IRON MAKING

Post 540 — by Gautam Shah

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First iron used by ancient people was of a meteoric source, an iron alloy with nickel. This was used for everal millenniums before the actual iron age. It was a natural Iron in metallic state and so required no smelting of ores. This nearly pure iron is softer than bronze, and therefore tools formed of it had soft wearing edge.

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Primitive age iron was smelted by mixing iron ore with charcoal, and burning in bloomeries, a type of furnaces where bellows were used to force in the air. The carbon monoxide produced by the burning charcoal, reduced the iron oxide ore to metallic iron. The apparatus, however, did not achieve a temperature of 1540° C, to completely melt the iron. The metal collected in the bottom of the furnace remained as a spongy non homogeneous mass or bloom. It had high proportion of intermingled slag. The blooms were repeatedly heated, beaten and folded to remove the slag. This produced wrought iron (=worked iron), a malleable, but fairly soft material. Iron age Irons were not castable products but required hot forming (forging). This was mainly due to inability to fully melt the material. Hot forming was a labourious process, requiring skill and experience. In comparison to bronze, iron ore was procurable everywhere and cheaper to process.

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Wrought iron shows high resistance to corrosion due to the trapped slag in the metal. The presence of slag in the iron helps fusion joining by hammering or forging. Wrought iron is no longer produced commercially, because low-carbon steel is less expensive and is of more uniform quality. Wrought iron, however, is still produced for certain craft-based uses such as making intricate craft objects balustrades, gates, garden accessories, etc.

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Simulated form of wrought iron is made by melting scrap mild steel in small furnaces, blowing air through the melt to remove carbon, and pouring the molten metal into a ladle containing molten slag, which is usually prepared by melting iron ore, mill scale, and sand together. When the molten iron carrying a large amount of gas in solution, is poured into the molten slag (kept at a lower temperature than iron), the metal solidifies almost instantly, releasing the dissolved gas. The force exerted by the gas shatters the metal into minute particles that are heavier than the slag and settle at the bottom of the ladle, agglomerating into a spongy mass.

Silla iron armor, en:Three Kingdoms of Korea, 3rd century Wikipedia image

It was Chinese (1200 BC or earlier) who designed kilns that could raise the temperature for iron making. These kilns, used upgraded coal and had high volume air supply for efficient burning. Chinese were able to melt the Iron and cast it into desired forms. Casting was less labourious, and allowed multiple items with same die form. It was accurate than forging each piece. Chinese smiths melted wrought iron and cast iron together to produce steel -a material of controlled carbon content. The process was called ‘harmonizing the hard and the soft’. This was widely used for casting cooking pots and iron statuettes. A cast iron is harder than wrought iron, but maintains the cutting edge.

Casting pig iron, Iroquois smelter, Chicago, between 1890 and 1901. Wikipedia image

Perhaps as early as 500 BC, although certainly by 200 AD, high quality steel was also produced in southern India by the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in a crucible and heated until the iron melted and absorbed the carbon.

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Carbon content of iron is a major factor that creates harder material. It was necessary to absorb more carbon in the iron. This required higher ratio of fuel to ore, and push in a lot more volume of air. The strength of iron begins to increase with carbon contents of 0.5 percent. To heat treat iron a carbon content of 1.2% was necessary. Wrought iron which contained less than this proportion had no qualitative effect due to heat treatments. A higher carbon content creates a brittle material but allows heat hardening. ‘Iron hardening by quenching was not practised because it made iron very brittle, unless followed by tempering, or reheating at a lower temperature, to restore toughness’. Simple fire, 600-700° C, based technique of repeated cold forging and annealing was used.

cast iron columns line the Albert Dock’s quayside Wikipedia image

In the pre-Christian portion of the period, the first important steel production was started in India, using a process called Wootz steel. It was prepared as sponge (porous) iron. This was hammered while hot to expel slag, broken into smaller pieces, and placed with wood chips in clay containers, and heated. On melting, an iron composition containing 1 to 1.6% carbon was produced. The pieces were reheated to form articles that required a hard body and sharp edge. Such steel products were exported to Middle East and other countries. It was known as Faulad (Persian). (Faulad or wootz steel has a Kannada term, ukku, a Language of Indian region of Karnataka).

Elevator screen from the Chicago Stock Exchange cast iron electroplated with copper. Wikipedia image by Joe Mabel

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Nowadays commercial steel plants produce ingots or pig iron. It has very limited use. It goes to casting foundries or to steel mills. At both the places it is remelted to reduce its carbon content and for allying by adding various elements such as manganese and nickel. Often scrap steels are also added for the same purposes.

Melting points for various forms of Irons

Iron, Wrought     1482 – 1593

Iron, Gray Cast   1127 – 1204

Iron, Ductile       1149

Steel, Carbon      1425 – 1540

Steel, Stainless   1510

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WROUGHT IRON LATTICES

Post 226 ⇒   by Gautam Shah 

WROUGHT IRON LATTICES

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Wrought Iron lattices have been used primarily over windows, doors and other gaps. Latticed structures of wrought iron are used for balconies, as space dividers, church screens, vine climbers, stair railings, estate gates and barricades, frames for furniture items, lintels, beams, brackets, columns and for garden structures like orangeries and pavilions.

Iron forming reflects man’s innovative and craft skills. It has been a very difficult material to work with, as it presents different behaviour in its various forms. Yet, it has been cast, resealed, joined, spliced, chased and engraved. It has been reformatted with hot and cold treatments. Wrought iron has been used for household utilities, tools, vessels, arms, building elements, architectonic entities, decorative items and statuettes. It has replaced wood for its stability, strength and malleability.

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Before the Middle Ages, wrought iron was used primarily for weapons, tools and utilities that only could be made with a metal. Unlike Cast iron, Wrought iron has a lower carbon content. It is stronger, non-brittle, and could be forged to any shape, and join by beating. Literally, Wrought iron means an iron that can be worked, both in hot and cold forms.

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One of the most creative forms of wrought iron manifests in trellis, grills, and other hollowed or pierced-out planner forms. Earlier trellis or grills were formed of wood, bamboos, vines, and cast of copper or bronze, or even of ceramics. These materials were not amenable to plastic shaping. Wrought iron has been used as a plastic material to form variety of trellis, in simple or multi-curved planner forms and also mould sub-elements differently.

The first lattices were functional elements like the protective cover within gaps, and in doors and windows. Simple linear cast or forged elements were inserted in side structures of masonry or wood. These, however, soon became interlacing or entwined entities of bars, hot-forged or riveted forming a grill. Same techniques were used for creating grills for hearths and sieves.

Wrought Iron lattices began to be used 13 and 14th C windows of mansions and cathedrals requiring high security. Same structures were used as barricades and partitions. The lattices were designed with variegated shaping of bars’ profiles, and in terms of angle and spacing. Hot-forging and cold working methods were used to alter the sections and shapes of the linear elements. Round and square rods and bars were twisted, coiled and beaten into complex foliated forms. Iron pieces were chiselled, chased, riveted, shape forged. Iron plates were also used for plate like tracery elements. Ends, finials and cresting were cast from other materials like brass or bronze and mounted over steel roods. Riveting and hot forging was chief techniques of joint making. Joints, However, were so skilfully concealed that the grill seemed like one cast or formed piece.

Wrought ironwork began to serve other decorative purposes. Famous cathedrals and other public buildings ( Canterbury and Winchester Cathedrals of England and Notre Dame de Paris) have extremely crafty pieces wrought iron works.

Wrought Iron lattice work, began as a rough surface entity, but by end of middle ages, the surfaces were well formed, ground and joints were concealed. Surfaces were often chased, engraved, inlayed with materials. Finials, caps and other elements of brass, copper, bronze and gold were added. Ornaments were forged out as separate parts, and assembled with riveting, or welding. Decorative elements, such as of flowers, leaves, vines, birds, names, and coats of arms, were bunched or heaped to provide a composition language.

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