Nondestructive Microanalysis of Thin-Film Coatings on Historic Metal Threads

A new standards-based scanning electron microscopy with the energy-dispersive X-ray spectrometry (SEM-EDS) quantification method was used to analyze the thin-film coating of an 18th century French textile decorated with metal threads in variable pressure conditions. This analytical technique can allow for nondestructive quantitative characterization of the near surface of cultural heritage objects small enough to be placed in an SEM chamber that may contain corrosion products, without applying a conductive coating. A multivoltage analysis consisting of measurements taken at a series of electron beam energies was obtained and input into a film thickness and composition (FTC) computational model to characterize a layered Au on Ag reference material, in addition to a historic metal thread. Using the FTC computation, the thread coating was determined to be an alloy ≈ 80% Au 20% Ag on a nominally pure Ag substrate, and this composition matches a minimum gold standard allowed for goods around the time of manufacture. The computed gilding thicknesses range from single digit nm to 300 nm depending upon surface inhomogeneities formed during the production of the thread. Interaction volumes and X-ray spectra generated by Monte Carlo modeling are consistent with the measured gilding thicknesses and compositions. Validation of the FTC-computed gilding composition and thickness variations were obtained by cross-sectional analysis.