Forsøg på rekonstruktion af en fortidig jernudvindingsproces

Resumé

Trial reconstruction of an early process of iron extraction

In KUML 1962 were presented the results of certain new investigations of prehistoric iron extraction plants 1) which have made it possible to put forward a new basis for reconstruction of the plants which were used for iron smelting in the first centuries A. D. It has unfortunately not been possible to demonstrate by excavation every detail in the proposed reconstruction, as, for example the height of the shaft and details in the construction of the connection between the shaft and the slagpit.

All the remains of furnaces hitherto investigated have been more or less altered by the actual process of the smelting, and we are therefore without evidence of the appearance of the furnace at the start of the process. Only a fortunate combination of circumstances could allow us to discover such evidence, but it should be possible to gain a degree of knowledge if the smelting process could be successfully imitated in a reconstructed furnace. The chief importance of such successful experiments would, however, be to provide a reliable basis for calculations of production and consumption.

Experiments in smelting were begun at Drengsted in the autumn of 1963 by the Aarhus University Institute of Prehistoric Archeology and Ethnography and were later carried further at the Varde Steelworks Limited. The following is an account of the experiments at the latter place.

Considerations before the start of the experiments

To convert iron ore into iron is a very easy task. All that is needed is a suitable temperature and a suitable reducing agent, that is, a substance which is able to remove the oxygen which is in chemical combination with the iron of the ore. One of these reducing agents is the gas carbon monoxide.

The chemical process in the reduction of iron ore may in simplified form be written

FeO +CO à Fe+ C02.

The reaction begins to take place at about 650° C, and speeds up at higher temperatures.

Now, as luck will have it, burning charcoal produces high temperatures at the same time as the smoke from the charcoal has a suitable carbon monoxide content. There will also be nitrogen, carbon dioxide and perhaps oxygen in the smoke. Nitrogen takes no part in the chemical reaction, but carbon dioxide and oxygen hinder the process, as these gases cannot remove oxygen from the ore, but on the contrary can oxidize already smelted iron.

The quantity of carbon monoxide which will be produced in the course of combustion depends on many factors, including the speed of the draught through the furnace, the height of the charcoal layer and the size of the pieces of charcoal.

A good reduction may be expected if there is four times as much carbon monoxide as carbon dioxide in the smoke, and the whole course of the smelting depends upon whether it is possible to check and to regulate the mixture of gases. We have checked the composition of the smoke gas by taking analyses, but there is little doubt that with some experience it would be possible to estimate its composition by watching the flame at the top of the furnace.

One might very well feel content with having carried out this first and most important part of the process. The lumps of slaggy spongy iron which are the result of the reduction process can with care be forged. In the process a lot of the impurities are hammered out of the iron, and the resultant small pieces of iron can thereafter be forged together into larger pieces.

The second part of the process consists of the small pieces of iron sintering together and of the slag being melted out.

For the slag to separate out from the iron it must be melted to a liquid state. Experiments with the Iron-Age slag from Drengsted 2) show that the slag first became sufficiently liquid at 1200-1300 °C. In a reducing atmosphere, i. e. an atmosphere containing sufficient carbon monoxide, the slag becomes liquid at 1100-1200° C.

As the specific gravity of iron is almost double that of the slag it could be expected that the iron sank to the bottom and collected below the lump of slag. No iron, however, was found under the lumps of slag which have been excavated 1), and the iron must have collected somewhere else in the furnace, the slag then dripping from it, just as water will drip from a sponge hung up on a string.

These preliminary considerations can be summarized into the following experimental conditions.

In the upper part of the furnace there must be a temperature of above 650 °C and an atmosphere containing four times as much carbon monoxide as carbon dioxide.

In the lower part of the furnace the temperature must be 1100-1200° C. Here too the highest possible carbon monoxide content must be aimed at; not only because the slag is more liquid in a reducing atmosphere, but also on account of the risk of oxidizing iron already formed if the carbon monoxide content it too low.

The conduct of the experiments

In the autum of 1963 three experiments were carried out, each with two furnaces of exactly the same construction as those used in the experiments in Drengsted 3). The one furnace was equipped with four thermocouples for measurement of temperature, and with four vents for gas sampling for analysis. The other furnace had no instruments but was treated exactly the same as the first. Fig. 2 shows the placing of the thermocouples. Samples of the smoke gas were drawn off besides the four measuring points for temperature. Analysis of the gas is very easy to carry out. 100 cu.cms. of the smoke gas is measured off in a pipette. The gas is then passed through a liquid which can absorb carbon dioxide. The gas, now free of carbon dioxide, is led back and measured. If 97 cu.cms. of the gas remains, then there has been 3 % carbon dioxide. These 97 cu.cms. of gas are thereafter passed through other solutions which can absorb respectively oxygen and carbon monoxide, and the percentage content of these gases is determined in the same way as with the carbon dioxide.

In the first series of experiments charcoal of pinewood was used, and the highest temperature measured was about 1000 ° C. In the second experiment an attempt was made to raise the temperature by using charcoal of hardwood, and temperatures up to 1160° C were measured.

In the third experiment hardwood charcoal was also used, and the temperature was further raised by increasing the draught. The draught is most easily regulated by a change in the degree of filling the furnace. In the last experiment the upper third of the furnace was kept free of charcoal and ore. As a result of this the air resistance was less, and the empty portion of the furnace acted as a chimney. Temperatures up to 1330 °C were measured; but this temperature was reached at the expense of the carbon monoxide content of the smoke gas. And the final result showed that a considerable part of the iron was in fact burnt away.

As only minor alterations were made in the carrying out of the last two experiments it will be sufficient to describe the first of these.

The furnace was stoked with charcoal until, in the course of 24 hours, the temperature had reached 860 °C in the lower half of the furnace. Ore and charcoal were then added in the proportion of ½ kilo ore to 1 kilo charcoal. The ore had been heated red-hot first in order to remove water and organic impurities, which had comprised 21 % water and 13 % organic matter. After heating the ore contained 50 % iron, 22 % SiO2 (sand), 0.5 % manganese and 1.3 % phosphorus.

3½ hours after the first charge of ore the temperature had risen over 1000° C, and the slag began to run out of the air holes. The composition of the smoke gases was satisfactory, with 7-9 times as much carbon monoxide as carbon dioxide. After 8 hours each furnace had been charged with a total of 20 kilograms bog ore. The proportion of ore to charcoal in the charge was changed to 1 kilo ore to 1 kilo charcoal.

The experiment was terminated 15 hours after the first ore was inserted, at which time it was no longer possible to keep the air holes free of slag. Each furnace had by then been charged with a total of 50 kilos of ore.

On dismantling the furnaces there was found in the botton a layer of ash and of unconsumed charcoal. Among this there was a number of lumps of sponge-iron of the same shape and size as the bog ore. It can be assumed that they come from the ore last added; they had been reduced in the upper part of the furnace, but the temperature in the lower part has not been high enough to cause the lumps to sinter together.

The straw plug 4), which was to block the hole to the slag pit at the beginning of the experiment, was almost undamaged despite the high temperature, and there was accordingly no slag in the slag pit. On the other hand, just above the slag pit there was a lump of slag, as large as a clenched fist, with the same surface and cleavage appearance as the Iron-Age slags. It was found on the inner surface of the furnace wall close to the air holes. Close to this lump, smelted fast to the underside of the furnace, there was another lump consisting, closest to the furnace wall, of slag, followed by a layer of sponge-iron mixed with charcoal. The surface of the lump closest to the centre of the furnace consisted of compact coarse-crystalline iron. The amount of iron can be roughly estimated as 1.5-2.0 kilograms for each furnace. Fig. 3 shows the position of the iron in the furnace. Figs. 4, 5 and 6 are, respectively, loose lumps of sponge-iron, the lump of iron which was attached to the furnace wall, and the lump of slag.

All three forms of iron, the individual pieces of sponge-iron, the larger fused lumps of sponge-iron, and the coarse-crystalline iron, are capable of being forged. To begin with it is necessary to maintain a very high temperaure during the forging, until a considerable proportion of the slag is hammered away. If the iron is hammered while it is only red-hot or at yellow heat it will split. However, after it has been swaged several times it can be treated as normal iron, and is even easier to forge than modern commercial iron.

Iron with a carbon content as low as this is very soft, and is only suitable for the production of nails and less important implements. That even Iron-Age man understood how to produce hard steel, i. e. iron with a relatively high carbon content, can be seen from the chemical analyses made of prehistoric implements, including objects from weapon hoards. As an example can be quoted three swords found at Nydam, which contained respectively 0.52 %, 0.42 % and 0.52 % carbon.

Such high carbon content can hardly have been achieved in a furnace of the type described here, but has more probably been produced in the course of the subsequent swaging. Here, on the other hand, several different methods can be envisaged. To study these methods in greater detail it would be necessary to carry out metallographic examinations of the whole cross-section of the iron object, which requires the object to be sawn across. The snag with such an investigation lies in an unwillingness on the part of museums to have their Iron-Age specimens sawn in two.

We have, however, been fortunate enough to acquire two specimens of prehistoric iron, the microstructure of which gives very interesting information concerning the carburising process.

The specimens are derived from a board of about 20 iron bars, which were found at Hedeby in February 1964. The specimens of iron were sent to us by Dr. H. Hingst of the Landesamt für Vor- und Frühgeschichte von Schleswig-Holstein, in order that we might determine by chemical and microscopic examination whether the iron was of prehistoric date, which our investigations confirmed. The carbon content in the two specimens was 0.15 % and 0.19 %; but the greater part of the cross-section consisted of completely carbon-free iron with a structure and a slag-type exactly corresponding to the iron which we had produced in our experimental furnaces. On the surface of one of the specimens areas of hard steel were to be found, while the inner core consisted entirely of soft carbon-free iron. On the polished and etched cross-section surface a particle of hard steel about 2 mms. in size can be clearly seen (Fig. 7). The microphotograph (Fig. 8) shows the same particle enlarged 200 times. In the upper left hand corner a white area can be seen, consisting of pure iron, thereafter a slaglike layer which is certainly an oxide scale, and finally the particle of hard steel.

It is necessary to envisage the carburisation process carried out by heating a bar of soft iron to welding temperature together with the particle of hard steel. These particles are then forge-welded onto the surface of the iron. At the present moment it is impossible to give a more detailed description of the process, while the origin of the particles of hard steel is a matter of pure guesswork.

The carbon content of the other specimen lay in a strip across the section surface (Figs. 9-12). In the centre of this strip there is a thin crack. This piece of iron must have been produced by welding two pieces together.

This would be an appropriate place to describe forge welding, which has now been superceded by various forms of fusion welding, but which was very widely used even up to 25 years ago. Two pieces of iron which are to be welded together are heated in the forge to about 1100° C. The temperature is estimated, partly by the colour of the iron, partly by the fact that it begins at that temperaure to throw off small sparks like a "sparkler". The two pieces are then hammered together on the anvil with quick light strokes. A correctly carried out forge welding is in no way inferior in quality to the welding achieved by modem methods.

Before the two pieces are welded together it is necessary to remove the oxide scale. In recent times this was achieved by sprinkling the objects to be welded with sand mixed with clay. The sand combined with the oxide scale to form a liquid slag which falls off when the two pieces of iron are struck against the anvil. It would be possible to prevent the formation of oxide scale by introducing a layer of charcoal between the pieces of iron, and that is probably what the Iron-Age smith wished to achieve by adding carbon. In addition he found that his iron had been converted to steel, and could not fail to note that by welding two pieces of iron together he obtained a harder material. We have tried to reproduce this method by forge­welding two pieces of the iron we won from our first "Iron-Age furnace" with a layer of charcoal between. The method is easy to carry out. The carbon content which we reached (Fig. 14) was smaller than that of the Hedeby iron; but a higher carbon content could have been achieved by introducing a thicker layer of charcoal between the pieces of iron.

15-16 arrowheads were forged from the material from the first experiment. They are exact copies of arrowheads from the Third Century A. D. found at Ejsbøl.

The iron differs in composition from modem steel in its total freedom from carbon (Fig. 13), and in its phosphorus content, which is 25 times as high as in the modem product. On the other hand its composition resembles closely that of Iron-Age iron.

 

Produced at

Varde Steelworkd

Found at

Jevenstedt 5)

Modern

steel

C…..less than

0.01%

0.02%

0.15%

Si

0.76%

0.045%

0.3%

Mn

0.04%

0.07%

0.7%

P

0.67%

0.81%

0.03%

S

0.03%

0.05%

0.03%

It should be remarked that the relatively high Si content in our iron is accounted for by slag in the mixture.

We were not successful in producing lumps of slag of an appearance similar to those found in the excavations. As the formation of such a lump of slag would be the surest sign that the method described above is the actual method which was in use in the Iron Age, it is our intention to carry out a further series of experiments.

Robert Thomsen