From a lump to a stream
A bloomery makes iron as a solid lump. A blast furnace is built to melt the iron. It is a tall shaft, filled from the top with ore, fuel and a flux, usually limestone, while a strong blast of air is blown in near the bottom. The iron takes up a good amount of carbon, which lowers its melting point, so it runs as a liquid to the bottom of the furnace. Furnace workers can tap it out into molds or channels and continue charging materials from the top, which makes the process continuous rather than a batch run.
The product has a high carbon content, typically somewhere around two to four percent or more. The carbon makes it castable but brittle.
How it works
Inside the furnace, hot gases rise as the charge descends:
- Burning fuel at the bottom produces carbon monoxide and intense heat.
- The gas rises through the charge and strips oxygen from the iron ore.
- Limestone combines with impurities in the ore to form a molten slag, which floats on the iron.
- At the bottom, liquid iron and slag collect and are tapped separately.
The high temperature needed depends on a strong air supply, which is why bellows and, later, water-powered blowing machines were so important.
Early beginnings in China
Evidence indicates that Chinese ironworkers were casting iron in the first millennium BCE, centuries before it was common in Europe. They used furnaces with strong bellows, sometimes operated by water power by around the early centuries CE according to written sources. Cast iron was used for tools, cooking vessels and farming equipment such as plow parts, and also for bells and pagodas later. The exact development of the Chinese furnace and the relationship, if any, with Europe’s later furnaces are topics where scholars disagree.
Europe
In Europe, larger furnaces producing liquid iron are generally dated to the late Middle Ages, with evidence from regions such as present-day Belgium, Germany and Sweden from about the fifteenth century, and possibly some earlier. By the sixteenth century, the technology spread in England, notably in the Weald of Kent and Sussex. Cast iron was used for cannon and cannonballs, firebacks, pots and, eventually, many other goods.
Pig iron, the crude casting of the furnace, got its name from the way it was cast into rough bars in a channel with side molds that were said to resemble piglets feeding from a sow.
The fuel problem and coke
Charcoal furnaces consumed large quantities of wood, and charcoal is fragile, which limited how tall a furnace could be. In Britain, concern grew about timber supplies. The turning point is usually associated with Abraham Darby, who in 1709 successfully smelted iron with coke at Coalbrookdale in Shropshire. Coke is coal that has been heated to drive off impurities, and it is strong enough to support a tall column of ore. Darby’s success did not spread overnight, since coke iron suited some uses better than others and different ores and furnaces responded differently, but over the following decades the method became common. The same shift to coke fuel eventually supported the larger industrial scale of iron production.
Later improvements included steam-powered blowing cylinders and, in 1828, James Beaumont Neilson’s hot blast, which heated the air before it entered the furnace and saved fuel. Furnaces grew taller and wider, and gas leaving the top was eventually captured and used to heat the blast.
What cast iron was used for
Cast iron resists compression well but is weak in tension and can crack under sudden shock. Despite that, it was useful for:
- Columns, beams and arches. The Iron Bridge over the River Severn, completed in 1779 at Coalbrookdale, is a famous cast iron structure.
- Pipes for water and gas supply in cities.
- Machine frames, gears and engine parts, including cylinders for steam engines.
- Cookware, stoves and fences, as well as ornate decorative items.
- Rails in early tramways, although brittleness made them less than ideal.
From pig iron to better iron and steel
Because pig iron was brittle, it was often refined for forging. Henry Cort’s puddling process in the 1780s stirred molten pig iron in a reverberatory furnace so that carbon burned off, producing a pasty mass of wrought iron that could be shaped by hammering and rolling. Later, the Bessemer process and its successors converted pig iron to steel on a much larger scale.
Modern blast furnaces
Modern blast furnaces can be several stories tall and produce thousands of tons of iron per day, and they remain central to primary steelmaking, although scrap-based electric arc furnaces now handle a growing share. The basic idea, liquid iron tapped from a continuously charged shaft, is the same one developed centuries ago.