Composite sodium alginate-ion exchangers as cleaning systems for the removal of gypsum efflorescences

摘要

Film forming systems constituted of an MgAl layered double hydroxide (MgAlCl) and half sodic zirconium phosphate (ZrPNaH) ion exchangers, sodium alginate (SA), polyvinylalcohol (PVA) and water (exchangers, polyvinylalcohol, alginate: EPA) were investigated as a cleaning tool for the removal of gypsum efflorescences on fresco paintings. The high water content in EPA assures the gypsum solubilization which also benefits from the capture of calcium and sulphate ions by the exchangers. Moreover, calcium ions cause the SA reticulation thus allowing an easy detachment of the mixture from the surface without leaving residues. PVA acts as medium to disperse the inorganic exchangers in SA. Before formulating the EPA, the rheological properties of calcium alginate were investigated as a function of the ZrPNaH content in order to assess the conditions leading to film forming systems with suitable consistency and surface adhesion. To test EPA efficiency, gypsum efflorescences were artificially generated on fresco mock-ups prepared using the distinctive fresco technique and painted with five pigments (Hematite, Ultramarine Blue, Naples Yellow, Malachite and Manganese Black). The mock-ups were treated with EPA and the efficiency of the treatment in the gypsum removal was investigated by several techniques including X ray diffraction (XRD), X ray Fluorescence (XRF), Infrared spectroscopy (MIR), Raman spectroscopy, SEM-EDX and colorimetric analysis.