Wootz Steel - Raw Material for Damascus Steel Blades

Exploring the 2,400 Year Old Crucible Process of Steel Making

Electron Micrograph of Etched Wootz Steel Sample
Electron micrograph of a deeply etched wootz sample illustrating the fine precipitation probably formed by self-tempering of martensite during the final cooling. Published in Durand-Charre et al. 2010. Courtesy Institut National Polytechnique

Wootz steel is the name given to an exceptional grade of iron ore steel first made in southern and south central India and Sri Lanka perhaps as early as 400 BC. Middle Eastern blacksmiths used wootz ingots from the Indian subcontinent to produce extraordinary steel weaponry throughout the middle ages, known as Damascus steel.

Wootz (called hypereutectoid by modern metallurgists) is not specific to a particular outcrop of iron ore but is instead a manufactured product created by using a sealed, heated crucible to introduce high levels of carbon into iron ore.

The resulting carbon content for wootz is reported variously but falls between 1.3-2% weight

Why Wootz Steel is Famous

The term 'wootz' first appears in English in the late 18th century, by metallurgists who conducted the first experiments trying to figure out the content. The word wootz may have been a mis-transcription by scholar Helenus Scott of "utsa", the word for fountain in Sanscrit; "ukku", the word for steel in the Indian language Kannada, and/or "uruku" (to make molten) in old Tamil. However, what wootz refers today is not what the 18th-century European metallurgists thought it was.

Wootz steel became known to Europeans in the early Middle Ages when they visited Middle Eastern bazaars and found blacksmiths making amazing blades, axes, swords, and protective armor with gorgeous water-marked surfaces. These so-called "Damascus" steels may be named for the famous bazaar in Damascus or the damask-like pattern that was formed on the blade.

The blades were hard, sharp, and were able to bend up to a 90-degree angle without breaking, as the crusaders found to their dismay.

But the Greeks and Romans were aware that the crucible process came from India. In the first century AD, the Roman scholar Pliny the Elder's Natural History mentions the importation of iron from Seres, which may refer to the southern Indian kingdom of Cheras.

The 1st c AD report called Periplus of the Erythraen Sea includes an explicit reference to iron and steel from India. In the 3rd century AD, the Greek alchemist Zosimos mentioned that the Indians made steel for high-quality swords by "melting" the steel.

Iron Production Process

There are three main types of pre-modern iron manufacture: bloomery, blast furnace and crucible. Bloomery, first known in Europe about 900 BC, involves heating iron ore with charcoal and then reducing it to form a solid product, called "a bloom" of iron and slag. Bloomery iron has a low carbon content (0.04% by weight) and it produces wrought iron. Blast furnace technology, invented in China in the 11th century AD, combines higher temperatures and a greater reduction process, resulting in cast iron, 2-4% carbon content but brittle.

With crucible iron, blacksmiths place pieces of bloomery iron along with carbon-rich material into crucibles. The crucibles are then sealed and heated over a period of days to temperatures between 1300-1400 degrees centigrade. In that process, the iron absorbs the carbon and is liquefied by it, allowing the complete separation of slag. The produced wootz cakes were then allowed to cool extremely slowly.

Those cakes were then exported to arms manufacturers in the Middle East who carefully forged the fearsome Damascus steel blades, in a process which created the watered-silk or damask-like patterns.

Crucible steel, invented in the Indian subcontinent at least as early as 400 BC, contains an intermediate level of carbon, 1-2%, and compared to the other products is an ultra-high carbon steel with high ductility for forging and high impact strength and reduced brittleness.

Age of Wootz Steel

Iron making was part of Indian culture as early as 1100 BC, at sites such as Hallur. The earliest evidence for the wootz type processing of iron includes the fragments of crucibles and metal particles identified at the 5th century BC sites of Kodumanal and Mel-siruvalur, both in Tamil Nadu. Molecular investigation of an iron cake and tools from Junnar in Deccan province and dating to the Satavahana dynasty (350 BC-AD 136) is clear evidence that crucible technology was widespread in India by this period.

The crucible steel artifacts found at Junnar were not swords or blades, but rather awls and chisels, tools for everyday working purposes such as rock carving and bead making. Such tools need to be strong without becoming brittle. The crucible steel process promotes those characteristics by attaining long-range structural homogeneity and inclusion-free conditions.

Some evidence suggests that the wootz process is older still. Sixteen hundred kilometers north of Junnar, at Taxila in present-day Pakistan, archaeologist John Marshall found three sword blades 1.2-1.7% carbon steel, dated to 5th c BC-1st c. AD. An iron ring from a context at Kadebakele in Karnataka dated between 800-440 BC has a composition close to .8% carbon and may be crucible steel.

Sources

This article is part of the About.com guide to Iron Age, and part of the Dictionary of Archaeology. See the article on Damascus steel for further information.

Dube RK. 2014. Wootz: Erroneous Transliteration of Sanskrit “Utsa” used for Indian Crucible Steel. JOM 66(11):2390-2396.

Durand-Charre M, Roussel-Dherbey F, and Coindeau S. 2010. Les aciers damassés décryptés. Revue de Métallurgie 107(04):131-143.

Grazzi F, Barzagli E, Scherillo A, De Francesco A, Williams A, Edge D, and Zoppi M. 2016. Determination of the manufacturing methods of Indian swords through neutron diffraction. Microchemical Journal 125:273-278.

Heimann RB, Kreher U, Spazier I, and Wetzel G. 2001. Mineralogical And Chemical Investigations of Bloomery Slags from Prehistoric (8th Century BC to 4th Century AD) Iron Production Sites in Upper And Lower Lusatia, Germany. Archaeometry 43(2):227-252.

Kumar V, Balasubramaniam R, and Kumar P. 2012. Microstructure Evolution in Deformed Ultrahigh Carbon Low Alloy (Wootz) Steel. Materials Science Forum 702-703(802-805).

Park J-S, and Shinde V. 2013. Technology, chronology and the role of crucible steel as inferred from iron objects of the ancient site at Junnar, India. Journal of Archaeological Science 40(11):3991-3998.

Pearson G. 1795. Experiments and Observations to Investigate the Nature of a Kind of Steel, Manufactured at Bombay, and There Called Wootz: With Remarks on the Properties and Composition of the Different States of Iron. By George Pearson, M.D. F.R.S. Philosophical Transactions of the Royal Society of London 85:322-346.

Srinivasan S. 2013. Indian iron and steel, with special reference to southern Indian. In: Humphries J, and Rehren TH, editors. The World of Iron. London: Archetype Books. p 83-90.

Srinivasan S, Sinopoli CM, Morrison KD, Gopal R, and Ranganathan S. 2009. South Indian Iron Age Iron and higher carbon steel: with reference to Kadabakele and comparative insights from Mel-sirivalur. In: Mei J, and Rehren TH, editors. Metallurgy and Civilisation: Eurasia and Beyond: Proceedings of the 6th International Conference on the Beginning of the Use of Metals and Alloys (BUMA VI). London: Archetype Books. p 116-122.

Sukhanov DA, Arkhangelsky LB, Plorniknova NV, and Velousova NS. 2016. Morphology of Excess Carbides Damascus Steel. Journal of Materials Science Research 5(3).