Multi-analytical approach to the study of mecca gilding technique

Meccatura is a type of historical gilding technique, which consists in the application of a coloured varnish on burnished metal surfaces (silver or tin foil) with the aim to both imitate the golden surface and to protect the metal from tarnishing. Since the medieval period in Europe, it has been applied in paintings, carved wood altarpieces, frames, polychrome sculptures, and decorated leather. There is a variety of mecca historical recipes, which report on different organic substances, e.g. siccative oils, resins (sandarac, colophony, shellac etc.), organic dyes (saffron, dragon's blood, gamboge, aloe etc.), plasticizers (beeswax or carnauba), often unknown, subject to degradation phenomena and past interventions. More recently, the classical mecca antica has been substituted with mixtures of synthetic colourants charged with pigments and inert materials, the so called mecca moderna. Due the complexity of present materials, this represents an analytical challenge for heritage scientists and conservators. With more attention dedicated to the traditional gilding techniques, available scientific results on the meccatura are rather fragmentary, being part of a larger multi-analytical study on decorated artworks. This paper focuses on meccatura research issues, outlines the gaps in their scientific study and evaluates several methodologies to overcome them. In specific, the limitations and contributions of complementary Raman and Fourier-Transform Infrared (FTIR) Spectroscopies to the study of main traditional mecca components are analyzed and discussed, supported by structural information from Optical Coherence Tomography (OCT) and colorimetric measurements by Fiber Optics Reflectance Spectroscopy (FORS). The experimental study is carried out on mock-ups and on a real case study. The paper highlights potentials, limitations and future perspectives in the development of non-invasive analytical approach to the investigation of historical and modern mecca varnish recipes, their alteration and ageing.