Blast furnaces in the Bergslagen region were first used sometime at the beginning of the 13th century, which was the period during which miners started to extract the iron from rock ore, instead of from bog or lake ore. The new blast furnaces soon replaced the old pit furnaces.
The blast furnace had a tall structure, with a central vertical shaft several metres in length and having a diameter at its widest point of 1–2 metres. The shaft was lined using a selected natural stone of clay mortar, reinforced with a further encircling wall of heavier stone and, finally, timber boxing. The area between the lining and the timber boxing was sealed using a fine-grained material—often sand and soil. Two vaults or passages, running in towards the centre, were created at the bottom: one, the tuyère, for blowing in air by means of water-driven bellows, and the other for tapping the molten iron and slag at regular intervals.
For practical reasons, blast furnaces were built alongside a fast-flowing watercourse, on which a water-wheel could be installed to drive the bellows. Ideally, the site would also be on the side of a hill, as the ore and charcoal could then readily be carted to the top of the furnace from the storage site on the level above. This type of blast furnace was built and in use up until the mid-19th century.
The remains of the ancient medieval smelting works are today no more than slag heaps and mounds. However, they could prove to be fruitful sites for archeological digs.
Professional ironworkers, know as the Walloons (French-speaking Belgians), came to Sweden during the 17th century onwards, and introduced a new construction for blast furnaces, one without timber boxing. In the new design, the carefully sealed foundation wall of dressed stone continued all the way up, secured by iron ties. These were known as French-type or stone furnaces.
Various improvements, brought on by increasing competition, took place in blast-furnace engineering in the mid-19th century. The main areas concerned were fuel economy and output. In the past, the blast furnace was open to the skies and the waste gases were burnt off in open flares. So the first job was to fit a cover over the top, which meant that the unburnt gases could be diverted to other heating needs.
For instance, the gases could help to heat a hot blast stove, which was used to preheat (by some 200°C) the air blown in through the nozzle at the bottom of the shaft. Another option was to burn the gas in a separate roasting house adjacent to the blast furnace. In the past, the ore was roasted using large wood fires in open furnaces, a method that was both slow and imprecise. In later years, it became possible to use the recycled gas to fuel the boiler in a steam-driven blower machine when the capacity of the water-wheel was inadequate.
The blast furnaces also grew taller and wider, creating a demand for a greater volume of air and a higher blast pressure. Examples of these later smelting houses and roasting houses are the Engelsberg and Flatenberg smelteries, which are just outside Smedjebacken.
Towards the end of the 19th century, the benefits of overseas techniques were at last adopted in Sweden and special refractory bricks, instead of natural stone, were used to line the furnaces. The timber boxing disappeared, the heavy shaft was retained by means of iron strapping, and the entire assembly was made freestanding, supported on pillars in an external framework—a “Scottish” type furnace. One such blast furnace can be seen at Klenshyttan, which lies between Ludvika and Grängesberg.
Gradually, the production of iron became increasingly concentrated, with fewer but larger blast furnaces located close to the new steelworks, with the charcoal fuel being replaced with coke produced from imported coal. Today, there are just two active blast furnaces in Sweden, one at Luleå and the other at Oxelösund.
A single day’s output of iron at a modern steelworks is more than would have been produced in an entire year in a 17th-century smelting house!