are dry, put them in a fire and blow vigorously until the skin is burnt. Then remove them quickly from the clay, quench them evenly in the water, withdraw them and dry them at the fire.190
The lard and skin provided the carbon which, when encased in the clay and heated in the forge, migrated into the iron to create the steel layer. Keeping the handles
uncovered insured that they remained soft and malleable, reducing the chance of breakage during use. While medieval knife blades could be produced by forge welding thin strips of iron and steel together, Theophilus’ text suggests that case hardening was the preferred method for file making, perhaps because it would have been easier to cut the teeth into the softer iron.
188
Starley, ‘Medieval Iron and Steel Production’, pp. 43-44. 189
Theophilus, De Diversis Artibus, trans. by Dodwell, p. 72. 190
Partial fining is essentially the same as the fining of cast iron to produce wrought iron, with the difference that the loup is removed before all the carbon is removed. The Brescian process, by which solid wrought iron is mixed with melted cast iron so that it absorbs the carbon, is first mentioned in Biringuccio’s Pirotechnia and was not used during the Middle Ages.191