Viking Age Metal-casting. - Anders Söderberg. Strona 1
Blowing new life in old technology - Viking Age Metal-casting.
By Anders Söderberg.
In the last couple of decades, a few early medieval workshops have been excavated in Sweden; in the Migration Age setlement at Helgö
in lake Mälaren (5-8th C), in the towns of Lund and Sigtuna (11-12th C), in the Viking town of Birka (8-10th C), and several places in
Denmark, of which Ribe (8th C) holds an important position. Here we have been able to create and understanding of the craftsmen's
conditions and - more tangebly - and understanding of the technical aspects of the different crafts themselves.
Early medieval founders cast using technology with roots deep in the Bronze Age. The process looked almost the same as it had done for a couple
thousand years. If the Bronze Age was the golden age of bronze casting, the craft didn't die with the coming of iron. Casting in bronze and silver
still played an improtant social part through the production of jewellery and prestige objects, a production of social codes expressing identity and
belonging; sworn fidelity and social ranking. All confirmed by a system of gifts, sometimes almost strong enough to give each object a life of it's
own; at least this may have been the way contempory man regarded it.
I have devoted the last three years to examining some of these crafts in detail. I have worked particularly with reconstructed bronze and silver
casting, supportedby excavation publications and earlier experimental projects. There have been many of them in recent decades, with varying
ambitions and results, but the most extensive are probably the Danish and English. These projects are also the best documented.
The heart of the workshop
The very heart of the foundry is the charcoal hearth. The hearths I have been working with have been modelled on the small bronze-casting
hearths from Helgö workshops, excavated in the fifties and sixties.
A pair of bellows in conneced to the hearth. Like the hearth itself, these need not be large. The bellows I built are made from the pattern on the
Sigurd runic carving in Ströland, Sweden. Since the carmver seems to depict Sigurd's tools relatively true to proportion, I have used the carving
almost directly as a patern for my pair of bellows.
This hearth and bellows makes a small but efficient system. With a steady, actually quite relaxed, pumping it is easy to attain the necessary
1100-1200?C and a crucible filled with bronze can be melted in 15 minutes.
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Viking Age Metal-casting. - Anders Söderberg. Strona 2
The Moulds
Since the Bronze Age, the common form of mould of clay tempered with fine sand and some organic material. There has been a discussion since
the forties about how the moulds were made and which casting technique they derive from. The main standpoints in the discussion have been that
either the moulds were formed by pressing an original metal object into the moulding material and thereafter placing the two mould halves
together or that there were derived from a lost-wax method.
It seems quite natural to me to associate a plastic, water-soluble mould material with, lost-wax casting. My hypothesis regarding ancient clay
moulds is that they generally derive from such a process. I don't expect every archaeologist to agree with me but I am presently working on trying
to confirm it. Perhaps I will have to surrender in the end, but in that case my hypothesis will at least have been good research platform in the
matter.
Bronze and brass
In archaeology we often carelessly speak of all copper alloys as "bronze". When speaking of Viking "bronzes", we are actually referring to alloys
more closely related to present day brasses. Modern brasses are alloys of 30-40% zinc added to copper. During the first centuries AD, the Romans
worked out methods of mass-producing brass. At this time it was not possible to smelt zinc ore, but it was possible to mix zinc ore with metallic
copper and smelt them together thus obtaining the alloy from this process, called a cementation process.
The Romans founded brass foundries in present Belgium and Germany in the region around Aachen. The metal was exported from the continent
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Viking Age Metal-casting. - Anders Söderberg. Strona 3
to Scandinavia, as ready-made objects. As scrap-metal, and probably as raw material in rods. It is possible that this production continued under
Germanic management after the fall of the Roman Empire.
Brasses, with a slight addition of tin or lead, are represented in Scandinavian casting all through the Migration, Vendel and Viking eras. Alloys
like this have the advantage of running well when casting or solidifying slowly.
Crucibles - keys to the workshops' social status?
Like the moulds, the crucibles were made of sand-tempered clay, Domestic Scandinavian clays of earthenware qualities are not very resistant to
heat of the melting hearths, especially since the charcoal contains compounds that act as fluxing agents dramatically lowering the melting point of
the clay. The craftsmen solved this problem by tempering the clay with large quantities of sand or ground quartz. Despite this, their crucibles were
made of an unsatisfactory material that lasted for only a few meltings at best.
In Sigtuna, Sweden (11-12th centuries AD) I have seen a crucible of a completely different kind of ware. These crucibles are tall and elegantly
thin-walled, in a material that looks more like stoneware. They are far from the poor condition in which we often find the earthenware crucibles. It
is possible to imagine an import from the continent, of raw material or ready-made crucibles for the finest workshops in northern Europe. Trade in
ready-made Rhineware crucibles, with its base in Lübeck, took place a couple of centuries later. Is this an indication of earlier similar trade? If so,
this trade would have been quite extensive.
Could the presence of these crucibles be a sign that the patrons of the finest workshops provided the craftsmen with the best materials? Could it
indicate that continental craftsmen were employed, provided with the tools they were accustomed to, and which their expertise gave them the
prerogative to demand?
It would not be surprising if the finest raw materials and tools were in use at Sigtuna, as this was royal town in the early Middle Ages - and a
stronghold of the early Svealand Christian church, Situne Dei. So far no analyses have been made of these crucibles to confirm the nature of the
goods, although this might be and interesting phenomenon, worth closer study.
A stoneware crucible, tempered as described above, is good for up to ten or twelve meltings. It is a very useful and durable thermoshock-resisant
material.
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Casting
After the metal has been melted, casting can take place. Prior to this, the mould is fired in a separate hearth. The hearth in which the moulds are
fired does not require blowing with the bellows. With good ventilation it easily reaches the 700-800?C required, firing the moulds and keeping
them hot until casting.
After some 300 castings in moulds of tempered clay, I have founds this to be and almost ideal mould material. Even if earthenware clay is not the
optimal material for crucibles, it is excellent for making moulds. It is easy to acquire - you can find the raw materials almost anywhere you happen
to be. It is thermoshock-resisant enough to stand the sudden shock of pouring metal at 1100-1150?C into it - I have never yet experienced a mould
cracking, even when casting in quite cold moulds. Finally it offers goods with very slow heat conductivity - especially after being pre-heated - a
valuable quality that allows casting of objects with thin details and fine ornamentation. The mould does not allow the liquid metal to cool rapidly.
This is important when casting intricate objects such as thin oval broaches.
Combined with the qualities of brass-based alloys, the qualities of the mould material explain why it is possible to cast even the upper shells of
double-shelled brooches, latticed shells no thicker than 1mm. Archaeologists have regarded these objects as near miracles. Since my
investigations into the subject I fully understand the possibility of mass-producing them. Even though I have no yet attempted to cast objects of this
sort, I am fully confident that the "basic concept" will apply quite well. The early medieval founder's achievements were no wonders, though
contemporary opinions would certainly have thought so. Their skills were dependent on knowledge based on long traditions and good methods and
materials. My experience, thus far, is that clay moulds tempered with sand and cattle dung, combined with good alloys, allow one to be most
creative. Sometimes producing even the most amazing articles.
Ongoing projects in this area.
With regards to my own work, I am presently working on the subject of reproducing and mass-producing bronzes. This includes questions about
the use of lost-wax methods in Scandinavian Iron Age/early medieval foundry.
The workshop material gleaned from the large Birka excavation in 1990-96 is still being examined. We are still awaiting these publications, well
worth waiting for, as this is possibly a very important foundry from the town's early days.
There are large amounts of material from early excavations in several places still awaiting closer examination. And, as if his isn't enough, many
new workshops sites are emerging from current excavations.
The situation regarding knowledge and material on this subject has changed dramatically since the early fifties. Prior to that all research had to be
conducted supported by only very fragmentary sources. Nowadays, there is possibly more material than time to take care of it.
This is mostly a short version of the main text in The Viking Bronze Casting Site (http://hem.spray.se/anders.sberg/index.htm).
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Some literature:
Bareham, T. 1994. Bronze casting experiments. In: Historical Metallurgy - The Journal of the Historical Metallurgy Society, Vol. 28 No 2,
1994. London.
Brinch Madsen, H. 1984. Metal Casting. In: Ribe Excavations 1970-76. Vol. 2. Bencard, M. (Ed). Esbjerg.
Hawthorne, J. G. Smith, C. S. 1979. Theophilus; On Divers Arts. The Foremost Medieval Treatise on Painting, Glassmaking and Metalwork.
New York.
Oddy, W. A. (Ed). 1980. Aspects of Early Metallurgy. Occasional Paper No. 17. British Museum. London.
This article has earlier been published in: Viking Heritage Newsletter 1/1999. O. Hoffman (ed.). Visby 1999.
Appendix:
Analyses of a couple of Scandinavian early medieval objects
This is a small collection of metallographical quantifications of the copper alloys in early medieval cast objects. There are more, spread over
available litterature, but unfortunately mostly written in Scandinavian languages. I have therefore put together this short list, in order to give a
picture of the nature of the alloys at use in ancient nordic foundries.
Note the absence of classic 90/10 tin bronze. This is actually more of a characteristic Bronze Age-alloy. One of the very few Iron Age or Medieval
objects I know, cast in this alloy, is a Buddah-figurine found at Helgö in Sweden. This one is of course cast in India, not in Scandinavia.
As shown in the list, the alloys used gives a rather "wild" impression, far from modern terms of industrial standards. This wild impression is
probably a mirror of the re-use of old worn-out objects and scrap-metal in the workshops. The material was possibly pretty expensive; mostly
probably originally imported from the continent. The founder had to be economical and use whatever raw-materials he could get and waste was
certainly not to be thrown away; being valuable as it could be re-used.
When melting an alloy over and over again, the alloying-metal with the lower melting point - in these cases zinc - will go away as fumes and the
alloy will be less usable as a casting-metal. As metallic zinc wasn´t available in these times, the craftsman would have to replace lost alloying
metal with metals available - tin and lead. Tin or lead could also be added in order to improve the alloy´s casting properties.
The five analyses deriving from Andreas Oldebergs work should be used carefully; they might merely serve as impressions rather than truths. The
analyses originally derives from an earlier work of the same author (1942-43) where they are presented as approximate quantifications. When re-
published in 1966, this reserved stand was removed.
There are always reasons to be suspisious of analyses of ancient brasses showing as high figures as 30 % concerning the zinc-content. The
cementation process - the method of making brass that was available - normally hardly gave brasses with higher zinc-contents than 28 %. A lot of
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the zinc in objectsmade of copper-zinc alloys will also be lost, while lying for a thousand years in the soil.
9th century bell from Hedeby: Cu 75, 33% Sn 17, 37% Pb 6, 56% (Roesdahl, E (red), Arnadóttir, L. Edgren, T. Hohler, E B. Liebgott, N-K.
Tegnér, G. 1992. Fr:
n Vikingar Till Korsfarare. Norden och Europa 800-1200. Stockholm.p 280.)
Animal-head brooch. Garda, Gotland: Zn 20% Sn 10% Pb 6% (+Cu) (Oldeberg, A. 1966. Metallteknik fr:
n vikingatid och medeltid.
Stockholm. p 57.)
Yellow ornamental inlay on a sword. Älvsby, Vänge sn, Uppl: Zn 30% Sn 10% (+Cu) (Oldeberg 1966:57)
Oval brooch. Ärsta, Runtuna sn, Sörml. Zn 20% Sn 10% (+Cu) (Oldeberg 1966:57)
Oval brooch. Br:
nnestad, St Joh sn, Ögötl. Zn 30% Pb 6% (+Cu) (Oldeberg 1966:57)
Animal-head brooch. Annexhemmanet, Hemse sn, Gotl. Zn 30% Pb 6% (+Cu) (Oldeberg 1966:58)
Buddah-statyette from Helgö: Cu 90% Sn 10% (Forshell, H. 1992. The Inception Of Copper Mining In Falun. Theses and papers in archaeology
B:2. Sthlms Univ. Sthlm. p 61.)
Brooch. Hejnum, Gotland. Cu 77,5% Sn 5,2% Pb 4,2% Zn 11,2% Fe 1.0% (SHM 10725:8) (Forshell 1992:61)
Brooch. Lilla Bjerges, Lau, Gotl. Cu 73,3% Sn 0.0% Pb 1,1% Zn 11,2% (SHM 187003:28) (Forshell 1992:61)
Brooch. Bjärs, Hejnum. Gotl. Cu 88,8% Sn 3,5% Pb 1,4% Zn 5,8% (SHM 8767:109) (Forshell 1992:61)
Brooch. Alands, Hogrän, Gotl. Cu 87,6% Sn 7,2% Pb 1,4% Zn 3,8% (SHM 6048) (Forshell 1992:61)
Brooch. Grötlingbo, Gotl. Cu 68,8% Sn 1,2% Pb 1,3% Zn 28,7% (SHM 10482:3) (Forshell 1992:61)
Brooch. Grötlingbo, Gotl. Cu 79,7% Sn 6,7% Pb 4,2% Zn 9,2% (SHM 10482) (Forshell 1992:61)
Brooch. Kylver, St:
nga, Gotl. Cu 69,8% Sn 1,5% Pb 3,5% Zn 25;3% (SHM 13436:5) (Forshell 1992:61)
Brooch. Sandeg:
rda, Sanda, Gotl. Cu 73,7% Sn 0,8% Pb 1,7% Zn 22,3% (SHM 7480) (Forshell 1992:61)
Brooch. Lilla Ihre, Hellvi, Gotl. Cu 76,3% Sn 0,0% Pb 3,2% Zn 20,1% (SHM 20826:280) (Forshell 1992:61)
Brooch. Hade, Hedelunda, Jämtland. Cu 76,3% Sn 4,2% Pb 2,8% Zn 19,1% (SHM 143) (Forshell 1992:61)
Brooch. Bjärs, Hejnum, Gotland. Cu 85,3% Sn 3,4% Pb 1,4% Zn 7,4% Fe 1,1% (SHM 10298:139) (Forshell 1992:61)
Brooch. Roes, Grötlingbo, Gotl. Cu 74,2% Sn 0,0% Pb 2,2% Zn 24,6% (SHM 7563) (Forshell 1992:61)
Clasp button, Helgö. Cu 50.7% Sn 5,0% Pb 47,7% (Kyhlberg, O. 1980:184 Vikt och värde. Sthlm)
Weight, Birka, the Black Earth. Cu 73% Zn 22% (Kyhlberg 1980:187)
Weight, Birka, the Black Earth. Cu 74% Pb 16% (Kyhlberg 1980:187)
Weight, Birka, the Black Earth. Cu 76% Pb 13% (Kyhlberg 1980:187)
Weight, Helgö. Cu 73% Zn 23% (Kyhlberg 1980:187)
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