Diagnostics and monitoring of frescoes using evanescent-field dielectrometry

Abstract

Moisture and soluble salts are the main causes of degradation of mural paintings, in particular, frescoes. Water is the ‘driving force’ of damage such as the detachment of the painted layer and the whitening of the painting due to the crystallization of salts (efflorescence). Indeed, the appearance of efflorescence itself is related to the alterations caused by moisture in the process of the evaporation of water through the surface of the wall. Early detection of the presence of moisture under the wall surface is therefore essential for avoiding such kind of damage. In this paper a non-invasive microwave system is described which allows the measurement of the moisture content and the detection of salts in frescoes and mural paintings. The system performs a sub-surface measurement with an investigated depth up to about 2 cm. The measurement system consists of an evanescent-field resonant sensor, a network analyser and a numerical code. The method has been validated by measurements performed on some reference materials with known dielectric characteristics. Several tests on moistened plaster samples, some of them containing calcium nitrate at different concentrations, have been performed in order to verify the effectiveness in quantifying the moisture and salt content. In situ investigations have been carried out by measuring both the moisture content and salt content on frescoes in several museums and churches. The preliminary results prove the usefulness of the method as a diagnostic tool for investigating the health status of frescoes.

State of Arts of Monumental Stones Diagnosis and Monitoring

“…This method has been usefully employed for determining by non-invasive way the moisture and salts content inside materials (Olmi R., et al, 2006). The theoretical concept is based on the measurement of the dielectric contrast between water (about 80) and the host material in the microwave frequency range.…”

Exploitation of the whole electromagnetic spectrum in art conservation studies

“…44 A much simpler device operating in the GHz range, which is able to map moisture and salts on walls, as well as frescoes and other artefacts, has been set up and is based on evanescent-field dielectrometry. 12,17 Finally, it is worth mentioning broadband dielectric spectroscopy operating from 10 -3 Hz to 10 9 Hz, which could be of particular interest for monitoring possible alterations in plastic artefacts.…”

“…14 Instead, the interval of low energies so far has been only exploited sparingly, even if important advances have currently been made. [15][16][17] One important point to be emphasised is that, very often, one single technique is not sufficient to answer the questions of conservators, art historians, or restorers. Therefore, it is useful to integrate the different techniques in order to give results that are as most complete as possible.…”

A multidisciplinary diagnostic approach preliminary to the restoration of the country church “San Maurizio” located in Ittiri (SS), Italy

“…The main problems of the UNI 11087: 2003 [22], concerning the salt content analysis, relies on the invasive analytical procedure and the lack of a sampling guideline. Alternative procedures were proposed by various research groups [25][26][27][28][29].…”