Bio-Inspired Fluorescent Calcium Sulfate for the Conservation of Gypsum Plasterwork

In this work, the potential of bio-inspired strategies for the synthesis of calcium sulfate (CaSO₄·nH₂O) materials for heritage conservation is explored. For this, a nonclassical multi-step crystallization mechanism to understand the effect of calcein- a fluorescent chelating agent with a high affinity for divalent cations- on the nucleation and growth of calcium sulfate phases is proposed. Moving from the nano- to the macro-scale, this strategy sets the basis for the design and production of fluorescent nano-bassanite (NB-C; CaSO₄·0.5H₂O), with application as a fully compatible consolidant for the conservation of historic plasterwork. Once applied to gypsum (CaSO₄·2H₂O) plaster specimens, cementation upon hydration of nano-bassanite results in a significant increase in mechanical strength, while intracrystalline occlusion of calcein in newly-formed gypsum cement improves its weathering resistance. Furthermore, under UV irradiation, the luminescence produced by calcein molecules occluded in gypsum crystals formed upon nano-bassanite hydration allows the easy identification of the newly deposited consolidant within the treated gypsum plaster without altering the substrate's appearance.