MISURA DA REMOTO DELLA TEMPERATURA SUPERFICIALE NELLA DIAGNOSTICA DEI BIENI CULTURALI: PROBLEMATICHE E NUOVI SVILUPPI

To conserve the work of arts (paintings, sculptures, etc..) in a preventive mode, a careful monitoring of the environment around these artifacts, as well as of their surface temperature, is necessary. The latter is the only physical variable which can be measured in a non-invasive way, following directly the thermal conditions and variations of the work of arts due to the dynamics of the microclimate. Considering that the works of art are often untouchable, an automated and accurate remote sensing could be very useful to prevent dangerous processes of deterioration. For these reasons a new sensor has been developed by a spin-off of the ISAC – CNR. This sensor allows to check in real-time the surface temperature changes of the artifacts both over time and at different predefined points. This automated sensor is a radiometer sensible to wavelengths ranging from 7,5 µm to 13,4 µm. A system rotating over three dimension “pan and tilt” allows to make multiple measures on a grid of points previously defined on the surface of the work of arts. The accuracy, obtained by means of a carefull calibration process, is  0,5 °C, more precise than the usual remote sensing (thermal camera and commercial radiometers), characterized by an accuracy value of  2°C. In order to obtain accurate measures of the surface temperature for a real body, the correct emissivity values need to be integrated in the calculation. Hence, an easy to use management software has been developed allowing to set the emissivity value in each point of the grid. For rejoinable points of the surface, the exact emissivity value could be determined comparing the measurements recorded by the new infrared sensor with the ones obtained by a very sensitive sensor (0,02 – 0,03)°C manually placed on the surface for a short time. In case of work of arts placed at great distance from the sersor, the emissivity values must be determined previously. The emissivity depends on a lot of variables and one of them is the surface roughness. Since the artifacts are often charaterized by a high surface roughness, such dependence has been studied in order to obtain accurate temperature measurements. The results obtained indicate an increase of the emissivity with increasing surface roughness. In conclusion, this study has allowed to develop a reliable, accurate and automatic control system, as well as a low cost sensor that, unlike the thermal camera, can also be used by less experienced operators. Besides, in order to support the museum managers in the preventive conservation of the artifacts, an alarm system is automatically activated, through the managent software, when dangerously large thermal variations - both spatial and overtime - are detected on the surface. In order to introduce in the software the temperature ranges tolerable for a good conservation of the wooden paintings, the dimensional variations of the poplar wood sample have been studied in function of the temperature changes. Poplar is the most used wood in Italy in the Cultural Heritage field. The results have shown an exponential trend of the deformation of the sample wood with the temperature, highlighting a non negligible thermal influence on the wood deformation. In fact a variation of 30 µm has measured has been measured on the samples, having a initial size of 10,25 cm in radial direction, for a thermal excursion of 30°C. These results have opened new prespectives in the Cultural Heritage field for a preventive conservation of the wooden paintings. In fact the thermal influence on the dimensional variation of the wood until now was considered negligible, and not much studied, in comparison to hygroscopic dimensional variations. Finally, in three case studies: Museo dell’Opera di Santa Croce (Florence – Italy), Longobard temple (Cividale del friuli- Udine- Italy) and the Ratto delle Sabine clay model (Florence-Italy), the issues related to conservation of the work of arts have been assessed in the field. In addition it was possible to define, directly in the field, the applicability limits of the traditional remote sensing methods for a better conservation of the artifacts.