The Ħal Millieri church : a study of physical and chemical factors affecting the conservation of its frescoes

Abstract

A number of physical parameters obtaining for Hal Millieri church, were monitored over a period of two years. Air and wall temperature, relative humidity/vapour pressure, wall moisture content and air current speed and direction were recorded. Samples were taken from the fabric of the church, consisting of efflorescences, plaster, modern repairs ("cement") and mortar. Their composition was analysed both qualitatively and quantitatively. Analysis of the data showed that air temperature and water vapour pressure fluctuate seasonally in response to external changes. The wall surface temperature follows the air temperature inside the church. The walls seemed to be drier in summer and autumn and wetter in winter and spring. Weak air currents were present; these decreased once the windows were closed. The soluble salts which are concentrated in the lower areas of the walls were identified to be mainly sodium chloride and sodium nitrate. The crystallisation of these salts as efflorescences· was found to be directly related to the relative humidity inside the church. At high. relative humidity inside the church, the soluble salts remain in solution and do not cause damage to the fresco layers.