Terahertz Time-Domain Reconstruction of Coating Microstratigraphy on Gilded Surfaces.

Cacciari Cacciari,Baija Baija,Siano Siano,Ciofini Ciofini

doi:10.3390/ma12233822

Abstract

Here, a systematic study in order to assess the potential of THz time domain reflectometry for measuring the thicknesses of overpaint layers applied on original gilded surfaces was carried out. The work is part of a thorough characterization campaign, which is going on at the Rijksmuseum for addressing the conservation problems of a set of 19th century gilded picture frames on which heavy coatings were applied in previous undocumented restoration interventions. To perform such non-invasive thickness measurements, an analytical protocol based on Gaussian fits of the THz pulse-echo temporal profiles was optimized through the preparation of suitable technical samples and the comparison with direct thickness measurements. Finally, the methodology was validated by characterizing the microstratigraphy of an original sculptural element from a gilded picture frame in the Rijksmuseum collection. The results achieved show the effectiveness of the present approach in revealing multi-layered dielectric microstructures with a spatial resolution of about 30 µm when using a spectral range up to 1.5 THz.

Highlights

Terahertz (THz) technology has been shown to be increasingly useful in a plethora of practical applications ranging from biosensing [1], to imaging [2], as well as for wireless communications [3].One of the most intriguing features of THz waves is that they are well suited for non-destructive characterization of multi-layered dielectric materials [4]
Thickness measurements of paint layers are generally accomplished in the frequency domain, and plenty of works in different applicative fields are known
In time domain, few studies are devoted to thickness measurements of metal paint layers in particular in automotive industries

Summary

Introduction

Terahertz (THz) technology has been shown to be increasingly useful in a plethora of practical applications ranging from biosensing [1], to imaging [2], as well as for wireless communications [3].One of the most intriguing features of THz waves is that they are well suited for non-destructive characterization of multi-layered dielectric materials [4]. A THz spectroscopy setup, both in transmission and in reflection, can be used as an excellent tool for non-destructive measurement of dielectric thicknesses In this case, the application fields may cover pharmaceutical coating inspections [19,20], automotive-paint quality check-ups [21,22], and cultural heritage [23,24]. Besides THz time of flight, existing non-destructive techniques for direct thickness measurement may include ultrasound and photoacoustic probes, ellipsometry, optical coherence tomography (OCT), NIR confocal laser scanning microscopy. Within such an analytical set, the THz approach extends the potential of optical techniques, which can be used only on sufficiently transparent layers, and allows avoiding the strict mechanical contact needed for pressure wave probing. The THz probing can be used as a valid complementary technique in micro-stratigraphic characterization of multi-layered dielectric materials in transmission or reflection configuration

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Conclusion