Material Appearance for Conservation and Restoration- Capturing and modelling the appearance of gilded surfaces

Abstract

The human visual system is extremely efficient at identifying and recognising different materials. Rarely do humans mistake one material for another, and the appearance of a material tells us many important cues about the environment we live in. However, the mechanisms behind material appearance and perception are not fully understood. Cultural heritage objects exist in a wide range of materials, each with its own particular appearance. While it is common to evaluate the colour of cultural heritage objects before and after restoration, this is not always representative of perceptual differences that the restoration may cause. Gilding is a form of polychromy commonly used in the Middle Ages which poses many technical challenges to acquire its appearance. The gold leaf, of metallic nature, creates a particular appearance which changes depending on the fabrication method and has a strong angular-dependence. This thesis aims to address some of these issues, by acquiring, modelling and analysing the appearance of complex cultural heritage materials. Focus is given to gilded surfaces since they exhibit interesting appearance properties such as gloss, metallicity, and in some cases even translucency. The research is divided into two sub-objectives, one concerning methods for appearance capture, and the second deals with applying material appearance analysis for conservation and restoration. The first research domain of this PhD thesis deals with methods for material appearance capture. First, the challenges and limitations of using conventional methods for appearance capture are explored. As an alternative, an imaging-based methodology is developed and evaluated for bi- directional reflectance measurements, catered for cultural heritage materials of challenging appearance. This method is then applied to satisfy the second sub-objective. The second research domain concerns the application of material appearance analysis for conservation. For this, the appearance of different types of gilding is acquired, modelled and evaluated. By using perceptual gloss metrics the different types of gilding can be described and characterised in terms of perceptual differences. Moreover, varnish removal methods are evaluated in terms of appearance change, to guide the conservation of a 15th century painted panel.