Colloidal Particles of Ca(OH)₂:  Properties and Applications to Restoration of Frescoes

Abstract

We studied the synthesis and characterization of Ca(OH) 2 colloidal particles and their application to the restoration of wall paintings. Submicrometer Ca(OH) 2 crystalline particles have been obtained by mixing equal volumes of NaOH and CaCl 2 aqueous solutions with different degrees of supersaturation (from 2 to 10) and in the temperature range 60−90 °C. The Ca(OH) 2 particles are crystalline and have the appearance of hexagonal platelets with thicknesses of a few nanometers, as determined by scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and atomic force microscopy techniques. The kinetic stability of dispersions of Ca(OH) 2 particles in water and propan-1-ol has also been studied in view of their possible application in wall painting conservation. A successful example of this application to the restoration of Santi di Tito's wall paintings (16th century) in the Santa Maria del Fiore Cathedral in Florence is also reported.