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Iron Into Steel

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Iron Into Steel Iron to Steel : Talk the Talk Introduction : In preparation for the demonstration I gave at the October 2019 OABA meeting (held at Bob Young's workshop) I had prepared a formal lecture, and an even longer set of notes. I was asked afterwords if these notes could be made available to the membership. There will be two versions, here as a pair of articles for the Iron Trillium, plus a more lavishly illustrated version created for addition to the OABA web site. I bring an experience and viewpoint to the topic of historical metalworking which is unique, being involved in practical, research and academic communities. (1) I am constantly observing problems with consistency in language between these groups, where the same word may have differing meanings, or the same concept using differing words. This is further blurred by the inclusion of many terms that are in fact culturally specific. Increasingly with in blacksmithing itself, there is a marked drift from technical use into mere marketing. (2) The definitions offered here represent my own research and (of course!) opinions. Part One - Iron into Steel Meteoric Iron (material) Rare finds of extra-terrestrial iron indicated by high nickel content, in the range of 7 – 15%. (Basically) the only naturally occurring metallic iron, the first iron worked by humans. Initially only worked COLD. (3) Beads of meteor metal, the oldest known human worked iron : c 3200 BC / Egypt Iron Age (Archaeologists) The start of human made iron, reduced from natural iron oxide ores. Date will vary by culture - estimated ‘Old World’ date c. 1200 BC, in the Middle East. Note that iron does not become a primary material until the Romans and Celts (c 600 BC). (4) Although there are some small knives as artifacts, the majority of the earliest iron objects are jewellery items like pins, bracelets and rings, in keeping with the high status as a material. Early iron objects as jewellery : Anatolia, 1200 – 1100 BC Bloomery Iron (process / material) Strictly speaking, refers to a *method* of iron making. Made in a direct process furnace, where ore is reduced to collect into a spongy iron mass (bloom). This then needs to be compressed (welded) to eliminate voids and slag impurities. Most typically easy to forge soft / low carbon content was desired. (Just how to modify for hardness / carbon was 'uncertain'!). Although the science is fixed, regional materials (ore quality) and cultures shape the exact process used. Most typically in ancient times, small amounts, suitable to hand working methods, were produced (5 - 10 kg). (5) Starting extraction of an iron bloom (seen as the brighter mass on top side of furnace). Wrought Iron (1 material) Literally ‘Hammered Iron’. A historic / antique material, mainly created by the bloomery process, but also can refer to iron metal from other historic processes. • The metal has little or no carbon, (reddish 'balls' on spark test) • A distinctive fibrous / linear texture, (result of slag inclusions as consolidated) These combine to make it especially easy to forge (same impact gives greater effect) The texture sometimes determines working methods (frequent re-welding, consideration of grain). The quality of the Wrought Iron from a given furnace firing was commonly quite random, with the quality of the ore and the skill of the individual Iron Master often critical. No new wrought iron has been made industrially since 1973. (Some still available from re-cycled, largely Victorian, objects.) https://en.wikipedia.org/wiki/Wrought_iron A piece of wrought iron, nicked 3/4 through and broken – reveals distinctive texture. Wrought Iron : (2 marketing) A very problematic term! It is also used today to describe: • Any object that has been forged, regardless of the material used . So modern mild steel, then hammered into shape by blacksmithing processes. • Any object that looks like it might have been forged - a purely modern marketing description. (As example, Home Depot sells ''wrought iron' railings, actually made of machine cast aluminum, painted black!) Cast Iron : (material) As carbon content increases, the melting point is reduced. At roughly 1.8 % carbon, the melting point drops below the burning temperature. This allows for liquid cast iron to be effectively poured into moulds (mass production). The resulting metal is brittle however, and can not be forged (depending on exact additional alloying elements). The earliest direct use of cast iron is from China, c 400 BC. Europeans would not develop this technology (for objects) significantly until the 1600’s. Cast iron would become a dominant production method roughly after the development of steam power (into the early 1800’s), most especially during the Victorian era. https://en.wikipedia.org/wiki/Cast_iron Illustration of a historic Chinese blast furnace Pig Iron : (process) Simply another name for the product of a cast iron furnace. Molten cast iron was ran out into channels formed in sand at the base of the furnace. The individual blocks, attached to a central channel, had the look of piglets suckling, hence the name. https://en.wikipedia.org/wiki/Pig_iron Pouring cast iron to form 'pigs', c 1900 Coal : (material) Although there is some limited use of mineral coal by the Romans, this was confined primarily to heating systems for large buildings. Charcoal was the primary fuel for all forging operations in Europe through to roughly the 1300’s (first use of coal is in glass making, later 1200’s). With coal used in the forge, the ‘traditional’ great bellows also comes into use. Coal would not be used in iron refining until about 1600 - and charcoal would remain the primary fuel for bloomery furnaces producing wrought iron into the Modern era. Note that there can be some confusion with European writers, who will use 'coal' when actually meaning 'charcoal'. Woodcut of a working Blacksmith : likely 1500's. Side blast (charcoal?) forge, twin chamber bellows, block anvil. Finery Forge : (process) With the introduction of water power (c 800 - 1000, from the Arabs), higher furnace temperatures could easily be attained (iron absorbs carbon almost instantaneously in its liquid state). A mechanism to take advantage of this was not perfected until about 1450 (in Europe). The furnace was ran 'hot' to create a high carbon cast iron, which was poured out into thin plates. The plates were then placed into a secondary furnace, the Finery Forge, with a high volume air blast. This would then effectively burn out the excess carbon, until the resulting metal was at a point it could be correctly forged. This would remain an alternate to the direct bloomery process into the early 1800’s. https://en.wikipedia.org/wiki/Finery_forge Puddled Iron : (process) With the introduction of both coal fuel and later higher air blasts possible via steam power, an alternative method to the Finery system, basically into the early 1800’s. In a puddling furnace, a pool of liquid cast iron had the driving air blast channeled over the top. A worker then would gather up a mass of iron on to a long iron rod (like rolling honey on a stick), and manipulate this inside the blast. The air would burn out the excess carbon to a level desired. https://en.wikipedia.org/wiki/Puddling_(metallurgy) Blister Steel / Cementation / Shear Steel : (process to material) In this process, thin plates of low carbon wrought iron are packed with fine charcoal into air tight clay or stone containers, then baked at high temperatures for many days. The carbon then slowly migrates into the iron surfaces. (Note, in the solid state, carbon will diffuse into iron, but at avery slow rate!) The process (at an industrial level) dates to about c 1600. Note the importance of sealing out oxygen during the process. The resulting smaller plates where then gathered and (carefully!) forge welded into larger billets. This final product was sold as Shear Steel. see also Case Hardening. https://en.wikipedia.org/wiki/Cementation_process Slitting Mill : (process) Starting in the 1600’s more elaborate water powered machinery allowed for the production of a wider number of shapes to starting bar stocks. A combination of compressing rollers and cutting blades resulted in not only simple flats and squares, but also for round, D or lens, hexagon cross sections. These new available profiles quickly effect the designs seen in architectural work especially – as the working smith no longer had to spend effort shaping the starting profiles desired. Bessemer (Steel) : (process to material) This new process (England or USA - 1856) allowed for conversion of cast iron into a new material, Mild Steel. The same raw material and labour resulted in between 5 - 7 times the metal produced, also with vastly more control over the quality. The new metal also had a different physical texture, making it ideal for machinery. Combined, the new Mild Steel quickly replaced the older Wrought Iron effectively by about 1900. The primary difference with the Bessemer process was that the air blast was forced through a container of (previously refined) molten cast iron, burning out the carbon to a desired level. https://en.wikipedia.org/wiki/Bessemer_process Steel (1 material) Technically an alloy of Iron and Carbon (in small %). The modern industrial material, which has some different working methods (working temperatures and shape generations) and quite different texture than antique Wrought Iron. The addition of carbon allows for hardening of the metal. Some typical types : Mild Steel (0.2%) / Spring Steel (0.5%) / Tool Steel (1.0%) Typically this carbon is expressed as ‘Points’, equal to 1/100% carbon. Steel (2 Archaeologists / Art Historians) Any iron metal with some carbon content. Note that before 1855, this means carbonized Wrought Iron - the material retains the fibrous texture, but maybe possible to harden.
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