Detaching frescoes

Frescoes differ from murals in that murals are painted on a dry wall, while frescoes are painted in wet plaster. After roughly plastering a wall, a drawing (the sinopia) is done on the wall. This drawing is covered with a finer layer of plaster. Pigments are ground and mixed with water before being applied directly to this wet layer of plaster.

When conserving frescoes it is sometimes necessary to remove them from their original location. The oldest method involves cutting out the entire wall. In the distacco method, the painted plaster and a layer of mortar are removed, avoiding demolition of the wall. Another method used removes only the paint layer, often revealing the sinopia still on the wall. This method is good if there are imperfections in the adhesion between the paint layer and the fresco and when the restorers wish to save the sinopia. The final decision of which technique to use depends on the source of the fresco's decay, but it is often difficult to determine this simply by studying a fresco. [1]

An article published in 2005 by Italian scientists suggests a portable X-ray instrument for diagnosing detachments and inhomogeneities in frescoes. The instrument utilizes Compton back-scattering of X-rays. The Compton effect occurs as a result of the interactions between the X-ray incident photons and the electrons in the fresco materials. The secondary photons produced scatter. The energy of these photons is less than that of the incident energy. The intensity of these photons depends on the electron density of the fresco materials. Because the electron density is proportional to the physical density, changes in intensities can be used to detect holes and inhomogeneities in the frescoes. These devices have been used in the past in industry to study buildings and other structures. Application of this technique in art conservation could decrease the chances of damaging a fresco during the removal process. [2]


Sources:
[1] L. Tintori. Methods used in Italy for detaching frescoes. Studies in Conservation, 6(4). Nov 1961, 131-132.
[2] A. Castellano, et al. Detection of detachments and inhomogenieties in frescos by Compton scattering. Nuclear Instruments and Methods in Physics Research B, 234(2005). 548-554.