Infrared termography as a tool for the stained glass windows characterization

Abstract

Stained glass windows are complex artworks composed by glasses, which can be painted with grisailles and enamels, mounted on lead cames to form window panels. These cames are usually soldered to each other using a melted tin–lead alloy. To characterize the materials of the stained glass windows, it is habitual to disassemble the different pieces, thus the characterization is commonly carried out during their restoration. In recent years, thermography has been successfully applied to characterize in-situ building materials, mosaics, sculptures, paintings and archaeological materials; however studies on glassy materials are scarce and they are focused on the evaluation of windows for industrial purposes. In this study we present the results of the characterization of glasses, enamels, grisailles, lead cames and tin–lead welds by infrared thermography in a contemporary stained glass window and in a 20th century stained glass window from the CSIC headquarters in Madrid. The thermographic analysis was carried out with a FLIR ThermaCAMTM B4 (7.5 to 13 µm wavelength range, -20 to +130 ºC temperature range and 0.08 ºC of temperature accuracy) in transmission and reflection modes. The study was carried out by active thermography illuminating the stained glass windows with a spotlight of 500 W during 10 min. Each material has a different behavior vs infrared radiation. Glasses presented an increase of the apparent surface temperature due to the radiation transmission through them. This temperature abruptly decreased after the illumination in transmission mode. Nevertheless, in reflection mode, the cooling of the glass surface after the illumination was slower due to the surface warming. Metallic materials presented an almost constant apparent temperature in transmission mode because of their low heat capacity; although in reflection mode, tin–lead welds were the most reflective material, increasing their apparent temperature up to three times the original value. In contrast, lead cames presented the lowest variation, which consisted in a slow apparent temperature increasing due to the progressive warming. Grisailles and enamels presented a noticeable apparent temperature increase in both modes due to their warming. The temperature difference between these materials and the supporting glasses can induce the formation of thermal stress, and finally fissures and cracks could arise. Both the glass bubbles and cracks were better observed in transmission mode.