Preparing the foundation for stable gilding: Scientific evaluation of the durability of Baroque gesso gilding grounds

This paper follows two earlier papers about Portuguese gesso gilding grounds, a typical decoration from the 13th to the 18th century but with a special focus on the Baroque period. It concentrates on understanding the reasons why these gilded surfaces are so durable. The main concerns of the people involved in the production of the gilded surfaces, as revealed in contemporary historical documents, are the quality and durability of the decorations. The investigation of ‘durability’ involved the study of factors not explored before regarding materials and practices commonly used to produce gilded wooden surfaces in South Europe. The paper discusses the probable effects on durability of loading a binder with a filler, of the shape and size of the filler particles, of the interaction of filler and binder, and of using a multi-layered system; it discusses the science underlying the use and behaviour of particular gilding materials and practices. This paper is followed by a second paper focused on technological choices. Together they aim to contribute to understanding why Portuguese gilders clearly chose double-structured gesso grounds in preference to other possibilities, and to aid on conservation decision-making and the design of new strategies for the treatment and preservation of these historical gilded surfaces.