Fig. 1

a Jan van Eyck, The Arnolfini Portrait (NG186), 1434, oil on oak, 82.2 × 60 cm; b paint cross-section from the man’s plum-coloured robe, showing a thick layer of red lake, ultramarine and colourless powdered glass, on top of which is a thin ultramarine layer which appears light because it is degraded and blanched; c backscattered electron SEM image of the cross-section, with the glass particles circled in red; d silicon EDX map of the cross-section with the glass particles circled in red (the other silicon-containing particles are ultramarine); e EDX spectrum of one of the glass particles showing the characteristic series of elements. Images © National Gallery, London