Fig. 6

a Fragment from the Oseberg find, recently broken. The alum-rich layer, ca. 5 mm thick, surrounds an inner alum-poor core. Inner voids are also evident (‘V’). Tension between the alum-rich and alum-poor boundary regions has resulted in crack formation (arrow). b X-ray image of the same fragment revealing minimal alum penetration across the grain, and better penetration along the grain (ca. 5 cm). Transverse inner cracks have been formed by drying stresses in the alum-poor core (arrows). These are not visible on the surface, shown in the photograph (c). Xray and photo SB