The potential of Reflectance Transformation Imaging in Architectural Paint Research and the study of historic interiors: a case study from Stowe House, England

Reflectance Transformation Imaging (RTI) has been increasingly 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 synthetised using Polynomial Texture Mapping or the Hemispherical Harmonics fitter, resulting in polynomial texture maps or RTI images. These visualisations capture colour and texture information, reveal low relief detail and emphasise surface topography. RTI has found several applications in the study of painted surfaces, yet it has not been utilised for the study of historic interiors, which are more commonly evaluated through Architectural Paint Research, a process of empirical examination of the painted finishes to an historic structure to establish its decorative history. Typically, this is achieved through a combination of archival research, paint and substrate cross-section analysis, and through the careful layer-by-layer exposure of the accumulated paint stratigraphy so that the texture, sheen and colour of each paint layer can be observed. This study evaluates the use of RTI as an alternative means 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 its use in texture comparisons between exposed, restored and covered areas of the wall painting as a diagnostic tool and for appraising condition.