torsdag den 27. oktober 2011

Experiments with iron smelting, Lejre

Experiments with iron smelting
In Denmark several experiments have been conducted with the extraction of iron from bog ore. The smeltings have been carried out in copies of Espevej furnaces, Drengsted furnaces, slag tap furnaces and pit furnaces. Many experiments have been conducted by scouts, evening school classes or staff from the country’s many ‘experience centres’, where iron smelting and forging are activities that attract large numbers of visitors. A typical Danish iron smelting experiment goes as follows: the furnace is built of clay tempered with straw, sand or gravel. The solid bog ore is obtained from West Jutland and contains between 60% and 75% iron oxide and, like most Danish bog ores, a considerable amount of phosphorus oxide. First the ore is roasted over an open fire so that it undergoes a number of physical and chemical changes which benefit the subsequent reduction process. The water evaporates and the organic components are charred. This increases the porosity. Roasted bog ore is dark reddish-brown, porous, light and magnetic. At the same time the furnace is pre-heated. Then it is filled with charcoal and bog ore and the temperature is increased by artificial or natural blowing-in of air. Once the temperature has risen to close to 1200oC, the slag begins to flow. The furnace is filled repeatedly with charcoal and roasted bog ore – often in the weight ratio 1½:1 or 1:1. The process lasts between 10 and 30 hours depending on the furnace type and progress.

Iron sponge
Several attempts have been made to give an objective assessment of the stages of the iron processing. One useful method is to measure the density of the pieces. The unprocessed iron sponge (4.5- 5.5 g/cm3) has a lower density than the slightly processed iron sponge (5.1-5.7 g/cm3) – which is correspondingly lower than the density of blocks (5.5-6.6 g/cm3) and bars (6.5-7.5 g/cm3) and substantially lower than in the finished iron implements (around 7.5 g/cm3).

Iron blooms
The only furnaces from which we in Denmark know the result of a reduction are the small, late slag tap or pit furnaces. One has to imagine that the red-hot iron sponge was taken directly from the furnace, laid on a stone and pressed lightly together so that in shape it resembled a large, round cheese; then it was raised on edge and split from one to three times with an axe. From Denmark we know of two whole blooms split in four, as well as ‘fingers’ of at least four more blooms. They are all stray finds.

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