Metallurgical Clay Packages

This artefact group, only fairly recently recognized and still not fully researched, is characterized by

a) internal impressions of the object which was being treated

b) poor-quality clay with a high degree of vitrifi-cation

c) high degree of fragmentation as the packaging had to be broken up when the process was fi-nished and the metal artefact under treatment was removed.

Wilhelm Gebers touched on this artefact group in his reports on excavations in the Slavonic fort of Bosau, Germany. Hans Drescher dealt with such objects in an article on the finds from Hedeby, and Helge Brinch Madsen published them as ‘sintered clay’ in the report of excavations at Ribe, Denmark. Subsequently the artefact group has been noted in finds from Birka and Sigtuna, where two variants, melting bowls and brazing packages, have been identified as packaging used when brazing or hard soldering iron objects.

Tube-shaped or cigar-shaped clay packagesThe artefact group contains at least one other type of ceramic packaging in addition to brazing packa-ges and melting bowls: tube-shaped or cigar-shaped clay packages, several fragments of which were found in Building Group 3. Similar objects were discovered at Bosau (Gebers 1981; 1986) and in Viking Age Hedeby (Drescher 1983). In Sweden they have been found in a Viking Age workshop

context at Bastubacken, Tortuna parish, Västman-land (Reisborg 1996), in Viking Age Birka (Jakobs-son Holback 1999), in the Gotlandic Viking Age harbour of Fröjel (Gustafsson & Söderberg 2005) and in Medieval Sigtuna.

The tube-shaped packages are characterized by their tubular shape – elongated and with an internal cavity often displaying impressions of a soft, pliable material wrapped in a single or double layer around an inner rod-shaped object (Figures 1, 2 and 3). Externally they are more or less heavily vitrified through exposure to extreme heat (Figure 4).

In the Helgö examples the ware varies from 2 mm to 15 mm thick, and sometimes the whole of this range occurs within a single fragment. Thus, the ware thickness of the packages is generally very uneven.

Metallurgical clay packages Anders Söderberg

A group of fragments found among the technical ceramics from Building Group 3 and very similar in appearance to sherds from crucibles, have proved to derive from ceramic packaging used in various metallurgical processes such as brazing and case hardening. The different forms discovered at Helgö are described and their function discussed. Comparisons are made with examples from elsewhere in Sweden and abroad.

Figure 1. Fragments of tube-shaped clay packages. Photo ATA

2 Anders Söderberg

Figure 3. Fragment from no. 9809b. A closed end of a tube-shaped clay package

Figure 4. Fragment from no. 9889, showing the heavy vitrification of the ware

Figure 2. Fragment of tube-shaped clay package from no. 9809. In the text it is called no. 9809a as the find number comprises many fragments kept in separate boxes in the museum store. The illustration shows the interior with traces of diagonal binding

Figure 5. Cross-section with shape of cavity, no. 9889

Figure 6. Cross-section with shape of cavity, no. 12154

3Metallurgical clay packages

There is also a wide variation in the size of the tubes. Cross-section diameters of between 15 mm and 38 mm are represented, but the original lengths are more difficult to calculate because of the frag-mentation. Some idea can be gained, however, by comparison with the find from Viking Age Bastu-backen in which the total length could be recon-structed as 170 mm. Its greatest diameter was 33 mm (Reisborg 1996, 241).

The cross-section of the cavities differs greatly, from oval to rectangular, square, rhomboid and trapezoid (Figures 5-7). The sizes of the cavities are also very variable, from 7 mm to 22 mm wide. Similarly the widths of the textile impressions vary from 1mm to 11 mm.

The exteriors of the tubes vary in colour from grey through greenish-grey to blackish-grey, some-times with red flecks. The surfaces are more or less glazed, and often cracked. The colour of the cavi-ties varies from light to dark grey, sometimes with rust-red layers, which are not the result of oxidizing firing of the clay but probably a consequence of iron

precipitation from a source different from that of the clay raw material.

The ceramic ware is untempered, is often of a slightly silty character rather reminiscent of the clay commonly used for moulds from the Iron Age, and often with isolated grains of quartz scattered through it. The ware is mainly reduced, and thus in shades of light and dark grey, but in some cases there are also small patches of oxidation.

Impressions of the bindingMany different binding patterns are discernable in the impressions preserved in the cavities of the tubes. No. 9809a shows a strip 9-11 mm wide, wound diagonally, and separated by a 1-2 mm gap (Figure 2). No. 9408 displays a similar but slightly narrower strip (Figure 8). In no. 9809b there are marks of a strip 8-11 mm wide and wound diago-nally, with gaps 5-6 mm wide. The gaps show im-pressions of an inner binding wound the opposite way to the outer (Figure 9). No. 8812 also carries impressions of inner layers of bindings (Figure 10).

Figure 7. Cross-section with shape of cavity, no. 10124.The shape of the edges of the cavity indicate that the metal object enclosed in the clay was wrapped in longitudinal binding

Figure 8. Pattern of binding, no. 9408

Figure 9. Pattern of binding, no. 9809b, with presumed underlying diagonal binding

Figure 10. Pattern of binding, no. 8812

4 Anders Söderberg

In general, in those cases where it could be measu-red the binding seems to have been c. 1 mm thick.

Several examples differ from the norm. No. 10147 carries clear impressions of a two-ply cord c. 2 mm thick (Figure 11); nos 9889a and 10127 show impressions of fibres or animal hair (Figure 12). The impressions on no.12984 show that the binding had a longitudinal fibrous structure like grass or reed (Figure 13), and a coarser longitudinal fibrous structure is present in the binding on nos 9809a (Figure 14).

Nos 10982, 12452 and 12872 all display thinner and slightly wider bindings. No. 10982 carries a diagonal binding c. 2 mm wide, and vague indica-tions of an inner binding running crosswise to the outer (Figure 15). Nos 12452 and 12872 both dis-

play criss-cross bindings using 1-2 mm wide and c. 1 mm thick strips (Figure 16-17).

A secondarily fired fragment from no. 12453 shows impressions of a folded cloth bag knotted at the top, as in recent finds of melting bowls used in the brazing of Viking Age weights (Söderberg 1997 and 2004).

Common to the fragments from nos 12452, 12872 and 12453 is that they differ from others in not deriving from tube-shaped packages. Rather, they show signs of having originated from box-shaped brazing packages. This package type differs from tube-shaped packages in shape and cavity impres-sions, but are identical in their ware and degree of vitrification.

No. 9890 differs markedly from all the above, but

Figure 11. Cord impression, no. 10147 Figure 12. Impression of animal hair or fur, no. 10127

Figure 13. Impression of grass-like fibrous material, no. 12984

Figure 14. Longitudinal structures in the binding impres-sions, no. 9809a

5Metallurgical clay packages

probably has some connection with the packages. It is a mushroom-shaped clay plug, 22 mm high and 30 mm in basal diameter, while the inner plug is 19-22 mm in cross-section and terminates in a bro-ken surface. The base is vitrified, olive green shading to red, whereas the inner plug is a dull grey in colour.

Distribution, find context and datingThe fragments’ distribution pattern agrees well with the distribution of many other workshop-related

artefact types from Building Group 3 (Figure 18). The tube-shaped packages were concentrated in the west and centre of the building group, mainly in the large rubbish deposit in Area III and in the deposit in Area VI thought to have been thrown out from workshops in Areas IV and VII. The artefacts found in the deposits in Areas III and VI attributed the areas mainly to the Migration Period (Lamm 1984, 85–6).

Both refuse deposits contained large quantities of manufacturing waste. In addition to crucibles and moulds, Area III contained numerous iron tools, such as punches, scrapers, files, a hammer and a sickle, as well as rods, sheet iron, nails and rivets (Wigren 1984, 27). Area VI contained fragments of moulds and iron tools (Wigren 1984, 49), and rods and scrap iron (Knape 1981, 63–73). Areas III and VI both contained dense concentrations of iron slag (Hall-inder et al. 1986, 147–8), crucibles (Blidmo 1982, 116) and tuyères (Brinch Madsen 1981, 100).

Figure 15. Pattern of binding, no. 10982 Figure 16. Pattern of binding, no. 12452. Narrow criss-cross binding in a fragment of a brazing package

Figure 17. Pattern of binding, no. 12872. Narrow criss-cross binding in a fragment of a brazing package

Figure 18. Distribution of metallurgical clay packages within Building Group 3. Each symbol indicates one find number

6 Anders Söderberg

The divergent finds nos 12452, 12872 and 12453, deriving from brazing packages, were all found in Area VIII. This area contained rubbish deposits with contents similar to those in the central areas, including great quantities of iron tools, but additionally locks and keys (Wigren 1984, 66) and a high incidence of iron slag (Hallinder et al. 1986, 147–8). Area VIII displayed the densest concentration of cupellation trays in Building Group 3 whereas the distribution of crucibles was less (Blidmo 1982, 112–6) and tuyères were rare (Brinch Madsen 1981, 100).

The hearths in Area VIII have the early C14-dates of the Roman Iron Age and early Migration Period, but the overlying refuse deposits are later. The cast-ing waste is of a distinctly Migration Period charact-er, but the Viking Age is also represented by finds of Viking Age pottery and Arabic silver coins. Some of the refuse in Area VIII seems to have been thrown out from the activities in Area IX, dated to the Mig-ration and Vendel Periods (Lamm 1984, 86). Nos 12452, 12872 and 12453, however, belong to what Roger Blidmo called ‘waste deposit area I’ (avfalls-område 1) in his chorological study, interpreted as refuse thrown out from the unexcavated Foundation 19 east of the excavated area (Blidmo 1982, 207). The Viking Age finds which were concentrated around that foundation suggest that the Viking Age phase of occupation of Building Group 3 was centred there.

Find recording at Helgö was not always precise. Thus, where the same find number incorporates finds from many squares, the symbols on the dist-ribution map (Figure 18) have been placed in the SW (lower left-hand) corner of the squares. Thus, Figure 18 gives a generalized impression of the distribution rather than a completely objective picture.

Interpretation of functionThe hard-burnt, often glazed, appearance of the fragments shows that their function must be sought in metalworking. This interpretation is supported by the context of the fragments, not only at Helgö but also at the other sites where similar fragments have been discovered.

As early as the second century AD, spherical Schmelzbomben, similar to Drescher’s Schmeltz-kugeln, were in use at the provincial Roman Danube castellum of Straubing-Sorviodurum (Walke 1965, 61). Brinch Madsen (1984, 30–1)

described similar objects of sintered clay from Ribe, interpreting the 63 fragments as packaging from processes in which metal had to be heated without coming into direct contact with flames. Two more fragments from Ribe have been published more recently, along with similar material from Birka (Jakobsson Holback 1999). Drescher described röhrenförmige Tiegeln and Schmelzkugeln in an article on finds from Hedeby and Bosau, but without attributing their function to actual metallurgical processes (Drescher 1983, 183). In 1986, Gebers (1986, 67–8) cautiously interpreted the obvious fragments of brazing packages and clay tubes from Bosau (1981, figs 8-19) as moulds and bellows pipes. A find from the Scandinavian occupation level at the Brough of Birsay, Orkney, Scotland, has been interpreted as a blowpipe, but the binding visible in the illustration indicates that it was the open end of a tube-shaped package (Curle 1982, 42, fig. 406). Fragment 288 from Lagore Crannóg, Co Meath, Ireland, interpreted as a tuyère, is suggestive, even though the analytical drawing does not show the appearance of the interior (Comber 2004, 193).

It has been possible to show that two groups of objects – Drescher’s Schmeltzkugeln or melting bowls and box-shaped brazing packages – both derive from brazing processes. The melting bowls were used in a process whereby Viking Age spher-ical iron weights with flat poles were covered with thin shells of copper alloy (Söderberg 1997 & 2004), whereas the brazing packages derive from a process in which iron objects such as padlocks and bells were soldered together with brass in a process si-milar to the one described by Theophilus in Book III, chapter 92 (Hawthorne & Smith 1979, 186; Jakobsson Holback 1999; Gustafsson 2003). In both these processes the soldering took place inside an enclosing and reducing clay shell. The tube-shaped packages are very reminiscent in many ways of these enclosing shells, but they do not come from a soldering process.

The fragments of tube-shaped packagesA possible interpretation of the tube-shaped clay packages is that they were used as wrapping during case hardening or box carburization of steel (Gustafsson & Söderberg 2005). This interpretation goes back to Theophilus chapters 18 and 19 in which he described the hardening of iron files, ‘... smear

7Metallurgical clay packages

them with old pig fat and wrap them around with leather strips cut from goat skin and bind them with linen thread. After this cover them individually with kneaded clay, leaving the tangs bare. When they are dried, put them into the fire, blow vigorously, and the goat skin will be burnt. Hastily extract them from the clay and quench them evenly in water. Then take them out and dry them at the fire’ (Hawth-orne & Smith 1979, 94–5).

The Norwegian archaeologist Sigmund Jakobsen has described a similar method of carburizing edging steel, but without referring to Theophilus (Jakobsen 1991, 23–4).

He suggests that the iron should be enclosed in a clay package together with a nitrogenous material such as bone or horn meal, leather waste and wood ash. Carburization should then be carried out for between two and eight hours at 850-950°C.

When tested against Theophilus’s text, the frag-ments from Helgö fit very well in the process as he described it. The interior of the fragments carry impressions of strips, which could well have been strips of skin, wound around long objects. Clearly, both no. 9889a and no. 10127 display impressions of probable animal hair; in these instances there may have been a binding of skin with the hairy side against the clay packaging. The clear impression of a binding of two-ply cord c. 2 mm thick on 10147, fits well with the instruction ‘bind them with linen thread’ after the skin was wound around the iron bar.

The purpose of the fibrous strip in no. 12984 is more uncertain; perhaps in this example strips or strands of some vegetable origin replaced the twisted cord. The same phenomenon has been observed on similar artefacts from Birka (Jakobsson Holback 1999, 8).

No. 9809b comprises an end fragment, showing that the clay packaging was not closed off at the end but that the metal object to be treated stuck out from it. An analytical drawing of the tube from Bastu-backen suggests the same thing (Reisborg 1996, 242), and both examples may illustrate Theophilus’s instruction, ‘cover them individually with kneaded clay, leaving the tangs bare’.

It is worth remembering that in Area III of Build-ing Group 3 tube-shaped packages were discovered in deposits containing metalworking refuse and iron tools such as punches, scrapers, files, a hammer and

a sickle. Tools of hardened carbon steel may have been made there.

The clay packages may also have been associated with the numerous rod-shaped iron blanks found at Helgö. In 1978, Hallinder and Tomtlund published 242 examples from Building Groups 1-4 (Tomtlund 1978, 59–80). In ten of the 38 blanks that were analysed, the carbon content was in excess of 0.6%; the carbon content in six blanks was between 0.8% and 1.2 % (Modin & Lagerquist 1978, 110–50). According to Jakobsen (1991, 52), a carbon content of 0.8 % is ideal for edging-steel. The clay packages, then, may be interpreted as evidence that Building Group 3 served as a smithy for weapon manufac-ture. The shapes and sizes of the cross-sections of the rod-shaped blanks found there show that, when wrapped in strips of skin, they would have fitted snugly into the cavities in the clay tubes.

The fragments of box-shaped packagesNos 12452, 12872 and 12453 are distinguished from the rest by being the remains of packages with flat

Figure 19. Fragment of a box-shaped brazing package from the Svarta jorden, Birka. Photo Torbjörn Jakobsson Holback

8 Anders Söderberg

sides, and by having impressions of thin criss-cross bindings or a cloth bag.

The external shape of each clay package was dictated by the basic shape of the metal object en-closed in it for heat treatment, meaning that a package with flat sides would have been built up around an object with flat sides.

The 1990-1995 excavations of the metal work-shop in Birka’s Black earth (Svarta jorden) un-earthed 9.5 kg of fragments of clay packages, mainly used in the soldering of padlocks. For solde-ring, the components of the lock-housing were bound together with cord, and moulded into the clay along with the solder. Then the dried packages were placed in the smithing hearth and heated up until the solder melted and ran out (Jakobsson Holback 1999). Many fragments from Birka display internal impres-sions of thin bindings, and often also impressions of the lock’s ornamentation, in the spaces between the bindings, herringbone patterns for example (Figure 19) (cf. Jakobsson Holback 1999, 8–9).

The soldering of padlocks and bells in enclosing clay packages has a long history in Sweden. In Dalarna, for instance, the method was still in use in the eighteenth and nineteenth centuries (Björklund 1982, 342).

The fragments from nos 12452 and 12872 could well have derived from the soldering together of box-shaped padlocks, although the small fragments display no obvious impressions of decoration bet-ween the bindings, even though no. 12452 has vague impressions of two thin, flat ribs (Figure 20). The flat interiors of the fragments, the straight, broken edge of fragment 12872 that suggests that it had enclosed an object with straight edges (Figure 21),

and the fact that both no. 12452 and no. 12872 were discovered in a workshop context where remains of locks and keys were also discovered, support the above interpretation.

The fragment from no. 12453 was discovered in the same waste deposit as the two probable lock-soldering fragments nos 12452 and 12872, but it shows internal impressions of a cloth bag with drawstring, just like those in melting bowls used for brazing iron weights. However, the fragment (33 x 56 mm in size) is too big for a weight, and it also preserves the shape of the artefact that it enclosed: one flat side and at least one corner. Fragment 12453 shows that cloth bags rather than cords could also be used for holding together objects larger than weights when they were being brazed.

The clay plugNo. 9890 (Figure 22) resembles fragments from Medieval Sigtuna – gable fragments of brazing packages in which barrel-padlocks were soldered (Söderberg & Gustafsson in prep.). But the Sigtuna fragments show internal surfaces with clear impres-sions of the gables of padlocks, whereas the Helgö example has a rough, broken surface at the inner end of the plug.

Another similarity with the packages for barrel-padlocks is the circular impression around the base of the inside of the plug. If this were a package for a barrel-padlock, this impression would indicate the thickness of the material used in the lock cylinder, but in this case it may reflect the thickness of a dif-ferent type of material. This material would have been 2-3 mm thick, and in two places there are imprints of seams or overlaps lengthways along the plug.

Figure 20. Fragment of a box-shaped brazing package, no. 12452

Figure 21. Fragment of a box-shaped brazing package, no. 12872, with straight broken surface

9Metallurgical clay packages

It is impossible to tell whether the circular impres-sion is from metal or from organic material. A metal cylinder may indicate that the plug is an end plug from a brazing package for a pipe-shaped metal object; if it were of organic material the plug could have been the end of a tube-shaped package. Hypothetically, the plug could have been formed by a longitudinal leather wrapper surrounding the end of a metal rod of circular cross-section (Figure 23).

The Bosau publication contains an analytical drawing of a clay plug similar in appearance to the Helgö example (Gebers 1981, fig. 4). The Bosau plug may have been a gable plug from a package used in brazing a padlock, like other fragments illustrated there which seem to show fairly clear impressions of the gables of padlocks (Gebers 1981, figs 1-3).

ConclusionsTwenty-four find numbers from Building Group 3 at Helgö contain fragments of metallurgical cer-amics different from the usual types of crucibles. These fragments are the remains of ceramic packa-ging used in processes whereby metal objects were heat-treated at high temperatures under reducing conditions; for example, in brazing processes and possibly in processes involved in the carburization of iron.

These finds are filling out our picture of the Iron Age smiths as skilled, specialized potters. The smiths from Helgö carried out advanced metal- lurgical processes using simple methods and raw materials. The manufacture and use of ceramics formed an important part of the activities of smiths and, judging from the material discussed here, this was true not only of the smiths working in precious metals, but also of blacksmiths.

The distribution of the ceramic packages shows that the possible production of carbon steel for tools and for edges took place within the western and central parts of Building Group 3, primarily in the Migration Period. On the other hand, the sparse remains of padlock manufacture were found in the eastern rubbish heaps and may have derived from craft activities in the unexcavated Foundation 19. The ceramic fragments suggest that box-shaped padlocks were made, and the dating of this lock type indicates that this took place during the Vendel Period or Viking Age (cf. Tomtlund 1978).

AcknowledgementsMy thanks are due to Torbjörn Jakobsson Holback for help and valuable discussions during my exami-nation of the material.

Figure 22. Fragment of a clay plug, no. 9890 Figure 23. Clay plug, no. 9890, reconstructed interior of a clay package with cylindrical shape. The enclosed object or wrapping material is shown in black

10 Anders Söderberg

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