Externally the blast furnace looked like a short square chimney, standing straight on the ground. It was built of brick or stone—whichever happened to be available on the particular site—and internally it had a lining of bricks or stones chosen for their ability to resist fire. The furnace, at 3–5m (10–16ft) tall, was much bigger than anything used previously for ironmaking, though still tiny by today’s standards.
The blast furnace brought several changes, technical, economic and social. Technically it introduced a new product, cast iron, an alloy of iron and carbon which, unlike wrought iron, is quite easily melted. When molten it will flow into a cavity where it solidifies to produce a faithful copy of the mould. It can, in short, be cast—hence the name—and moulds can be made in sand or certain other materials to produce simple or complicated castings as required. Cast iron is very different from wrought iron. It is strong in compression—that is, it will support heavy loads resting on it—but it is comparatively weak in tension— it will not carry heavy loads suspended from it. In addition it is relatively brittle and cannot be forged or shaped by hammering, so its uses were limited in comparison with wrought iron. But cast iron could be made in much larger quantities in the blast furnace, and it can be converted into wrought iron by a second process, so the needs of the market were met.
A blast furnace, like a bloomery, needs a continuous blast of air to keep the fire burning, but its greater size demanded mechanical power to work the bellows. This meant, at the time, a waterwheel, and blast furnaces were built alongside streams where water was available. Nature does not always put streams and rivers close to deposits of iron ore—and blast furnaces needed both, plus forests to provide timber for making charcoal. One district in Britain which had all these requirements was that part of Surrey and Sussex known as the Weald, and it was also close to an important market, London. The iron trade became important there, and many signs of its former importance survive in place names like Furnace Mill or Farm, Forge Wood, Minepit Field and Hammer Brook. Several of the old waterwheel ponds remain, some of them now used for commercial watercress growing.
Economically the introduction of the blast furnace meant that ironmaking took the first real steps towards becoming an industry as distinct from a craft. It also brought about a change in the organization of the trade. A bloomery worker needed little more than his skilled knowledge: everything else he required to work his furnace he made himself. Stonemasons and bricklayers were needed to build and maintain a blast furnace; millwrights were necessary to make the waterwheels and keep them in repair, and numbers of other specialized workers were also required. All this called for a new type of organization, and investment. Some landowners were able to finance the building of blast furnaces themselves; otherwise groups of men formed partnerships, sharing the funding and the profits. Partnerships in business were not new, but they were novel in the iron trade.
The blast furnace also brought about social change. It had to work continuously, twenty-four hours a day, seven days a week, and the workers had to be organized accordingly. Two teams of men, each working twelve hours, were needed to operate the furnace, so shift working, now general in many industries became common. These men would have to adjust their home lives to a programme which meant that sometimes they worked all night and at other times all day.
The blast furnace was in some respects like a bloomery (though much bigger) and it still used charcoal as its fuel. The major difference was that, because the furnace operated at a higher temperature and the ratio of charcoal to ore was greater, the iron absorbed a greater amount of carbon; therefore it produced, instead of a spongy piece of wrought iron ready for forging, molten cast iron. This was allowed to accumulate in the bottom (or hearth) of the furnace and taken out, or tapped every twelve hours or so. The molten iron was allowed to run into channels moulded in a bed of sand, where it solidified. To produce pieces a man could lift, a main channel was made, with others branching off it at right angles and from these a number of short, dead-ended channels branched off again, looking from above not unlike a number of combs. The side channels also looked, some people thought, like a litter of pigs lying alongside a sow: pig iron is now made by machine, but the individual pieces are still called pigs.
As the charcoal and iron ore were used up in the furnace, more were tipped in at the top. The earthy materials and other rubbish mixed with the iron ore also melted and, being lighter than the molten iron, floated on top of it; they were also run off at intervals. Some limestone was also charged into the furnace, along with the iron ore and charcoal, to act as a flux, that is, to combine with the waste materials and help to form a molten waste called slag. At first, and for very many years, slag had no real use—except perhaps to fill up holes in the ground—so it was tipped in heaps and forgotten. Old furnace sites could often be traced by slag heaps or the remains of them, but this is becoming more difficult as the heaps are bulldozed away to tidy up the area, or for use as hard core in road or motorway construction. Slag was also left by bloomeries and some slag heaps are known to result from Roman or even earlier ironworking. The presence of the right kind of slag will always indicate that there has been ironworking of some kind nearby, but interpretation of slag heaps calls for expertise. Other metals besides iron produced slag, and some so-called ‘slag’ heaps are not of slag at all—colliery waste heaps are an example.
The blast furnace spread gradually; there was no dramatic change, and a number of bloomeries still remained in use; some survived into living memory in remote areas of Africa and Asia. A few uses were found for cast iron as it came from the blast furnace—cast iron cannons were being made in Sussex by 1543, and decorative firebacks for domestic fireplaces are among the oldest existing forms of iron castings—but most of the iron made in blast furnaces went through a second process for conversion into wrought iron. This apparent duplication of effort was in the interests of increased production: even in the early days of the blast furnace it would make as much iron in a day as a bloomery could make in weeks. Conversion of the cast iron from the blast furnace into wrought iron was done in a furnace called a finery, which also used charcoal for fuel and was blown by waterwheel-driven bellows. Here the solid pieces of cast iron were remelted in the fire and the carbon, which had combined with the iron in the blast furnace, was driven off, leaving wrought iron. Because this iron was made in two stages instead of one as in the bloomery, the name indirect reduction is given to the process.
Wroght Iron: The Prehistoric Era to AD 1500
