Iron was the primary metal used for armor throughout the Middle Ages, with steel gradually taking over as metalworking techniques improved. Early medieval armor was almost entirely wrought iron, while later centuries saw armorers increasingly working with low- and high-carbon steel to produce the iconic plate armor of the 14th and 15th centuries.
Iron: The Foundation of Medieval Armor
For most of the medieval period, wrought iron was the go-to material. It was relatively easy to produce in bloomery furnaces, which heated iron ore with charcoal to create a spongy mass that smiths hammered into usable metal. Wrought iron is soft and tough, meaning it bends rather than shatters under impact. That made it well suited for chainmail, the dominant form of body armor from roughly the 6th through 13th centuries. Metallurgical analysis of surviving mail links confirms this: of samples tested from the high medieval period, the majority were wrought iron rather than steel.
The tradeoff was hardness. Wrought iron measures around 90 to 120 on the Vickers hardness scale, which means it dents and deforms more easily than steel. A blade or arrow could push through iron rings or plates more readily than through hardened steel. As weapons grew more powerful, armorers needed something tougher.
Steel and the Rise of Plate Armor
Steel is simply iron with added carbon, and that carbon changes everything. Even a small percentage makes the metal harder and more resistant to cutting and piercing. Medieval armorers didn’t have the industrial tools to measure carbon content precisely, but they understood through experience that certain smithing techniques produced stronger metal. By the 14th and 15th centuries, armorers in major production centers like Milan and Augsburg were regularly producing steel plate armor.
Analysis of surviving armor pieces shows a range of quality. In one study of 42 samples, 20 were wrought iron, 6 were low-carbon steel (too little carbon to harden significantly), and 12 were high-carbon steel, some of which had been deliberately hardened. A 14th-century helmet from Slovenia, for instance, measured 261 to 274 on the Vickers hardness scale, more than double the hardness of plain wrought iron. A late 14th-century great bascinet in the British Museum measured around 206 Vickers. These numbers reflect real variation in quality: not all medieval armor was created equal, and the steel available to a wealthy knight differed dramatically from what a common soldier could afford.
How Armorers Hardened Steel
Producing steel was only half the equation. To get the most protection from it, armorers used heat treatment. The basic process involved heating the steel until it glowed red, roughly 820 to 870°C, then plunging it into water or oil. This rapid cooling, called quenching, transformed the metal’s internal structure into an extremely hard form. The problem was that fully quenched steel becomes brittle, almost glass-like. A breastplate that shattered on impact was worse than useless.
To fix this, armorers reheated the quenched steel to a much lower temperature, around 200 to 340°C, in a process called tempering. This sacrificed a small amount of hardness in exchange for toughness, giving the finished plate the ability to absorb a blow without cracking. Skilled armorers could even apply differential hardening, making the outer face of a plate harder while leaving the inner surface softer and more flexible. Not every armorer had the skill or resources for this, which is why metallurgical studies of surviving pieces show such wide variation in hardness and carbon content.
From Chainmail to Full Plate
The transition from mail to plate armor unfolded over roughly two centuries. Between 1200 and 1400, armorers began reinforcing chainmail suits with individual plates at vulnerable points like the knees, elbows, and shins. A knight in 1250 might wear a full mail shirt with small steel plates strapped over the limbs. By the mid-1300s, the breastplate had appeared, and helmets were being shaped more closely to the head, with a curtain of mail protecting the neck.
By around 1420, the full plate harness had arrived: a complete suit of interlocking steel plates covering the body from head to toe. Plate thickness varied by location on the body. Breastplates, which needed to stop lances and swords, were typically 3 to 4 millimeters thick. Less critical areas like the backs of the limbs might be only 1 to 1.5 millimeters. A complete field harness weighed 15 to 25 kilograms, distributed across the whole body, comparable to what a modern soldier carries in gear. Tournament armor, which didn’t need to allow free movement, could reach 50 kilograms.
Crucible Steel in the Middle East and Asia
European armorers weren’t the only ones pushing metallurgy forward. In Persia and India, smiths produced crucible steel, the earliest form of liquid steel in history. The process involved sealing iron and carbon sources inside a clay crucible and heating them to extreme temperatures. This created a more uniform metal with fewer impurities than the bloomery process used in Europe, because the slag separated completely from the molten steel.
Persian crucible steel, known as pulad, dates to at least the 10th century in present-day Uzbekistan and Turkmenistan. Some Persian smiths even added chromium minerals to their crucible charges. Analysis of 11th-century steel from the site of Chahak in Iran found about 1% chromium content, a deliberate addition nearly a thousand years before chromium steels became common in industrial metallurgy. Indian wootz steel, produced in southern India and Sri Lanka, was another crucible tradition that created high-quality metal for weapons and armor. These steels were prized across the medieval world and traded along routes stretching from Southeast Asia to North Africa.
Copper Alloys and Decorative Metals
Iron and steel did the heavy lifting, but other metals played supporting roles. A small number of armor components, about 4 out of 42 in one sample group, were made from copper alloys like bronze or brass. These appeared more often in decorative elements, buckles, hinges, and fittings than in protective plates. Brass borders and rivets were common on helmets and gauntlets, adding visual appeal without significantly increasing weight.
Wealthier knights and nobles decorated their armor with gold, silver, and copper alloy inlays. Armorers used techniques like engraving grooves into the steel surface and hammering in thin wires of precious metal, creating intricate patterns that signaled status and identity on the battlefield. The functional armor underneath, though, was always iron or steel.