Fig. 2: Surface and cross-sectional optical micrographs of concretions attached to the outer surface of the porcelain fragment.

a Surface of concretions on the exposed body at the rim of the artefact. A longitudinal crack is present at the rim (box 1). To the left of the crack, a brown concretion covers the outermost layer, and to the right, the pores in the yellow region are filled with black–brown material. b Surface of concretions on neighbouring glaze surface. A skeleton of porous structure with a yellow skeleton is present, with the smallest pore having a diameter of 3 μm (indicated by the red arrow), which represents the minimum pore size. c Cross-section of concretions on the exposed body at the rim of the artefact. Box 1 shows the top area of the concretions, with the outermost brown coating visible. Box 2 shows translucent crystals embedded within the yellow skeleton. Box 3 shows a brown interface layer (~30-μm thick) between the concretions and porcelain body, with pits on the body. d Cross-section of concretions. The red line marks a large translucent crystal embedded in a pore of the yellow skeleton, with a diameter of about 300 μm, which represents the maximum pore size. Box 1 shows the presence of pits on the glaze covered by the concretions. The red boxes mark the locations of the SEM images in Fig. 3.