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.
Changes of temperature and relative humidity of the environment where the artefact is stored can cause deformations of the surface that can harm the object or affect the repeatability of the measurement. Currently, there are very few techniques and approaches that allow studying non-invasively the surface deformations of objects of different sizes maintaining accuracy of the order of micrometres. To address this problem, three different systems based on a conoscopic holography sensors have been tested on hygroscopic samples. Moreover, we implemented a low-cost apparatus for controlling relative humidity. Eventually, we investigated procedures that use reference standards, insensitive to thermal and hygrometric variations with the scope to assess short and long-term drifts of the instrumental set-up. We tested the approach on samples of paper and lambskin parchment and on a wooden icon.
In recent years, smartphones have developed tumultuously; in fact, they have increasingly taken on the role of portable and compact personal computers equipped with a range of sensors and applications capable of monitoring, for example, physical activity, heartbeat and sleep. Their ability to manage and transmit data makes them a very interesting tool even in the scientific field, where the concept of “lab in a phone” is gaining ground. In this work, after briefly summarizing these new approaches, we will consider the application of smartphones to the diagnostics of Cultural Heritage.
The characterization of the surface morphology and the material layers stratigraphy is a pivotal step in several stages of the conservation process of the artwork. Generally, optimal characterization of the surface cannot be achieved using a single technique but is not always possible to find techniques that can be coupled effectively. Surface maps were retrieved using acoustic microscopy and conoscopic laser holography over a set of reference samples. The surface maps compared for showing possible way to better characterize the surface morphology when their information is combined. The final data give us a more complete surface and subsurface digitalization while the results obtained are discussed highlighting the main pros and cons of the two techniques. Thus besides having insights on the effectiveness and on the performance of the processes, it is also evident that uncertainties associated with the measurement can be overcome and a single more accurate representation of the surface can be derived using both scanning methods.
Surface metrology of artworks requires the design of suitable devices for in-situ non-destructive measurement together with reliable procedures for an effective analysis of such non-engineered variegate objects. To advance the state-of-the-art it has been implemented a versatile optical micro-profilometry taking advantage of the adapt- ability of conoscopic holography sensors, able to operate with irregular shapes and composite materials (diffusive, specular, and polychrome) of artworks. The scanning technique is used to obtain wide field and high spatially resolved areal profilometry. The prototype has a modular scheme based on a set of conoscopic sensors, extending the typical design based on a scanning stage and a single probe with a limited bandwidth, thus allowing the collection of heights data from surface with different scales and materials with variegate optical response. The system was optimized by characterizing the quality of the measurement with the probes triggered in continuous scanning modality. The results obtained on examples of cultural heritage objects (2D paintings, 3D height-relief) and materials (pictorial, metallic) demonstrate the versatility of the implemented device.
Thermal Quasi Reflectography (TQR), e.g. imaging in the thermal band 3-5 μm (MWIR), is discussed as innovative tool
for the noninvasive analysis of pictorial surface layers in artworks, and its potential is demonstrated in some applications.
The results encourage further developments in this field. The novel experimental technique, which has been recently
introduced by the authors, is reviewed here giving focus to current research and potential applications.
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