Hundred-Refined Steel: How Chinese Smiths Folded Steel

Hundred-refined steel (百炼钢, bǎiliàngāng)百炼钢 bǎiliàngāngSteel repeatedly heated, folded and hammered to drive out slag and even out carbon, recorded from the Han dynasty.Glossary → is steel that Chinese smiths heated, hammered, folded and forge-welded again and again until the slag was driven out and the carbon spread evenly. It was the premium material of traditional Chinese sword forging from the Han dynasty onward, but never a single secret recipe. It was the last link in a chain only China had: cast iron first, then steel made from cast iron, then folding, then blades that wrapped hard steel around soft iron.

China’s head start: cast iron first

Most of the ancient world made iron in a bloomery, a furnace too cool to melt it, and hammered the spongy lump into shape. China went another way. By the 5th century BCE its furnaces were hot enough to melt iron, which soaks up carbon as it liquefies and can be poured into moulds like bronze. Some of Europe’s earliest cast iron comes from Swedish furnaces such as Lapphyttan, around 1150–1250 CE.

Cast iron, 生铁 (“raw iron”), carries more than 2% carbon. That makes it castable and hard, but brittle: a cast-iron sword would shatter on the first parry. So Chinese smiths learned to take the carbon out again.

From the Western Han dynasty (206 BCE – 9 CE) they practised puddling (炒钢, chǎogāng), literally “frying steel”: cast iron was heated until pasty and stirred in the open air so that oxygen burned off its carbon. Stopped early, the result was steel; pushed further, it became soft wrought iron, 熟铁 (“cooked iron”). Song Yingxing’s Tiangong Kaiwu (1637) still describes the method: molten iron runs into a shallow square pit while several men stir it with willow poles and another scatters dried, finely sieved mud over it.

Chinese sword forging, step by step

Puddled steel was only the raw material. What turned it into a sword was a sequence of operations that changed remarkably little between the Han and the Ming. Scroll through the explainer below; the sections that follow unpack the steps that matter most.

What “hundred-refined” actually means

The character 炼 (in Han inscriptions also written 湅 or 練) means one cycle of heating and hammering. Puddled steel left the hearth full of slag and uneven in carbon, so smiths reheated it, drew it out, folded it back on itself and welded it again. Every cycle squeezed out more slag; every fold blended high- and low-carbon zones.

By the Eastern Han (25–220 CE), refining had become a boast engraved on the blade. A ring-pommel sword about 110 cm long, excavated from the Tōdaijiyama tomb mound in Tenri, Nara, carries a gold-inlaid inscription dated to the Zhongping era (184–189 CE) that calls it 百練清剛, “hundred-refined pure steel”. A Han blade that ended up in a Japanese grave, it is now a National Treasure in the Tokyo National Museum.

How many refinings make “a hundred”? Not a hundred folds. The best evidence comes from the polymath Shen Kuo, who watched smiths at the forges of Cizhou (in today’s Hebei) and recorded their test for real steel:

So “hundred” counted heats, not folds, and the real endpoint was physical: you stopped when the bar stopped losing weight, because the slag was gone. In Chinese 百 also simply means “a great many”, as in 百姓, “the hundred surnames”, the common people. By the early 4th century the phrase was proverbial: the Western Jin poet Liu Kun asked how “hundred-refined steel” could turn “soft enough to wind around a finger” (何意百炼刚，化为绕指柔).

The layer arithmetic

Folding is exponential. Each fold doubles the layers, so n folds give 2ⁿ:

Fifteen folds, the number the explainer counts up to, already give 32,768 layers. A literal hundred folds would give 2¹⁰⁰, about 10³⁰ layers, each far thinner than an atom: proof that the word was a grade, not a recipe.

There is also a practical ceiling. After about ten folds, a billet a few centimetres thick holds layers only microns to a few tens of microns thick, thinner than the distance carbon diffuses during a single forging heat at around 1,100 °C. From then on the carbon evens out completely, and more folding mainly burns metal off as scale. Song Yingxing put a number on the waste: fresh wrought iron loses three parts in ten as scale and flakes when it is forged (十耗其三).

Co-fusion steel: the shortcut

Folding was slow and wasteful, so smiths looked for a faster route to the right carbon content. The answer was co-fusion steel (灌钢, guàngāng): melt high-carbon cast iron into low-carbon wrought iron and let the two meet in the middle. The earliest description is in the biography of Qiwu Huaiwen (綦毋怀文), a 6th-century metallurgist and Daoist adept who served Gao Huan, patriarch of the Northern Qi ruling house:

Not everyone approved. Shen Kuo called co-fusion steel 伪钢, “false steel”: in his view the cast iron only hardened the metal for a while, and after two or three refinings it reverted to soft iron. Yet by Song Yingxing’s day it was standard practice. His 1637 recipe: beat wrought iron into thin strips about a finger wide and one and a half cun (some 5 cm) long, bind them tightly, lay cast iron on top, cover the stack with old straw sandals caked in mud, and blow the furnace hard. The cast iron melts first and “seeps into the wrought iron, the two uniting”; then the smith hammers, refines and hammers again. He calls the result 团钢, “lumped steel”, or 灌钢.

Building the blade: wrap, sandwich, quench

Uniform hard steel makes a sharp but brittle sword; soft iron makes a tough but dull one. Chinese smiths solved this with composites, and Song Yingxing spells out the hierarchy:

Ordinary knives and axes, he adds, merely have steel inlaid along the cutting face, and even a costly blade that can chop nails will, after thousands of sharpenings, wear through its steel until the iron shows. Every good blade, wrapped or inlaid, was then quenched in water; its keenness came from the whetstone. Smiths use three main names for these builds:

• 包钢 (bāogāng, wrapped steel): a steel skin around a soft iron core, Song Yingxing’s 钢表铁里, “steel outside, iron inside”.
• 夹钢 (jiāgāng, sandwiched steel): a hard steel core between two softer cheeks, exposed only at the edge.
• 贴钢 / 嵌钢 (tiēgāng / qiàngāng, applied or inlaid steel): a steel strip forge-welded onto, or into, the edge of an iron body; the cheapest way to put steel where it cuts.

Song Yingxing was candid about one rival. Japanese sabres, he wrote, had backs under two fen (about 6 mm) yet balanced on a fingertip: “We do not know what hammering method they use; China has not received its transmission.”

Quenching: water, urine, fat and oil

Quenching, 淬火 (cuìhuǒ), is the moment a blade is made or ruined. Heated to a cherry-red glow (roughly 780–850 °C for a typical blade steel) and plunged into liquid, carbon steel turns hard. Song Yingxing’s instruction is plain: take the forged steel straight from the fire and plunge it into clean water, and only then is it 健钢, “strengthened steel”.

Older recipes were more exotic. Qiwu Huaiwen’s urine-then-fat treatment is often read by historians of technology as a two-stage quench: urine, salty like brine, cools fast; fat, like oil, cools more gently. That reading is an interpretation, but the physics holds: water and brine give maximum hardness and the most risk of cracking, oil less of both. Song Yingxing also reported that foreigners quenched swords in 地溲, “earth urine”, a petroleum-like oil not produced in China, and claimed the steel could cut jade, adding drily: “I have never seen it.”

Hundred-refined steel vs Japanese and Damascus steel

Chinese folded steel is constantly compared with the katana and with Damascus steel. All three solved the same problem, a hard edge on a tough body, with the iron their regions could make.

None is simply “better”. Chinese smiths had cheap cast iron and the puddling and co-fusion processes it made possible; Japanese smiths made the most of a patchy bloomery steel; Indian crucible steel was so rich in carbon that it needed no folding at all. To see these metals in real weapons, read about the jian, the dao, the bronze Sword of Goujian that came before them, and the forges of Longquan that still fold steel today.

《天工开物》 古法制造传习之旅 第一集 | CCTV

In Phantom Blade Zero

S-GAME’s fondness for iron goes beyond props. Besides the replica swords, its 2025 S-Party event in Beijing was staged at the Shougang industrial site beside the No. 3 blast furnace, and its location scouting took in “old steel factories in Beijing”. Kungfupunk is built from the same smoke and slag as Chinese swordsmithing, which suits a game whose arsenal draws on traditional Chinese weaponry: every jian and dao in it descends, at least in spirit, from a bar of steel that someone weighed after every heat.