Most iron and steel contains a high proportion of the element iron, a small proportion of carbon, and trace amounts of material such as silicate. The shapes of the embedded carbon particles also affect the iron or steel produced.
 
This story concentrates on iron salvaged from the 20th century and earlier building construction - wrought iron, cast iron, malleable iron and steel - the type of material commonly found in reclamation yards.
 
Generally, more carbon makes iron and steel products harder and more brittle but allows it to be melted and poured into moulds as cast iron and steel. Conversely, the softest iron has the least carbon, cannot be poured, and can only be worked by beating as wrought iron.
 
Pure iron is wrought iron with a 99.8% iron content and 0.02% carbon, it can be worked cold, is too soft for everyday use, and can be cut with a knife - yes, it is as soft as butter.
 
Typical wrought iron, the type from which horseshoes, gates and railings were traditionally made, can only be worked red hot and contains more than 99% iron with traces of carbon, silicon, sulphur, and phosphorus.
 
The proportions of carbon, the crystal structure of that carbon, the production methods, and small amounts of other chemicals make the difference between a brittle cast iron bath which can be smashed with a hammer, an old wrought iron or modern steel gate, which deforms but does not break under hammer blows, and a malleable iron manhole cover which is tough but not brittle.
 
In 1875 Britain was the leading iron and steel producer with an annual output of 4m tonnes - half the world's output at the time. China now leads global steel production with 1.1b tonnes, around half the world's annual output, of which 100m tonnes is made from scrap. The UK is ranked 24th with 700,000 tonnes annually and exports 8m tonnes of scrap.
 
Embodied carbon, production methods, reuse, and durability of products made from wrought iron, cast iron and mild steel make a difference in saving resources, carbon emissions and pollution avoidance when reusing iron and steel products.
 
This article irons out some of the differences.
 
Wrought Iron
 
Definition
Wrought iron is a semi-fused mass of iron, with inclusions of fibrous silicate, silicon oxide, and slag, giving it a wood-like grain. It was commonly used for making anchors, chains, gates and railings before the advent of commercial steel production in the early 20th century,
 
Identification tests
There are four identification tests for wrought iron, all of which make the silicate 'grain' more obviously visible.
These tests are:
- fine polishing followed by acid-etching
- visual signs of delaminated corrosion caused by rust
- sawing a cut in a bar and bending it until the fracture shows a fibrous silky lustre - see https://www.youtube.com/watch?v=un9sCXExl2Q
- blacksmiths spark test using a grinder - there are various videos and photos, but it may make sense to see this https://www.youtube.com/watch?v=z-JOaQM5kCM&t=4s by an old spark test sceptic.
 
History
Wrought iron, known and worked by ancient civilisations, was originally sourced from meteorites, many of which were pure iron because the solar system is 25% iron. Wrought iron working began around 2000 BCE with Egyptians, Mesopotamians, and Hittites heating and hammering iron ore for tools, weapons and ornament.
 
Chemistry
Slag, the stoney waste from the ore, can be beneficial as the silicate inclusion act as a flux to assist in joining pieces of wrought iron when forge welding. The silicate filaments protect the iron from corrosion and reduce metal fatigue caused by shock and vibration. There are 100,000s of silicate inclusions in a cubic centimetre of wrought iron. Due to the variations in ore origin and iron manufacture, wrought iron may be better or worse in resisting corrosion than other iron alloys. When scrap wrought iron is melted and cast, the slag 'stringers' disappear, and the finished product resembles impure cast Bessemer steel, which results in no engineering advantage compared to cast iron or steel, both of which are cheaper. Incidentally, in 2010 a traditional smelt was worked into a record 99.7% pure iron with no carbon present.
 
Embodied energy
The refining of iron ore used coal or coke to produce billets of pig iron: 1 tonne of pig iron took 1.5 to 2.5 tonnes of coal to produce, and emitted 2.5 to 4.5 tonnes of CO2e/t. The refinement of pig iron into wrought iron used charcoal which, as a renewable, did not introduce additional fossil carbon into the atmosphere. As a result, the carbon emissions associated with wrought iron production using charcoal were relatively low. However, with the planet deforesting which reduces its capacity to sequester carbon, the use of wood charcoal nowadays is less eco-friendly. It takes around 2 to 3 tons of charcoal to make a tonne of wrought iron.
 
Technical
Until the 1790s, wrought iron was smelted from ore using charcoal, but in 1800 wrought iron began to be 'puddled' from pig iron that was cast into billets, or pigs, using coal and coke, which resulted in metal which was variable, both in chemistry and slag content. Puddled iron caught on fast. The puddling process took the pig iron and refined it in a reverberatory furnace to prevent iron contamination from the sulphur in the coal or coke. The molten pig iron was stirred to expose it to atmospheric oxygen, which reduced the carbon content of the iron, during which globs of iron collected by the stirring rod were periodically removed by the puddler. Puddling, widely used after 1800, produced a better structural material that became the preferred material for bridges, rail tracks, ships and buildings. Historically, a modest amount of wrought iron was refined into steel, which was used mainly to produce swords, such as Japanese swords in the 1300s, cutlery, chisels, axes, and other edged tools, as well as springs and files. The demand for wrought iron peaked in the 1860s era of ironclad warships. With the advent of mass steel manufacture in the early 1900s, wrought iron production gradually ceased on a commercial scale. The last working wrought British ironworks was the Atlas Forge of Thomas Walmsley & Sons in Bolton which closed in 1973. Their 1860s-era equipment was moved to Blists Hill at Ironbridge, where the museum preserves it, and wrought iron is still produced from reclaimed wrought iron scrap for heritage restoration.
 
Usage
Items traditionally produced from wrought iron included rivets, nails, wire, chains, water pipes, steam pipes, nuts, bolts, horseshoes, handrails, wagon tires, straps for timber roof trusses, ornamental ironwork, baker's racks, wine racks, pot racks, etageres, table bases, desks, gates, beds, candle holders, curtain rods, bars, and bar stools.
 
Misleading use of the phrase 'wrought iron'
The renaming of mass-produced mild steel products as wrought iron started in the second half of the 20th century with items such as garden furniture and gates. This was a marketing ploy because wrought iron sounds more antique and, therefore more valuable than steel. Arguably, modern steel products may have been wrought traditionally by a blacksmith and made to resemble objects which in the past were wrought. Technically these should be called 'wrought steel'.
 
Market
The biggest 19th century wrought iron structure was the Eiffel Tower. A tour-de-force built for the Paris Expo from 7,300 tonnes of wrought iron, sourced in eastern France, built by Gustave Eiffel (1832-1923), it took two years to build and was completed in 1889. Apparently, he did not trust steel. Eiffel's parents ran a coal or charcoal business in eastern France. Many of the haute fourneaux, including the famed Val d'Osne foundries, were located there, using local wood and charcoal to make the cast iron garden ornament.
 
Top auction prices: A pair of antique wrought iron garden gates attributed to Jean Tijou made in the late 17th century for Hampton Court were sold at auction in 2005 for approximately £1.02m.
A pair of wrought iron gates created by Samuel Yellin sold for $692k at Sotheby's in 2005, and an ornate wrought iron balcony from the Eiffel-designed Samson Building in Paris fetched $430,000 at a Christie's auction in 2014. A wrought iron spiral staircase from the Eiffel Tower sold for $550,000.
 
Bottom trade prices: Scrap historic wrought iron is currently considered scrap steel with no premium for its historical value, nor for the original low embodied carbon of wrought iron compared to scrap modern steel. Price June 2023:
Iron (Heavy) £70 - £120/t - cast iron baths, machinery, iron pipe, steel sections
Iron (Light) £60 - £100/t - machinery, iron pipe, engine blocks, white goods
 
Cast Iron
 
Definition
Cast iron is a hard, relatively brittle alloy of iron and carbon which can be readily cast in a mould and typically contains 2.0% - 4.3% carbon. There are various types of cast iron, with grey cast iron being the most common and the most brittle.
 
Identification test
A basic test for grey cast iron is to hit it hard with a heavy hammer. Grey cast iron will crack due to structural weaknesses created by the arrangement of the carbon flakes within the metal. If smashing it is too destructive then this video https://www.youtube.com/watch?v=zn5QMt-byyY shows a small drill and centre punch test. The centre punch raises a burr on ductile iron but not cast iron, and drilling results in powdered metal with cast iron but shavings with ductile iron.
 
History
Cast iron is created by melting ore at 1500°C and then pouring it into a mould. It is believed to have been made in China as early as the 5th century BCE for weapons, ritual vessels, ploughshares and pots. Annealing of castings was developed by keeping them hot in an oxidising atmosphere for a week or longer to burn off surface carbon and make the objects more malleable. The Iron Lion of Cangzhou, a 44t iron sculpture cast in 953, is the largest and oldest surviving cast iron artwork in China at 6m high and 6.5 m long. Around the same time, in the central African Congo, high temperature iron furnaces melted iron in crucibles which were poured into moulds to make composite tools and weapons with an outside layer of harder cast iron and a softer, more flexible wrought iron interior. This was similar to the Japanese Katamura sword technology of the 1300s. Some researchers believe Central African ironworking to be radiocarbon, dated as early as 3600 BCE.
 
In the West, where it did not become available until the 15th century, the earliest use of cast iron was to make cannon and shots. Henry VIII initiated the casting of cannon, after which English iron workers using blast furnaces developed the technique of producing cast iron cannons, which, while heavier than the prevailing bronze cannons, were much cheaper and enabled England to arm her navy better. The technology of cast iron was transferred from China. Al-Qazvini in the 13th century and other travellers subsequently noted an iron industry in the Alburz Mountains to the south of the Caspian Sea. This is close to the silk route, so using technology derived from China is conceivable. Ironmasters from the Weald of Kent continued producing cast iron until the 1760s.
 
Chemistry
Iron (Fe 94% - 97%), carbon (C 2.5% - 4%), plus silicon (Si 1% - 3%), manganese (Mn), sulphur (S) and phosphorus (P). Higher carbon content results in more brittle cast iron. Grey cast iron is characterised by its graphitic microstructure, which causes fractures of the material. It has less tensile strength and shock resistance than steel, but has high compressive strength. The size and shape of the graphite flakes present in the microstructure control mechanical properties.
 
Technical
Common cast iron is known as grey cast iron, a general purpose material used for many items, for example, garden ornament, fireplaces and cast iron baths. It is one of a group of iron-carbon alloys with a carbon content greater than 2%. It has a relatively low melting point of 1200°C and good casting properties.
 
Early cast iron structures
The 385t Iron Bridge in Shropshire was the first cast iron bridge built during the 1770s by Abraham Darby III. The bridge is immensely strong in compression, but cast iron bridges perform less well than steel and wrought iron when subjected to tension or bending moments.
London's 1851 Great Exhibition was housed in an enormous 4,000t cast iron structure, Crystal Palace, designed by Joseph Paxton. The 1866 4,041t dome of Washington's Capitol building was built of cast iron.
 
Usage
Cast iron is brittle but has excellent compression strength, making it ideal for applications like engine blocks, pipes, and cookware. Because cast iron is comparatively brittle, it is not suitable for purposes where a sharp edge or flexibility is required. It is strong under compression but not in tension.
 
Highest prices:
Set of four Val d'Osne cast iron urns modelled by Pierre-Louis Rouillard, price realised $288,000, late 19th Century, Christie's 11 April 2007.
A Royal Arms fireback dated 1742 made by the Oxford Furnace in Warren County, NJ, sold for $22,680 at Sotheby's, 21 January 2023
A Griswold "Erie No. 1" cast iron skillet, manufactured in the late 19th century, sold for $9,000 at an auction in 2018.
An antique cast iron mechanical bank, such as the J&E Stevens "Girl Skipping Rope," reached a price of $19,550 at a Morphy Auctions sale in 2019.
 
Malleable and ductile iron
 
Definition
Malleable iron is a hybrid cast iron which is tough and durable but considerably less brittle than grey cast iron. It starts as a white iron casting that is heat-treated at about 950 °C and gradually cooled to transform the embedded carbon into spheroid particles rather than the long stringer flakes found in grey cast iron, which are relatively short and far from one another, and this reduces crack propagation. They also have blunt boundaries, as opposed to carbon flakes, which alleviate stress concentrations. In general, the properties of malleable cast iron are more like those of mild steel. There is a limit to how large a part can be cast in malleable iron.
 
Developed in 1948, nodular or ductile cast iron has its graphite in the form of very tiny nodules, with the graphite in the form of concentric layers forming the nodules. As a result, the properties of ductile cast iron are that of spongy steel without the stress concentration effects that flakes of graphite would produce. The carbon percentage present is 3% to 4%, silicon is 1.8% to 2.8%, and small amounts of magnesium and cerium slow the growth of graphite precipitates, which allows the carbon to separate as smaller spheroidal particles than malleable iron as the material solidifies. The properties are similar to malleable iron, but parts can be cast with larger sections.
 
Embodied energy
Around 1.5 to 2 tons of coal or coke are used to make a ton of malleable iron. The estimated carbon emissions can range from approximately 2.5 to 3.5 tons of CO2e/t of malleable iron.
 
History
The discovery of malleable iron occurred during the 18th century with notable contributions by Abraham Darby in England.
 
Usage
Malleable and ductile iron has good tensile strength and is commonly used in applications requiring high strength and resistance to wear. Manhole covers, gullies, road drains, and underground pipework are typically made from malleable or ductile iron.
Malleable iron is used to make reproduction black rustic door furniture, old iron gas and water pipe, scaffold fittings, crank shafts, vices and piano frames.
 
Steel
 
Definition
Steel is an alloy of iron and carbon with improved strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Modern steel is identified by various grades defined by assorted standards organisations. 
 
Identification test
It is possible to eliminate wrought iron and cast iron by using the tests above. If the metal is neither wrought nor cast, then it could be either malleable iron, ductile iron or mild steel. 
Ductile iron is stronger than mild steel in tension and compression. It may also be harder, so one test would be to use a hacksaw or file and compare a piece of ductile iron, such as a bench vice, with mild steel to compare them with the mystery metal that needs identifying. If the cast iron test above is too destructive, this video shows a small drill and centre punch test https://www.youtube.com/watch?v=zn5QMt-byyY
 
There seems no way of simply distinguishing malleable iron and mild steel, so here is a Chinese video showing the manufacture of malleable iron fittings set to soothing background music resembling the Red Army chorus https://www.youtube.com/watch?v=UxuyTNv1eqg
Please get in touch with us if you know a simpler ID method.
 
History
Crucible steel was produced in ancient times in India and the Middle East with early advancements in steel production, dating back to 300 BCE. Steel had been produced in bloomery furnaces for thousands of years. Still, its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the introduction of the blast furnace and the production of crucible steel, followed by the open-hearth furnace and then the Bessemer process in England in the mid-19th century. The Bessemer process started a new era of mass-produced steel, spurred on by the railways whose iron rail track needed frequent replacing compared to steel from 1865 in Britain.
 
Cast and wrought iron remained dominant for structural applications until the 1880s, because of problems with brittle steel caused by nitrogen, high carbon, excess phosphorus, excessive temperature, or too-rapid rolling, which began to be resolved. By the 1890s, steel started to replace iron for structural applications. Further refinements continued in the steelmaking process, such as basic oxygen steelmaking, largely replacing earlier methods and further lowering the cost of production and increasing the quality of the final product. Today, steel is one of the most commonly manufactured materials in the world, with more than 2b tonnes produced annually. 
 
Usage
The modern steel industry is one of the largest manufacturing industries in the world, but also one of the most energy and greenhouse gas emission intense industries, contributing 8% of global emissions. Steel is also very reusable and is vaunted by the industry as one of the world's most-recycled materials, with a recycling rate of over 60% claimed, but according to World Steel statistics, the actual level of recycled steel in new steel output is currently around 10%.
 
Embodied energy
The carbon emissions associated with steel production vary depending on the production method. In the case of the basic oxygen furnace (BOF), assuming 0.6 to 1.2 tons of coal per ton of steel, the estimated carbon emissions can range from approximately 1 to 2 tons of CO2e per tonne of steel. Electric arc furnaces can reduce carbon emissions significantly; the mix of energy is mainly from renewable sources.
 
Chemistry
Iron is the dominant constituent with carbon between 0.2% and 2.1% and alloying elements such as manganese, chromium, nickel, molybdenum, and others added to achieve specific properties.
 
Market
The sale of a 1931 Bugatti Type 41 Royale Kellner Coupe was $9.8m at auction by Gooding & Company in 2019.
Jeff Koons' stainless steel Rabbit sculpture sold for $91.1m at Christie's in 2019, becoming the most expensive work by a living artist. The sculpture weighed 1,300kg and is 104cm high. Chromium is a key component that gives stainless steel its corrosion resistance. The minimum chromium content in stainless steel is typically around 10.5%. Nickel is often added to stainless steel to enhance its corrosion resistance and provide additional strength. Stainless steel may contain other elements, such as manganese, molybdenum, nitrogen and others.
 
Coatings and treatments
Rust-prevention coatings and treatments used for steel include:
- galvanising by coating steel with a layer of zinc which acts as a sacrificial anode, providing corrosion protection to the underlying metal;
- powder coating is a dry finish powdered polymer electrostatically applied to steel and cured by heat
- electroplating deposits a layer of another metal on steel using electrolysis, commonly chrome, nickel, and zinc;
- epoxy coatings consist of a two-part resin and hardener system that creates a durable and chemically resistant coating;
- ceramic and vitreous enamel coatings are applied in thin layers to provide corrosion resistance, high temperature tolerance, and chemical resistance.