Smelting Iron the Viking way is a remarkable simple process. It only takes some charcoal, some bog iron ore, roasted and crushed, plus a simple clay oven.
Bog ore may be found where streams flow from mountains and into peaty land. The streams carry the iron, which is then acidified because of the low level of oxygen in the bog. Bog ore is typically identified through the presence of an iridescent oily film floating in the bog. The bog ore may then be harvested in the form of pea sized nodules of bog iron, the raw material which may then be melted down.
Extracting iron from bog ore is a traditional summer activity, which may be experienced at any decent Viking museum in Scandinavia.
By experimenting, the have found that they are able inside six to eight hours to extract 4.5 – 5.5 kg bog iron from app. 35 kg roasted and crushed bog ore. The bog iron might then be forged. In practice you need a furnace, which may be constructed of clay or stones, lined with a mixture made of sand, fiber (horse manure) clay, and water.
The furnaces might be built into the earth. The construction of such furnaces have been based on finds from archaeological excavations from the Iron Age. They usually measure app. 20 – 30 cm in diameter and a height of 80 – 90 cm. They are dug 40 – 50 cm into the ground.
One of the challenges is of course to get the 1200 – 1300o celcius, needed to smelt the iron from the bog ore. This is done by continually feeding the oven from the top with charcoal. This implies a process whereby any Vikings would need to first burn a cartload of charcoal, which takes about five days.
When completed, the oven is turned over and the bloom is extracted. This consist of a mixture of low-carbon iron, slag and charcoal. This bloom has to be processed afterwards in order to cleanse it for residue. This is done by hammering at it for a while until the dross and slags falls away.
Beating the bloom at Ribe Viking Centre