Optimising DHSPI-SIRT detection diagnostics through combined thermal and humidity excitation procedures on fresco mock-ups

This study investigates the impact of relative humidity (RH%) and temperature values, simulating real-life environmental conditions, on assessing the internal structural condition of fresco wall paintings using Digital Holographic Speckle Pattern Interferometry (DHSPI) and Stimulated Infrared Thermography (SIRT). Freshly carbonatized fresco mock-ups, containing in their subsurface known simulated defects such as void and filled detachments and cracks, located at various depths, were subjected to controlled thermal and thermal humidity stress impacts. These specially designed fresco mock-ups simulate real composition scenarios of conservation states, such as those encountered in the restoration of late medieval frescoes by Giotto, Italy. A series of DHSPI-SIRT full-field monitoring experiments are conducted on these laboratory mock-ups, exposing them to controlled RH% and T conditions. The RH values are simulated in a climate chamber, while surface T values are slightly increased using infrared lamps. The systematic correlation of data from both diagnostic methods allows us to assess the influence of environmental/ microclimate changes in humidity and temperature variations on diagnostic outcomes. In summary, this research contributes to refining structural diagnostic procedures of remote real-time monitoring by the DHSPI-SIRT methodology for non-invasive subsurface damage detection. By advancing the application of DHSPI-SIRT under humidity stress conditions, this study enhances its potential for early risk detection, long-term preservation monitoring, and informed conservation of historical fresco wall paintings exposed to changes in the environmental conditions.