In order to reconstruct the different types of temper several sand and crushed granite samples were studied and compared with the original temper to find a similar match. The organic temper was also studied by a dendrochronologist in order to determine species. When investigating the different samples and original minerogenic temper, focus was laid on the grain size distribution (See Graph 1). This resulted in three types of reconstructed temper labelled Temper Types 1, 2 and 3 (Table 2). Temper 4, represented in one of 10 sherds, was never reconstructed due to the time frame of the project. However, it was concluded that it most probably consisted of crushed granite and pine/spruce charcoal.

Based on these studies and using the material presented above, seven new crucibles were made. Since Temper 3 was present in five of 10 original crucible sherds Temper 3 was used in four of the seven reconstructions. Temper 2 was used in two replicas and Temper 1 was used in one of the replicas. These reconstructed crucibles were labelled 1A, 2A, 2B, 3A, 3B, 3C and 3D according to the type of temper used (See Figure 5).

The experiments

Using earlier studies of Bronze Age workshops (Johansson 1993; Thrane 1994; Rönne 1996) a small hearth was built in order to conduct the experiments in an as accurate way as possible (See Figure 6). A Cu-alloy consisting of 10%Sn and 90%Cu (bronze) was melted in the crucibles. The metal was put inside the crucible that was situated on top of a layer of charcoal directly under the airflow from the tuyére. After that it was covered with charcoal, air was pumped trough the tuyére using bellows and the melting began. When the metal was melted the crucible was taken out of the hearth and the metal was poured out. No mould was used since the casted object not was in focus for this study, instead a cavity in a soapstone was used to dispose the metal. Each crucible was used as many times as possible during one day and then left in the hearth to cool off during the night. The day after, the crucibles were inspected for visible cracks, and if cracks were not present the experiments continued (See Figure 7). When using a crucible it is exposed to enormous changes in temperature. The hearth must hold a temperature of around 1100^oC at the lowest in order to melt the copper alloy and to make sure it stays in liquid state during the whole casting process. Outside the hearth the temperature, in this case, is on average around 10 to 15^oC. When moving the crucible from the hearth to the mould in order to do the casting it is vital that the crucible does not break due to the enormous temperature fluctuations. Ceramics have a tendency to break when cooling off or heating up too quickly. One of the critical points in the heating / cooling process is 572^o C. This is sometimes referred to as the quartz point, where the quartz crystal goes from a-stage to ß-stage and back (Blackburn and Dennen 1988, 74f & 311f) something that results on heavy strains on the ware. During the experiments it was clear that the crucibles endured such ordeals. In spite of this, no visible cracks were detected when inspecting the crucibles at the beginning of the workday. Even though the crucibles were used several times and also by an unskilled craftsman (the author) they did not crack or fall apart into sherds after being used up to ten times as is indicated by earlier studies. On the contrary, all the crucibles (with the exception of crucible 1A which lasted for six castings) were still in one piece when the experiments were concluded. When studied both during the experiment and after it was clear that each crucible had traces of use depending on how many times they were used. The sintering of the ware started from the top/rim and on the inside and slowly spread as the crucible was used repeatedly times. A red layer of copper oxide started to cover the inside of the crucibles and got more intense as they were used. A crucible used for 20 castings showed an even buff red layer of copper oxide mainly on the rim and inside, the ware had sintered mostly on the parts facing the airflow from the tuyére. Due to the traces of use on the replica crucibles, when comparing the replicas with the archaeological material it is clear that the Broåsen crucibles were used for minimum 20 castings.

Conclusions

The first conclusion that can be made from these experiments is that we have to reconsider the statements of earlier studies. In some cases crucibles might have had a low sustainability, but evidently there are also sites that show the contrary. So far there are too few studies focusing on the connection between choice of clay/temper and sustainability in order to get a good overview of the difference between the quality of crucibles. It is highly plausible that the sustainability of crucibles varied between sites and craftspersons depending on factors such as production rate, level of knowledge and available resources (as discussed by among others such as Stilborg 2008). In the literature concerning Scandinavian and European Bronze Age the focus is on who the bronze caster and the people connected to the production of bronze objects were (for example Goldhahn 2007; Kristiansen & Larsson 2005). This is intimately connected to the study of crucibles because there may have been a difference in who made the crucibles at different sites and different types of production. A highly skilled group of craftspersons would probably have both needed and used a sustainable crucible to conduct their work in an effective way. In another scenario, if only one casting was needed, the crucible did not need to have sustainability higher than one time. In conclusion, we now have evidence that crucibles could have had a high level of sustainability, however the sustainability of crucibles from other sites may vary in comparison to the Broåsen material.

Experiments yet to be made

Continuing studies will be conducted on archaeological materials assumed to represent low scale production. The purpose is to investigate whether the high sustainability among the crucibles from Broåsen is also present in that type of material even though it is assumed not to have been necessary.

Acknowledgements

Special thank to Anders Lindahl, Stella Macheridis and Andreas Nilsson for reading and commenting on the text.