Archeometallurgy: iron making with a low shaft roman-celtic furnace

Inspired by the second siderurgy phase of Piani d’Erna (LC) -Ist century bc- Ist century ac-

Cesare Alippi, Gianpaolo Barindelli

There is evidence that meteorites were used as a source of iron before 3000 BC, but extraction of the metal from ores dates from about 2000 BC. Production seems to have started in the copper-producing regions of Anatolia and Persia, where the use of iron compounds as fluxes to assist in melting may have accidentally caused metallic iron to accumulate on the bottoms of copper smelting furnaces.

In celtic Europe iron making appeared quite late in history compared to the middle east (e.g., around 8-700 BC). The complexity of the metallurgical process was mainly related to the reachable temperature which was well below the nominal melting temperature of 1535° C. However, above 800° C the iron reduction process activates and the slag (mainly composed of silicon and other oxides) melts around 1150° C. This gives the possibility to iron atoms to get close each other hence supporting a sintering process (for a proper iron melting we have to wait for the high shaft furnace which appeared only in recent times). Iron sintering was possible thanks to a low-shaft furnace, a tronco-conic clay and fibre-made structure with tuyeres at the base for air conveyance. Charges (mineral and coal) are inserted from the top over time while air is inflated from the bottom. During this procedure, provided that all steps are carried out properly, iron sintering occurs within the furnace in its hottest segments.

Once the process is over, the low-shaft furnace is destroyed and the iron extracted for further processing. The outcome is a bloom, a sponge-like iron structure which needs to be hammered to remove the slag and gain a compact shape for subsequent artifact making.

In principle smelting seems easy but it is surely not. As a matter of fact no precise recipe is available (being left in our ancestor’s memory) and obtaining a bloom is everything but simple.

In the following we synthesise our smelting experiments in the different phases without any quantitative analysis or technological details.

The iron ore

Well, it was not easy to find the raw material. At first you have to find a mine then you need to identify a good ore. The first attempt was a failure: we brought 30 Kg of potential ore from an Esino’s mine. Unfortunately, they were stones since the iron content was negligible...

We moved then to the St.Aloisio mine in Val Trompia (BS) thanks to Salvatore and Piergiuseppe. The ore was there, in the heart of the mountain. An excellent 45% pure siderite (see Cesare active on the ore in figure 1 and Paolo in the mine, figure 2).

Figure 1 : Cesare and the ore Figure 2 : Gianpaolo

The roasting phase

The mineral is then hammered to walnut size pieces. The roasting phase aims at transforming siderite FeCO₃ in hematite Fe₂O₃ and magnetite Fe₃O₄ . It can be done by interleaving layers of wood with ore and burn up the stack (figures 3 and 4). Even if expected the result is astonishing: the outcome can be collected with a magnet (see figure 5 for the roasted ore).

Figure 3 : Roasting furnace Figure 4 : Roasting furnace

The charge A charge is a mix of roasted ore and wood charcoal (we reasonably kept equal parts). We also added shells to provide some additional calcium content (calcium, e.g., 5%, is beneficial in smelting since it reacts with the slag leaving more Fe to generate the bloom).

Figure 5 : Roasted ore

The furnace

The furnace required 300Kg of clay that we collected in a mountain cave (and carried on our backs!). The clay, once cleaned from stone impurities was mixed up with fine sand (10% in our experiment). This mix can be somehow thought as that needed for making ceramics: it gives resistance, toughness and plasticity to the furnace and it helps to reduce the formation of cracks during the heating process. Clay loafs were then made out of the mixed and constituted the basic “bricks” of the furnace.

The furnace construction started by digging the soil and creating a stone basement onto which deploy the furnace. Then we constructed the base (see figure 6) and the slag canal.

Figure 6 : Creating the shaft Figure 7 : Building the furnace

After having collected river straws, we stacked them to configure a cylindrical shape with the largest diameter at about 35cm from the base (figure 7 and 8). De facto, straws constituted the scaffolding of the structure and clay bricks were piled them up having the straw structure as a side.

Figure 8 : Building the furnace Figure 9 : Building the furnace

Then we made the two tuyeres (oriented at about 25^o) and stopped the construction at about 60cm to allow the furnace to dry up (otherwise the fresh clay would not support the weight of upper layers and the furnace collapses).

The final structure was about 110cm in height, a diameter at the base of around 70 cm and 25 at the top (see figure 9). We left the furnace to dry naturally for two weeks after which we cooked it to give more stability (figure 10). The resulting was a low shaft furnace ready to undergo a smelting process.

Figure 10 : Furnace cooking Figure 11 : Furnace pre-heating

The reduction process

The reduction process is rather interesting and it worths a more detailed description. In particular, around 900-1000^oC, combustion of charcoal tends to produce CO instead of CO₂ , i.e., the reaction 2CO+O₂ ↔2CO₂ is active on the left. CO plays an active role in the reduction process (here simplified to consider only hematite Fe₂O₃ and magnetite Fe₃O₄ presence)

Fe₂O₃ +CO ↔ 2FeO+CO₂

FeO+CO ↔ Fe+CO₂

3Fe₂O₃ + CO₂ ↔ Fe₃O₄ + CO₂

Fe₃O₄ + CO ↔ 3FeO + CO₂

Fe particles float in the slag and, through a sintering process, get together to constitute the bloom.

Smelting

Smelting required a full working day. Errors in the charging frequency and air flow immediately affect temperature, then the slag quality and, finally, the formation of the bloom. No precise written recipes are given but only experimental trials, quite often leading to unsuccess.

The first phase requires the furnace to be pre-heated, e.g., with wood first and then char coal (figure 11) to allow all humidity to go out.

Once the temperature within it is high enough (continuous sparks were coming out with fumes), the slag door was closed with tuyeres receiving all needed air (figure 12).

Figure 12 : Cesare and Gianpaolo: Smelting

Charges (figure 13) where then continuously inserted from the top with a frequency depending on the charge consumption (we experienced about 7 minutes).

We opened twice the slag door to allow some of it leaving the furnace to keep under control the quantity inside and allow more charges to be added; then the door was closed again with fresh clay.

An interesting view of the inside, i.e., the slag, can be seen from the tuyeres (figure 14).

Figure 13 : Charge Figure 14 : View from tuyere

Bloom extraction

After the last charge was inserted the furnace operated for a good hour to allow all charges in the chimney to be consumed. At this point the furnace was destroyed to extract the –hopefully existing- blooms. Furnace destruction is necessary since the slag solidifies with the bloom and the two must be separated by hammering (figure 15).

Figure 15 : A bloom

Hammering is also important to compact the bloom (figure 16) and the result is an iron ingot (figure 17) ready for being processed to improve its features and/or making manufacts: les jeux sont faits.

Figure 16 : Hammering Figure 17 : The iron

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