Thickness Measurements of Clear Coatings on Silver Objects using Fiber Optic Reflectance Spectroscopy

Polymer films have been used extensively to coat heritage metal objects to provide protection against atmospheric oxidation and tarnish. Coating performance is directly proportional to coating thickness, but current methods used to measure coating thickness for quality control during treatment are qualitative at best. This paper demonstrates how fiber optic reflectance spectroscopy (FORS) was used to accurately determine coating thicknesses on a number of nitrocellulose-coated silver objects at Winterthur Museum. FORS appears to be significantly more accurate than other common methods used to measure film thickness, such as magnetic/eddy current techniques, but is limited to transparent or semi-transparent coatings. FORS is a simple, accurate method for measuring the thickness of protective coatings on a wide range of cultural heritage metal objects.