INOM EXAMENSARBETE TEKNIK,GRUNDNIVÅ, 15 HP

, STOCKHOLM SVERIGE 2019

From Bloomery Furnace to Blast FurnaceArcheometallurgical Analysis of Medieval Iron Objects From Sigtuna and Lapphyttan, Sverige

ANDREAS HELÉN

ANDREAS PETTERSSON

KTHSKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT

Abstract During the Early Middle Ages, the iron production in Sweden depended on the bloomery

furnace, which up to that point was well established as the only way to produce iron.

Around the Late Middle Ages, the blast furnace was introduced in Sweden. This made it

possible to melt the iron, allowing it to obtain a higher carbon composition and thereby

form new iron-carbon phases.

This study examines the microstructure and hardness of several tools and objects

originating from archaeological excavations of Medieval Sigtuna and Lapphyttan. The aim is

to examine the differences in quality and material properties of iron produced by

respectively blast furnaces and bloomery furnaces. Both methods required post-processing

of the produced iron, i.e. decarburization for blast furnaces and carburization for

bloomeries. These processes were also studied, to better understand why and how the

material properties and qualities of the items may differ. The results show that some of the

studied items must have been produced from blast furnace iron, due to their material

composition and structure. These items showed overall better material quality and

contained less slag. This was concluded because of the increased carbon concentration that

allowed harder and more durable structures such as pearlite to form.

The study also involved an investigation of medieval scissors, also known as shears, made

from carburized bloomery furnace iron. Here, one specific aim was to find out if the

different sections of the shears had different properties, and if so, if these properties

correlated with the functions of the different parts of the shears. Our microstructure

analysis showed that the blade indeed was the hardest part due to intentional carburization

and forming of martensite. The blade is connected to a softer ferritic handle, which in turn is

connected to a ductile bow, also ferritic but with a larger grain size.

Keywords: bloomery furnace, blast furnace, iron production, Middle Ages

Sammanfattning Den svenska järnproduktionen var under medeltiden beroende av blästerugnen som då var

väl etablerad i hela landet. Under denna period introducerades även masugnen i Sverige,

vilket gjorde det möjligt att smälta järn. Den nya tekniken gjorde det möjligt att uppnå en

ökad kolhalt och därmed bilda nya järn/kol-faser.

Den här studien undersöker mikrostrukturer och sammansättningar i medeltida järnföremål

från arkeologiska utgrävningar i Sigtuna och Lapphyttan. Syftet är att undersöka vilka

egenskaper och materialkvaliteter som gick att uppnå i järn som tillverkats med masugn

respektive blästerugn. Båda tillverkningsmetoderna kräver efterbearbetningar som

förbättrar järnets egenskaper. Även dessa efterbearbetningsmetoder studeras, för att kunna

dra slutsatser angående hur det slutligt producerade järnets egenskaper och

materialkvalitet skiljer sig mellan de två produktionsmetoderna. Analyserna visade att järn

som tillverkats i masugn innehöll mindre slagg och generellt var av bättre kvalitet. Detta järn

innehöll även mer kol vilket tillät perlit att bildas. Därmed blev järnet betydlig hårdare.

Slutsatsen är att dessa järnföremål hade övergripande bättre mekaniska egenskaper och

materialkvalitet än de järnföremål som tillverkats med järn från en blästerugn.

I studien undersöks även medeltida fårsaxar, tillverkade av järn från en blästerugn som

sedan genomgått uppkolning. Syftet är att utifrån mikrostrukturen i materialet ta reda på

om de olika sektionerna i en fårsax har olika egenskaper, och om dessa egenskaper i så fall

är anpassade till den aktuella sektionens användning.Analysen visade att bladet var hårdast

på grund av avsiktlig uppkolning och martensitbildning. Därefter följde ett mjukare handtag

med en ferritisk struktur. Den böjda delen av saxen var den mest duktila och fjädrande,

eftersom den uppvisade en rent ferritisk struktur med större kornstorlekar än i handtaget.

Nyckelord: Blästerugn, masugn, järnproduktion, medeltiden

Table of Contents

1. Introduction ........................................................................................................................................ 1

1.1 Purpose ......................................................................................................................................... 1

1.2 Limitations ..................................................................................................................................... 1

2. Background ......................................................................................................................................... 2

2.1 The importance of carbon in steel ................................................................................................ 2

2.2 Bloomery Furnace ......................................................................................................................... 3

2.3 Blast furnace ................................................................................................................................. 4

2.4 Osmund process ............................................................................................................................ 5

2.5 Carburization ................................................................................................................................. 6

2.6 Finery forge ................................................................................................................................... 6

3. Methods and material ........................................................................................................................ 7

3.1 Knives ............................................................................................................................................ 7

3.2 Shears ............................................................................................................................................ 7

3.3 Ingots............................................................................................................................................. 8

3.5 Sample preparation .................................................................................................................... 10

3.6 Microstructure and analysis ........................................................................................................ 10

3.7 Vickers test .................................................................................................................................. 10

3.8 SEM ............................................................................................................................................. 11

4. Results ............................................................................................................................................... 12

4.1 Light microscope images ............................................................................................................. 12

4.2 Vickers hardness test .................................................................................................................. 21

4.3 Sem analysis ................................................................................................................................ 21

5. Discussion .......................................................................................................................................... 25

5.1 Ingots........................................................................................................................................... 25

5.2 Knives .......................................................................................................................................... 26

5.3 Shears .......................................................................................................................................... 26

5.4 Vickers hardness test .................................................................................................................. 27

5.5 Ethical aspects ............................................................................................................................. 28

5.6 Sources of error .......................................................................................................................... 28

6. Conclusion ......................................................................................................................................... 29

7. Further investigation ......................................................................................................................... 30

8. Acknowledgement ............................................................................................................................ 31

9. References ........................................................................................................................................ 32

1

1. Introduction During the middle ages Sweden experienced many technological advances in the field of

metallurgy, one of which was the ability to melt iron and achieve a much higher carbon

content. This was made possible thanks to the blast furnace which could reach a much

higher temperature than the bloomery furnace that had been used before. In conjunction

with this came the Osmund process, a process which allowed the smith to make iron from a

blast furnace forgeable. This advancement resulted in a significant increase in iron

production where both the blast furnace and the bloomery became more common [1].

Excavations in both Lapphyttan and Sigtuna have yielded thousands of objects from this

time, allowing future generations to study these objects to get a clearer picture of the past.

Lapphyttan was during the middle ages a small industry town where finds of an old blast

furnace has been made [2]. To the contrary, Sigtuna was during that time a large town with

lots of trading and an important religious seat in Sweden [3].

Few metallurgical studies have been made on the iron produced during the earliest days of

the blast furnace in Sweden. As far as we can tell no metallographic comparison which also

includes carbon composition between products made using a bloomery and blast furnace

during the late middle ages has been conducted.

1.1 Purpose

The focus of this study is to present a metallurgical analysis of medieval findings to

determine the quality of objects made using a blast furnace in contrast to a bloomery

furnace. The Blast furnace allowed the medieval smith to achieve a much higher carbon

content than what had previously been possible using a bloomery furnace. The increased

carbon content was of great importance as this allowed for different microstructures to

form within the iron. Therefore, the difference between objects made using a blast furnace

and a bloomery will be determined by analysing the microstructure as well as the carbon

content. The change in microstructure also changes the properties of the iron. Therefore,

the hardness will be tested for different phases in each sample. To complement this and to

investigate any irregularities, the composition will be analysed for some of the samples.

This study will also examine two single bow blade shears by comparing the metallographic

structures and hardness in the blade, shaft and bow to determine whether the properties

for each respective part have been altered to fit their intended purpose.

1.2 Limitations

To be able to answer these questions some limitations had to be set. The objects that are

examined are all dated to between AD 1100-1250 which correlates to the earliest days of

the blast furnace. The exact interaction that different elements have on iron, other than

carbon, will not be investigated as this is most likely not something that was actively altered

by the smith but rather the result of which ore was used.

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2. Background To be able to understand the difference between iron made using a blast furnace and a

bloomery furnace, it is important to understand how they operate and what parameters

affects the metallic structures. The following background is presented to elaborate on this

as well as the most important aspects of the Osmund process and carburization.

2.1 The importance of carbon in steel The microstructures and phases that are achieved from the manufacturing process depends

on several factors such as temperature, cooling rate and carbon content. The phase diagram

in figure 1 illustrates the relationship between temperature and carbon content in iron and

how these parameters interact to form the final phases.

The melting temperature of the iron is highly dependent on the amount of carbon. Pure iron

has a melting temperature of 1538 ˚C, whereas iron with a composition of 4.30 wt.% C has a

melting temperature as low as 1147 ˚C.

At low carbon content up to 0.02 wt.% C the iron will form ferrite(α) which is considered a

rather soft phase. Important to note here is that the solubility for carbon in ferrite is at its

highest at 727 ˚C. The carbon solubility becomes close to zero as the temperature drops.

One thing that is not apparent in fig 1 is the formation of martensite, as it is not directly

dependent on the Fe-C composition but rather on the presence of austenite and the rate of

cooling. This means that a martensite structure can be achieved at both low and high

carbon content. However, the amount of carbon present in martensite does alter the

achieved hardness.

Figure 1. Fe-C Phase diagram, Carbon composition in wt.% as X-axis and temperature as Y-Axis [4]

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Looking at higher carbon content there is an eutectoid point for austenite at 0.76 wt.% C

where pearlite can be formed on either side of this eutectoid point. Pearlite is composed of

ferrite and cementite (𝐹𝑒3𝐶) in a lamellar fashion. The amount of ferrite increases at lower

carbon content and cementite therefore increases with a higher composition. Cementite is

also called iron carbide and has a carbon solubility of 6.67 wt.% C.

As iron reaches 2-4 wt.% C it allows for cast iron to be produced. Cast iron comes in several

forms, including grey cast iron where the carbon is present in the form of graphite scales in

a ferritic matrix. Secondly, there is white cast iron where the carbon is bound to the iron in

the form of cementite (𝐹𝑒3𝐶) and pearlite. Formation of white cast iron benefits from a high

cooling rate and a lower amount of silicone (Si) compared to grey cast iron. White cast iron

is more brittle and less ductile than grey cast iron.

2.2 Bloomery Furnace

A bloomery furnace was commonly used to process iron ore to get an iron bloom that could

then be forged. The bloomery furnace is typically built between 70-100 cm high with a

diameter of 30-40 cm. The walls could be built using clay, stones or other media that could

withstand the heat. There is an opening at the top for feeding additional material and an

opening at the bottom that creates airflow and can be used to remove slag. This design can

be seen in figure 2 where an intake hole for air is seen on the right side. A common source

for the iron ore was in bogs and swamps where bog iron ore could be gathered. The bog

iron ore mainly consisted of iron oxide in the form of goethite that through roasting was

converted to hematite through formula 1 [5].

2FeO(OH)→Fe2O3+H2O (formula 1) The roasted iron ore is then crushed and added to the preheated bloomery furnace together

with charcoal. From there the reaction shown in formulas 2-4 [5] occurs to reduce the iron

ore to the desired iron bloom that can be collected at the bottom. More iron ore and coal

can be added during the process to keep it going for as long as desired. The exact

temperature achieved in a medieval furnace is hard to determine but probably stayed below

1250 ˚C [6].

Figure 2. Bloomery furnace [5]

4

3 Fe2O3 + CO – – – > 2 Fe3O4 +CO2 (formula 2)

Fe3O4 + CO – – – > 3 FeO + CO2 (formula 3)

FeO + CO – – – > Fe + CO2 (formula 4)

When the iron bloom is removed from the furnace it is then hammered while it´s still hot to

force the liquid slag out of the porous structure of the iron bloom. At this point the quality

of the iron becomes noticeable as inclusions and impurities will alter the characteristics of

the product. The presence of high carbon or phosphorous can alter the iron bloom to such a

degree that it becomes too hard or too brittle to handle the smithing process without

breaking [7].

2.3 Blast furnace

The blast furnace is believed to have its origin during the late middle ages and the oldest

one found in Europe might be found at Lapphyttan in Norberg. The furnace at Lapphyttan

has been dated to be in use as early as 1100 AD. During the middle ages the furnace was

about 3-4 meters high with a square shaped shaft that later got a more rounded shape

which can be seen in figure 3. The shaft is constructed with rocks and gets narrower closer

to the bottom where an intake hole for air is located. Air was forced through this hole with

the use of bellows that were driven with the help of a waterwheel.

Figure 3. Older Blast furnase made using stone [8]

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Contrary to the bloomery process, the iron in the blast furnace reaches melting

temperatures which allow the iron to get a much higher carbon content, typically between

2.1-4% carbon. It is believed that the temperature in a medieval blast furnace would reach

temperatures of 1400-1500 ˚C. The high carbon content in the iron makes it too hard and

brittle to forge after it has solidified but from a liquid state it can be cast to form pig iron,

intended to be processed further to make it malleable [9].

2.4 Osmund process

The osmund process was the first industrial process that made commercial products from

iron created by a blast furnace. A typical osmund piece is seen in figure 5 and has been cut

from a slab similar to that seen in figure 4. The osmund industry started in Sweden in the

late 12th century and was still in production all the way up to the 18th century. One of the

earliest places where they were made was in Lapphyttan in Norberg [10].

An osmund is a piece of forgeable iron that has been made in a blast furnace and then has

been decarburized to lower the carbon content. The carbon content varies a lot but could

be as low as 0,1%. Osmunds, being a commercial product, typically had a set size and weight

of around 280 grams. They were also tied to the Swedish monetary system [10]. Osmunds

were an important export product in Sweden. In the 14th century the yearly production is

estimated at around 2000 tonnes, and almost all of it was traded or bought by the Hanseatic

league [11]. the Hanseatic league was a large trading union during the middle ages,

consisting of many cities along the Baltic and North Sea [12].

The osmund pieces were chopped out with an axe from a larger round billet, which had

been flattened out by hammers, usually driven by a waterwheel while the billet was still

malleable and glowing hot [13].

Today there exists a replica of the old blast furnace in Lapphyttan, called Nya Lapphyttan.

Here enthusiasts and metallurgists have tried to recreate the old process of creating steel in

a medieval blast furnace using old techniques [14].

Figure 5. Piece from which osmunds are cut Figure 4. Osmund with visible groove from axe cut

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2.5 Carburization

Iron made from bloomeries had a very low carbon content, about 0,1-0,2 %C. This made the

material soft and suboptimal for using as cutting tools. To counteract this the medieval

smiths could carburize the tools. Carburization occurs when a low-carbon iron is heat-

treated in a carbon rich atmosphere at temperatures up to 900 C˚. This lets the carbon

diffuse into the iron, both increasing the surface hardness and making it more wear

resistant [15]. The diffusion depth varies, depending on time and temperature, from 0,5 to

1,5 mm. This is made possible by the size difference of Fe and C atoms, where Fe atoms are

much larger, which lets the carbon diffuse interstitially in iron. The Carburization can raise

the carbon content on the surface by up to 0,8-1,2%. If the iron is then quenched the core

will remain ductile with a harder surface [16].

2.6 Finery forge

The cast iron that came directly from the blast furnace had a carbon content of about 2,1-

5% [17]. This made it brittle and therefore unforgeable. To make it forgeable the carbon

content had to be lowered and to do this an early variant of a finery forge was used. Even

today very little is known about the first methods of decarburizing pig iron in Sweden. As for

what we do know, during the excavation in Lapphyttan the bases of seven finery forges

were found. They had a rectangular base with the dimensions of roughly 0,5x0,3 meters

wide and about 0,3 meters tall, although due to collapses the original height could have

been taller [18].

The earliest descriptions of the process are given by a monk named Peder Månsson, who

during the 16th century describes the method [19]. He describes it as hammering the pig

iron into smaller pieces and putting it into the forge while continually using the bellows. Slag

products such as scale, which is mostly iron oxides, was put into the forge to help reduce

the carbon. Later descriptions of the process describe the slag as “boiling” because of the

CO2 being released. After the process is done the remaining slag is left in the forge and the

billet is removed and cut into osmunds [9].

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3. Methods and material The samples examined are borrowed from Sigtuna museum and from Dr. Gert Magnusson

and are show in table 1. All items are archaeological finds and come from excavations made

in Sigtuna and Lapphyttan. Their original item numbers, given to them during the

excavations have been kept but with the addition of a unique name to simplify

identification.

Table 1 List of all the items examined with corresponding item numbers.

3.1 Knives

Both knives were found in Sigtuna. Knives are a common tool to find in these kinds of

excavations as they were easy to make and were used for a lot of chores in everyday life.

The purpose of examining the knives is to determine if any have been made using material

from a blast furnace. Another interesting aspect was to see how they had been forged by

the smith and if that could be seen in the microstructure. Knife 1 (3774) is to the left and

Knife 2 (8415) to the right in figure 6. Knife 2 (8415) is heavily corroded and has not been

conservation treated as opposed to Knife 1 (3774).

Figure 6.Knife 1 (3774) left Knife 2 (8415) right

3.2 Shears

The shears are also both from Sigtuna and are bit more uncommon to find then knives. They

were also examined to see if any were made with material from a blast furnace. Another

thing examined was the theory regarding the differing properties of the different parts of

the shear. Such as the possibility of a harder blade and a softer bow, the bow being the

round piece holding the handles together as seen in figure 7. Unfortunately, the bow on

Shear 2 (23276) had been fully corroded and was therefore not possible to examine.

Shears Knives Ingots

Shear 1 (27182) Knife 1 (3774) Ingot 1 (1199)

Shear 2 (23276) Knife 2 (8415) Ingot 2 (2078)

Ingot 3 (2599)

Ingot 4 (3689)

Ingot 5 (27791)

Ingot 6 (40019)

Ingot 7 (8636)

8

She shears examined are similar in shape but of different size. Shears 1 and 2 can be seen in

figure 8. Shear 1 (27182) is the smaller one. Its blades are bent, and the bow is broken.

Shear 2 (23276) is about twice as large. The bow is broken, possibly due to use-wear. Two

other breaks might have happened after the excavation, as there is no visible corrosion on

the surface of the breaks.

Figure 8. Shear 1 (27182) to the left and shear 2 (23276) to the right

3.3 Ingots

The ingots came from both the excavation in Sigtuna and Lapphyttan. Ingots are understood

as pieces of iron of various size and shapes, for us the most commonly being a rectangular

bar, that do not resemble a tool or other finished item. These are very common finds in

excavations, especially in places like Lapphyttan where they were produced but also in

Sigtuna where they were traded and used. Ingots 1-6, seen in figures 9-11, are all from

Sigtuna. They all have a rectangular shape but display different sizes. Ingot 7 in figure 12 is

from Lapphyttan. It has a round shape and was probably cut from a sphere.

Figure 7. Shear used for cutting [18]

9

Figure 9. Ingot 1 (1199) to the left and Ingot 2 (2078) to the right.

Figure 10. Ingot 3 (2599) to the left and Ingot 4 (3689) to the right.

Figure 11. Ingot 5 (27791) to the left and Ingot 6 (40019) to the right.

Figure 12. Ingot 7 (8636)

10

3.5 Sample preparation

All the items were fist photographed for documentation purposes in their original state

before being cut.

The first two samples, made from shear 1, were prepared at the Division of Biophysics at

Stockholm University. For the mould a two-part epoxy compound called Epoxicure resin was

used which hardened at room temperature overnight.

The other samples were prepared at the Department of Materials Science and Engineering

at KTH campus. A Discotom saw, which is a water-cooled cutting machine, was used to cut

the samples to size. It worked well but was not large enough to cut through some of the

largest items. After cutting, a metallographic sample preparation was made.

3.6 Microstructure and analysis

When the preparation of the samples was done, they were all etched using a 4% Nital

solution except for Ingot 6 (40019) and Shear 2 (23276) which were etched using a weaker

2% Nital solution. This because some samples were prone to over-etching and because

some samples contained more than one piece, which did not etch at the same speed. The

etching is done so that the morphology of the microstructure becomes more visible in the

light optic microscope. This makes it easier to see differences in phases, grainsize and flaws

such as cracks or inclusions. Pictures were then taken using a LEICA DMRM light optic

microscope connected to a computer using a software called LEICA QWIN V3. The pictures

have varying magnification from 5, 10, 20, 50 and a 100-times magnification.

In total twelve samples from eleven objects were made and examined using the light optic

microscope. Some of the images were used to calculate the proportions of phases present

by counting the points within a 20*20 grid layover for each phase and divide by the total

amount of points used. The total amount of carbon in the examined area can then be

estimated by multiplying the % of each phase with C wt.% for respective phase and then

adding together the given value for each phase as in formula 5 below, where A, B, C… are

different phases.

C wt. % =Points (A)∗𝐶 𝑤𝑡.% (𝐴)

Number of points examined (400)+

Points (B)∗𝐶 𝑤𝑡.% (𝐵)

Number of points examined (400) (Formula 5)

The carbon composition used for the different phases are, ferrite 0,02 C wt.%, pearlite 0,8 C

wt.% and cementite 6.7 wt.% C.

3.7 Vickers test

A Vickers hardness test was made to determine the hardness of our samples, allowing them

to be compared with one another, but also with data from other sources. Vickers Hardness

tests are measured in HV as seen in formula 6 [20]. The equipment had a microscope to aim

with, after which a pyramid shaped diamond tip was pressed with a given load to make a

square indentation in the sample as seen in figure 8. A light optic microscope was then used

to measure the diagonals of the indentations. Two different loads were used for the test,

11

200 g and 500 g, to be able to see the indentation. Because some samples contain both

ferrite and martensite which differ greatly in hardness, the indentation would have been too

difficult to detect if a 200 g load would have been used on martensite. The load used for

each test can be seen in table 6.

𝐻𝑉 ≈ 0,1891𝐹

𝑑2 [𝑁

𝑚𝑚2] (Formula 6)

Where F is the force applied and measured in Newton and d is the length of the diagonal of

the indentation measured in mm.

Depending on the differences in phases and microstructure, such as grainsize and inclusions,

each sample was tested between one to four times.

3.8 SEM

A scanning electron microscope, or SEM, uses a high-energy beam of electrons to penetrate

and refract off a samples surface residing in a vacuum chamber. The interaction with the

sample creates various signals, which are then analysed and can give the atomic

composition and the surface topography of the sample [22]. A SEM analysis was conducted

on sample Shear 1 (27182), Ingot 7 (8636) and Ingot 6 (40019) to see if there were any

irregularities in the composition of the samples. Ingot 6 (40019) also had another SEM

analysis made on its corrosion due to its unusual clear blue appearance, which might

indicate that the composition differs from the other samples.

Figure 13. Vickers test tip angle and dimensions [21]

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Figure 14. Sample:1199, edge of sample. Figure 15. Sample:1199, Middle of sample.

4. Results

The following results are comprised of images gathered using a light microscope, a Vickers

hardness test for individual phases in each sample and a SEM analysis.

4.1 Light microscope images

The result from the light microscope is composed of 37 images seen in figure 14-50. Each of

the samples are presented with 2-4 images and a clarification of visible structures.

4.1.1 Ingot 1 (1199) Found at kv.Trädgårdsmästaren 9 & 10 i Sigtuna 1988-90. The speckled white and grey

structure seen in figure 14 is martensite with some inclusions visible as black spots.

Martensite is visible in figure 14-17 but is less prominent towards the middle of the sample

as seen in figure 15. The darker areas surrounding the martensite in figure 15 is pearlite and

is also present in figure 16-17 as dark patches. Figure 16-17 also contains some cementite

seen as clear white uneven lines.

Figure 16. Sample:1199, between middle & edge of sample.

Figure 17. Sample:1199, Close to the edge.

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Figure 20. Sample:2599, edge of sample

4.1.2 Ingot 2 (2078) Figure 18 and 19 shows that this sample is made entirely of a ferritic structure with some

inclusions seen as dark spots. Two large inclusions are seen in the upper right corner of

figure 18 with several smaller inclusions below these. The grainsize in figure 18 is of varying

size.

4.1.3 Ingot 3 (2599)

The most prominent phase is pearlite, with small amounts of martensite and

widmanstättenferrite. In figure 20 a slight colour gradient is seen in the grey pearlite, with

some martensite seen as white patches in the darker background. Figure 21 shows a closer

look at the right most area in figure 20 where widmanstättenferrite is seen as white streaks

in the darker pearlite. In figure 22 some martensite surrounded by pearlite can be observed,

similarly to what can be seen in figure 20 but at higher magnification. What looks like a dark

feather in figure 23 is bainite and was not found anywhere else.

Figure 18. Sample:2078 Figure 19. Sample:2078

Figure 21. Sample:2599, center of sample

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4.1.4 Ingot 4 (3689) The majority of the sample is made from the grey speckled phase martensite, seen

throughout the entirety of figure 25. Martensite is also seen surrounded by pearlite in figure

24. Small patches of dark pearlite can also be seen in the lower half of figure 27. The white

uneven stripes seen in figure 27 are cementite, while the more even white stripes in figure

26 are widmanstätten ferrite embeded in a pearlite matrix.

Figur 22. Sample:2599 Figur 23. Sample:2599

Figure 24. Sample:3689 Figure 25. Sample:3689

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4.1.5 Ingot 5 (27791) The darker lines within this sample are made of graphite and are contained within a lighter

background consisting of ferrite seen throughout figure 28-30. Comparing figure 28 and 29

shows that the structures varies where the graphite scales are found as patches in figure 29.

The graphite scales are more densely packed towards the centre of most clusters and can be

seen in figure 30.

Figure 27. Sample:3689

Figure 29. Smaple:27791 Figure 28. Sample:27791

Figure 30. Sample:27791

Figure 26. Sample:3689

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4.1.6 Ingot 6 (40019) Figure 31 and 32 shows that most of the sample contains martensite with some pearlite

seen as dark spots in figure 31 as well as some inclusions of slag seen as dark lines. Figure 31

shows a composition of 7% pearlite and 93% martensite.

4.1.7 Ingot 7 (8636) The sample contains mostly pure ferrite which is seen in both figure 33 and 34 as a light grey

phase. Figure 34 is seen at higher magnification which reveals dark spots embedded in the

ferrite. These inclusions are further examined in the SEM analysis. The size of the ferrite

grains is quite inhomogeneous as seen in both figures.

Figure 34. Sample:8638 Figure 33. Sample:8638

Figure 32. Sample:40019 Figure 31. Sample:40019

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4.1.8 Knife 1 (3774) The knife contains mostly ferrite as seen in figure 36 and 37 as a light grey phase. Pearlite is

seen as a darker phase at the edge of the blade in figure 35 and at higher magnification in

figure 38. The amount of pearlite is highest at the very edge of the blade which can be seen

at the bottom of figure 35. There are few slag inclusions visible as dark spots in figure 36

and as a dark line going vertically in figure 37. At the edge of the knife seen in figure 38 the

composition is 32% ferrite and 68% pearlite.

Figure 35, Sample:3774, edge of knife Figure 36. Sample:3774, back of knife

Figure 37. Sample:3774, back of knife Figure 38. Sample:3774, edge of knife

18

4.1.9 Knife 2 (8415) The knife was cut into two samples to be able to analyse both the handle and the blade. The

handle sample is seen in figure 39 and 41 and contains ferrite except for a localized darker

area at the top corner being pearlite. Figure 40 shows an overview of the blade with ferrite

seen at the top and bottom, with a core that has a much finer grain size and some pearlite.

The core can be seen at higher magnification in figure 42 where two inclusions are visible in

the top right corner. Figure 42 contains 81% ferrite and 9% pearlite. Inclusions can be seen

in both the handle and the blade as irregular darker spots.

Figure 39. Sample:8415, edge of shaft Figure 40. Sample:8415, back of knife (core)

Figure 41. Sample: 8415, edge of shaft Figure 42. Sample:8415, core of blade

19

4.1.10 Shear 1 (27182) The shear was cut into two samples so that the blade, handle and bow could be examined.

The blade as seen in figure 44 and 46 is constructed of a ferritic core. The edges have a

higher carbon content that is present in the form of pearlite as well as some horizontal slag

lines in figure 44. The size of the ferrite grains varies throughout the shear. The bow in

figure 43 shows the largest grains with the handle in figure 45 having slightly smaller grains

size. The smallest grain size is found in the blade. The composition of the edges of the blade

in figure 46 is 17% pearlite to 83% Ferrite.

Figure 46. Sample:27812, edge of blade

Figure 43. Sample:27182, shear spring Figure 44. Sample:28172, blade

Figure 45. Sample:27182, core of handle

20

4.1.11 Shear 2 (23276) The handle sample is seen in figure 47 and 49 and the blade sample in figure 48 and 50.

Most of the handle contains ferrite with an area of darker pearlite in the upper left corner of

figure 47. Figure 49 shows a closeup of the most pearlite rich area of the handle where the

composition is 76% pearlite to 24% ferrite. Some of the ferrite in figure 49 is in the form of

widmanstätten ferrite. The blade as seen in figure 48 is almost entirely made of martensite

and small amounts of slag in the lower left corner. Smaller amounts of ferrite and pearlite

can be seen at some areas of the blade such as the one seen on the left side of figure 50.

The bow of this shear could not be examined due to heavy corrosion that made it too

brittle.

Figure 47. Sample:23276, edge of handle Figure 48. Sample:23276, blade

Figure 49. Sample 23276, edge of handle Figure 50. Sample:23276, blade

21

4.2 Vickers hardness test

The diagram in figure 51 represents the hardness of each sample. The phases where the

tests were made have been color-coded in blue, orange, grey and yellow for martensite,

pearlite, ferrite, and cast iron respectively. The samples have also been divided into

separate columns if there was a large difference in hardness or microstructure of the same

phase. The exact values used in figure 51 can be found in table 5.

4.3 Sem analysis The SEM analysis was made for sample 40019, 8636 and 27182 to investigate the composition and

some irregularities in the form of inclusions.

4.3.1 SEM Analysis: 40019 The SEM analysis for sample 40019 was done on the oxidized surface layer and is represented in

table 2 and figure 52, showing high amounts of phosphorus with some magnesium and sulphur.

Figure 52. Sample 40019, Data from Table 3 with varying colours assigned to individual elements.

0100200300400500600700800900

1000

HV

Sample number

Vickers Hardness Test

Martensite Pearlite Ferrite Cast iron(white)

Figure 51. Vickers hardness for different phases within each sample, Vickers hardness as Y-axis.

22

A secondary analysis was done on sample 40019 to examine the composition of an inclusion

as presented in figure 53. This data is illustrated in figure 54, showing high amounts of

silicone within the inclusion.

4.3.2 SEM Analysis: 8636 The SEM analysis for sample 8638 was done to examine the spherical inclusions that can be

seen close to the top edge in figure 55. The composition presented in table 3 shows that the

inclusions consist of carbon in the form of spherical graphite particles.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0keV

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

cps/eV

C Si Al K K

Ca Ca

O Fe Fe

Spectrum: Point

Element AN Series Net unn. C norm. C Atom. C Error

[wt.%] [wt.%] [at.%] [%]

--------------------------------------------------------

Oxygen 8 K-series 5815 25.36 34.60 60.98 3.2

Iron 26 K-series 6112 33.69 45.98 23.21 1.0

Phosphorus 15 K-series 3583 7.99 10.90 9.92 0.3

Magnesium 12 K-series 351 1.21 1.65 1.91 0.1

Sulphur 16 K-series 653 1.56 2.13 1.88 0.1

Manganese 25 K-series 254 1.11 1.51 0.77 0.1

Zirconium 40 L-series 487 1.78 2.43 0.75 0.1

Calcium 20 K-series 208 0.59 0.81 0.57 0.0

--------------------------------------------------------

Total: 73.29 100.00 100.00

Table 2. Sample:40019, Data from SEM-analysis of oxidized surface layer presenting at.% for included elements.

Figure 53. Sample 40019, SEM analysis of inclusion with varying colours assigned to individual elements.

Figure 54. Sample 40019, Data from SEM Analysis Presenting the Composition

23

4.3.3 SEM Analysis: 27182 The SEM analysis of sample 27182 was performed on the ferrite phase in the blade. Table 4

shows that the sample is made up of pure iron with the area tested seen in figure 56.

Spectrum: Point

Element AN Series Net unn. C norm. C Atom. C Error

[wt.%] [wt.%] [at.%] [%]

-----------------------------------------------------

Iron 26 K-series 1780 82.25 76.30 43.01 2.4

Carbon 6 K-series 176 17.09 15.86 41.56 4.3

Oxygen 8 K-series 201 8.45 7.84 15.43 2.1

-----------------------------------------------------

Total: 107.80 100.00 100.00

Table 3. Sample 8636, Data from SEM-analysis of oxidized surface layer presenting at.% for present elements.

Figure 55. Sample 8638, Image of inclusion

Spectrum: Point

Element AN Series Net unn. C norm. C Atom. C Error

[wt.%] [wt.%] [at.%] [%]

------------------------------------------------------

Iron 26 K-series 39174 97.10 100.00 100.00 2.9

------------------------------------------------------

Total: 97.10 100.00 100.00

Figure 56. (SEM) Sample :27182

Table 4, Sample 27182, Data from SEM-analysis of oxidized surface layer presenting at.% for included elements..

24

4.3.4 Summary of investigated objects The different properties of the samples are summarized in Table 5, i.e. the excavation

background, the different phases observed in the samples’ microstructures, the Vickers

hardness values for these phases, the amount of C wt.% in the samples, and various

comments about the samples and their composition. The amount of C wt.% are calculated

according to formula 5.

Sample Excavation site Structure Vickers test

C wt.% Notations

Knife 1 (3774)

Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Pearlite (edge, carburized)

Ferrite (edge, little

carburization)

211 HV200 145 HV200

Edge of knife ≈ 0.78 C wt.% Back of knife < 0.02 C wt.%

Carburized edge, Much slag.

Knife 2 (8415) Sigtuna 2006 Kv. Humlegården 3

Ferrite (edge, carburized)

Ferrite (centre) Pearlite Ferrite

147 HV200 107 HV200 190 HV200 108 HV200

Core of blade < 0.09 C wt.% Sides of blade < 0.02 C wt.%

Carburized Core and handle,

Shear 1 (27182) (handle/bow)

Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Ferrite Ferrite

131 HV200 120 HV200

Handle & bow < 0.02 C wt.%

Shear 1 (27182) (blade)

Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Ferrite Ferrite (edge,

carburized)

132 HV200 209 HV200

Core of blade < 0.02 C wt.% Sides of blade ≈ 0.14 C wt.%

(SEM) Carburized blade

Shear 2 (23276)

Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Martensite Pearlite Ferrite

893 HV200 241 HV200 105 HV200

Handle ≈ 0.19 C wt.% Blade Uncertain

Martensite in blade, Carburized handle

Ingot 1 (1199) Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Pearlite Martensite

545 HV200 806 HV500

> 0.78 C wt.% Uneven structure

Inhomogenous Structure

Ingot 2 (2078) Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Ferrite 112 HV200 < 0.02 C wt.%

Ingot 3 (2599) Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Martensite Pearlite

696 HV500 290 HV500

< 0.78 C wt.% Bainite found in localized area

Ingot 4 (3689) Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Martensite Pearlite

871 HV500 452 HV500

Uncertain

Ingot 5 (27791)

Sigtuna 1988–1999 Kv. Trädgårdsmästaren 9&10

Grey cast iron (edge)

Grey cast iron particle

535 HV500 166 HV500 205 HV500

Uneven carbon composition ≈ 1.8-3 C wt.%

Uneven carbon composition

Ingot 6 (40019)

Sigtuna 1999 Professorn 1

Martensite

556 HV500 Unsertain

(SEM) Unusual composition

Ingot 7 (8636) Lapphyttan 1978–1983 Ferrite Ferrite

112 HV500 116 HV500

≈ 0.03 C wt.% (SEM)

Table 5. Properties of the investigated objects: excavation background, observed microstructures, Vickers hardness values for different phases, estimated C wt.%, and notations to highlight individual factors that are of interest for each sample.

25

5. Discussion The introduction of the blast furnace made it possible for smiths during the middle ages to

achieve a much higher temperature than that of the bloomery furnace. The high

temperatures achieved in the blast furnace now allowed the iron to melt, which significantly

alters the solubility of carbon and the metallic structures. The blast furnace used in

conjunction with a finery forge could now produce forgeable iron in the form of osmunds.

Our hypothesis is that the osmunds had significantly better properties compared to what

was produced in a bloomery furnace in conjunction with carburization. This hypothesis is

here explored by comparing the differences in chemical and structural composition together

with the hardness for several archaeological finds.

A secondary hypothesis is explored regarding the two single bow blade shears that are

believed to have properties that differ in the blade, shaft and bow. To investigate whether

this hypothesis holds up, a metallographic analysis and Vickers hardness test was conducted

for each individual part.

The exact values derived from the Vickers hardness test and estimated carbon composition

can be found in table 5.

5.1 Ingots The ingots that have been analysed show a wide variety of structural difference and carbon

content. Ingot 2 (2078) is made almost entirely out of ferrite which indicates a very low

carbon content below 0.02 C wt.%. The sample contains some form of unwanted inclusions

and is of rather poor quality as well as uneven grain size. It is forgeable due to the soft and

ductile nature of ferrite were the Vickers hardness test show a hardness of 112 HV200,

which can be found in the table 5 with all other Vickers hardness results. Due to the low

carbon content and low quality this ingot is determined to have been produced using a

bloomery furnace. Ingot 6 (40019) most likely falls under this category as well due to the

high amount of slag seen throughout the entire sample. This could also be due to the

unusual composition which is seen in the SEM analysis in chapter 4.3.1. The SEM analysis on

the surface layer for this sample shows very high amounts of phosphorus at around 10 at.%

as well as noteworthy levels of magnesium, sulphur and zirconium between 0.5-1.9 at.%.

The data collected from the Vickers hardness test show that the martensite in this sample is

significantly softer in comparison to the other martensite phases and is most likely due to

the high amount of silicone seen in the second SEM analysis which examines the core. In

Ingot 7 (8636), spherical graphite particles surrounded by ferrite can be observed. This is a

form of ductile cast iron which is less brittle than the typical cast iron and is therefore

forgeable.

Within Ingot 1 (1199), Ingot 3 (2599) and Ingot 4 (3689) high amounts of pearlite can be

observed. This indicates a high carbon content estimated to be around 0.78 C wt.% and

could not have been achieved with the use of a bloomery furnace. Therefore it is concluded

that they were made using a blast furnace and then they have been decarburized using a

26

primitive finery forge. Just tapping the iron from the blast furnace without further process

would otherwise yield cast iron which is seen in Ingot 5 (27791). The main difference here is

that the cast iron is not forgeable due to the high carbon composition which can be

observed in the form of graphite scales in figure 28-30.

Judging by appearance alone these samples do not show the typical shape and weight of an

osmund. However, it is most likely that Ingot 1 (1199), Ingot 3 (2599) and Ingot 4 (3689)

have been forged from an osmund judging by their carbon content and included phases.

They might have been discarded due to defects such as cracks or just be a residual piece

from the forging of another tool. When looking at the Vickers hardness for these samples

they present a much higher hardness when compared to the previously named ingots that

originate from a bloomery furnace. The cast iron from Ingot 5 (27791) also show a high

hardness but is on the other hand not forgeable, making it less practical and harder to

process.

5.2 Knives

Ferrite is the most present phase in both knives which is accompanied by pearlite in

localised areas. Slag inclusions can be seen throughout both samples. They can both be

classified as low carbon steel where the pearlite is the result of carburisation to increase the

hardness and wear resistance of the blades. The carburisation differs quite a lot between

the two blades as to where it has occurred and amount of carbon that has been achieved.

Highest amount of carburization can be seen at the edge of Knife 1 (3774) in figure 35,

where the biggest effect is seen at the very edge. The clear pearlite structure is a sign that

the blade has been heated to allow for austenitization during or after the carburization.

Knife 2 (8415) presents carburization in the core which has most likely been achieved by

laminating three pieces together.

The amount of pearlite present in the material is an indicator to how well the item has been

carburized as it is localised to specific areas and greatly affects the hardness. This can be

seen by comparing the very edge of each blade were Knife 1 (3774) contains 68% pearlite

with a hardness of 211 HV200 and Knife 2 (8415) with less than 9% pearlite and a hardness

of 147 HV200. The use of carburization to increase the carbon content and the amount of

inclusions indicates that the iron used for these knives has been produced using a bloomery

furnace.

5.3 Shears

With a quick glance at the microstructure in figures 43-50 it becomes obvious that the

shears are rather different from one another.

Shear 1 (27182) is made entirely with a ferritic structure where the bow shows a much

larger grainsize that makes it more flexible than the rest of the handle. The results from the

SEM analysis in chapter 4.3.3 shows that the ferrite phase contains pure iron. Yet again

carburization can clearly be seen in the blade but in contrast to the knives it is now seen at

27

the sides of the blade with a hardness at 209 HV200, surrounding a softer more ductile core

ranging between 120-133 HV200. This should allow the blade to be quite flexible yet able to

retain a sharp edge. The slag lines visible in this area might be the result of folding the iron

between each carburization session to further increase the amount of carburization

achieved.

The second shear called Shear 2 (23276) has similar structure in the handle that is mostly

ferrite, apart from some pearlite seen in the top left corner of figure 41. The blade differs

from the other shear where the entire blade now contains nothing but martensite. The

martensite structure has been acquired by rapidly cooling the blade from the austenite

phase which can be done even if the carbon content is low. This makes this blade much

harder than the previous shear which is highlighted by the Vickers test at 894 HV200. The

carbon content of martensite cannot be accurately estimated from the structural analysis

but can be considered as low in carbon due to a low carbon composition in the shaft and by

comparison to the other shear.

The hypothesis put forth was that the properties of different sections of the shears would

vary depending on what properties would be beneficial. By comparing the grainsize found in

the blade and bow of shear 1, which can be observed in figure 43-44, it is clearly seen that

the grain size in the blade is drastically smaller. This difference in grainsize together with the

carburization that has been observer, shows that the properties indeed was altered to

benefit each section and its intended use.

Even though the two shears differ from a structural standpoint it´s safe to assume that both

have been forged from iron produced in a bloomery furnace. The two shears differ in size,

as Shear 2 (23276) is around 20cm in length which makes it about twice the length of Shear

1 (27182). It´s rather interesting that the bigger shear has a much harder blade which might

imply that the intended purpose for each shear differ from each other. To put this into

context the smaller shear seems to be most suited for fine work such as cutting hair or

different fabrics where the strain on the tool is limited. The bigger shear is comparably more

suited for work that includes tougher materials such as cutting twigs or extended repetitive

use that otherwise would dull the edge of a softer blade.

5.4 Vickers hardness test The data from the Vickers hardness test in figure 51 show some variations that should be

highlighted. The exact values obtained from the Vickers hardness test can be found in table

5. Ingot 4 (3689) and Ingot 1 (1199) both contain pearlite which is much harder than what is

observed in the other samples. This is believed to be due to the high carbon composition in

both samples as well as the presence of martensite. The martensite might have been just

below the tested martensite and thereby affected the result. The grey cast iron in Ingot 5

(27791) also show a rather significant difference even though they all share the same phase.

They key here is the amount of graphite present in the area that has been tested, the

28

Vickers hardness of 535 HV500 has been found at the edge of the sample where the amount

of graphite was highest.

5.5 Ethical aspects

It is a bit of a moral dilemma we are facing when it comes to destroying these centuries old

finds and objects. On one hand destroying them will make it possible for us to research and

understand how they were made and how they were used. It is also a piece of history that

tells us of our past and preserving it will enable the past to be remembered. Even though,

when visiting the vast collections of items from this period, we feel it is morally justified to

destroy, examine and experiment on just a small part of it. Because, just looking at an object

does not give the whole story, and instead of collecting dust these objects may help us learn

more about our history which we feel is worth it.

Due to the nature of this project being arkeological, the environmental effects of our studies

are nominal.

However, the environmental effects of the invention of the blast furnace and the increase of

iron production during the middle ages as well as the start of mining had a large

environmental effect. Because of the difference in scale between the blast furnace and the

bloomery, and because the blast furnace could run nonstop, the demand for wood coal

must have increased drastically. Leaving the areas around these sites to become deforested

if production was upheld for a long period of time. The slag, which sometimes contained

toxic elements, was left as a biproduct and could contaminate the water and surrounding

environment if not handled properly.

5.6 Sources of error

When studying objects that are this old there are going to be a lot of areas where

uncertainties and insecurities may occur. Because the samples have been under the ground

for hundreds of years the objects studied have been corroded, some heavily, to the point of

having no solid core left in some places. This makes it impossible to determine the

microstructure or if there have been exposed to surface hardening, such as carburization, as

that would have corroded away. It can also mean that some of the results may be faulty.

Another aspect is measuring errors. When doing the Vickers Hardness test the diagonals

made to determine the size of the indentation are made by hand and will have a small

margin of error, leading to a slightly different result.

The locations of the finds also bring uncertainties. Especially the finds from Sigtuna, as that

place was, during the time these objects were made, a larger town with lots of trading of

different items and materials. This makes it very hard to determine where the objects could

have originated from, if the iron ore came from the same place or if the objects had been

manufactured elsewhere and then bought or traded. Lapphyttan does not have the same

level of uncertainties because it was a production facility which made steel.

29

6. Conclusion

From the samples that have been analysed it can be concluded that the ones thought to

have been produced with the use of a blast furnace show very good properties, especially

increasing the hardness of the pearlite. The amount of pearlite within these samples are

also much higher and more widespread which increases the strength throughout the entire

object significantly. As it was theorized these samples show a much better consistency and

better properties compared to the objects made using a bloomery furnace.

With that said, the objects made using a bloomery furnace should not be overlooked as they

can still be made into tools with good quality and properties that are modified to better suit

their intended use. This is clearly proven by the single bow blade shears where the

properties can be directly corelated to the intended purpose. The theory that was put forth

was that the shear vas constructed with a ductile bow, a slightly harder shaft and a much

harder blade which was concluded to be in line with this conclusion.

30

7. Further investigation There are several topics in this report that can further be analysed and researched. The first

being the continuation of this project by examining more objects to better understand the

variation of properties and quality. A more in-depth SEM analysis could also be made to

better understand the irregularities in composition that is quite common in medieval iron

objects. This could also be used to determine which ore that has been used and how this

affected the properties and quality. Another thing is to examine a piece of confirmed

osmund iron and compare it to the pieces that we believe originate from an osmund. This

should be done to see if our assumption is correct and if the iron has been altered by any

later forging process.

There is also the case to further investigate the first production techniques in creating iron

from a blast furnace and more specifically the finery forge as very little is known about this

process. An experimental physical recreation of the decarburization process will be

conducted at Nya Lapphyttan in the summer of 2019.

All the tools examined in this report comes from iron made in a finery forge. It would

therefore be useful to examine tools that have been made using iron originating from a

blast furnace to see how the properties and quality vary in the final product.

31

8. Acknowledgement We would like to thank everyone who helped us along the way for making this project

possible. First of all, we would like to thank Sigtuna Museum for lending us the items and

letting us examine them. To Gert Magnusson, Docent in archaeology, we want to extend a

big thanks for letting us borrow and examine samples and for providing us with valuable

insight and assistance. We would also like to express our gratitude to our mentors in this

project, Sebastian Wärmländer at the Biochemistry and Biophysics Department at

Stockholm University and Anders Eliasson at the Department of Materials Science at the

Royal Institute of Technology. We would like to thank Wenli Long at the Department of

Materials Science at the Royal Institute of Technology for assisting us with the preparation

an analysis of our samples. Thanks to Eva Hjärtner-Holdars, Docent in archaeology, and Lars

Bentell, Licentiate in metallurgical chemistry, for taking the time to discuss our project and

provide valuable insight.

32

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