Reworking paintings has been a common practice throughout art history, with artists modifying their own work at various stages and occasionally altering pieces by others. Advances in chemical imaging and increased access to traditional imaging techniques have facilitated the documentation of such interventions. Initially focused on Old Masters, research on reworking practices has expanded to nineteenth- and twentieth-century artists. For the first time, a classification system for reworkings is introduced, based on the oeuvre of Belgian Modernist painter James Ensor (1860–1949). Five representative case studies each illustrate one of the proposed types of reworking: (1) pentimenti, (2) post-factum revisions, (3) recycled works, (4) metamorphoses, and (5) appropriations. Using advanced imaging and spectroscopic techniques, including Macro X-Ray Fluorescence (MA-XRF) and instrumentation from the Iperion HS consortium, the study also provides material-technical evidence of Ensor’s experimental studio practice while shedding new light on anachronisms in his oeuvre.

The discoloration rate of chrome yellow (CY), a class of synthetic inorganic pigments (PbCr_1−xS_xO₄) frequently used by Van Gogh and his contemporaries, strongly depends on its sulfate content and on its crystalline structure (either monoclinic or orthorhombic). Macroscopic X-Ray powder diffraction imaging of selected areas on Van Gogh's Sunflowers (Van Gogh Museum, Amsterdam) revealed the presence of two subtypes of CY: the light-fast monoclinic PbCrO₄ (LF-CY) and the light-sensitive monoclinic PbCr_1−xS_xO₄ (x≈0.5; LS-CY). The latter was encountered in large parts of the painting (e.g., in the pale-yellow background and the bright-yellow petals, but also in the green stems and flower hearts), thus indicating their higher risk for past or future darkening. Overall, it is present in more than 50 % of the CY regions. Preferred orientation of LS-CY allows observation of a significant ordering of the elongated crystallites along the direction of Van Gogh's brush strokes.