Clinker in our walls

The beautiful clinker bricks are a typical feature of the Bergslagen region. They are very popular and much appreciated for their special, slightly scruffy appearance. They have a rough and suggestive form, and the bluish, glassy surface and multifaceted patina attracts attention. Many people know that they are a type of building material and use them for such things as garden decoration but few probably know that clinker bricks are in fact slag that is a waste product when making iron.

The slag from the blast furnaces was often used in the past as a building material, especially in Bergslagen. Before it had had time to solidify, it was cast in block form. These were for the most part slightly larger than a brick. Both houses and industrial buildings constructed from the clinker bricks such as Verket can be seen today near the old iron works. Since the slag was otherwise relatively worthless, it was discarded something which slag piles stand witness to in several mining districts and the like throughout Sweden.

The chemical process!

Slag is a by-product formed during various types of metallurgical reactions. It mainly consists of the oxides of unwanted substances and metals, and metals with a level of impurities that is far too high. It is separated as blast furnace slag for example. Slag is also formed during the oxidation of metals in the heating and smelting processes. When molten metal solidifies alloy substances (e.g. aluminium) with a high affinity for oxygen separate as oxides and form slag inclusions in the solidified metal. The sintered lumps of incombustible materials and ash that are formed during combustion of solid fuels like coal and coke are also called slag.

Slag is not just a substance that has to be separated and removed in many metallurgical processes; the reaction between slag and metals is used during sulphur and phosphorus removal. An oxidising slag can be used to remove coal from a carbonaceous smelt, so-called pig boiling. The solidified slag is normally vitreous in appearance and can be used as a raw material in other industries such as the production of cement, construction elements and mineral wool as well as for road aggregate. Slag from basic steel production contains phosphates and is used to make fertiliser.

Foto: Therese Asplund
Foto: Therese Asplund