Optical surface metrology for heritage science: proof of concept and critical-constructive discussion

Abstract

In the engineering field, surface metrology is a valuable tool codified by international standards that enables the quantitative study of small-scale (down to micrometer) surface features, i.e., the surface topography. However, it is not recognized as a resource in heritage science. In literature we find a large use of qualitative inspection of surface morphology or of single-parameter roughness analysis, which confirms the need and potential of such diagnostics. Reasons of the gap are variegate; artworks are hand-made peculiar targets with heterogeneous surfaces, a multiscale approach is necessary, lack of guidelines and unclear meaning of surface roughness descriptors. We propose a critical-constructive discussion through Proof-of-Concept (POC) applications, on the use of surface metrology based on ISO descriptors. Exemplary case studies include: 1) In situ and in-process monitoring of painting microtexture in a Venetian masterpiece: wide and in-band roughness analysis is performed through the complementary use of amplitude, spatial, and hybrid parameters. 2) Multiscale roughness analysis for treatment monitoring in highly reflective metal artworks, requiring high micrometer accuracy in both depth (0.1 µm and lateral (5 µm) directions: surface analysis is performed on scale-limited components to discriminate different surface processes. Surface data are acquired using a prototype of a laser scanning profilometer based on conoscopic holography, with a versatile setup and a surface data pipeline tailored to artwork applications.