Charcoal Iron

With the introduction of blast furnaces in the 15th century the availability of wrought iron increased. The earliest wrought iron is known as 'charcoal iron', a highly carburised form of iron which was made by constant reworking in the fire.

Early processes of obtaining iron from its ore (iron oxide with various impurities) required heating the ore with a substance that had a high carbon content, such as charcoal, in small furnaces called 'bloomeries'. As temperatures within the furnace increased, the ore began to release oxygen and the charcoal released carbon. These two elements then combined to form carbon monoxide. The charcoal reduced the iron oxide to iron, giving off carbon monoxide in the process, leaving behind a spongy mass of iron. If the temperature was allowed to rise, this mass would absorb small amounts of carbon, forming wrought iron.

After smelting, the 'blooms' of iron were forged by heating and then beating them out into long bars. After that, each bar was cut into shorter length, bundled together and reforged. The process was repeated many times. With each forging, more of the slag was removed, and the fine residues left behind became integrally incorporated with the iron, together forming the unique fibrous microstructure which gives wrought iron its tensile strength.

Charcoal iron contained impurities, mostly in the form of silica or carbides. Iron silicate is it's predominant impurity. The silicate gave the charcoal iron more workability. This allowed the blacksmiths to create ironwork which just cannot be accomplished with modern iron and steel.

Charcoal iron sheet is soft and ductile, when tempered, so that a good depth of cold working and sharp detail is possible without cracking and it can withstand corrosion for hundreds of years.