The iron of the early Iron Age

The iron of the early Iron Age could not be melted and cast. Pure iron melts at 1535°C (2795°F), far too high for the first iron foundries. Early blacksmiths took advantage of iron ores that smelt into iron at moderately low temperatures without melting. The result was a mass oddly called a bloom from an old English word for “lump of metal.” But nonmetals from ore are still mixed with iron in the bloom after the reduction. The impurities have to be worked out of the bloom by pounding until wrought iron is free of most of them (wrought is an old past participle of work) . While pounding, the iron can be shaped into useful forms, such as blades or shields, but since wrought iron is never melted, the shapes must be simple. An iron pot is possible but difficult, and such a pot is about as far as one would want to go in shaping wrought iron.

Ironworkers in China

About 300 BCE ironworkers in China discovered that burning iron ore mixed with charcoal produces a thick metallic liquid instead of a bloom. We now know that carbon from the charcoal mixes with the iron to produce an iron alloy with a melting point that can be as low as 1130°C (2066°F). This temperature is exceeded by the hot-burning charcoal. The hot liquid can be poured into a mold where it cools into hard, durable (but brittle) cast iron.

Cast iron is not really iron, and not all iron that is melted and cast is cast iron

Cast iron is not really iron, and not all iron that is melted and cast is cast iron. In today’s usage, cast iron is one of a number of alloys of iron with carbon (and less importantly silicon, manganese, and other elements). Steel is also partly iron combined with carbon, but the carbon in steel has entered
into chemical combination with iron. In cast iron there is more carbon than in steel –– in general, more than one part in fifty –– and the carbon exists in flakes or other forms embedded in the iron. When the metal ancients knew as iron became completely melted, it could be cast, hence the name
cast iron.

Produce cast iron with different qualities

Although they did not know that the new method worked largely because charcoal is almost pure carbon, the Chinese quickly appreciated the advantages of cast iron over wrought iron. They also soon learned to vary the recipe to produce cast iron with different qualities, such as forms that were less brittle when cooled. The ability to cast the new alloy meant that all of the techniques previously developed for casting bronze or gold could be adapted. Complex shapes could be made in one operation. With wrought iron much working was required for even simple shapes.

An important part of Chinese life

Cast iron became an important part of Chinese life and public works. Even before cast iron became available in Europe, Chinese production was reaching levels of over 150,000 tons a year.