Optical sensors for evaluating environmental acidity in the preventive conservation of historical objects

Since their creation, appearing or recovering, historical objects are sensitive to environmental corrosion. Although the pollutants level is nowadays strictly controlled, new pollutants appear every year both indoor and outdoor the buildings, due to the industrial development. The increasing environmental acidity and the risk of acid rain under highly polluted and humid areas are one of the most dangerous chemical attack on the surface of historical materials. A wide variety of preventive conservation tools and routines have been searched and used to protect historical and cultural objects against deterioration and degradation. In this research new optical sensors for evaluating environmental acidity have been developed. The sensitive optical material is based on sol-gel thin silica film doped with an organic dye. The experiments were conducted by using a chamber with controlled SO2 concentrations in air. Optical absorption changes of the sensors are recorded for the different SO2 concentrations. The results indicate that it is possible to detect in the air variations of some tenths of pH and SO2 concentrations at about 10 ppm.