How did medieval blacksmiths make carbon steel, and add carbon into the steel?

This was usually not done by the blacksmith, but when the iron was smelted in the first place, or by a specialist steelmaker in a second step, before the blacksmith buys the iron/steel and makes things with it.

The most common Medieval European method was to make steel during smelting, in a bloomery furnace. The raw materials put into a bloomery furnace are iron ore and charcoal. The charcoal serves 2 or 3 purposes: fuel to provide the energy needed for the chemical reactions and to raise the temperature to the point where these reactions will happen quickly, carbon to bond with the oxygen in the iron ore so as to convert the iron oxide to metallic iron (i.e., the key process in smelting), and if making steel, carbon that will diffuse/dissolve into the iron to produce steel.

• If the temperature in the furnace is only just high enough for smelting to take place, then very little carbon will diffuse into the iron, and the product will be (low carbon) iron. Since the temperature is less than the melting point of the iron, the smelting takes place in the solid state (rather than liquid), and the iron will be in the form of a spongy lump called a bloom, which will need to be hammered to consolidate it and expel the slag.

• If the temperature is higher, a significant amount of carbon will diffuse into the iron, producing bloomery steel. The product will still be a bloom. Since the temperature varies within the furnace, the carbon content of the bloom will vary, from very low to potentially well above 1%.

• If the temperatures are even higher, enough carbon can dissolve in the iron to produce cast iron, with 3-4% carbon. The melting point of cast iron is lower than that of iron and steel. As more carbon diffuses into the steel, the melting point keeps getting lower. When the melting point is reached, carbon can rapidly dissolve in the steel, producing cast iron. Since the melting point is further reduced as carbon dissolves in the molten metal, it stays molten. This was often undesirable, since cast iron is too brittle for many uses, (and simplifying a bit) can't be forged. It was often considered an unusable waste product.

The efficiency of direct steelmaking during smelting can be improved by raising the temperature by blowing air into the furnace (with bellows) - more oxygen = more combustion = higher temperature. This is the basis of the blast furnace. Blast furnaces like this were used in Medieval Europe to produce large blooms with relatively high yields of steel. This also allows temperatures sufficient for the production of cast iron, and cast iron was deliberately produced in late Medieval furnaces in Europe, and various items cast from it.

Cast iron can be used for steelmaking. Steel is iron with a significant amount of carbon, but below 2% (steels with carbon contents between 1% and 2% are often described as ultra-high carbon steels (UHC steels)). Above 2%, you have cast iron (3-4% is the carbon content of the saturated solution of carbon in iron formed during smelting, so cast iron from the smelting furnace is usually over 3% carbon). Thus, if carbon can be removed from cast iron, one can make steel. To remove carbon, one can:

• Keep the cast iron produced during smelting molten, and the oxygen in air can bond with the carbon, reducing the carbon content. This is the basis of the modern Bessemer process, where air is blown through the molten cast iron. Similar processes, on a smaller scale, were being used in Medieval China. Rather than air being blown through the molten cast iron, it can be blown over it, with the iron being stirred to keep high carbon content cast iron in contact with the air. This is the process used in a finery, which was used in late Medieval Europe (15th century), and in China from the Han Dynasty.

Cast iron has too much carbon to be steel. Low-carbon iron has two little. Can the two be mixed? If both are placed in a closed crucible (perhaps a 50/50 mix of them), the crucible can be heated, and kept hot enough so that the carbon will diffuse from the cast iron into the low-carbon iron, producing steel. Typically, this is done for many such crucible at once. This is the crucible steel process, and was used in much of Medieval Central Asia and India, producing crucible steels often known as wootz, pulad, and bulat.

Typically, a blacksmith or a specialist armourer would buy steel made by one of the above processes.

However, there are two methods which a blacksmith might use:

1. Case-carburisation of iron. If iron is kept in a reducing high-carbon atmosphere (e.g., a low-oxygen region in a charcoal fire), carbon will diffuse into the iron, producing a steel skin. The skin is thin, but it's possible to hammer one's initial chunk of iron flat, and cut it into thin strips, and carburise these, and then weld them back together.

2. The smith can also use a small-scale version of the crucible steel process, by wrapping some wrought iron (or low-carbon steel) around cast iron pieces, and forge-welding the lot together, and folding, welding, folding, etc. until it is sufficiently homogenised.

Has modern methods changed much since medieval times?

One one level, the modern methods can be considered developments of Medieval/Ancient methods such as the finery. Variants of the Central Asian crucible steel method were also used in the Industrial Revolution.

While there is a lot in common, much has changed. There is less use of muscle-power, and wind/water power; steam took over much of the work in the Industrial Revolution, and more recently, electricity. The modern industry also sees the use of electric arc furnaces for melting iron. Today, we understand the chemistry involved, and have studied the various processes, and can control processes such as heat treatment of steel much better (we now have thermometers and thermostatically-controlled ovens). We can analyse samples of steel, e.g., by using mass spectrometers to determine their composition, polishing, etching, and microscopy to view their crystal structure, and procedures such as tensile testing, hardness testing, etc. to measure their physical properties.

Furthermore, how much more effective was carbon steel armour over iron armour, and iron armour over bronze armour?

Different irons and steels are compared in Alan Williams, The Knight and the Blast Furnace, Brill, 2003:

Type	Effectiveness
Iron munition armour	0.5
Low-carbon steel	0.75
Modern mild steel	1
Medium-carbon steel, unhardened	1.1
Medium-carbon steel, hardened	1.5

The number means how much more or less energy is required for an arrow to penetrate the armour, compared to mild steel sheet of the same thickness. That is, if an arrow requires 100J to penetrate a given thickness of mild steel, the same arrow would require 150J to penetrate hardened steel. The low carbon iron/steel performs worse than modern mild steel since it has a higher slag content and correspondingly lower toughness.

For iron vs bronze: bronze will typically outperform iron for the same thickness. The properties of bronze vary from alloy to alloy, depending on composition, but tin bronzes of about 10-12% tin would provide close to the protection of modern mild steel. Hardness and tensile strength are typically about 10-20% lower than modern mild steel, so a reasonable estimate is about 20% less protective for a given thickness, or about 30% less protective for a given weight per unit area (since bronze is denser than iron/steel). Adding bronze to Williams' table:

Type	Effectiveness
Iron munition armour	0.5
Bronze, by weight	0.7
Low-carbon steel	0.75
Bronze, by thickness	0.8
Modern mild steel	1
Medium-carbon steel, unhardened	1.1
Medium-carbon steel, hardened	1.5

Bronze has better corrosion resistance than iron. The main advantage of iron and steel for armour is lower cost (Iron Age and later).