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Iron Production in the Viking Age

Raudanes bog

Although Norse people knew of mining and mined some iron ore in a variety of locations throughout Scandinavia, most Viking era iron was smelted from bog iron. The photo to the left shows the bog at Rauğanes in Iceland, where Skallagrímur Kveldúlfsson, one of the early settlers in Iceland, had his smithy. Chapter 30 of Egils saga describes him as a "great smith" who "smelted a lot of bog iron in winter".

Anvil stone at Raudanes

The saga says that Skallagrímur couldn't find an anvil stone to suit him, so he rowed out into Borgarfjörğur one evening in his boat. He dove to the bottom and brought up a suitable boulder for his anvil stone.

The anvil stone, with its hammer marks, still stands at Rauğanes (left), but it has been moved away from the shore. Ruins (right), which are the probable site of Skallagrím's smithy, are located closer to the shore.

At the Norse settlement in L'Anse aux Meadows in Newfoundland, Canada, there is clear evidence that Norsemen harvested and smelted bog iron to use as the raw material for the iron rivets they fabricated to repair their ships there 1000 years ago.

Where streams run from nearby mountains through a peat bog, bog iron can almost always be found. That combination was probably visible to Norse explorers even from on board their ships in the cove at L'Anse aux Meadows. It occurs in glaciated regions throughout the world, and so would have been very familiar to the Norse explorers at Vínland.

water flows from mountains
oily film from bacteria

Streams carry dissolved iron from nearby mountains. In the bog, the iron is concentrated by two processes. The bog environment is acidic, with a low concentration of dissolved oxygen. In the acidic environment of the bog, a chemical reaction forms insoluble iron compounds which precipitate out. But more importantly, anaerobic bacteria (Gallionella and Leptothrix) growing under the surface of the bog concentrate the iron as part of their life processes. Their presence can be detected on the surface by the iridescent oily film they leave on the water (left), another sure sign of bog iron. In Iceland (right), the film is called jarnbrák (iron slick).

iron slick

When a layer of peat in the bog is cut and pulled back using turf knives (right), pea sized nodules of bog iron can be found and harvested. Although the iron nodules are reasonably pure, there aren't many of them. They are, however, a renewable resource. About once each generation, the same bog can be re-harvested.

turf knives
red earth

In some regions (particularly Sweden), iron ore, rather than bog iron, was the raw material for smelting. The ore was in the form of "red earth" (rauği), a powdery ocher. Regardless of the source, the raw materials were usually roasted to drive off moisture before being smelted. The roasting also served to "crack" the surface of the iron ore, making it more porous so that the gases in the smelting furnace could enter and react with the iron more easily. Once the dry nodules or ore was in hand, the lengthy process of smelting the iron began.

The smelting operation was performed in a bloomery furnace, a small clay or clay-lined shaft shown schematically to the right. The furnaces were roughly circular, and about 80cm (32 in) tall and about 30cm (12in) in diameter.

The clay was actually a homogenous mixture of sand, fiber, clay, and water. The fiber was introduced into the mix as horse dung, in which the undigested hay provided the fiber. This mix had the needed structural properties to contain the smelting materials, as well as appropriate insulating properties. The walls must radiate enough heat to keep the inside surface from melting into the slag during the smelt.

furnace
drying the furnace walls with a wood fire

The furnace was built around a tapered wooden form. As the clay dried, the form was removed, and the drying was completed with a wood fire inside the furnace.

To begin the smelting operation, a fire was started inside the furnace with a natural draft, to prevent the thermal shock from rapid heating from damaging the furnace. One warmed, an air blast was supplied through the side of the furnace with a bellows, and the bore was filled with charcoal. (The photo to the right shows a modern replica furnace during smelting operations.)

The air was adjusted to control the burn rate, and ore and charcoal were regularly added to the top of the furnace in roughly a 1:1 ratio (by weight). Inside the furnace, the temperature reached 1100-1300ºC (2000-2400ºF) at the bottom of the furnace near the iron. A reducing atmosphere was created, rich in carbon monoxide. The gas scavenged the oxygen from the iron compounds in the ore, converting them to elemental iron:

bog iron chemistry

Not surprisingly, the reactions are considerably more complex than this simple equation would indicate. The temperature in the furnace ranged from 300ºC near the top to as high as 1300ºC near the bottom. Different chemical reactions occurred in the different temperature zones of the furnace.

bloomery furnace

adding fuel to the furnace

Throughout the smelting process, additional fuel (left) and ore (right) were added at the top of the furnace. The process was tended constantly, adjusting the fuel, ore, and air to optimize the results. The smelting operation lasted for many hours. Slag (consisting of oxides of iron, silicon, and other elements) was the waste product of the smelting process and was drained through a port at the bottom of the furnace.

adding ore to the furnace

iron bloom
photo: Jeff Pringle

When completed, a section of the lower furnace was removed, and a frothy iron sponge-like material called a bloom (blástrjárn) was levered free of the furnace walls. The bloom (left) was a mixture of low-carbon iron, slag, and charcoal. The surrounding slag and charcoal were knocked off, and the remaining material was compacted with sledges to consolidate the material.

The bloom was refined by folding it, which mechanically served to homogenize the material and to drive out the impurities, such as slag. The folding process was repeated multiple times to create cleaner, more highly-refined material.

The desired end result was a malleable low-carbon iron, ready to be forged to fabricate the required articles. In the past, I believed that the quality of the iron obtained was highly variable because the smelting process was difficult to control. Yet modern smiths using replica Viking-age bloomery furnaces are routinely turning out high quality iron, which suggests they have good control of the process. Regardless, the process was very inefficient; a lot of iron was left in the slag.

The labor and skill required to make an iron bloom meant that they were valuable commodities. The distribution of iron finds at the Viking-age house at Hólmr in Iceland suggests that iron blooms were used as cult offerings there.

At L'Anse aux Meadows, the iron was probably used to make rivets and washers for ship repair. The wrought iron was rich in silicate impurities, which formed a glassy surface on the iron. This is visible on the parts even today (right). The surface helped protect the iron from rust, even when immersed in sea water.

iron rivets
trade iron

Most domestically produced iron in the Norse era was tediously produced from bog iron. Because of the time consuming processes used to create it, smelted iron was valuable. Roughly worked iron bars were used as trade goods (left). These bars are from Norway and are about 30cm (12 inches) long.

The axe head blanks shown to the right were also trade goods. They were probably exported from Norway and were on their way to a trading center in Denmark when they were lost. They are about 20cm (8inches) long and were found threaded on a wooden stave for convenient handling during shipping. 

blank axe heads for trade
tool chest

Because of their expense, iron tools and weapons were highly valued. The loss of an iron tool from a Norse farmstead was a disaster, especially if it were a major tool like an axe or scythe. A typical farm in the Viking age probably owned no more than 40-50kg (100lbs) of iron, in the form of tools, weapons, and cooking equipment.

The Mästermyr chest (a reproduction is shown to the left) is an oak tool chest filled with tools that  belonged to a master smith in the 11th century. The chest and its contents were found in Mästermyr, Sweden in 1936. The chest contained an assortment of tools for working wood and metals (including hammers, axes, adzes, saws, spoon-borers, tongs, shears, files, and others), along with raw materials and finished products. It's been suggested that its owner lost the chest while crossing the mırr (bog) that gives Mästermyr its name. The loss must have been a devastating blow to its owner.

The ruins of the smithy at Stöng in Iceland are shown to the right. The foundation stones are visible around the periphery, and the dished stone in the center held water to cool the workpiece.

However, the interpretation of Viking age smithies remains troublesome.

smithy ruins at Stong
anvil stone at Adalbol

Surviving anvil stones are quite low to the ground, too low for comfortably working on a piece. The anvil stone at Ağalból in East Iceland is shown to the left. At Reykholt, a depression was found next to the location of the anvil stone. It has been suggested that the smith sat on the ground next to the anvil with his legs in the hole on the ground as he hammered at the anvil. Modern smiths I've talked to don't find this interpretation convincing.

On the other hand. the wood carving from a 12th century church in Norway (shown to the right) illustrates a scene from a Norse heroic myth in which Reginn reforges a sword for his foster brother Sigurğr. As depicted in this carving, the anvil seems uncomfortably low to the ground.

Perhaps smiths in the Viking age routinely worked at heights that modern smiths find awkward.

Regin forges a sword

It seems likely that iron arms and armor were treasured and passed from generation to generation. Lifetimes of hundreds of years seem possible for some iron articles such as helmets. When one compares the probable date of manufacture with the probable date of burial of some archaeological finds, one is forced to conclude that some iron items were used for a century or more before being buried with their final owner.

helmet

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