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A folio fragment attributed to the Fatimid period in Egypt was found to bear tufts of white crystals associated with the orange-brown and yellow paints. Raman spectroscopy identified a mixture of arsenic sulfide-based pigments in the orange-brown and yellow areas, along with vermilion in the outlines of the figures. X-ray microdiffraction, Raman spectroscopy, and energy-dispersive X-ray spectrometry identified the white crystals as hörnesite [Mg3(AsO4)2·8H2O]. Synthesis of this compound at ambient temperature and elevated relative humidity over a period of 3 years, on paper painted with arsenic sulfide pigments and treated with magnesium carbonate, suggests the possibility that a magnesium-based deacidification treatment may have contributed to the formation of this phase on the folio fragment. This work highlights the potential damage that may be sustained by arsenic sulfide-based media through exposure to deacidifying suspensions such as the ones often used in the past to treat works of art on paper and historic documents. [ABSTRACT FROM AUTHOR]