blister steel

Blister steel is a crude form of steel that was historically produced by carburizing wrought iron. This process involved heating iron with charcoal in sealed clay pots within kilns. Carbon from the charcoal would diffuse into the iron, increasing its carbon content to convert it into steel.12

During reheating, internal gas formation caused blisters to appear on the steel's surface, giving it the name "blister steel". This method was labor-intensive, requiring multiple cycles of carburizing and forging, and was used to produce items like swords. While its production declined with the advent of cheaper cast iron and more efficient steelmaking processes, blister steel was made on a small scale into the 20th century.2