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ISIJ International, Vol. 54 (2014), No. 5, pp. 1024–1029

Metallurgical Evaluation of Farmer’s Steelmaking in Finland

Eiji YAMASUE,1)* Kazuhiro NAGATA2) and Tadahiro INAZUMI3)

1) Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto, 606-8501 Japan.2) Graduate School of Fine Arts, Tokyo University of the Arts, 12-8 Ueno Kouen, Taito-ku, Tokyo, 110-8714 Japan.3) Inazumi Professional Engineer Office, 3-16-1 Kimitsudai, Kimitsu City, 299-1143 Japan.

(Received on November 30, 2013; accepted on March 3, 2014)

In this study, we have documented the reconstruction of Finnish traditional steelmaking methods usingfarmer’s furnace in Möhkö by a local blacksmith, and metallurgically analyzed the materials and productsassociated with this technology. The steelmaking was successfully recreated in 2007, including mining,furnace construction and operation, followed by forging. Compared with Swedish lake ores, the Finnishlake ore used contains higher iron and lower silicon, aluminum and phosphorus. The produced spongeiron, bloom (luppe), had a yield ratio estimated to be 43–67%. The carbon and oxygen contents near sur-face areas of luppe were C: 0.32 ± 0.27 mass%, O: 1.20 ± 0.79 mass%, respectively, and those at thecenters are C: 0.06 ± 0.06 mass%, O: 0.045 ± 0.030 mass%, respectively. It lacked impurities excludingunexpectedly contaminating ore and slag debris. The reasons for higher yield and lower carbon contentwere discussed metallurgy.

KEY WORDS: farmer’s furnace; Finland; luppe; lake ore.

1. Introduction

It is said that a steelmaking method was developed in theAnatolian-Iranian region, namely, the Hittites Empire dur-ing the period of 1500–1000 BC, the knowledge and tech-nique of which were unique and national secret. After thedeclination of the Hittite Empire around 1190 BC, theknowledge and technique began to spread into surroundingcountries and later into considerable wide area. For exam-ple, toward the east, it reached as far as India and China in500 BC and 300 BC, respectively, through Silk Road, fol-lowed by the diffusion to Japan through Korean Peninsulain the 6th century.1) Toward the southward, the knowledgeand technique were widely diffused over Africa. Accordingto Tylecote,1) it spread to Nigeria, where the Iron Age Nokculture was producing iron by about 400–300 BC. It alsoreached Egypt through Greek or Carian traders, as there isreal evidence of smelting from the emporium of Naukratis.In Sudan, iron smelting started in about 200 BC and thisknowledge was introduced into Ethiopia. Central and EastAfrica received the knowledge and technique around 500AD from Nigeria with the migration of the Bantu tribes.This route finally ended in South Africa about 1000 AD.

Also, toward the north, it reached as far as United Kingdomthrough Donau and/or Roma in 500 BC. The Iron Age beganin the 8th century BC in Central Europe and the 6th centuryBC in Northern Europe utilizing locally minable iron ores.Especially, Sweden was blessed with leading high-gradeiron ore and hence it drastically developed before 7th cen-tury AD and is famous for steelmaking. Around the 13th

century AD, farmers produced farming tools from bog ironore (lake ore) by using so-called “farmer’s furnace”. Manyresearchers reported the farmer’s steelmaking in Swedenfrom viewpoints of archaeology and metallurgy, etc. Farmer’ssteelmaking using the lake ore was also reported not only inSweden but also in other Scandinavian countries.2) However,less has been reported on that in Finland.

In this study, we could carry out interview investigationon the farmer’s steelmaking in Möhkö, Karelian area withmany lakes and marshes in Finland and could successfullyrestore using the farmer’s furnace with same structure asancient one and the lake ore. Thus, the aim of this study isto analyze the steelmaking method and to clarify its charac-teristics metallurgically.

2. Fieldwork

2.1. About Möhkö and Experimental MethodsThe fieldwork for this study was carried out in 2007 in

Möhkö locating in Eastern Finland, Karelian area with manylakes and marshes. In Möhkö, about 3 300 ton of pig ironwas annually produced from charcoal and lake ore usingblast furnaces from 1 849 to 1 907, which was said to be thebiggest steelmaking plant using lake ore in the 19th centuryin Finland. Similar to Scandinavian countries, sponge bloom(luppe) was also produced using the farmer’s furnace inMöhkö by farmers to produce farming tools since MiddleAges. However detail techniques used in Finnish farmer’sfurnace are seldom known.

During our survey, we could investigate a steelmakingusing a restored farmer’s furnace by Prof. Lauri Holappa,Aalto University, with the supports by a blacksmith and twoassistants. In this investigation, although modern materials

* Corresponding author: E-mail: yamasue@energy.kyoto-u.ac.jpDOI: http://dx.doi.org/10.2355/isijinternational.54.1024

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were used, the basic structure of furnace, blower and usedore were same as those in Middle Ages. The steelmakingoperation consists of ore mining, furnace construction, steelproduction and forging. Each of these processes is explainedin the following section.

In this study, the lake ore and obtained luppe were ana-lyzed by using scanning electron microscope (SEM) withelectron probe micro analyzer (EPMA), X-ray diffractome-ter (XRD), X-ray fluorescence analyzer (XRF), oxygen-nitrogen analyzer and carbon-sulfur analyzer.

2.2. Restoration of Farmer’s Steelmaking in Finland2.2.1. Ore Preparation

Lake and bog ores were used in Finland to produce bloom(luppe). The lakes around Möhkö were very rich in ore.Average depth of the lakes or marches in these areas is about2–5 meters and the ores were mined at the bottom by a bas-ket with wire net at the tip of pole with the length of 5 m.The ore mining was done in summer or in winter from thetop of the ice. The lake ore is said to regenerate in a fewdecades. Figure 1 shows pictures of lake ore mined in ourfieldwork. It is coin-shaped with the thickness of 5–10 mmand diameter of 3–4 cm.

The ore was then calcined in the following procedure;three logs of white birch with length of 2 m are laid in par-allel, on which many logs with same length as lower logswere closely placed to be perpendicular to lower longs. Thelake ores were spread on the upper logs. The same structurewas further constructed on the bilayer. In total a lattice struc-ture with four tiers with the height of 1 m was composed. Itwas then burned for the calcination.

2.2.2. Structure of FurnaceFigure 2 shows the picture of furnace connected with a

man-powered bellows. The type of furnace is so-called“shaft furnace” which is in the form of a vertical cylindermade from steel. As illustrated in Fig. 3, the furnace consistsof two-tiered shafts with a diameter of 40 cm and totalheight of 108 cm (the heights of upper and lower shafts are63 cm and 45 cm, respectively), inside of which was cov-

ered with clay mixed with ashes made from burned fire-wood. The thickness of the clay wall at upper shaft is about5 cm. The bowl-shaped inner bottom of the furnace with a

Fig. 1. Pictures of lake ore mined.

Fig. 2. Picture of furnace connected with a man-powered bellows.

Fig. 3. Schematic diagram of crosssectional view of the furnace.

Air from bellows

15cm30cm

20cm

5cm

108cm

Upper shaft

Lower shaft

30cm

63cm Clay wall

Tuyere

40cm

Slag outlet

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diameter of 20 cm was also made from the clay with thick-ness of 15 cm. The heights from inner bottom and tuyere areabout 93 cm and 78 cm, respectively, which is about 20 cmlower than Japanese Tatara steelmaking furnace. The fun-nel-shaped steel tuyere was placed at 15 cm upper from theinner bottom. The diameter of tuyere is 37 mm, and its tip(about 5 cm) was inserted into the furnace. A slag outlet(with a diameter of 5 cm) from inner bottom was positionedon the right side of tuyere.

The bellows was composed of two pear-shaped thickboards sewed with moose leather. The narrow sides of theboards were connected with rectangular outlet made fromtimber using a hinge. The outlet was further connected withsteel pipe with a diameter of 10 mm. Appropriate length ofthe pipe contributes to increase in ventilation resistancewhich prevents the bellows from backflow. The oppositesides can be moved up and down by using a lever, the strokeof which is about 1 m. Ballast with a weight of about 25 kgwas placed on the upper board. In the lower board, an inletvalve with a diameter of 10 cm was prepared. The inner vol-ume of bellows is estimated to be 220 L. Since one stroketook 20 sec, it is estimated that about 660 L/min of air ismanually blown. It should be noted that the steel pipe wasnot directly connected with the tuyere but just placed infrom of the tuyere as shown in Fig. 4.

2.2.3. OperationBefore a steelmaking operation, the furnace was dried and

heated for 3 h using charcoal made from a pine tree. Althoughfist-sized charcoal was actually used, no special attention waspaid for the size of charcoal. That is, all pieces of broken char-coal were charged into the furnace. Burning rate was about 10cm for 10 min in charcoal layer, and it is almost same as Tatarasteelmaking. It should be noted, however, that since Tatarasteelmaking used high density charcoal, the heat value per unittime of Tatara steelmaking is thought to be higher.

After drying the furnace, the charcoal was charged intothe furnace every 10 min in the same manner as the dryingand heating processes. The pulverized lake ore (1–2 mm)was charged on the charcoal after every charge of charcoal.A proper amount of lime powder was also charged. Theamount of each charge was measured not by a scale but by

blacksmith’s eye measure. The total charges of charcoal andore for four hours were observed to be 25 kg and 15 kg,respectively. This indicates that average charges for char-coal and ore are 1 kg and 600 g, respectively. The blowingrate was controlled on demand. Slag was occasionally dis-charged. The amount of discharging lime powder was variedaccording to slag viscosity. The color of slag was black andits composition is estimated to be FeO–SiO2–CaO. It wasobserved through a light shielding glass that when reducediron particles came to contact with charcoal in front of tuyereinside the furnace, the iron particles were carburized, followedby melting and falling into furnace bottom, which phenomenawas also observed during Tatara steelmaking operation.

2.2.4. ForgingAfter 1 hour from stoppage of charging, remaining char-

coals were removed from the furnace, and then “luppe” wastaken out of furnace using fire tongs. The luppe was immedi-ately subjected to a forging process; the blacksmith held theluppe on an anvil using the tong, and two assistants alterna-tively hammered the luppe using hammers with several kilo-grams to obtain rectangular bloom. It took about 10 min forforging. During the forging, a relatively small amount of mol-ten slag was pressed out. The dimension of the forged luppewith rectangular shape was about 3–3.5 cm (T) × 11–13 cm(W) × 16–18 cm (L). Although the weight could not be mea-sured, it is estimated to be 4.1–6.4 kg from its size and density.

3. Results

3.1. Characteristics of Lake OreTable 1 shows the averaged composition in mass% of

lake ore by XRF compared with those mined in Sweden.3)

It was assumed that iron exists as goethite from X-lay anal-ysis to be shown later and the other elements exist as oxides.The analyses were carried out at different five positions.Compared with Swedish lake ores, the content of iron ishigher, while the contents of silicon and aluminum are low-er. Although the content of phosphorus is relatively lower,it is still high compared with current commercial iron ore;0.04–0.07 mass-P%.4)

Figure 5 shows cross-sectional view of near-surfaceregion of lake ore by means of SEM. It is found that the oreincludes a lot of pore. Figure 6 shows a backscattered elec-tron image (upper right) of the internal region of lake orewith elemental mappings for aluminum, iron and oxygen.Zonal structures indicate a possibility of existence of differ-ent phases, some of which contain aluminum. X-ray diffrac-tometry analyses indicate that the lake ore mainly containsthree phase, FeOOH (goethite), SiO2 (quartz) andAl2O3·SiO2 (kyanite), as shown in Fig. 7.

Fig. 4. Picture of tuyere.

Table 1. Composition of the Finnish lake ore compared with Swed-ish ones excluding oxygen (mass%).

Mined place Fe2O3 SiO2 Al2O3 MgO CaO MnO P2O5

Hutingen (Kalmar county) 79.8 8.6 9.2 0.2 0.7 0.7 0.8

Orsjo (Kronoberg county) 65.6 12.3 2.5 0.2 0.8 18.6 0.5

FeOOH SiO2 Al2O3 MgO CaO MnO P2O5

Mohko (this study) 92.3 2.5 2.9 0.4 1.0 0.5 0.3

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3.2. Characteristics of LuppeCompositional analyses using XRF show that the con-

tents of iron, aluminum and silicon are 99.4, 0.35 and 0.28mass%, respectively, and no other elements excluding car-bon and oxygen were detected. Considering common tem-perature and oxygen potential in the furnace, it is thoughtthat silica and alumina are not reduced into silicon and alu-minum, respectively. Thus the aluminum and silicon con-tents would be due to unexpectedly contaminating oredebris into the luppe.

Figure 8 shows the cross-sectional view of obtained lup-pe and the contents of carbon and oxygen near surface andat the center areas. It is found that the product is relativelydense. It is statistically said that the carbon and oxygen con-tents near surface areas of sample are C: 0.32 ± 0.27 mass%,O: 1.20 ± 0.79 mass%, respectively, and those at the centersare C: 0.06 ± 0.06 mass%, O: 0.045 ± 0.030 mass%, respec-tively. Although the values are scattered, it can be said that thesurface area is hypoeutectoid steel and the center is soft steel.

Fig. 5. Cross-sectional SEM image at near-surface region of lake ore.

Fig. 6. Backscattered electron image (upper right) of the internal region of lake ore with elemental mappings for alumi-num, iron and oxygen.

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4. Discussion

4.1. Higher Yield of Finnish Steelmaking UsingFarmer’s Furnace

As has been already pointed out by Nagata,5,6) the heights(from the bottom) of ancient steelmaking furnaces produc-ing luppe range more or less 1.2 m. The higher furnace eas-ily attains higher temperature and lower oxygen potential,resulting in higher yield (a percentage of weight of obtainedluppe to weight of elemental iron in iron ore). The lower fur-nace height of the Finnish farmer’s furnace must be disad-vantage in terms of yield. Further, the coarser lake ore withsizes of 1–2 mm must be also inadequate in terms of kineticsof reduction. It is interesting, however, that a large mass ofluppe was produced; the yield is estimated to be 43–70%both from the iron content in the ore as shown in Table 1and from the estimated luppe weight (4.1–6.4 kg). Evenconsidering estimation error, the yield is thought to be sim-ilar or higher than other ancient steelmaking methods; 42%for Japanese Tatara steelmaking7) and 40% for Ethiopiansteelmaking.8) The one of the possible reasons is the highersurface area ascribed to the porous structure of lake ore asseen in Fig. 5. Another possible reason may be its crystal

structure of Fe2O3 formed from calcined FeOOH, which iseasier to be reduced than Fe3O4 seen in sand iron for Tatarasteelmaking. There is a possibility that relatively large sizeof lake ore (1–2 mm) compared with sand iron (20–100 μm)prevents from unexpected scattering of the ore by blowing.

4.2. Mechanism of Finnish Steelmaking UsingFarmer’s Furnace

As was stated above, it was interestingly observed in frontof tuyere in the furnace during the operation that reducediron transformed from solid to semi-liquid state on charcoalsdue to carburization. Considering that common temperaturein front of tuyere is at least 1 300 C, it can be said that thecarburized iron, rather steel, must contain more than 1 mass-% of carbon. This speculation is, however, inconsistent withthe results of carbon content as shown in Fig. 8.

This discrepancy can be explained by a hypothesis thatthe semiliquid steel was again decarburized in the bottom offurnace, resulting in the production of steel with lower car-bon content. This is supported by the speculation that oxy-gen potential must be higher because the tuyere and bloweris not directly connected and further the furnace height islower. That is, so-called indirect steelmaking reactions pro-

Fig. 7. XRD profile of lake ore.

Fig. 8. Cross-sectional view of obtained luppe and the contents of carbon and oxygen near surface areas and at the centers.

0

500

1000

1500

2000

2500

3000

3500

4000

10 20 30 40 50 60 70 80 90

Rela

tive

inte

nsity

(a.u

.)

2-theta

SiO2 (Quartz)

Al2O3 SiO2 (Kyanite)

FeOOH (Goethite)

C: 0.80wt%O: 1.4wt%

C: 0.096wt%O: 1.9wt%

C: 0.33wt%O: 0.29wt%

C: 0.43wt%O: 0.21wt%

C: 0.13wt%O: 1.3wt%

C: 0.12wt%O: 2.1wt%

C: 0.63wt%O: 0.009wt%

C: 0.004wt%O: 0.018wt%

C: 0.037wt%O: 0.086wt%

C: 0.071wt%O: 0.028wt%

C: 0.14wt%O: 0.047wt%

C: 0.88wt%O: 0.091wt%

1cm

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ceeded in Finnish farmer’s steelmaking furnace. Based onabove hypothesis there is a possibility that the steel temper-ature partly reaches as high as 1 450–1 500 C, which corre-sponds to the solidus of iron-carbon phase diagram at thecomposition range of luppe (0.1–0.5 mass% carbon), due tothe decarburization by so-called bessemerizing. Consideringthe fact that the state of Tatara steel with carbon content ofmore or less 1 mass% was observed to be semi-molten,7)

temperature of Tatara steel in the furnace is estimated to beas high as 1 350–1 470 C at maximum in the same mannerabove, which is lower than Finnish steel.

The reason for lower oxygen and carbon contents at thecenter part would be that the decarburized steel was notexposed to surrounding carbon and oxygen sources. Thereason for higher carbon and oxygen contents at surface partwould be the re-carburization through the contact with char-coal at the furnace bottom and the re-oxidation during forg-ing process, respectively.

4.3. Reason for the SteelmakingOne of the characteristics of Finnish steelmaking using

farmer’s furnace is the operation under relatively higheroxygen potential. Why do they prefer such the operation?One of the possible reasons would be ascribed to highercontent of phosphorus in the lake ore. Even lower contentof phosphorus (~0.04 mass%-P) reduces the ductility ofsteel which is well-known as cold-shortness; the tendency ofsteel to crack is increased when it is worked at temperaturesbelow the recrystallization temperature (about half of melt-ing point, 450–650 C for steel). In order to prevent the steelfrom the contamination by phosphorus, the steelmakingoperation under higher oxygen potential is effective. Hence,it is thought that Finnish farmers empirically improved theintroduced steelmaking to meet with their minable ore.

5. Conclusion

In this study, we have documented the reconstruction ofFinnish traditional steelmaking methods using farmer’s fur-nace in Möhkö. Compared with Swedish lake ores, the lakeore used contains higher iron and lower silicon, aluminumand phosphorus. The produced sponge iron, luppe, had ayield ratio estimated to be 43–67%. The carbon and oxygencontents near surface areas of luppe were C: 0.32 ± 0.27mass%, O: 1.20 ± 0.79 mass%, respectively, and those at thecenters are C: 0.06 ± 0.06 mass%, O: 0.045 ± 0.030 mass%,respectively. It lacked impurities excluding unexpectedlycontaminating ore and slag debris.

AcknowledgementWe thank professor Miyuki Hayashi Tetsuya and profes-

sor Takashi Watanabe, Tokyo Institute of Technology, fortheir support in preparing the SEM-EPMA data.

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