S. Turcott November 23, 2020
Metallurgy During the Iron Age And its Legacy upon Modern Metallurgy
Figure 1: Photograph of an axe head found at the bottom of a lake. Its analysis began the journey of exploring the metallurgy of the past.
INTRODUCTION
No profession has been more affected by the Iron Age than that of metallurgy. Yet with decades of education and experience as a metallurgical engineer, never had its history been mentioned or made relevant to me - until one day I was handed an old axe head that began a fascinating journey into a realm I never appreciated existed (Figure 1). Now I realize that the metallurgical practices of the Iron Age literally forged the world of modern metallurgy.
Although this axe was probably made in the 1700s or early 1800s, it incorporated concepts that defined metallurgy in Europe for thousands of years. There are Roman, Viking and Medieval artifacts that exhibit comparable metallurgical features. This is because several of the core principles of the Iron Age remained the same until the invention of the Bessemer process in 1851 which began modern steelmaking.
As a disclaimer, a few months ago I had never even heard of the term “archaeometallurgy” – the metallurgical study of historical artifacts. As an amateur on the topic, I have written this to share concepts of interest from the books, articles and conversations experienced since. I will use this axe head as the foundation to demonstrate and weave through some of the core ideas within the Iron Age. I apologize in advance to the real experts if I oversimplify or overgeneralize. Yet I think some of the ideas introduced through the study of historical metallurgy will interest many.
IRON – THE BASICS
Archaeometallurgy uses the term “iron” very differently than we do in modern industry. Whereas today iron contains 2-4wt% carbon and graphite, in archaeology wrought iron is essentially that, iron with no carbon. Think of “iron” as low carbon steel with less than 0.1wt% carbon. Soft, malleable and not very useful for cutting edges such as weapons or axes. Yet it still had many practical applications such as nails, hinges, etc.
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For example, the wedge used to hold the axe head onto its wooden handle comprised of wrought iron. Under a microscope, it can be seen to have an entirely ferritic structure with no notable carbon/carbides (Figure 2). You can also see the high density of slag inclusions that identify it as having been made by reduction without the metal having been entirely melted. This matched with either bloomery furnaces used until around the 1400s, or older-style blast furnaces used afterwards, which failed to separate the slag from the final iron product. Due to the smaller furnace sizes, they could reduce ore into iron but did not have enough time/temperature to fully melt and/or soak in the carbon like that of modern, massive blast furnaces.
Slag inclusion Ferrite (white)
Wedge used to hold axe head onto wooden handle
Slag
Optical Microscope (50-1000x magnification)
Slag inclusion
Ferrite (white)
Wedge Structure, 100x Wedge Structure, 400x
Figure 2: Micrographs displaying the microstructure of the wrought iron wedge formerly used to fasten the axe head onto its wooden handle. Its predominantly ferrite structure with slag inclusions was classic for iron throughout the majority of the Iron Age. Incomplete melting during the reduction in bloomery furnaces or smaller/older blast furnaces left in slag inclusions. Etched using 3% nital. Metallurgy During the Iron Age and its Legacy upon Modern Metallurgy Page 2 of 12 S. Turcott November 23, 2020
One of the interesting things about iron is that it remained essentially the same for thousands of years. Figure 3 compares the microstructure and slag inclusions within our axe wedge (1700-1851AD) with a Roman nail estimated to be 2000 years old (0-100AD). Other than luck associated with the quality of ore used, my understanding is that there really were no large improvements in the quality of European iron over thousands of years until 1851.
Cross-sectioned
a) Axe Wedge, 1750-1851AD b) Roman Nail, ~100AD
Slag inclusions
Ferrite Ferrite (white) (white)
Slag inclusions
c) Wedge Core, 100x d) Nail Core, 100x
Figure 3: Micrographs comparing the structures of the (a,c) axe wedge produced sometime between 1750-1851AD and (b,d) a Roman nail from before 100AD. Because the structure of wrought iron had not changed for thousands of years, their ferritic structures with slag inclusions appeared similar to one another. Only subtle differences in the inclusions gave hints of being produced from different ore sources. Etched using 3% nital.
To be honest, I bought this nail on eBay from a seller that claimed it was Roman from the 1st century and I cannot verify its age. Yet the point remains the same, that from a metallurgical perspective, iron had not changed for thousands of years.
Believe it or not, iron existed in reasonable abundance during the Bronze Age. Yet the problem with iron was that it was soft, weak, pliable and not very useful for cutting edges
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such as weapons or axes. Cold worked, tin-alloyed bronze could be made into swords and tools with hardness values between 200 and 220 HV (archaeometallurgy uses Vickers hardness for everything). Yet bloomery iron was typically between 80 to 130 HV – this axe head wedge had a hardness of 130 HV. So iron was not a militarily strategic metal until the invention of steel.
STEEL – ADVANTAGES, LIMITATIONS AND DESIGN
Its “Ironic” (ha!) that the Iron Age actually began when steel was invented and long after iron had been in use. Yet the discovery of steel changed everything and quickly spread throughout Europe between 800BC and 600BC. Steel strengthened by 0.3 to 1.5wt% carbon content provided the opportunity for weapons and tools to surpass the usefulness of bronze. Plus iron ore was more available than the tin needed for weapon-grade bronze and overall, steel was cheaper to produce. It quickly became the dominant metal. Established countries and empires with militaries built upon bronze weaponry quickly fell to the taste of steel by once lesser competitors if they did not adapt fast enough.
Throughout the Iron Age, there were two factors that greatly affected (and limited) how steel was used: 1) Steel was pricey – usually 3-5 times more expensive than iron. 2) The carbon content could not be controlled or measured. (ranged between 0.3wt% to 1.5wt% carbon)
There were several different ways to make steel. Most of the methods involved carburizing iron to increase its carbon content. The higher the carbon content, generally the better the steel performed. Yet the additional processing meant that steel was inherently much more expensive than iron and carburizing limited the size of steel pieces that could be produced. To keep costs down and to work with the small pieces of steel, the majority of tools and weapons were made of steel and iron fused together. Steel would be selectively used in areas where its properties were most needed while the remainder would comprise of iron. The quality of a part included steel quality, craftsmanship and the design used by the blacksmith in its construction.
This axe head was a perfect example of this. The majority of its body comprised of iron yet a high carbon steel strip had been fused onto its cutting edge (Figure 4). The steel had a hardness of 427 HV (42 HRC), providing a wear resistant, hard cutting edge. Yet the design used the minimum amount of steel to keep the costs down.
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Iron body
Steel fused onto cutting edge
Iron body (Soft yet affordable) Steel fused onto cutting edge (Hard, wear resistant but expensive)
Figure 4: Photographs and macrograph displaying a cross-section taken of the axe head. The axe body comprised of soft iron. The blacksmith had fused steel onto the iron body at the cutting edge. This was a smart design that provided a hard, wear resistant steel that would hold a sharpened edge yet also was cost-effective. Etched using 3% nital.
Although this axe had been made in the 18th or 19th centuries, the concepts used in its design spanned over thousands of years. Axes excavated around Europe and dating back throughout different time periods used variants of this steel-iron design (Figure 5). Blacksmiths would selectively use steel only at the most critical portions of the tool. Labour and the availability of iron seemed less restrictive.
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Iron
Steel a) 3rd BC, Ireland Steel insert at cutting edge
Iron body b) 5th-6th AD, Ireland
Steel fused onto cutting edge
Iron body
c) 5th-6th AD, Baltics (Vikings!!)
Figure 5: Schematics displaying different axe designs used in different places/time periods. The blacksmiths used different designs to minimize the use of steel, iron and skill.1,2
Taking an even closer look at our axe’s cutting edge, Figure 6 displays optical micrographs of its steel. You will notice its structure comprised entirely of unresolvable, fine pearlite with carbides/cementite. Although this steel was hypereutectic, having over 0.8wt% carbon and readily hardenable, its structure showed that it had not been quenched. This was due to the second complicating factor haunting the Iron Age ̶ the inability to control or measure carbon content. This greatly delayed the widespread use of quenching.
Today, the properties of medium and high carbon steels can be optimized during heat treating. For many applications, this includes heat treatments which use quenching methods/media adjusted to the steel’s composition (switching from water to oil quenching for higher carbon steels). Yet hundreds of years ago, quenching was like playing Russian roulette – the part may crack during quenching in a seemingly random manner. With the carbon content uncontrolled and unknown, water quenching was risky business. Think of it – after a blacksmith had paid for the material costs and belaboured crafting the part, it might have been for nothing if the part cracked during quenching. Even a few parts cracking might prevent a blacksmith from quenching again.
1 R. Pleiner. Iron in Archaeology. Early European Blacksmiths. Praha: Archeologický ústav AVČR (2006). 2 R. Saage, K. Kiilmann, A.Tvauri. Manufacture Technology of Socketed Iron Axes. Estonian Journal of Archaeology (2018). Metallurgy During the Iron Age and its Legacy upon Modern Metallurgy Page 6 of 12 S. Turcott November 23, 2020
b) Axe Head Tip, 10x
Soft, low carbon steel core
Ferrite and slag inclusions (iron)
137HV c) Head Body, 400x
Fine pearlite and carbides (air cooled >0.8wt% steel)
427 HV
Hard, high carbon steel tip
a) Fused Metals, 100x d) Cutting Edge Metal, 1000x
Figure 6: Micrographs displaying the (a,d) steel cutting edge fused onto the (c) iron body of the axe head. The steel exhibited a fine, unresolvable pearlitic structure with cementite/carbides along the prior-austenite grain boundaries. This structure was consistent with air cooled hypereutectoid steel (>0.8wt% carbon). Still, with a decent hardness of 427 HV (42 HRC), this cutting edge would have been as durable as many of the axes produced today. Etched using 3% nital.
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Also, if successfully quenched without cracking and if tempering were not understood/applied, the steel would be brittle and may fail quickly upon use. Therefore, for much of the Iron Age, the benefits of quenching and tempering were eluded and not used systematically. Instead, the final heat treatments applied were often decided upon by the specific blacksmith and who can fault them for favouring air cooling over quenching.
EUROPEAN SWORDS
The significance of steel was far greater than it being used to make better axes and tools. Steel forever altered the course of history because of its military advantages, producing better weapons at lower cost than bronze. A reality of humanity is that, where applicable, the latest technologies are adopted into weaponry. Therefore, to really understand the metallurgy of the Iron Age, one must take a glance upon the construction of their weapons – specifically swords.
Laminated steel Iron sword (soft, poor quality) (numerous pieces of steel laminated together)
Carburized iron sword Laminated steel and carburized
Layers of iron-steel Iron Carburize d
Steel
Figure 7: Schematics displaying five Roman sword designs. The most common swords contained mixtures of iron and steel. The rarest, highest quality of swords were made of pieces of steel fused/welded together, sometimes carburized for even higher hardness. The heat treatments would vary between air cooled and quenched.
Figure 7 displays schematics of five known Roman sword designs that were produced about two thousand years ago.3 Note how even amongst a state as organized as Rome, the design used by their blacksmiths varied significantly. The poorest quality swords comprised entirely of soft iron which would have been nearly useless against metal armour. They would have dulled quickly and sometimes even bent during combat. The next step up in quality were swords made of iron with carburized cutting edges.
3 A. Williams. The Sword and the Crucible. Brill (2012). Metallurgy During the Iron Age and its Legacy upon Modern Metallurgy Page 8 of 12 S. Turcott November 23, 2020
Although modern carburizing could produce an excellent sword, carburizing of that era sometimes formed shallow cases and did not always introduce overly high levels of carbon. Constant resharpening of the sword could wear through the carburized layer and these swords were still at risk of bending.
Higher quality Roman swords included steel. Like our axe, many Roman swords incorporated hybrids of steel and iron. Swords have been found of intermixed layers of iron and steel forged together. Yet the best (and most expensive) swords comprised entirely of steel. Romans could not produce pieces of steel large enough to make an entire sword from so these swords comprised of several steel pieces forged/welded together. The cutting edges of steel swords were sometimes carburized.
As mentioned before, as well as the design of the swords, the finishing heat treatments varied quite a bit. Swords tended to be air cooled yet there were some that exhibited quenched structures. Remember, the worst thing that could happen in combat was the brittle fracture of a sword. Even a bent sword was more useful. So as well as the fear of quench cracking, many blacksmiths seemed to have erred on the side caution and air cooled to ensure combat durability.
One might think that looking forward over one thousand years, from the Romans to the late Medieval Age, that there would have been great improvements in sword design or at least a better understanding of steel heat treatment. Yet that does not seem to have been the case. Swords of all these designs and heat treatments were still being produced throughout all of the Iron Age. Yet a few additional designs had been added. These included (a) iron bases with steel added to the cutting edges, (b) steel sandwiched between iron and (c) pattern welded swords. The latter comprised of iron and steel twisted and turned together to create robust, beautiful swords.
Iron body, steel at cutting edge Steel sandwiched between iron
Pattern welded swords Iron
Steel
Figure 8: Additional sword designs used throughout the Iron Age.
It was finally in the 17th century that steelmaking in Europe was able to produce steel nuggets large enough to forge an entire sword from. Plus steelmaking using “finishing” furnaces had become repeatable enough for them to begin quenching more routinely. At that point, the most expensive swords made by the finest blacksmiths were repeatedly of higher quality, hardened steel.
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THE VIKING VLFBERH+T SWORD
Although this article has focused upon the European Iron Age, do not be fooled into thinking that Europeans were leaders in metallurgy. Quite the contrary. Vastly superior “crucible” steel was being produced in the Middle East and India from as early as 300BC. Ingots from crucible steel were large enough to produce entire swords, contained far less slag inclusions and were consistent enough in carbon content to master quenching. Therefore, high quality swords were being produced in these regions long before Europe had the technology to do so. The most famous of these were the Damascus swords which, beyond their excellent quality, were made beautiful by patterns associated with variations in chemistry throughout the steel (i.e. chemical segregation).
Bringing this story back to European swords, there was a legendary series of Viking swords produced during the 11th to 13th centuries, marked by the stampings “VLFBERH+T” or similar lettering.4 These swords were made of fine quality, high carbon steel which was far better than anything Europe had been producing at that time. The steel was thought to have been imported from Iran through trade along the Volga River and then forged by the Viking blacksmiths. Yet like other Europeans swords, whether the final heat treatment incorporated quenching or air cooling depended upon the blacksmith. Regardless of their heat treatment, the high carbon steel produced a vastly superior sword to their European counterparts and were properly feared wherever the Vikings went.
Once trade along the Volga River collapsed, these swords disappeared into history. It took nearly five centuries before Europe began making crucible steel and themselves producing steel of the same quality as that found within the VLFBERH+T swords. And this axe, the axe that began my journey into archaeometallurgy, was made with that European crucible steel which finally matched the quality to the VLFBERH+T swords (Figure 8g).
4 A. Williams. A Metallurgical Study of Some Viking Swords. Gladius (2009). Metallurgy During the Iron Age and its Legacy upon Modern Metallurgy Page 10 of 12 S. Turcott November 23, 2020
Viking Sword from 1000-1300 AD
The best Viking Swords were marked with “VLFBERH+T”
a) Axe (1740-1851) b) Viking Sword!!!
c) Axe Steel Edge, 400x d) Viking Sword, ~200x
Carbides Carbides (white) (white)
Pearlite (black) Pearlite (black) e) Axe Steel Edge, 1000x f) Viking Sword, ~1000x
Figure 8: For three hundred years, the Vikings produced the best swords Europe had ever seen. The most famous, best quality and most feared were the swords marked with “VLFBERH+T” made between 1000 and 1200AD. These swords were made of steel imported from Iran.
If you look at the micrographs, you’ll see the axe’s cutting edge structure matched perfectly with this Viking sword.4 It took England over 500 years to make steel as good as this sword. (a,c,e) Etched using 3% nital. (b,d,f) Etchant not reported.
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SUMMARY
The late archaeometallurgist, C.S. Smith (made famous for his work on the Manhattan Project) reminds us that the majority of metallurgical practices used today were discovered by artisans and blacksmiths – not scientists or metallurgists.5 Science had to catch up and try to explain what people were already doing. Essentially practical metallurgy was developed through historic, hands-on workers rather than in a laboratory. To understand metallurgy today, one must appreciate its past.
And the history of metallurgy was not a straight path. The introduction of steel forever changed the way tools and weapons were made. Yet its advancement was hampered for millennia by what could not be understood, controlled or seen. That the inability to explain why steel sometimes cracked during quenching prevented the widespread optimization of its properties. Yet even with these limitations, mankind’s ingenuity was applied to produce remarkable tools and weapons using what they knew.
Throughout the intellectual journey that had begun by being handed an axe head, I could not help but be amazed by the cleverness, resourcefulness and engineering of people of the past. How many of the designs and concepts used millennia ago resemble designs being used today. Lastly, I could not help but ponder that the greatest advances in metallurgy had been made by someone observing and exploring phenomena that all others failed to notice its significance. I wonder what we today are failing to notice amongst the science of metallurgy.
5 C.S. Smith. A History of Metallography. The MIT Press (1960). Metallurgy During the Iron Age and its Legacy upon Modern Metallurgy Page 12 of 12