Challenges in colorimetric evaluation of femtosecond laser cleaning on historical leather: influence of surface porosity and microstructure.

Abstract

This study critically evaluates femtosecond (fs) laser cleaning of historical vegetable-tanned goat leather, addressing quantification on porous, heterogeneous surfaces. Artificially aged, graphite-contaminated samples were treated by fs-laser irradiation with varying fluence and shot number. Cleaning performance was assessed by high-resolution CIE L*a*b* colorimetry, complemented by quantitative porosity analysis using optical microscopy and ImageJ. Pore-size measurements showed a mixed distribution of small and large pores affecting cleaning uniformity. Fs-laser cleaning reduced visible contamination, with ΔE* approaching the perceptibility threshold of 2.0. However, the leather microstructure led to significant reflectance variability, underscoring the limitations of area-averaged colorimetry for cleaning assessment. Comparative measurements on graphite-contaminated parchment, with negligible porosity but pronounced micro-relief, showed that pore structure and micro-topography govern optical consistency. Overall, these findings underscore the need to interpret colorimetry in relation to surface morphology and support integrating spatially localized optical diagnostics into fs-laser cleaning protocols to preserve visual integrity.