Monitoring the non-local surface response using microprofilometry techniques

In engineering, surface metrology is a codified tool employed to gain insight into the study of small-scale quantitative features of surface topography. While this information is valuable in complementing qualitative morphological inspection, it is not currently considered useful in heritage science and is not fully exploited. The objective of this thesis is to address some of the existing gaps in the utilization of the surface metrological approach in the field of cultural heritage. The underlying approach involved investigating authentic cases, focusing on the non-local surface response at the micrometer scale across various materials and stress conditions. The fundamental premise is a reliable instrument, namely a prototype of an optical microprofilometer developed in the OpDATeCH laboratory of the University of Verona, capable of scanning macroregions (tens of centimeters) with micrometer accuracy. Based on the data acquired by this instrument, the principal aim is to demonstrate the efficacy of surface metrology in artwork analysis, with a specific effort on statistical analysis based on ISO areal field parameters. The journey begins with evidence of the potential of surface metrology for quantitative analysis of surface topography, which extends beyond the prevalent practice of qualitative inspection of surface morphology and the calculation of a single roughness parameter (typically Sq). The methodology is then advanced to account for the heterogeneity of the surfaces in question, recognizing that it is of significant importance to overcome the use of area-averaging parameters, as this may result in a loss of local information. Thus, the multiscale surface analysis is proposed as a method to examine hand-processed surfaces in artwork diagnostics. Nevertheless, the optical microprofilometer lacks reference points in the height map, which presents a challenge in accurately interpreting the data. To address this, a novel workflow is proposed to register microsurfaces of artwork using the raw intensity signal collected by the interferometric sensor. In light of this intensity-based information, a feature-based analysis of the height map can be performed, thereby enabling reliable surface metrology. This aspect is further deepened by spatially registering a 2D multispectral imaging stack with the height data. The distinct spectral signatures of the various materials in the imaging bands allow for the identification of the features of interest and the corresponding region in the height map. This enables a comprehensive analysis of the integrated response between the materials and the microsurface. The artwork surfaces also exhibit a temporal dimension as they are complex multiscale structures, shaped by centuries of history. The concluding part interprets short- and long-term climatic variations as forcing factors acting cumulatively on artworks. Subsequently, the developed data pipeline is utilized for monitoring the spatio-temporal surface response, thereby enabling an investigation into the extent to which the surface can be interpreted as an information archive.