Iron from red earth & lake ore
Two-thousand years ago, at about the time of the birth of Christ, iron was being produced from fine-grain red earth or bog ore in Sweden. The red earth was collected and smelted in pits, a metre deep, that were sealed with stones and clay. These furnaces were first dried and preheated by means of intense fire. Next, they were charged with charcoal, on top of which was placed the roasted red earth. Air was blown in by bellows through a hole in the ground. To start with, strong blasts of air were used but the bellows gradually reduced the blast as the red earth was seen to collapse. Many hours’ later, a porous, glowing lump of soft iron, probably weighing about 10 kg, could be extracted from the slag. The lump was then sledge-hammered on a stone into fine malleable iron, and was broken into smaller pieces. Finally, the slag was removed from the furnace and the process started again.
The yield of iron from this process was small, as much of it ran in with the slag and was lost, but the iron extracted was clean and malleable, even though the red earth often contained a large proportion of contaminants. This method of producing soft, malleable iron was practised in many places in Sweden where red earth or bog ore was available. In fact, the same method was still in use in northern Dalarna into the 19th century, where the iron, without further refining, was used by blacksmiths for making tools and implements. Remains of these furnaces are to be found in plenty in Älvdalen and Särna, and in the Ecomuseum at Röda Jorden in Riddarhyttan, and at Dunshammar just outside Ängelsberg.
From a chemical viewpoint, red earth is the same as rust. In some types of rock iron sometimes dissolves in the water percolating through the rock. You can taste the presence of iron in groundwater that contains iron. When the water then rises to the level of the soil, it reacts with the oxygen in the air and the bacteria, changing the chemical nature of the iron. Oxidation causes the iron to convert from divalent to trivalent iron, which becomes stable and is no longer soluble, but forms a rust-coloured sludge. This is a continuous natural process and can be observed in bogs, and along rivers, in the areas where it occurs. After hundreds of years have elapsed, thick layers of red earth are formed, and it was these that our forefathers were on the lookout for, so that they could dig up the earth and extract the iron from it.
“Lake ore”, ie, ore lying on the bed of a lake, has the same composition as bog ore or read earth, but a different structure. In some areas, particularly in the south of the country, the ore precipitates into hard, brownish crusts or grains that collect around pebbles and stones at the bottom of the lake. During the winter, the ore would be raked up through holes in the ice and, in the county of Småland, attempts were even made in the 19th century to smelt the ore in blast furnaces.