Abstract

Reflectance Transformation Imaging has been largely used for the recording, documentation, and analysis of cultural heritage. A series of images of a static object, taken in raking light at constant exposure, is synthetized using the Polynomial Texture Mapping or the Hemispherical Harmonics fitter, resulting in polynomial texture maps or Reflectance Transformation Images. These visualizations capture colour and texture information, revealing low relief detail and emphasizing surface topography. RTI has found several applications in the study of painted surfaces, yet it has not been utilized for the study of historic interiors. More commonly, this would be achieved through Architectural Paint Research, a process of examining the painted finishes to the interior or exterior of an historic structure, to establish its decorative history. Typically, this is achieved through a combination of archival research; sampling of the paint and substrate layers that are analysed at high magnification in cross-section; and through careful layer-by-layer exposures of the accumulated paint stratigraphy so that the texture, sheen and colour of the paint layer can be observed. The latter of these techniques is especially important if an underlying scheme is decorative or polychromatic.

This study evaluates the use of RTI for the study of historic wall paintings, with an emphasis on the revelation of overpainted designs. Considering the efficiency of RTI in recording and documentation of subtle surface variations, this research explores the use of texture comparisons between exposed, restored and covered areas of the wall painting as a diagnostic tool and for appraising condition.