Wednesday, May 6, 2009

The Viking Bronze Age and Furnace


The Bronze Age is roughly described as a time period where societies used advanced metalworking that included smelting cooper and tin, creating bronze alloy by melting cooper and tin together and casting bronze artifacts. The Bronze Age is regarded as part of a three stage system of prehistoric societies: Neolithic Age (Stone Age), Bronze Age and Iron Age. However, these time periods are not uniformly spread across different nations. For example, in sub-Saharan Africa copper was used predominantly, thus the Copper Age is considered first then the Bronze Age.
The oldest example of bronze was found in North Caucasus Mountains. The Maykop culture that lived in what is now modern day Georgia, Armenia, Azerbaijan and Russia used arsenic bronze dating to the mid 4th millennium B.C. Tin Bronze was developed much later since it requires two separate smelting processes. Tin ore was mined and smelted and then added to melted copper alloys to create the bronze alloy. The general Bronze Age time period dates from 3300-1200 B.C. and spans from the Mesopotamia-Urk period, Ancient Egypt’s Proto-dynastic period, Levant-Caanan Period, Anatolia-Hittite Empire, the Persian Plateau, Central Asia-China and the Indus Valley [1]. The Nordic Bronze Age (Northern Bronze Age) was given this name by Oscar Montelius. He divided the Nordic Bronze Age into six sub-periods in his book On Bronze Age Dating with a Particular Focus on Scandinavia [2]. For Central Europe, which consisted of modern day Germany and Poland, a system for dating the Bronze Age was created by Paul Reinecke and established the Bronze Age. These two systems overlap each other and cover most the dating for the Bronze Age in Northern Europe [3].

Oscar Monteilus-Nordic Bronze Age Paul Reinecke-Central Europe Bronze Age
I: 1800 B.C. -1500 B.C Bronze Age (BR) An Early Bronze Age
II: 1500 B.C-1300 B.C. BR B
III. 1300 B.C.-1100 B.C. BR C
IV: 1100 B.C.-900 B.C. BR D Late Bronze Age or Urfield Period
V: 900 B.C.-700 B.C.
VI: 700 B.C.- 500 B.C.

The Bronze Age started later in Nordic Bronze Age in comparison to the other countries during the Bronze Age. Oscar Montelius described the Bronze Age periods as follows:
Period I: Metal production was still light and significant numbers of objects were imported from Central Europe and the Carthpathean Basin. Bronze items had a distinct Mediterranean style.
Period II: Inhumation burials were at its height and burrows and cairns graves were richly furnished with personal items and household goods.
Period III: Cremation instead of inhumation burials became more common and fewer artifacts are found in graves but bronze items are still found in other sites.
Period IV: Bronze Age was the dominate feature in Nordic Bronze Age.
Period V and VI: Later Bronze Age and the beginning of the Iron Age. Mixture of both bronze and iron are found in graves [4].

In comparison to the rest of Europe Scandinavia entered the Bronze Age late and in the beginning through trade only. It is theorized by historians that the early Nordic Bronze Age was heavily influenced by trade with Mycenaean Greece, the Villanovan culture, Phoenicia and Ancient Egypt. Early bronze pieces found in Scandinavia and the Baltic have distinct Mediterranean influences. Baltic amber has been found in Mycenaean graves and Tutankhamen’s grave from the Early Bronze Age which clearly shows that early Scandinavia and the cultures in the Mediterranean traded on what has been labeled the Amber Road [5].
In 1940’s and 1950’s archeologists discovered workshops in Sweden and Denmark that date to the Viking Migration Age. These workshops were found in the settlement of Helgö in Lake Malaren (5th -8th Century), in the towns of Lund and Sigtuna (11th-12th Century), and in the Viking trading town of Birka (8th-10th Century). Workshops were also found and excavated in Ribe (8th Century) Demark. These excavated workshops included furnaces for smelting and casting and forges. These workshops have been so well preserved that historians have been able to study the technical aspects of the Bronze and Iron Age craft in Scandinavia. These early workshop founders have their start in the Nordic Bronze Age. The process for iron smelting and casting is the same process used in bronze smelting and casting. The Nordic Bronze Age did not end with the advent of the Iron Age. Casting in bronze and silver was an important cultural aspect in the Nordic/Viking culture with status being represented by jewelry, personal items and household goods. Social codes, expression of identity, fidelity and social ranking were marked casted items from these workshops [6].

Bronze is an alloy that is 85-95% copper, with the other 5-15% tin or arsenic. Other metals have also been found present in small amounts in some bronze examples. This combination of copper and tin makes the alloy stronger and makes harder tools and weapons. Tin bronze melts at 950 C (1742 F.) which makes it an easy alloy to melt and cast. During the “Viking Age” the bronze that has been found in many sites actually is “brass” [7]. Cooper mixed with zinc makes brass. Copper and zinc smelting wasn’t new to the Nordic Bronze Age. Romans during the 1st Century A.D. discovered methods of mass producing brass. The Romans discovered that smelting zinc ore with copper alloys would make brass alloys called “cementations process.” The Romans built brass foundries in what is now Belgium and Germany. The brass alloy was cast into ready made objects, scrap metal and rods for future use. Trade between Germany and Scandinavia continued after the fall of the Roman Empire and artifacts begin to take on a distinct Germanic touch. Brasses alloys with the addition of tin or lead are found through out Scandinavian castings through the Migration Period, Vendel and Viking Eras [8].
Before a discussion of a Nordic Bronze Age furnace can be entertained, terminology needs to be clarified. In discussion “forge” and “furnace” are often used to describe the same thing. By definition and use a forge is a: 1. hearth where metals are heated or wrought; a smithy. 2. A workshop where pig iron is transformed into wrought iron [9]. In comparison and for the purpose of this project a furnace is: “is used to melt metal alloy in order to cast it” [10].

The bellows are the key to the furnace and the forge. The bellows push the temperatures up over 3000 degrees. A single bag bellows provides an intermittent supply: strong air flow as the bellows bag is compressed, and then no air flow as the bellows bag is drawn open to be refilled. This pause, allows the combustion rate to drop which drops the temperature. Each compression of the single bag bellows must first increase the combustion rate; the desired temperature increase will soon follow, only to start cooling again as the bag reaches empty.
The double bag bellows are capable of providing a steady stream of air when pumped correctly. Without the pause of the single bag bellows, the temperature is sustained and can more efficiently be raised.
Wood and stone carvings from the Viking Age frequently depict double bag bellows with metal workers. The art of making bellows, like many other arts, was passed from master to apprentice with only the wood and stone depictions as sparse documentation. Modern reproductions vary widely in their construction details, each based on the builder’s interpretation of the available drawings and on the builder’s ability to glean tips and ‘things to avoid’ from other builder’s stories.
Deceptively simple elements like the valves and the cut of the bags can have dramatic performance impacts. One of the most common errors is patterning the sections of the bags as simple triangle instead of direct patterning the complex curves from the assembled bellows structure [11].

Archeologists and historians have had difficulty finding Bronze Age furnaces and molds. Anders Söderberg of Viking Bronze states that, “medieval founders cast using technology with roots deep in the Bronze Age.” Experimental archeologists and groups like Umha Aois in Ireland, Hammer Out Bits in Canada and Sagnlandet Lejre “Land of Legends” in Demark have experimented with different styles of furnaces, bellows and molds to catch a glimpse into what may have occurred in these Nordic Bronze Age workshops. Anders Söderberg of Stockholm, Sweden has successfully re-created a charcoal pit furnace modeled after one discovered in Helgö, Sweden. He designed his bellows after the pattern seen in Sigured runic carving in Sörmland, Sweden. He has been able to cast small bronze pieces based off designs found in Birka, Sweden. Umha Aois (“Bronze Age” in Irish) is an ongoing experimental project that explores different styles of furnaces and molds for casting bronze. During the week of June 20th and July 2nd of 2005, Umha Aois gathered for a week symposium to experiment with stone and clay molds and three different styles of furnaces construction and use. The three styles furnaces technologies were studied. 1. Middle Eastern Upright types. 2. Pit Furnaces 3. Bowl Furnaces.
In an email correspondence with James Hayes of Umha Aois he explained that “the key to casting bronze in a Bronze Age approach is maximizing and controlling the efficiency of a very small scale fire. Large fires are not the answer, even for smelting. The furnace should have an inside volume only slightly larger than the size of the crucible. The space between the wall of the furnace and the crucible should be just large enough to pack some charcoal in, around two inches all around. Any larger cavity is simply a waste of fuel and leads to an inefficiently burning fie which hovers at temperatures too low to melt bronze.” I have based my furnace design on this description and the hearth on Anders Söderberg Helgö discovery.

Experimental archeology is not for the faint of heart. The Society for Creative Anachronism historical group, Drafn, recreated bread ovens for the Son’s of Norway Viking Festival based off bread ovens found in York, England and Colonial Williamburgs,Virgina. For these bread ovens the groups used refractory clay, sand and “organic matter.” James Hayes of Umha Aois confirmed this technique for the Bronze Age furnaces. In an email correspondence he described the use of horse dung as the “organic matter” used to mix with the clay and sand. Umha Aois had also experimented with mule, cow, peat and other forms of “organic matter” and discovered that horse dung was the best source.
Making of the furnace walls. The best ratio for the mixture was 2-1-1 ratio.
2 part refractory clay
1 part sand
1 part “organic matter- complete with horse hair and bugs” horse dung.
Roughly 1 cup of water.
Mix well. Hand mixing was the most successful method. The mixture became a sticky heavy consistency that clung to the walls of the mold and everything else it came into contact.
A coffee can was used as the mold for the furnace wall with the intention of removing the coffee can after the clay dried. To our surprise, the coffee can could not be removed without destroying the clay sides. The pictures above are the recreated furnace wall and chimney lid drying in the sun. For future experiments, ceramic clay is going to be substituted for the refractory clay. The refractory clay worked well for the furnace wall but was not appropriate for molds. The consistency of the clay was too sticky and required a structure or frame to hold its shape. In the evening coal was placed inside to help cure the walls.
In 2005, students from the University of London, Birkbeck College conducted a series of experiments that included recreating furnaces. The students wanted to monitor how weather and human abandonment influenced the destruction of clay furnaces. Archeologists have had difficulty finding complete furnaces even though they have found mines. The students constructed three types of early furnaces: 1. a clay lined hearth. 2. A pit hearth 3. A dish furnace a bottom of a narrow hole with a tuyere inserted. Students then smelted different combinations of cooper ore and studied the rate and speed of the smelting process. The students discovered and concluded that due to the high amount of heat the clay became fragile and vanished into dust. Rain, wind and erosion erased what remained. Unlike the Romans who wrote volumes about their engineering, very little was written on the construction of Bronze Age furnaces and smelting process. Fragments of hearths and molds only give anthropologists, experimental archaeologists and research scientists’ clues on the construction and use of these furnaces [12].
Archaeologists have only found fragments of molds at the different foundry sites for the Norse Bronze Age. These mold fragments are; soap stone, ceramic clay, cuttlefish bone, wood, and stone. Since the 1940’s there has been a great deal of discussion on whether the lost wax method was used or a metal alloy piece pressed directly into a mold material. It is agreed upon that a mold was made using clay tempered with fine sand and organic material, usually horse or cattle dung. According to Anders Söderberg, “The question of lost wax is difficult to deal with. The literature does not seem to record any findings of wax from workshops related to contexts except for one single case; a Viking goldsmith’s grave from Mysen in south east Norway. A man was found buried accompanied by crucibles, molds and piece of wax. In current literature both techniques are discussed [13].
From personal experimenting I discovered the following. Soap stone is excellent for pewter casting since pewter melts at a low temperature (700 degrees) and doesn’t harm the integrity of the design. For multiple pewter castings soap stone is an excellent reusable source. However, it does not handle the high temperatures needed for bronze (1,700-2,000 degrees) and the integrity of the design is damaged. Cuttlefish bone is good for small pieces about the size of a dime and is one time use. The pieces cast with cuttlefish were in silver which has a melting temperature of 1,760 degrees. I discovered that silica based investment or fine clay works best with bronze when using the lost wax method of casting. It can handle the high heat and weight of the bronze alloy.
The refractory clay used for the furnace walls was too sticky to be easily worked and required a firm frame to build upon and to hold its structure. In future experiments ceramic clay will be substituted for the refractory clay when making molds.

1. “Chapter 4: The Bronze Age” U.C Davis Geology Department.
2. Harding, A.F., European Society in the Bronze Age Cambridge University Press. 2000 pg.10
3. Harding, A.F., European Society in the Bronze Age Cambridge University Press. 2000 pgs. 11-12.
4. et al. pg. 12.
5. et. al. pg. 13
6. Söderberg, Anders, “Metal Casting. Blowing New Life in Old Teachnology-Viking Age Metal casting.” Viking Heritage Newsletter Number 6 1999 pg. 13
7. “Chapter 4: The Bronze Age” U.C Davis Geology Department.
8. Söderberg, Anders, “Scandinavian bronze casting in the Viking Age and the Early Middle Ages.” Viking Bronze Blowing New Life in the Ancient and Early Medieval Metalcraft
9. Merriam Webster’s School Dictionary
10. Hayes, James of Umha Aois “Personal email correspondence with the Umha Aois and James Hayes regarding the construction of a furnace and molds.”
11. Markwewitz, Darrell, “Viking Age Blacksmith Bellows” Hammered Out Bits
12. Viegas, Jennifer, “Ancient Furnaces Vanish to Dust” Discovery News
13. Söderberg, Anders, “Scandinavian Iron Age and Early Medieval Ceramic moulds-lost wax or not or both?” Tulp, C. Meeks, N. Paardekooper. Proceedings of the 1st International Workshop. Experimental and Educational aspects on Bronze Metallurgy, Wihlminaoord. October 18-22, 1999. (2001)

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Merriam-Webster School Dictionary, Merriam-Webster, Incorporated Publisher 1999
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Söderberg, Anders, “Scandinavian Iron Age and Early Medieval Ceramic moulds-lost wax or not or both?” Tulp, C. Meeks, N. Paardekooper. Proceedings of the 1st International Workshop. Experimental and Educational aspects on Bronze Metallurgy, Wihlminaoord. October 18-22, 1999. (2001)
Söderberg, Anders, “Metal Casting. Blowing New Life in Old Teachnology-Viking Age Metal casting.” Viking Heritage Newsletter Number 6 1999
Söderberg, Anders, “Metallurgic Ceramics as a key to Viking Age workshops Organization.” Journal of Nordic Archaeological Science Vol. 14, pgs. 115-124 2004
Söderberg, Anders and Gustaffsson, Ny Bjorn, “ A Viking Period Silver Workshop in Frojel, Gotland.” Viking Bronze Blowing New Life in the Ancient and Early Medieval Metalcraft
Smith, Michelle H., “Chapter 5: Breaking the Mould: A Re-evaluation of Viking Age Mould Making Techniques for Oval Brooches.” De Re Metallica: The Use of Metals in the Middle Ages. Ashgate Publishing Ltd., 2005
“The Bronze Age”
“Chapter 4: The Bronze Age” U.C Davis Geology Department.
Viegas, Jennifer, “Ancient Furnaces Vanish to Dust” Discovery News

Blood~ Sweat~ and Tears……..

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