Refining process & iron bar
When you encounter a place name in Bergslagen that ends with the suffix, “hammar”, you know that there was once a forge nearby for making bar—turning pig iron into malleable or wrought iron. Most forges producing bar had ceased working before the end of the 19th century.
Before the reign of king Gustav Vasa, refining was carried out by hand in small furnaces close to the blast furnace. Each batch of iron weighed only a few kilograms and it was usually divided into smaller units in accordance with long-standing customs. These units were known as Osmunds, which were packed in wooden kegs.
During the reign of king Gustav Vasa, the process advanced to making the more profitable wrought or malleable bar iron. Bar was much better suited to the large-scale production of various products. Thus, refining of pig iron also needed to be done on a larger scale, and the operation was therefore moved away from the blast furnace to special bar-iron forges.
The refining of pig iron involved smelting the iron in a furnace—an open charcoal fire through which, on one side, was directed a strong stream of air, and where molten slag floated at the bottom of the furnace. After the amount of carbon in the molten iron started to diminish, owing to the flow of hot air and the influence of the slag, the melting point rose, creating tough lumps in the melt. Under some conditions, the conversion took place at such a high temperature that the melt started to boil. Using a crowbar, the forge worker could bring the lumps together for remixing in the furnace. Once the charge was ready, the result was a large lump of fungus-like semi-molten iron with a low carbon content in the middle of the furnace. The melt was then removed from the furnace and transferred onto an anvil underneath a large water-driven forge hammer, which hammered the iron into a compact form, and drove all the slag out.
The melt was divided into several rectangular blooms. These then had to be reheated in “welding” furnaces for forging into long rods.
In the 16th century, the Crown started to establish forge hammers, but in the 18th and 19th centuries these were often run by the estate owner or foundry proprietor, or by a wholesaler in the town who would install his own foreman on the site. Some forge hammers were owned by homesteaders. All ironworks had to be licensed by the State in a royal charter,and were given a fixed limit on the amount of iron bar that they could export.
At the docks in Stockholm were inspectors who checked the quality of the iron and the ID stamp on the iron for the respective ironworks, and who weighed the load to determine the tax to be paid. This all took place on the southern side of the city, in the area now called Slussen. The iron was loaded onto the quay by sailing barges on the lake Mälaren side of the harbour and was then transferred onto ships tied up alongside the wharf at Skeppsbron. These ships then sailed out into the Baltic and on to various countries.
The export of pig iron was prohibited by law. All the iron had to be refined into bar (the most profitable way to sell the iron) before it would be allowed to leave the country.
Rolling mills were something of a rarity, and could only handle small dimensions. Steel plate was produced by hammering and forming rectangular billets, with plate sizes no bigger than one of today’s tabloid newspapers.
The Swedish iron industry was highly profitable in the 18th century. Swedish iron was of good quality—thanks to the purity of the iron ore—and therefore commanded high prices on the market. The grades of iron produced from the ironworks in the northern Upland district (Öregrund iron) were most sought-after, as this was carefully extracted from Dannemora ore using a process devised by the Walloons from Belgium. Thanks to the presence of manganese in the ore, the iron had a hard, steel-like quality, rendering it ideal for making swords and other weaponry. The same grade of iron was used in Britain for making precision tools and instruments.
If we look at the statistics of ironmaking, it is striking just how small the operations were up to the end of the 18th century. Iron was expensive and all the machinery was therefore made of heavy-duty timber. There were no railways, nor any iron bridges or iron ships. The annual output of a typical ironworks was 10–20 tonnes—the equivalent of half a lorry-load today. A large works might be licensed to produce 500 tonnes a year. Compare that with the output of a typical steelworks today, which is more than a million tonnes a year!
New technology for the large-scale production of simple commercial iron was developed in Britain in the 18th century. This involved the use of coke to fuel the furnaces. Both the export routes and the markets changed during the time of the Napoleonic Wars, and the structure of the Swedish iron industry faced a crisis in the years 1820–1840. Many small, weaker works were forced to close down and only those capable of investing in the new technology were able to survive.
This was also when the Lancashire furnace was introduced. In these new furnaces, a batch of iron ore could be smelted in just 45 minutes.
British-designed, water-driven mumbling hammers were installed to forge the lumps of iron into blooms. Special welding furnaces for reheating the blooms were built, and instead of hammering the iron into bar, high-capacity rolling mills that could produce smooth and even top-grade bar were installed.
Examples of Lancashire furnaces, together with smelting houses and forges in varying condition, can be seen at several of the Ecomuseum sites: the National Heritage Site at Engelsberg; and at Ramnäs, Karmansbo, and Trångfors. A very old rolling mill has been preserved at the Surahammar Ironworks museum, and more-recent machines can be viewed at Ramnäs and Karmansbo.
The next technological advance came soon afterwards, in 1870: the cast-steel process. With the use of refractory linings (capable of withstanding higher temperatures) in the furnaces, the pig iron could be produced in molten form and refined in homogeneous quantities of ten or more tonnes per charge. These were the Bessemer and Martin processes.
Much later, electric arc furnaces were introduced. This brought about an astronomical rise in the world production of steel and in an increasing number of countries. Sweden’s share of the world market declined steadily, despite its rising output. It was only a few decades ago that the output of steel in the industrialized world levelled out and started to fall.
The production of cast steel sounded the death knell for more and more small Swedish ironworks that still used the older methods. And more pain was to come in the economic crisis of the 1920s, which was largely attributable to the end of the First World War. All the remaining charcoal-fired smelting houses in west Bergslagen disappeared, all of them razed to the ground. There is just one exception: the ruin of the Klenshyttan smelting house.
After further amalgamation in the 1960s and 1970s, only a few steelworks now remain in Sweden, and these produce only highly specialized, high-grade, world class products.