Iron Smelting in Viking-age Iceland:
a study based on experimental archaeology

William R. Short and Reynir A. Óskarson
Hurstwic, LLC
©2023-2024 William R. Short and Reynir A. Óskarson

In the Viking age, a ready supply of iron was essential for survival. Iron was the raw material for a variety of weapons, tools, cookware, and more. For example, large quantities of iron rivets were required for the construction of the ships that allowed Vikings to sail on their voyages of trade, war, and exploration.

Archaeological excavations at graves, at house sites, and in maritime settings teach us about the artifacts made from iron, their uses and their distribution. Excavations at iron smelting sites where iron was made inform us about the furnaces used, structures at the site, along with the raw materials, waste materials, and processed materials relating to the smelting process.

What is missing are the details: furnace design and construction details; how the furnaces were operated; and how the raw materials were gathered and processed.

We at Hurstwic were curious to learn more about the iron smelting process in the Viking age, due to our focus on Viking combat and weapons. There was a clear disconnect between the experts' assessments of the quality of iron smelted in the Viking age and the quality of iron we saw in historic Viking weapons. To help us resolve this disconnect, we turned to the tools of experimental archaeology.

rivets in hull of Viking ship
Iron was used for many essential items in the Viking age, such as the rivets used in the construction of the hulls of Viking ships. Shown here is the hull of the replica Viking ship Íslendingur. A large ship needed upwards of 250 kg (550 lbs) of iron rivets.

• Experimental archaeology

Experimental archaeology is an approach that can help us fill in these missing details. It's a research tool that allows us to test hypotheses about an ancient culture.

To use the tool, we create a hypothesis and test it in the material world, using tools and techniques that would have been available to the ancient culture, as suggested by archaeological evidence. We use the scientific method to attempt to falsify our premise. We can't prove that our hypothesis is true, but we can strengthen our confidence in its validity by being unable to falsify it with our testing.

As a result of the testing, the hypothesis is no longer merely a theory in our heads, but something that has been tested and validated physically. This approach allows us to make deductive leaps and have confidence in the validity of our ideas.

Our organization Hurstwic has used experimental archaeology to study many aspects of the Viking people. Based in New England, much of our work has focused on Viking combat, yet we are interested in all things Viking. In our research and in this article, we use the term Viking generically, to refer to any of the northern European people in the early medieval period, about 1000 years ago.

• Motivation for our iron smelting research

Our interest in Viking-age iron smelting was sparked by a social visit to a farmer living at Eiđar in the east of Iceland, a major farm in ancient times. Evidence suggests that there was a massive iron-smelting operation at Eiđar in Viking times. Smelting is the process by which useless iron ore is converted into nearly pure elemental iron, suitable for making into a variety of tools, weapons, and other useful objects.

slag heap at Eiđar
One of many heaps of slag from Viking-age iron smelting operations at Eiđar in east Iceland.

The farmer at Eiđar showed us the numerous heaps of slag at the farm. Slag is a glassy waste product of iron smelting, and it was discarded on the site in tall mounds. Slag is essential to the smelting process, yet it contains a lot of good iron locked up in a form that can not be recovered, so it is wasteful.

The quantities of slag at the site were impressive. We were curious, so we looked at the archaeological report for the site. (Ţórarinn Ţórarinsson 1980) It was estimated that 1000 tonnes of iron was smelted at this farm in the centuries around the Viking age. This prodigious quantity of iron is far more than needed by the farm, and far more than needed by all the farms in the district combined. And, it is only one of a number of farms in the district known to have smelted iron in this period.

There can be little question about the nature of the activities on the site. In a subsequent visit to the farm, we looked at the soil surface in a slag heap where the vegetation had eroded away. We could easily identify and pick up iron ore, iron slag, and bits of charcoal fuel from the surface of the exposed soil. Additionally, clear signs remain today suggesting the raw materials for iron smelting were readily available on the lands surrounding the farm.

Slag found at Eiđar
Iron slag found on the soil surface of one of the Eiđar slag heaps.

Yet at the same time, we were aware of the widely held belief that iron smelted in Viking-age Iceland was of poor quality, a belief held by scholars and laymen alike and supposedly backed up by literary evidence. There are several episodes in the Sagas of Icelanders suggesting that the iron used for a sword blade was poor, resulting in the sword blade bending during combat and requiring it to be straightened out under foot. (Einar Ól. Sveinsson 1934a; 1934b) The idea that Viking-age Icelandic iron was poor, resulting in poor-quality weapons, is even expressed in modern novels, such as Nobel-laureate Halldór Laxness's novel, Gerpla. (Halldór Laxness 1952) We faced a strange conundrum. On one hand, there were reports of massive amounts of iron being smelted in Viking-age Iceland, and on the other hand, we saw a wide-spread belief that Icelandic iron was poor quality, supposedly backed up by literary evidence.  

Our curiosity continued to grow. First of all, why were such prodigious quantities of iron made at Eiđar in and around the Viking era, far beyond the needs of the farm? Additionally, why was iron made at all there if it was of such poor quality?

We decided to investigate further, since in our study of weapons, we did not find any strong evidence for such poor quality iron, either in our investigation of historic weapons, or in our studies of the literary sources. The trope of a sword bending in battle appears only twice in the Sagas of Icelanders out of the hundreds of sword battles mentioned. We wondered if researchers had fallen into the trap we call modern mindset, an umbrella term denoting instances where our modern-day thinking, ideas, and prejudices interfere with our investigation of ancient times. Perhaps the idea of poor quality iron was a problem we call ultimate authority, where a highly respected authority expressed the idea and it remained unquestioned, untested, and unchallenged subsequently, based on the reputation of the authority who first expressed it. (Short 2022)

We wanted to question that authority and break out of the trap that held back our thinking about iron smelting in Viking-age Iceland.

• Our hypotheses

We began by creating two hypotheses that could be tested using experimental archaeology, and if possible, falsified using the scientific method.

The first hypothesis: it is possible to smelt high quality iron using tools, techniques, and raw materials known to have existed in Viking-age Iceland.

The second hypothesis: high quality iron can be smelted using locally-sourced materials gathered close to known smelting sites in Viking-age Iceland.

This second idea comes from the realization that the smelting process uses enormous quantities of materials. It seems unlikely these materials would be imported from overseas or even transported from one part of Iceland to another. Because of the quantities involved, it seems more likely that smelting was conducted where the raw materials were locally available. Since evidence of smelting operations have been found in many parts of Iceland, it suggests that readily available local materials were used for smelting.

We began our investigation by looking at the archaeological evidence from Viking-age iron smelting sites in Iceland. (Tíminn 1926; Espelund 2007) We looked at available reports from the sites at Hals in west Iceland (Smith 1995; Smith 2007), at Vatnsfjörđur in the Westfjords of Iceland (Birch 2010), at Skógar in north Iceland (Guđmundur St. Sigurđarson 2013), and from the on-going excavation at Auđkúla in the Westfjords of Iceland. (Margrét Hallmundsdóttir 2017; 2022)

These excavations reveal the outlines of the smelting furnaces and the remains of structures on the site. They tell us about the raw materials, waste materials, and finished product on site.

Yet the excavations fail to reveal the process. How were the furnaces constructed? How were the furnaces operated? How were the raw materials processed to create the finished iron used in the manufacture of useful articles?

Earlier researchers, often using the tool of experimental archaeology, have created convincing hypotheses on iron smelting in Viking-age Scandinavia, and in many other ancient cultures and lands. (Hošek 2011; Espelund 1984; Sauder 2002) Was it different in Viking-age Iceland?

When we started our project, we had some convincing evidence that suggested it was. One of the surviving manuscripts of Landnámabók, a 13th century book detailing the settlement of Iceland, names Rauđa-Björn as the first person to smelt iron in Iceland, yet he was not one of the early settlers. (Landnámabók 1644) Given the importance to the Viking-age people of having a ready supply of iron, one might expect that the first settlers would already be smelting iron if the process was similar to what they were familiar with in their homelands.

It was this realization that sparked the idea in our minds at the start of the project that something was different in Iceland. After the project was completed, and it had become more clear that something truly was different in Iceland, this bit of evidence was called into question. Unpublished data from the ongoing excavation at Auđkúla suggest massive amounts of iron being smelted, yet dating of artifacts found at the site, although not yet verified, suggest that the farm may have been established in the very earliest period of settlement, well before the arrival of Rauđa-Björn. (Margrét Hallmundsdóttir 2022)

• Viking-age Iron Smelting

iron ore and elemental iron
The raw materials and the finished product of an iron smelt. Left: iron ore from excavated from the Viking-age iron smelting site at Skógar in north Iceland. Right: cuts reveal the interior of an iron bloom created in one of Hurstwic's modern reconstructions of a furnace.

Before discussing our experiments and research, it would be helpful to review how iron was smelted in the Viking age. Naturally occurring iron is in the form of oxides of iron mixed with other impurities in the raw iron ore. These oxides of iron, like the more familiar form of iron oxide called rust, is nearly useless, since it is weak and easily crumbles. The goal of smelting is to remove the oxygen from the iron oxide, leaving behind nearly pure elemental iron. In this form, iron is strong, capable of being formed into useful articles and honed to a fine cutting edge.

The smelting process takes place in a furnace heated to high temperatures and containing a reducing atmosphere with elevated levels of carbon monoxide (CO). The CO gas scavenges the oxygen from the iron ore, accelerated by the high temperatures, leaving behind elemental iron which drops into a structure that protects it from combining with oxygen once again.

Typical furnaces take the form of a structure made primarily of clay on the order of one meter high. A bore hole runs down the center of the structure and is the stack in which the smelting takes place.

Heat is generated by burning charcoal with a blast of air from a bellows at the bottom of the furnace. The fire is configured so as to generate both high temperatures (on the order of 1350°C) and excess CO gas in the stack.

smelting furnace schematic
Illustration of the key components of a medieval iron-smelting furnace.

charging the furnace with ore
Feeding iron ore into the stack of a modern reconstruction of a smelting furnace.

Iron ore is continuously fed into the top of the stack, along with additional fuel, so that the level of the mixture is maintained even with the top of the furnace. A series of chemical reactions take place as the ore falls through the stack, removing moisture, impurities, and eventually, the oxygen. The elemental iron drops down to the bottom of the furnace as small lumps.

At the same time, silicas from the walls of the furnace melt and fall to the bottom, forming glass-like slag. A portion of this material solidifies at the bottom, creating a bowl. The bowl fills with liquid slag which collects the falling particles of elemental iron. Contained within the liquid slag, the iron is protected and can't readily oxidize again.

excavated Viking-age iron slag
Viking-age slag excavated from the iron smelting site at Skógar in north Iceland.

The level of the liquid slag in the furnace needs to be controlled; too much and the process drowns. Occasionally, excess slag is drained through a hole made in the side of the furnace in a process called tapping the slag. The liquid slag flows like lava and solidifies, creating distinctive layers of solidified slag found at many iron smelting sites.

tapping the slag
Tapping the slag from one of Hurstwic's modern reconstructions of a smelting furnace.

extracting the bloom
Removing the iron bloom from one of Hurstwic's modern reconstructions of a smelting furnace.

The temperature inside the furnace is below the melting temperature of iron, so the elemental iron, while soft, is not liquid. As it falls to the bottom, the particles clump together into a sponge-like lump of elemental iron called a bloom.

The process takes many hours. At the end of the smelt, the hot bloom is removed from the bottom of the furnace.

The bloom is immediately worked while still red hot by hammering it to compact it and to drive out impurities, such as slag trapped between the particles of iron.

compacting the bloom
Compacting the bloom pulled from one of Hurstwic's modern reconstructions of a smelting furnace.

splitting the bloom
Splitting open the bloom with an axe at one of Hurstwic's modern reconstructions of an iron smelt.

In ancient times, the bloom was sometimes split open to examine the interior in order to judge the quality of the iron created. Raw blooms and split blooms are found in archaeological excavations of iron smelting sites.

ancient bloom
Partially split iron bloom from an ancient iron smelt at the farm of Mýnes in east Iceland. Ţjóđminjasafn Íslands 1965-139.

The bloom is still not ready for use; much more work is needed before the iron in the bloom can be turned into a useful product. The bloom must be heated, folded, and hammered repeatedly to drive out impurities, such as slag. The end product was an iron bar called a billet, the raw material for the smith. These raw iron bars were sometimes formed into currency bars and used as trade goods.

iron currency bars
Viking-age iron bars of the type thought to have been used for trade, UiO Kulturhistorik museum C24471. Creative Commons.

The size of the bloom varies from a few kilograms to 20 kg or more. Large blooms are thought to be impractical due to the difficulty of reheating and subsequent reworking of such a large lump of material to create the billet using the tools and forges available in Viking times.

Many things can go wrong during the smelt, giving low yields or poor quality material. Even when all goes well, the smelting process uses a large quantity of raw materials. In order to create a one kilogram finished iron bar, about 2 kg of bloom was needed, created using 4-8 kg of ore and 8-16 kg of charcoal. The ratios are highly variable and depend on many factors.

• Our research

We started our research project by gathering together an international team of experts: archaeologists excavating ancient iron smelting sites in Iceland; geologists; high temperature material experts; wood and forestry experts; smiths; and others.

We looked at the raw materials we would need to determine what would have been widely available around Iceland in the Viking-age.

source of Kringlumýri ore
Iron ore for many of Hurstwic's smelting experiments was dug from this slope at Kringlumýri in north Iceland.

Iron ore is found all over Iceland, often in or near bogs. Large quantities of ore are readily available. An analysis of samples from several sites, including ore excavated from ancient iron smelting sites, showed that generally the available ore was high quality, containing large amounts of iron oxides, and containing few impurities that might poison the iron smelting process, such as sulfur.

ancient ore from Skógar
Ancient iron ore excavated from the Viking-age iron smelting site at Skógar in north Iceland.

Kringlumýri ore analysis
Measured composition by weight of the modern ore from Kringlumýri derived from SEM/EDS analysis.

Skógar ore analysis
Measured composition by weight of the ancient ore from Skógar derived from SEM/EDS analysis.

Charcoal was readily available. (Espelund 2007; Church 2007) Extensive birch (Betula) forests existed in Viking-age Iceland that provided wood for charcoal. (Jakob Benediktsson 1986; Ţorleifur Einarsson 1962; Egill Erlendsson 2010)

The clay typically found in Iceland is bentonite, and that is a problem for the smelter. Unlike clays found in other lands, bentonite melts a temperature below that required for the iron smelting process.

Lastly, silica sand was likely a component in the walls of smelting furnaces to add strength to free-standing clay furnaces, and also to contribute silica for slag production. Yet little silica sand is found in Iceland, and only in a few places.

• Materials problems and speculative solutions

The materials needed for a free-standing clay furnace used in other Viking lands and by other cultures were not readily available throughout Iceland. Accordingly, we looked to find solutions that would have been available to Viking-age Icelanders and which were not contraindicated by available archaeological evidence.

One suggestion was a different furnace design. Excavations at smelting sites suggested Icelandic furnaces used turf as a component, a material used for many purposes in Iceland. (Guđmundur St. Sigurđarson 2013; Sigríđur Sigurđardóttir 2001) The details of the furnace construction are not clear from available evidence, but we speculated the furnace was a pile of turf blocks with a circular open central shaft lined with clay forming the stack. Much less clay is required than for a free-standing clay furnace, and the turf provides the structure for the furnace, so the sand in the clay becomes less important.

turf furnace
Modern reconstruction of a Viking-age turf-built iron smelting furnace built for Hurstwic's preliminary tests in Massachusetts.

furnace temperature measurements
Temperature measurements of one of Hurstwic's modern reconstructions of a turf-built iron smelting furnace in operation. The measurements demonstrate the ability of the turf to insulate and conserve the heat of the oven; the temperature inside the pile of the turf is only slightly raised over ambient.

Additionally, turf provides excellent thermal insulation, as shown by temperature measurements of experimental furnaces suggesting that a turf furnace might operate more efficiently than, for example, a free-standing clay furnace. Further, the turf helps to contain the fire should the clay lining in the central shaft be compromised. Lastly, the strong structure of the turf pile allows the furnace to be used again and again with little damage, unlike free-standing clay furnaces which suffer from cracking during repeated heating/cooling cycles and from the stresses of removing the bloom. Evidence at smelting sites suggests furnaces were reused many times. (Guđmundur St. Sigurđarson 2013)

The use of a turf-built furnace addresses some of the materials problems, but not the primary one: the readily available clay found in Iceland fails at the temperatures needed for smelting iron. What was needed was something that could be added to the clay to increase its melting point: some kind of refractory material. Two good candidates are materials containing either alumina or silica.

One possible source is materials from hot spring sites or volcanic sites that are rich in silica. It seems likely this material would serve the purpose, but some of the iron smelting sites in Iceland were far from volcanic areas. Since we postulated that the raw materials for smelting were obtained locally, we continued to look for suitable material that would be more widely available.

Organic material, such as ash from grasses and other vegetation, was suggested and tested. We created ash from several different varieties of grasses growing throughout Iceland and analyzed the ash. While several showed promise, we continued the search.

It was suggested we try ash made from horse manure, which tends to concentrate undigested grass fibers. Additionally, horse manure was widely used in Viking times as a structural material. We gathered horse manure samples in Iceland, burned them to ash, and analyzed the results. The horse manure ash samples were high in silica and appeared to show promise as a possible refractory material for a smelting furnace. Additionally, the manure has the benefit of being readily available all across Iceland.

composition of horse manure ash
Measured composition by weight of ash made from the manure of Icelandic horses derived from SEM/EDS analysis.

• Our Tests and Experiments

Hurstwic smelting furnace
A modern replica of a free-standing clay smelting furnace, one of many smelting furnaces built for Hurstwic's preliminary tests in Massachusetts.

We began our testing at Hurstwic's home in Massachusetts. For more than a year, we performed numerous smelts in order to hone our skills, test our protocols, and prove our measurement systems and analytical techniques.

A full accounting of the tests we performed at home would be the basis for a research paper on its own. Here is only a brief listing of the some of the areas we investigated in these Massachusetts-based smelts.

We tested several types of furnaces, including: traditional free-standing clay furnaces made with heat-resistant kaolin clay; stone ovens with a central clay-lined shaft; and turf ovens similar to those suggested by archaeological evidence at Skógar and other Viking-age iron smelting sites in Iceland. We tried various recipes for mixing kaolin clay and various construction techniques to strengthen the furnace so it could be reused more times.

We experimented with several types of charcoal, both commercially available product and home-made birch charcoal.

We tried variations of the smelting protocols, varying burn rate, airflow rate, and ore/fuel ratios. The tuyere, the pipe which admits the air blast into the furnace through the side, was tested using various positions and various materials for which there is evidence of use in ancient times, including ceramic, copper, and iron. We tested several different designs for the floor of the furnace to allow the bloom to removed quickly at the end of the smelt with minimal damage to the furnace. We tested several slag tapping protocols.

Hurstwic smelting furnace construction
Building a modern replica of a free-standing clay smelting furnace for Hurstwic's preliminary tests in Massachusetts.

Throughout these experimental smelts, we developed a team that could work together, keeping the needed materials flowing smoothly into the maw of the furnace. We also developed a working relationship with a local analysis lab, on whom we could count to deliver accurate analyses of some of the bizarre materials we were using. We are grateful to the staff of Sturbridge Metallurgical Services for their advice and suggestions.

Iron smelting furnace
Hurstwic's replica iron smelting furnace in operation at Járngerđar hátíđ, held at Eiríksstađir in west Iceland in 2019.

We conducted our final tests at an iron-making festival in Iceland, Járngerđar hátíđ, held in 2019 at what is perhaps the home of experimental archaeology in Iceland: the reconstructed Viking-age house at Eiríksstađir in Haukadalur in west Iceland. The house is a reconstruction of a Viking-age house on the site excavated in 1998 and thought to be the home of Eirík the Red (Eiríkr rauđi Ţorvaldsson), who explored and settled Greenland. (Einar Ol. Sveinsson 1934c; Guđmundur Ólafsson 1998) The reconstruction was built based on the principles and findings of experimental archaeology, so it seemed an apt site for our investigation of iron smelting using experimental archaeology. In a multi-day event open to the public, we did our final testing.

Viking house at Eiríksstađir
The reconstructed Viking-age turfhouse at Eiríksstađir in west Iceland.

• Our protocols

Many of the basics of smelting bloomery iron were developed by Sauder, and we based our protocols on his work and adapted them to suit our furnaces and raw materials. (Sauder, 2002; Sauder 2022)

We built two nominally identical turf furnaces, furnace #1 and #3. They were constructed of piled blocks of turf, which was cut in Haukadalur in west Iceland. The outside dimensions of the turf furnace were approximately 100 x 100 cm, and 75 cm tall.

smelting furnace
Hurstwic's replica of a turf-built smelting furnace in operation at Eiríksstađir.

The interior shaft in which the smelting took place was in the form of a truncated cone, 75 cm tall, with an approximate diameter of 40 cm at the bottom and 20 cm at the top. The tuyere was inserted into the shaft at a 22 degree angle, with the air blast approximately 20 cm from the bottom of the shaft.

The shaft was lined with a clay mixture approximately 2-3 cm thick. The clay lining was extended another 2-3 cm outward from the shaft in between each layer of turf, which we found made for a stronger structure and helped seal against any possible breakout of the heat.

turf-built smelting furnace construction
Construction of a replica turf-built smelting furnace in Massachusetts, showing details of how the clay liner was extended between the layers of turf. More layers of turf will be added on top of the visible layer to complete the furnace.

tapping the slag
Tapping the slag through the tunnel in the side of Hurstwic's replica turf-built furnace at Eiríksstađir.

A stone reinforced tunnel was created at the base of the furnace through the turf and into the central shaft, located 90 degrees away from the tuyere. The tunnel allowed the slag to be drained when required, and allowed the bloom to be removed at the end of the smelt with minimal damage to the furnace. A clay door sealed the tunnel at the central shaft, and the remainder of the tunnel was filled with turf during furnace operation.

The furnace was built over a pit approximately 30 cm deep and 40 cm in diameter, filled with stone, gravel, and sand sourced from Haukadalur in west Iceland. The pit aided in the removal of the bloom, since once the door into the shaft was open, the pit could be partially dug out, allowing the bloom to drop down and be pulled out.

The clay used was bentonite clay, sourced from the farm of Ytri-Fagradalur in west Iceland. The clay was sun dried.

Horse manure ash was created by burning horse manure over a fire. The manure was gathered in Haukadalur in west Iceland. Our advance calculations suggested that an ash to clay ratio of 10/90 by weight would suffice. Testing on site revealed that more ash was needed, and for the furnaces, a 40/60 ratio was used.

We also built a conventional free-standing clay furnace (furnace #2) using foreign clay that resisted heat. The clay was Belgian kaolinite clay. This furnace was used as something of a control furnace to help us determine if there was any aspect of the Icelandic ore that might affect the smelting process, in an attempt to separate the effects of the clay and ash from the effects of the ore.

clay furnace
Hurstwic's furnace #2, a free-standing clay smelting furnace built using more heat-resistant foreign clay for our tests at Eiríksstađir.

furnace and bellows
Replica hand-pumped bellows providing the air blast for a smelting furnace during Hurstwic's tests at Eiríksstađir.

Air to furnace #1 and #2 was supplied by an electric blower (a shop vacuum operating in reverse). Air to furnace #3 was supplied by hand-pumped bellows. Virtually no information exists concerning the nature of the bellows used in the Viking age. We created bellows similar to those used in the later medieval period.

Hurstwic reconstructed bellows
Replica air bellows based on later medieval bellows designs as used in Hurstwic's smelting tests at Eiríksstađir.

Using leftover materials, we built a half-size furnace (furnace #4) which was never charged with ore, but used to conduct some heat experiments in an attempt to understand the behavior of our clay/ash mixture at smelting temperatures in the reducing atmosphere generated by smelting conditions.

iron ore
Samples of iron ore sourced from Auđkúla as used in Hurstwic's smelting tests at Eiríksstađir.

The iron ore used in furnaces #1 and #2 was sourced from Kringlumýri in north Iceland. It was sun dried and broken into pea-sized pieces of roughly uniform size. The iron ore used in furnace #3 was sourced from Auđkúla in the Westfjords of Iceland, and likewise, was sun dried and broken into pieces of uniform size.

Samples of charcoal sourced from Vaglaskógar as used in Hurstwic's smelting tests at Eiríksstađir.

The charcoal for our experiments was generously provided by Skógrćktin (Forestry Service of Iceland). Larch (Larix) wood from the forest at Vaglaskógar in north Iceland was harvested as part of normal clearing and thinning operations in the forest. The wood was processed in a modern charcoal pit to create the fuel for the furnaces. While not identical to the birch charcoal more likely used in the Viking age, the differences were not thought to be significant. The charcoal was broken into walnut-size pieces of roughly uniform size.

charcoal pit
Charcoal for Hurstwic's smelting experiments being produced in a modern charcoal pit at Vaglaskógar in north Iceland

Each smelt began with a gentle drying fire inside the furnace shaft, burning wood kindling with no air blast to slowly raise the internal temperature of the furnace. Subsequently, the fuel was changed to charcoal and the forced-air supply turned on. The air flow was adjusted to burn 2 kg of charcoal in a 10 minute interval. Once the proper air flow was established, iron ore was added alternately with the charcoal. At first, 1 kg of ore and 2 kg of charcoal were fed into the furnace stack during every 10 minute interval, but once the smelting process was well-established, the ratio was increased to 2 kg of ore for every 2 kg of charcoal. Ore and charcoal were added as needed in the appropriate ratio to keep the furnace continuously full to the top of the stack.

Slag was tapped from the bottom of the furnace as needed during each run. A sight glass in the end of the tuyere allowed us to inspect the interior of the furnace during operations to better judge when tapping was needed and to assess furnace operation.

view down the tuyere
Interior view of Hurstwic's smelting furnace in operation at Eiríksstađir as seen through the sight glass of the tuyere. This view helped us assess the operation of the furnace and the level of the liquid slag.

extracting the bloom
Extracting the bloom from furnace #1 during Hurstwic's smelting tests at Eiríksstađir.

Furnace #1 yielded a 1.4 kg bloom, a yield of 4.7%. This yield was lower than we expected, and may have been due to the higher than expected melting of the clay furnace walls, resulting in excessive slag. This slag traps elemental iron, and much of our iron may have flowed out the bottom of the furnace contained within the liquid slag.

Furnace #2 yielded a 3.1 kg bloom, a yield of 10.3%. Again, this yield was lower than expected, and we do not have a ready explanation.

Furnace #3 yielded only a few small bits of iron totaling 0.61 kg, a yield of 2%. Our subsequent research and analysis of the ore from Auđkúla used for this furnace showed that it contained sulfur, a poison for the smelting process, which could explain its poor performance in the furnace. An approach used to purify ore is to roast it prior to smelting. Had we known of the presence of the sulfur, we would have taken this additional step. Our subsequent tests showed that roasting the Auđkúla ore dropped the sulfur content below the level of detectability, which likely would have improved our yield in furnace #3.

Auđkúla ore composition
Composition by weight of iron ore from Auđkúla derived from SEM/EDS analysis. This ore was used in furnace #3 during Hurstwic's smelting tests at Eiríksstađir.

• Our iron smelting results

iron bloom
A portion of the iron bloom extracted from Hurstwic's furnace #1 at Eiríksstađir.

In summary, we built several test furnaces based on evidence from Icelandic archaeological sites. We used all Icelandic sourced materials, including horse manure ash as a refractory material. And we made iron.

analysis of iron bloom
SEM-EDS analysis results of the iron bloom extracted from Hurstwic's furnace #1 at Eiríksstađir. Note that other impurities, notably carbon, may be present but below the threshold of the analytical tool to detect.

Analysis of the iron from furnaces #1 and #2 showed that it is excellent iron, nearly 100% pure, with an excellent crystalline structure and few inclusions of slag or other impurities.

grain structure of iron bloom
Microphotograph of an etched face of the bloom from furnace #1 at Eiríksstađir.

We subsequently used period techniques to form a part of one of the blooms into an iron billet, further proving the excellent qualities of the iron. It was more than good enough for making tools, weapons, or other useful products.

bloom and bar
A portion of the iron bloom extracted from Hurstwic's furnace #1 at Eiríksstađir, alongside a billet made from another portion of the same bloom.

iron exhibit
Museum guests at the opening of the Viking-age iron smelting exhibition at the National Museum of Iceland. The exhibit is based on Hurstwic's research.

Our research is now the basis of a special exhibit on Viking-age iron smelting at Ţjóđminjasafn Íslands (The National Museum of Iceland) entitled Úr mýri í málm (From mire to metal).(Ţjóđminjasafn Íslands. 2022) It is quite possible that Hurstwic's iron was the first iron smelted in Iceland for many centuries.

• Conclusions from our research

We were unable to falsify our two hypotheses, and so they still stand: evidence suggests that it is possible to make high quality iron with materials and methods known to have been available to Viking-age Icelanders; and that it is possible to make high-quality iron with locally sourced Icelandic materials.

Our work in no way proves that horse manure ash was the refractory material used, nor was that the intent of our research. That choice was speculative, and there may be other materials more likely to have been used. We simply do not know based on currently available evidence. Additionally, the horse manure ash did not work as well as we expected. Much larger quantities of ash were needed than was calculated, and even these additional amounts did not fully prevent partial melting of the furnace walls.

Yet what can not be questioned is that we made good quality iron. We were unable to falsify our hypotheses, thus giving them greater credence. We broke free of the trap of the modern mindset that has long suggested that only poor quality iron was possible. In a joyous multi-year adventure, involving experts in many fields from two continents, we achieved our goal using experimental archaeology: we changed history. Or, at the very least, we made a major correction to long-held beliefs about iron smelting in Viking-age Iceland.

• Acknowledgements

We are grateful for the support of the many people who made this project possible:

James Austin, Eric Beebe, Bjarki Sigurđsson, Bjarnheiđur Jóhannsdóttir, Matthew Card, Emiliano Carrillo, Michael Cicale, Fanndís Huld Valdimarsdóttir, Guđmundur Stefán Sigurđarson, Halla Sigríđur Steinólfsdóttir, Lee Jones, Kristine Krakowski, Gregory Lott, Josh and Staci MacNeil, Margrét Hrönn Hallmundsdóttir, Snćbjörn Guđmundsson, Valdimar Reynisson, Barbara Wechter, Ţröstur Ţorsteinsson.

The authors regret any omissions in this list.

• References

Birch, Thomas. 2010. Assessment and evaluation of the archaeometallurgical residues from Vatnsfjörđur, Ísfjarđardjúp (Westfjords, Iceland). Archaeomaterials Asessments, Report 2010/3 (2010).

Church, M J et al. "Charcoal Production During the Norse and Early Medieval Periods in Eyjafjallahreppur, Southern Iceland." Radiocarbon 49(2) 2007, 659-672.

Egill Erlendsson and Kevin J. Edwards. 2010. "Gróđurfarsbreytingar á Íslandi viđ landnám." Árbók fornleifafélagsins. 101 (2010), 29-55.

Einar Ól. Sveinsson, ed. 1934a. Laxdćla saga. Íslenzk fornrit 5. (Reykjavík: Hiđ Íslenzka fornritafélag, 1934.) ch. 49, p. 153.

Einar Ól. Sveinsson and Matthías Ţórđarson, eds. 1934b. Eyrbyggja saga. Íslenzk fornrit 4. (Reykjavík: Hiđ Íslenzka fornritafélag, 1934.) ch. 44, p. 122.

Einar Ól. Sveinsson and Matthías Ţórđarson, eds. 1934c. Eiríks saga rauđa in Einar Ól. Sveinsson and Matthías Ţórđarson, eds. Eyrbyggja saga. Íslenzk fornrit 4. (Reykjavík: Hiđ Íslenzka fornritafélag, 1934.)

Espelund, Arne, ed. 1984. Bloomery Ironmaking During 2000 Years. Vol. 2. (Trondheim: Budalseminaret 1984)

Espelund, Arne. 2007. "Ancient Ironmaking in Iceland, Greenland, and Newfoundland." Archaeologia Islandica 6 (2007), 48-73.

Guđmundur Ólafsson. 1998. Eiríksstađir í Haukadal: Fornleifarannsókn á skálarúst. Rannsóknaskýrslur fornleifadeildar 1998 no. 11 (Reykjavík: Ţjóđminjasafn Íslands).

Guđmundur St. Sigurđarson and Guđný Zoëga. 2013. Skógar í Fnjóskadal: Fornleifarannsókn 2011-2012. Byggđasafn Skagafirđinga Rannsóknaskýrslur 2013/140. (Sauđárkrókur: Byggđasafn Skagfirđinga, 2013)

Halldór Laxness. 1952. Gerpla. (Reykjavík: Helgafell, 1952.) Translated by Philip Roughton as Wayward Heroes (Brooklyn: Archipelago Books, 2016) ch. 17, pp. 134-135.

Hošek, Jiři et al, ed. 2011. The Archaeometallurgy of Iron: Recent Developments in Archaeological and Scientific Research. (Prague: Institute of Archaeology of the ASCR, 2011)

Jakob Benediktsson, ed. 1986. Landnámabók in Íslendingabók Landnámabók. Íslenzk fornrit 1. (Reykjavík: Hiđ Íslenzka fornritafélag, 1986.) ch. 1, p. 5.
"Í ţann tíđ vas Ísland viđi vaxit á miđli fjalls ok fjöru."
"In that time, Iceland was covered with woods from mountain to shore."

Landnámabók, AM 106, fol. 12r. 1644. Landnáma og ýmiss samtíningur. Ísland, 1644-1651. https://handrit.is/manuscript/view/is/AM02-0106/27?iabr=on#page/11v/mode/2up (accessed December 23, 2022).
"Hann blés fyrstr manna rauđa á Íslandi, ok var hann af ţví kallađr Rauđa-Björn."
"He was the first man to smelt iron in Iceland, and so he was called Rauđa-Björn."

Margrét Hallmundsdóttir and Sólrún Inga Traustadóttir. 2017. Arnarfjörđur á miđöldum: Framvinduskýrsla 2016. Náttúrustofa Vestfjarđa 01-17. (2017)

Margrét Hrönn Hallmundsdóttir. 2022. Conversations during site visits on August 17, 2022, and on other dates.

Sauder, Lee and Skip Williams. 2002. "A Practical Treatise on the Smelting and Smithing of Bloomery Iron." Historical Metallurgy 36(2) 2002 122-131.

Sauder, Lee. 2022 "Smelting Research," Lee Sauder. https://www.leesauder.com/smelting-research (accessed December 21, 2022).

Short, William R. and Reynir A. Óskarson. 2022. "Traps and Pitfalls in the field of Viking research," Hurstwic, https://www.hurstwic.com/research/text/traps_and_pitfalls.htm (accessed December 20, 2022).

Sigríđur Sigurđardóttir. 2001. Torf til bygginga. Smárit Byggđasafns Skagfirđinga VII. (Sauđárkrókur: Byggđasafn Skagfirđinga, 2001)

Smith, Kevin P. 1995. "Landnám: the settlement of Iceland in archaeological and historical perspective." World Archaeology 26, no. 3 (1995): 319-347.

Smith, Kevin P. 2004. "Ore, Fire, Hammer, Sickle: Iron Production in Viking Age and Early Medieval Iceland." in De Re Metallica: The Uses of Metal in the Middle Ages, ed. Robert Bark. 183-206 (Farnham: Ashgate, 2004).

Tíminn. 1926. "Ísland í erlendum rítum." Tíminn (Reykjavík), October 2, 1926.

Ţjóđminjasafn Íslands. 2022. "Úr mýri í malm." Ţjóđminjasafn Íslands. https://www.thjodminjasafn.is/syningar-vidburdir/sersyningar/syningar-i-gangi/ur-myri-i-malm (accessed December 21, 2022).

Ţórarinn Ţórarinsson. 1980. "Ísarns meiđur á Eiđum." Múlaţing 10 (1980): 31-55.

Ţorleifur Einarsson. 1962. "Vitnisburđur frjógreiningar um gróđur, veđurfar og landnám á Íslandi." Saga 3 (1960-1963): 442-469.

• Other video resources

Experimental archaeology: putting theories to the test, a lecture by William R. Short and Reynir A. Óskarson presented at the National Museum of Iceland on Hurstwic's use of experimental archaeology in our research, drawing examples from our iron-smelting research and our combat research. (Introduction is in Icelandic, but the lecture is in English.)

A video story by Gísli Einarsson broadcast on Landinn on Icelandic television RÚV. The story covers the events at Járngerðar hátíð, the iron-making festival sponsored by Hurstwic and Eiríksstaðir in 2019, the culmination of Hurstwic's research in Viking-age iron making in Iceland.


A guided tour of the special exhibit on Viking-age iron smelting now on display at the National Museum of Iceland. The exhibit is based on Hurstwic's research in this field that altered the vision of history by demonstrating that long-held and widely-taught beliefs were faulty.

Firing Up Ancient Secrets, a lecture by Dr. William R. Short on Hurstwic's research in to how iron was smelted in Viking-age Iceland. The lecture was presented on-line by the Massachusetts Archaeological Society Gene Winter chapter.


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