Primitive Iron production in "Blästan”.
Transcript from photo.

Historically and pre historical Sweden has produced iron after two methods, They are called direct and indirect iron production. The difference between these main methods are that the production first is direct make forgeable iron with low carbon content (<0,2-0.3%C) or instead first pig iron with high carbon content  (3,5-4,5C) and then make it forgeable with different methods by reduction the carbon content.

The indirect method is the which has been used since early Middle Ages up to modern time been used to first produce pig iron with the help of a furnace and then with different methods get the iron forgeable by lowering the carbon content.
The direct production of forgeable iron has been done since ancient times to produce forgeable iron by using blower ovens in different constructions over whole world, but the principle has been the same - in a cavity or a low shaft - warm up the ore together with wood or coal and raising the temperature with the help of blowers to initiate  a reduction of the ore changing to iron.

The raw material for these blower ovens have usually been different types of limonite ores. It might be lake or bog ores, but also red ground has for certain parts of Sweden been a significant raw material. The ore has been dig out during summer from the bogs, while the lake ore has been brought up from the bottoms of lakes during winter times.

Before this ore could be treated in the blaster oven it had to be rusted. This was done by heating the ore during several hours. The goal was by reduction the chemical bound water and reduce the for sea ore harmful sulfur in the ore.