Iron Gallic Ink: Additives and photoinduced dehydration of the sulphate

The mineral phases identified (SEM-EDAX) in the ancient iron gallic ink are resistate additives, i.e., chemically resistant minerals used to improve t he finish of medieval inks (haematite, rutile, magnetite) and in situ neofo rmed phases (pyrite, in anoxic environments; calcite formed by atmospheric reaction of the environmental CO2 and the lime (CaO) of the paper. Laborato ry driven experiments on synthetic iron gallic ink at room temperature (i.e ., from 15 degreesC to 30 degreesC) shows that the rate of the ferrous sulp hate heptahydrate-tetrahydrate transition is strongly temperature-time depe ndent, hence at 15 degreesC the step of the dehydration reaction is five ho urs while, at 20 degreesC the step is five minutes. In the case of the mixt ure melanterite and iron gallic ink the step is five minutes when the isoth ermal run heating reach up 25 degreesC. This small delay can be explained b y the organometallic rim (iron gallic ink) which encircles the iron sulphat e crystals and increases the water vapour concentration. Phase transitions have been controlled by a self Optically Stimulated X-Ray Diffraction (OSXR D), which allows an electronic temperature control of the sample and progra mming of sequential diffractograms while the sample is also irradiated with red-infrared light(halogen lamp). The original OSXRD prototype, which is w orking in the Museo Nacional de Ciencias Naturales (Madrid), is suitable to detect sensitive dehydration reactions as in iron sulphate case.