Delamination of wax-resin linings in oil paintings: Visualization and analysis using infrared active thermography and terahertz time-domain imaging

Wax-resin linings, which were widely used in the 20th century to reinforce oil paintings on canvas, are no longer commonly used owing to material-related issues. Although many treated artworks remain well-preserved, some exhibit delamination in the lining, a challenge with mechanisms underexplored. Gaining a deeper understanding of this phenomenon is essential to guide restoration efforts. This study investigates delamination in wax-resin linings using advanced nondestructive testing methods to visualize and assess its extent. Active infrared thermography with pulsed phase thermography (PPT) and terahertz time-domain imaging (THz-TDI), validated in previous mockup studies, were applied to museum artworks to assess their potential. Three oil paintings on canvas, exhibiting suspected delamination in their wax-resin linings, were selected as case studies from the collections of the University Art Museum, Tokyo University of the Arts, and the National Museum of Modern Art, Tokyo. Imaging results from PPT and THz-TDI, combined with traditional manual examinations, i.e., visual inspection and palpation, provided insights into delamination and its contributing factors. These findings from the three case studies indicate that the key factors contributing to delamination include unevenness on the painting’s reverse side, such as bulges or hidden irregularities beneath seemingly flat paint layers, and inadequate or uneven adhesive impregnation. Other factors, including environmental conditions, may amplify these effects, and dynamic studies that track changes over time could further elucidate the delamination process. The measurement results indicate that the chosen method should align with the investigation’s specific objectives: PPT offers rapid identification of delaminated areas in near real-time, whereas THz-TDI provides detailed, cross-sectional visualizations critical for deeper analysis. In conclusion, this study underscores the importance of early detection and continuous monitoring of delamination in wax-resin linings. Accurate assessment of these conditions enables conservators to implement more effective treatment strategies, ultimately improving the preservation of culturally significant artworks.