Over the centuries, enormous slag heaps grew up around the smelting houses. In the 18th century, we adopted the practice prevalent in other countries, namely, when the slag was tapped from the furnace, it would be poured straight into moulds to make building bricks. Many attractive buildings constructed using slagbricks can still be seen in Bergslagen.
A less expensive method was to use the slag in its natural state, in a crust or as cinders, or to simply build the walls with mortar but to use the slag as a filler. These building methods were widely used in Bergslagen, even as late as into the 20th century. We need to remember that bricks cost much more in those days. Slag has also been used to fill wall cavities, and as thermal insulation in buildings—in that case, it would be porous, granulated slag tapped from the furnace into water.
Building using slagbricks continued up to the beginning of the 20th century. But as blast-furnace technology advanced, the slag became an unsuitable building material.