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

Abstract

Revealing precious hidden features by a completely non-invasive approach is one of the crucial issues in the Heritage Science field. In this regard, concealed fresco paintings still represent an analytical challenge. This paper addresses the specific issue in wall painting diagnostics by the photoacoustic (PA) imaging technique, already proven to be efficient in revealing underdrawings and internal stratigraphy in movable paintings on paper and canvas. A newly set-up reflection PA prototype was applied here for the first time to probe the charcoal, graphite and sinopia hidden sketch drawings in concealed (gypsum, limewash, overpainted) wall paintings. The results presented here push forward the frontiers of the PA imaging technique and point to its potential effectiveness of revealing hidden underdrawings in historical wall paintings with complex stratigraphy.

Emerging photonic technologies for cultural heritage studies: the examples of non-linear optical microscopy and photoacoustic imaging

“…To overcome these restrictions, a novel optimized epi-illumination (reflection-mode) PA imaging system has been built, to provide universal diagnostic information in objects of arbitrary thickness or shape [55]. The capabilities of the epi-illumination PA apparatus were first demonstrated to reveal hidden charcoal, graphite or sinopia sketch drawings in thick concealed (gypsum, limewash, overpainted) wall painting mock-ups [56]. The obtained results provided experimental evidence that the PA signals may provide significantly improved imaging contrast when compared to the conventional NIR optical technique in similar spectral regions (Fig.…”

Materials and Technique: The First Look at Saturnino Gatti

“…This last technique involves the creation of a full-sized drawing on a preparatory cartoon; then, the contours of the different figures are subsequently perforated with a needle or another small tip. Next, the perforated cardboard is placed on the wall's surface, and the perforated parts are dabbed with a canvas bag filled with ash, charcoal, or other fine dust that may leave a trace on the wall [40]. A clear example of such dusting is visible in image 5B, in which the series of dark dots are clearly defined near the forearm and the folds of the dress of the woman identified as Mary Magdalene [24], who supports the head of the dead Christ.…”

The Ability of the EPR MOUSE to Study Underpaintings

“…The initial discovery of hidden images often utilizes imaging techniques, while the investigation may also take advantage of point sensitive techniques. The imaging techniques, such as macro X-ray fluorescence (ma-XRF) [6], X-ray laminography [7], reflectance imaging spectroscopy (RIS) in the visible and near infrared [8], and photoacoustic imaging [3,9], are noninvasive. Some point-sensitive techniques cannot see through a surface layer, so paint cross section analysis [10] is necessary to expose information underneath a surface layer of paint.…”