# Steel > Iron with just enough carbon - Date: c. 1,000 BCE - Era: Antiquity - Domain: [Materials & Chemistry](https://lastcodex.org/en/domains/materials/) - Rebuild stage: III (Crafts & cities) - Difficulty: 4/5 - URL: https://lastcodex.org/en/inventions/steel/ Steel is iron with a small, controlled share of carbon, usually between about 0.1 and 1.5%. It is forged hot; with enough carbon it can be hardened by heating and quenching, then tempered to make it tougher at some cost in hardness. ![Steel · Iron with just enough carbon](https://lastcodex.org/_astro/plate.CqU-mOHI_CdAUJ.webp) ## Legend 1. Bellows 2. Forge hearth 3. Hot steel held in tongs 4. Quench tank 5. Bessemer converter 6. Air tuyères in the converter bottom ## What it is Pure iron is soft: a knife made of it bends and soon goes blunt. Add a little carbon, a few tenths of a percent up to about one and a half, and the metal still forges hot. With enough carbon, heated and plunged into water or oil, it becomes hard enough to cut iron. That alloy is steel. By its carbon it sits between two other materials. Wrought iron has almost no carbon: it is tough and welds easily but will not harden. Cast iron holds 2 to 4% carbon: it pours into moulds but is brittle and cannot be forged. By heating and cooling it, the smith can move steel between hard and tough. It is now by far the most used metal. ## How it works Carbon atoms are small and sit in the gaps between iron atoms. Cold iron holds very little carbon, so the excess forms hard iron carbide. In slowly cooled steel it lies in thin layers between soft iron, a mixture that is strong but can still be filed. Above about 727 °C the iron crystal starts to rearrange into a form that dissolves carbon, and once the change is through the steel no longer attracts a magnet. How much hotter, and for how long, it must be held for enough carbide to dissolve depends on the steel. Cooled slowly, the carbon comes back out as carbide. Quenched, it has no time: it stays trapped in a strained crystal called martensite, very hard and as brittle as glass. ![A cold chisel showing bands of tempering colour, pale straw at the edge, then brown, purple and blue towards the shank](https://lastcodex.org/_astro/v1.CLXHzJ9B_Z2u2L27.webp) _As the shank is heated, the colours run towards the edge; the smith cools the tool when the right one reaches it._ So the smith tempers it: reheats it to between about 200 and 350 °C, which eases the strain and makes it much tougher at the cost of a little hardness. On polished steel a thin oxide film forms whose colour follows the heat, from pale straw near 210 °C through brown and purple to dark blue near 290 °C. These colours, which also depend on time and on the surface, were the smith's rough guide: straw for a razor or a wood chisel, purple to blue for a spring. Annealing, heating and then cooling very slowly, gives the softest state, easy to cut and file. Below about 0.3% carbon, steel hardens little; more carbon makes it harder and less forgiving. Iron packed in charcoal and held at red heat for hours takes up carbon at its surface. Quenched afterwards, it gets a hard skin over a tough core: this is case hardening. ## How to build Heat treatment is the part of steelmaking a small workshop can do. Start from a known plain high-carbon tool steel and keep its data sheet, which gives exact temperatures, soak times and quenching medium. Old files are the traditional source, but their steel is unknown and some cheap ones are only surface-hardened. **Soften and shape.** Anneal the bar as its data sheet says. The traditional way, bringing it to a dull red and letting it cool overnight in wood ash, is only approximate. Grind the softened bar into a small punch or cold chisel, keeping it cool. **Harden.** Heat the working end evenly in a charcoal fire or with a gas torch. The old sign is that a magnet no longer sticks, then a little more heat, to cherry red seen in shade, but the temperature and soak time of the data sheet are more reliable. Plunge the piece straight and fully into the quenching medium it names, usually oil for a small tool, in a bath of several litres kept well below its flash point. Move the piece up and down until cool. **Test.** A new file drawn across the hardened end should skid without biting. If it bites, the surface may only have lost carbon in the fire, or the hardening may have failed: grind a small spot bright and test again before heating once more. **Temper at once.** The data sheet gives the tempering temperature and time, best kept in an oven: a common cycle is an hour at temperature, a cooling to room temperature, then a second hour. The traditional method is quicker and only approximate. Polish the end bright with abrasive paper, heat the shank slowly and watch the colours travel towards the tip. Cool the tool when brown to purple, about 260 to 270 °C, reaches the edge of a cold chisel, or straw, about 220 °C, the edge of a wood chisel. > **Safety.** Forge heat causes deep burns, and steel below red heat looks cold when it is not: treat every piece as hot. Wear safety glasses, with a face shield over them when quenching, leather gloves for handling hot work but never at the grinder, leather shoes, and cotton or wool clothes. Work outdoors or with strong ventilation, because forges give off carbon monoxide and hot oil gives off smoke. Use a metal oil container with a metal lid at hand, and plunge the work fully and quickly, which limits flare-ups. If it catches fire, close the lid; never throw water on burning oil or let water into hot oil. Keep a dry powder extinguisher nearby. Never hammer, bend, drop or try to snap hardened, untempered steel: it can shatter and throw sharp fragments. ## History and evolution Fragments from Kaman-Kalehöyük in Anatolia, around 1800 BCE, contain steel, probably by accident. A quenched and lightly tempered pick from Mount Adir in Galilee was long dated to the twelfth century BCE, but that date is now questioned. No single find fixes the start of deliberate steelmaking. This plate uses around 1000 BCE as a convention, the time when iron came into common use in the Levant, between about 1100 and 900 BCE. ![A broken clay crucible beside the round steel ingot that solidified inside it](https://lastcodex.org/_astro/v2.BAII-_RH_15zMU9.webp) _A crucible broken open after firing, and the steel cake it held._ In southern India, iron sealed in clay crucibles with carbon-rich matter was melted into even cakes of 1 to 1.5% carbon; crucibles at Kodumanal date from around the second century BCE. This crucible steel, which Europeans later called wootz, is the kind used for the patterned blades known as Damascus steel. In Sri Lanka, hillside furnaces blown by the monsoon made steel from the seventh to the eleventh century CE, perhaps for trade across the Indian Ocean. In China, a grave of the state of Yan from the early third century BCE holds quench-hardened steel, though Chinese iron was then mostly cast iron. Held hot in air for days, or stirred in a fining hearth under a blast, cast iron lost carbon and became steel or wrought iron; excavated fineries date from the first century BCE. ![A cementation chest packed with iron bars in layers of charcoal, beside a bar of blister steel](https://lastcodex.org/_astro/v3.Cheh_ZgR_26yvzA.webp) _In the cementation chest, iron bars spend a week or more in charcoal and come out blistered._ Early modern Europe used cementation, first developed in Germany. At Derwentcote in England, from around 1730, Swedish iron bars packed in charcoal were held above 1,100 °C for six to ten days; the blister steel was then bundled and forge-welded into shear steel. Around 1740 the clockmaker Benjamin Huntsman melted blister steel in crucibles and cast it into uniform steel. In 1856 Henry Bessemer blew air through molten pig iron; the carbon and silicon burned, and their heat kept the metal liquid. Open-hearth furnaces followed in the 1860s. In 1952 at Linz, in Austria, converters blown with pure oxygen began work; they still make most steel, while electric furnaces, fed mostly with scrap, make about 29%. ## Key figures - Carbon: About 0.1–1.5%; wrought iron has less, cast iron 2–4% - Hardening heat: The change starts near 727 °C; each grade has its own quenching temperature and hold - Tempering colours: Straw at about 220 °C, purple about 270 °C, dark blue about 290 °C - Bessemer blow: About 15 tonnes of iron turned to steel in 10–20 minutes - World output, 2024: About 1.9 billion tonnes, 29% of it from electric furnaces ## Requires - [Iron Smelting](https://lastcodex.org/en/inventions/iron-smelting/): Wider adoption around 1200 BCE; ore, charcoal and a bloomery (c. 1,200 BCE) - [Charcoal](https://lastcodex.org/en/inventions/charcoal/): Wood cooked slowly under earth (c. 4,500 BCE) ## Leads to - [Internal Combustion Engine](https://lastcodex.org/en/inventions/combustion-engine/): Small explosions, steady power (1876) - [Magnetic Compass](https://lastcodex.org/en/inventions/compass/): A needle that points north (c. 1088) - [Printing Press](https://lastcodex.org/en/inventions/printing-press/): Movable type, copied by the thousand (c. 1440) - [Reinforced Concrete](https://lastcodex.org/en/inventions/reinforced-concrete/): Steel for tension, concrete for compression (c. 1848) - [Sewing Machine](https://lastcodex.org/en/inventions/sewing-machine/): A thousand stitches a minute (1830) - [Steam Turbine](https://lastcodex.org/en/inventions/steam-turbine/): Steam spinning the world's generators (1884) - [Water Turbine](https://lastcodex.org/en/inventions/water-turbine/): Falling water turned into electricity (1827) - [Wind Turbine](https://lastcodex.org/en/inventions/wind-turbine/): Wind turned into electricity (1887) ## Sources - [Iron and Steel in Early Europe](https://www.tf.uni-kiel.de/matwis/amat/iss/kap_a/backbone/ra_3_1.html), H. Föll, Kiel University - [A Brief History of Martensites](https://sethna.lassp.cornell.edu/Tweed/Martensite_History.html), Sethna group, Cornell University - [The Transition from Bronze to Iron in Canaan: Chronology, Technology, and Context (Yahalom-Mack & Eliyahu-Behar, 2015)](https://doi.org/10.2458/azu_rc.57.18563), Radiocarbon, Cambridge University Press - [Early iron in China, Korea, and Japan (Wagner)](https://donwagner.dk/KoreanFe/KoreanFe.html), Donald B. Wagner - [Crucible steel in South India: preliminary investigations on crucibles from some newly identified sites (Srinivasan & Griffiths)](https://www.cambridge.org/core/journals/mrs-online-proceedings-library-archive/article/abs/crucible-steel-in-south-indiapreliminary-investigations-on-crucibles-from-some-newly-identified-sites/0C7F9DB996F0DD5758502D8F62201947), MRS Online Proceedings Library, Cambridge University Press - [Wootz Crucible Steel: A Newly Discovered Production Site in South India (Srinivasan, 1994)](https://student-journals.ucl.ac.uk/pia/article/id/164), Papers from the Institute of Archaeology, UCL - [Wind-Powered Furnaces](https://archive.archaeology.org/9609/newsbriefs/srilanka.html), Archaeology, Archaeological Institute of America - [History of Derwentcote Steel Furnace](https://www.english-heritage.org.uk/visit/places/derwentcote-steel-furnace/history/), English Heritage - [A Crucible of Steel History](https://westyorkssteel.com/blog/a-crucible-of-steel-history), West Yorkshire Steel - [Bessemer Process](https://www.gracesguide.co.uk/Bessemer_Process), Grace's Guide to British Industrial History - [Bessemer Converter](https://www.sheffieldmuseums.org.uk/whats-on/bessemer-converter/), Sheffield Museums - [The history of LD steelmaking: back to the roots](https://www.primetals.com/en/metals-magazine/the-history-of-ld-steelmaking-back-to-the-roots/), Primetals Technologies - [60 years of the Linz-Donawitz (LD) process](https://www.voestalpine.com/stahl/en/Stories/60-years-of-the-Linz-Donawitz-LD-process), voestalpine - [The share of EAF in global steel production in 2024 increased to 29.1% (worldsteel data)](https://gmk.center/en/news/the-share-of-eaf-in-global-steel-production-in-2024-increased-to-29-1/), GMK Center - [Austenitizing, Magnetism, and Heat Treating](https://knifesteelnerds.com/2019/03/18/magnet-heat-treating/), Larrin Thomas, Knife Steel Nerds - [Steel Tempering Temperatures: Colour Chart](https://westyorkssteel.com/technical-information/steel-heat-treatment/tempering-temperatures), West Yorkshire Steel Fact-checked on 2026-10-14. --- The Last Codex · https://lastcodex.org/en/inventions/steel/ · © The Last Codex. Texts, plates and figures: CC BY-NC-ND 4.0.