Iron Smelting

When forging historical tools and weapons I use iron and steel made with the exact same
materials, tools, and techniques as in the iron age and viking era. The most important raw material is bog iron ore, which I dig out of the ground in my local area. The ore is smelted into iron in a clay furnace using charcoal as fuel. The result is a very unique metal that looks, feels and reacts very differently from modern steel.

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Bog iron ore from western Jutland. The bog from which it was mined is seen in the background.

 

What is bog iron ore?

Bog iron ore (in danish: myremalm) is very abundant in my home region, western Jutland. It is usually found as large, hard blocks or as a spaghnum rich, red earth in the bogs, fields, and meadows of the area. The colour can range between bright yellow, reddish brown, and coke grey, depending on its composition.

Bog iron ore is formed where iron-bearing ground water reaches the surface. The  dissolved iron compounds oxidize when they get in contact with the atmosphere, and settles as ochre in the top soil. After decades or centuries of accumulation, a thick layer of iron ore is formed.

 

How is iron extracted from ore?

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Yellow hot bloom beeing extracted from the iron smelting furnace.

First, the ore is roasted on a fire and crushed into powder or granulate. Carefully measured amounts of ore and charcoal are then poured into the smelting furnace, which is running for up to eight hours. The temperature of the furnace is kept very high by blowing in air with a pair of bellows. During the smelt, an iron bloom is formed at the bottom of the furnace. Wether this bloom consists of soft iron, carbon steel, or phosphoric iron depends entirely on the ore and how the smelt is performed. The impurities of the ore combines with iron oxide and forms a slag that melts and flows down into the furnace bottom. The iron or steel never becomes liquid.

When the bloom has reached the right size, it is pulled out of the furnace with a pair of tongs and hammered on a stone anvil. This is done in order to compact the metal and drive out the remaining slag while it is still liquid. Finally, the bloom is cut up into smaller pieces of a few kilograms each and forged out to bars that can be worked in the smithy.

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Why is iron smelted from bog ore so different from modern steel?

The main reason is that the metal never gets molten at any stage in the process from raw ore to finished blade. Thus it keeps the unique structure it was given during its formation in the clay furnace. The raw bloom is usually stretched, folded, and forge-welded a number of times to homogenise the material and drive out remaining slag. In this process, the grains of the metal are stretched out, forming the characteristic pattern often seen in very old iron, like pre-industrial anchor chains and fixing ties in medieval buildings. This structure makes every single bar of smelted iron unique, like a block of marble or wood.

Another important reason is the presence of phosphorus in the metal. It comes from the bog ores which are usually rich in this element. In the right amount, phosphorus has several positive effects on iron. It makes the iron hard like steel, and it gives the metal a bright silver lustre only seen in phosphoric iron. Phosphorus also has a strong ability to prevent iron from rusting, and even from corroding in sea water. Today phosphorus is considered a harmful impurity, and only very small amounts are allowed in commercial steels. In the iron age and viking era the blacksmiths knew how to take advantage of the hardness and shine of phosphoric iron, using it deliberately together with soft iron and carbon steel to make unique tools and weapons.

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