A new method for consolidating wall paintings based on dispersions of lime in alcohol

Abstract

Dispersions of calcium hydroxide. Ca(OH)2, in short-chain aliphatic alcohols were studied with the aim of establishing a new pre-consolidation and consolidation methodology for wall paintings. The choice of lime (a paste comprising calcium hydroxide plus water) as the consolidant was suggested by physicochemical compatibility criteria, as lime is the original binder in the wall paintings treated. Many problems are encountered when trying to apply calcium hydroxide to wall paintings. Saturated solutions are too weak to act as consolidant, due to the low solubility (1·7gl-1) of Ca(OH)2 in water. In addition, suspensions of Ca(OH)2 in water are too unstable to be applied to unprotected painting surfaces. The use of Ca(OH)2 dispersions in propan-1-ol is proposed; these dispersions were characterized, and their kinetic stability investigated, by ultraviolet/visible spectrophotometry. The alcohol dispersions were found to be much more stable than suspensions in water. The second part of the study evaluated the consolidating power of alcohol dispersions on specimens that simulate different degrees of decohesion. These were examined using X-ray diffractometry and Fourier transform infrared spectroscopy. Their surface properties were characterized by studying capillary rise, water permeability and mercury porosity. The mechanical properties of the specimens were determined using sclerometric resistance and the 'Scotch Tape test'. All the results indicated that the lime/alcohol dispersions produced excellent consolidation. Finally, the paper reports the positive results of tests using the lime/alcohol dispersion on wall paintings by Andrea da Firenze in the Cappellone degli Spagnoli, Chiostro Verde of Santa Maria Novella, Florence.