The Role of Varnishes in Modifying Light Reflection from Rough Surfaces - A Study of Changes in Light Scattering Caused by Variations in Varnish Topography and Development of a Drying Model
Résumé
To obtain a better understanding of the variations in the appearance of paintings brought about by different varnishes, their ability to form an optically smooth film over rough surfaces was studied using laser scanning confocal microscopy and stylus profilometry, as well as gloss and distinctness-of-image gloss measurements. Two different varnishes, one made with a low molecular weight resin and having low viscosity, and another, made with a polymer of high molecular weight and having high viscosity, were applied over glass plates that had been given surface roughness in the micrometer range. Measurement of the topography showed that the polymeric resin eliminated mid-to-high spatial frequency roughness of the ground glass but did not climinate roughness of lower spatial frequencies. The results are discussed in terms of the surface state of the varnish, using concepts such as root-mean-square roughness, total integrated scattering and the power spectral density. These observations indicate that both varnishes reduce the wide-angle light scattering by the rough substrates, but only the varnish made with the low molecular weight resin reduces the small-angle scattering about the specular reflection. This suggests that the smoothness of the varnished surface depends both on the molecular weight of the resin and on the spatial frequencies of the surface roughness of the surface beneath it. A model for the drying of varnishes is presented that takes the spatial frequency dependence into account.