We investigated the characteristics of a group of 13 Middle Iron Age egg-shaped crucibles and crucible fragments from Tilburg (The Netherlands). We used a combination of optical and chemical analyses, including hand-held XRF, microCT scanning and 3-D printing polarizing light microscopy and SEM-EDX. The chemical analyses confirmed that the crucibles were used for copper alloy metallurgy. Impressions in the lids of the crucibles turned out to be imprints of copper alloy scrap, including fragments of twisted wire and fibulae. Most remarkable, however, is the large proportion of sheet metal among the scrap.

In order to make crucibles from the local, non-refractory clays, a hitherto unknown ceramic-faience hybrid was used: A combination of clay and halophytic plant ash was mixed with silt into a paste, and this was used to construct the crucible. During firing, the flux would promote melting of the clays and probably prevent catastrophic failure of the crucibles. The resulting glassy groundmass – in which silt grains are embedded and partially dissolved – is rich in Al2O3 as well as in Na2O, K2O, CaO, MgO and Fe2O3.

It is likely that this technique of crucible manufacture was widespread in Late Prehistory in areas where no refractory clays were available.