Revealing Underdrawings in Wall Paintings of Complex Stratigraphy with a Novel Reflectance Photoacoustic Imaging Prototype

The relationship between the microclimate and efflorescence of revealed mural paintings and the later protection strategy

Throughout history, there were many fine mural paintings concealed within ancient buildings, hidden beneath layers of plaster, wall or other structure. In recent decades, research and practical efforts had primarily focused on nondestructive diagnosis of these hidden murals and the removal of their surface coverings. However, limited attention had been given to the consequences of overlay removal on mural preservation. This study aims to address this gap by examining the revealed mural paintings in the Prince Shi's Palace as a case study, employing an analysis of mural ontology, investigation into the preservation environment, and simulation experiments to comprehensively analyze the factors contributing to the rapid deterioration of these revealed murals. The findings indicated that certain carbonated lime particles adhered to the mural surface, and in a humid environment, these particles absorbed sulfur dioxide from the atmosphere and converted into calcium sulfate. The strong hygroscopic property of calcium sulfate enhanced the humidity on the surface of murals, thereby increasing their susceptibility to sulfur dioxide absorption. Over time, the murals suffered more sulphate and moisture damage. Additionally, historically inaccurate interventions such as non-breathable shading curtains installation and organic coatings reinforcement exacerbated this deterioration process. Therefore, safeguarding the integrity of these exposed concealed murals necessitated prioritizing the preservation of environmental humidity and acid gas levels.

Agar Gel as a Non-Invasive Coupling Medium for Reflectance Photoacoustic (PA) Imaging: Experimental Results on Wall-Painting Mock-Ups

The new reflectance set-up configuration extended the applicability of the photoacoustic (PA) imaging technique to art objects of any thickness and form. Until now, ultrasound gel or distilled water have been necessary as coupling mediums between the immersion-type transducer and the object’s surface. These media can compromise the integrity of real artwork; therefore, known applications of reflectance PA imaging have been limited to only experimental mock-ups. In this paper, we evaluate an alternative non-invasive PA coupling medium, agar gel, applied in two layers of different consistency: first, rigid—for the protection of the object’s surface, and second, fluid—for the transducer’s immersion and movement. Agar gel is widely used in various conservation treatments on cultural heritage objects, and it has been proven to be safely applicable on delicate surfaces. Here, we quantify and compare the contrast and signal-to-noise ratio (SNR) of PA images, obtained in water and in agar gel on the same areas, at equal experimental conditions. The results demonstrate that the technique’s performance in agar is comparable to that in water. The study uncovers the advanced potential of the PA approach for revealing hidden features, and is safely applicable for future real-case studies.