A Treatise on iron ore as found in the bogs and swamps of Norway and the process of turning it into iron and steel, 1790

[Although of an 18th-century date, the processes are basically the same as those used during the Viking Age.]

Where the bog has an influx of water either through the earth or flowing from a stream, where the bog forms a slope, where the bog is covered with grass, heather, trees, shrubs, or where there are several hillocks resembling small earth banks. In these places one will seldom have to search in vain for bog ore.

[...]To find the ore one uses a spit, termed ore-spit. The spit is pushed into the bog, [...] If the spit goes down easily there is no ore, but if it is difficult, ore is present. The ore [...] clings to the spit, which is the reason for it being difficult to turn it. One can judge if the ore is fine or coarse and what colour it has from what ore adheres to the spit.

From the way in which the ore clings, one can judge whether it is satisfactory. There is usually peat and iron pan above the ore, but this is rarely more than 5-6 in thick. All this should first be removed with a fork or a pick and spade [...] That (ore) which is coarse like sand, or gravel mixed with concretions like hens' eggs (round or flat), is good. That with sharp edges is bad. [...]

Once the ore has been collected, the next job is to roast it. [...]One pieces pies of wood, about 10 yards long and 13 inches thick[...] another layer of wood of the same length and thickness. [...] On top of that one places the ore. Between the two layers one puts small pieces of dry cut wood and twigs which lights easily and burns quickly. [...]

[...] When the small pieces of wood have turned into charcoal, one moves the logs a bit with an axe, so that the ore lumps fall down onto the charcoal where it stays until it becomes reddish brown, which shows that its roasting is completed. [...] The ore should be left alone until it is cold. At that point one rakes out the ore from the charcoal, which then should be stored quickly.

[...] After the ore has been roasted it must not become wet or moist and it must be kept clean.[...] This requires a hut where it can be stored, preferably on the spot where one is to build the smelting furnace.[...]