CHARACTERIZATION OF CRYSTALLINE PIGMENTS WITH LOW-FREQUENCY VIBRATIONAL SPECTROSCOPY AND SOLID-STATE DENSITY FUNCTIONAL THEORY

Although historical pigments are seldom found in the modern artist’s palette, their characterization is a critical aspect of designing effective conservation and restoration protocols, establishing provenance, and detecting forgeries. Ideal characterization methods are nondestructive, noninvasive, and able to distinguish between pure and mixed pigment samples. Spectroscopic techniques are commonly used to identify pigment composition because of their non-ionizing nature, rapid acquisition times, and safety. Unfortunately, the majority of these methods have difficulty distinguishing between pigments with similar chemical and physical properties. Recent advancements in instrument technology have increased the broader availability of terahertz time-domain spectroscopy (THz-TDS) and low-frequency Raman spectroscopy (LFRS). In this work, the capabilities of THz-TDS and LFRS for identification and characterization of historic and modern pigments were evaluated. These experimental studies were supported with solid-state density functional theory (ss-DFT) simulations of the pigment structures and vibrations to gain insight into the molecular and intermolecular origins of the observed spectral features. These results demonstrate the powerful combination of low-frequency (≤ 200 cm-1) vibrational spectroscopic methods and computational techniques for the identification and characterization of pigments and establish the compelling abilities of THz-TDS and LFRS as new tools for characterization of pigment components in artworks and artifacts

Time-domain imaging system in the terahertz range for immovable cultural heritage materials

In the field of cultural heritage science, the use of non‐destructive and contact‐free techniques has increased sharply over the past 10 years. Compared with conventional spectroscopic and imaging techniques such as X‐ray, ultraviolet, infrared, and laser spectroscopy, terahertz time‐domain imaging (THz‐TDI) is an innovative, non‐invasive, and safe technique, which provides good penetration depth (~1 cm) and broad spectral bandwidth (0.1–10 THz). This paper sets out the protocol and methodology for the application of THz‐TDI to immovable cultural heritage, illustrated by a series of case studies. The case studies demonstrate the efficacy of the technique in providing structural and material information for conservators

Cyclododecane as a contrast improving substance for the terahertz imaging of artworks

This paper presents measurements of the terahertz properties of the art conservation substance cyclododecane, demonstrating that it can act as a contrast improving agent in the terahertz imaging of concealed wall paintings. Results are presented which show that the terahertz optical properties of cyclododecane are dependent on the rate at which it has cooled from the melt. Based on the results, a theoretical explanation of the contrast enhancement mechanism is postulated. The findings presented here may lead to the development of novel coating materials that could improve the quality of terahertz images in a variety of fields and not just in art conservation

Confocal Imaging at 0.3 THz with depth resolution of a painted wood artwork for the identification of buried thin metal foils

A compact confocal terahertz microscope working at 0.30 THz based on all-solid-state components is used to locate buried thin metal foils in a painted wood artwork. Metal foils are used for decoration, and their precise localization under the pictorial layer is relevant information for conservation scientists and restorers, which can neither be obtained by X-ray radiography nor by spectroscopic imaging in the infrared, as we directly show here. The confocal microscopy principle based on the spatial pinhole concept is here implemented by positioning the first focus of an ellipsoidal reflector at the phase center of horn antennas coupled to Schottky diode detector and emitter mounted in rectangular waveguide blocks, together with an optical beamsplitter. The second focus of the reflector is mechanically scanned inside the sample in three dimensions. The predictions of diffraction theory for a confocal microscope at an imaging wavelength of 1.00 mm with numerical aperture of 0.53 are verified experimentally (1.2 and 2.8 mm for the lateral and the axial resolution, respectively). These values of resolution allow a precise determination of the position of buried metal foils in an ancient piece of art hence making restoration interventions possible

Plasmonic terahertz detectors based on a high-electron mobility GaAs/AlGaAs heterostructure

In order to characterize magnetic-field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the cyclotron resonance amplitude on the conduction-band electron filling factor which was explained within a model of the electron gas heating with the THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on high-mobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.Comment: 8 pages, 11 figure

Non-destructive investigation of paintings on canvas by continuous wave terahertz imaging and flash thermography

Terahertz (THz) imaging is increasingly used in the cultural heritage field. In particular, continuous wave (CW) and low frequency THz is attracting more attention. The first application of the THz technique inherent to the cultural heritage field dates back 10 years ago. Since 2006, tangible improvements have been conducted in the refinement of the technique, with the aim to produce clear maps useful for any art restorer. In this paper, a CW THz (0.1 THz) imaging system was used to inspect paintings on canvas both in reflection and in transmission modes. In particular, two paintings were analyzed: in the first one, similar materials and painting execution of the original artwork were used, while in the second one, the canvas layer is slightly different. Flash thermography was used herein together with the THz method in order to observe the differences in results for the textile support materials. A possible application of this method for the detection of artwork forgery requires some parameterization and analysis of various materials or thickness influence which will be addressed in a future study. In this work, advanced image processing techniques including principal component thermography (PCT) and partial least squares thermography (PLST) were used to process the infrared data. Finally, a comparison of CW THz and thermographic results was conducted

Terahertz pulse imaging in archaeology

The work presented in this article was performed at the Oriental Institute at the University of Chicago, on objects from their permanent collection: an ancient Egyptian bird mummy and three ancient Sumerian corroded copper-alloy objects. We used a portable, fiber-coupled terahertz time-domain spectroscopic imaging system, which allowed us to measure specimens in both transmission and reflection geometry, and present time- and frequency-based image modes. The results confirm earlier evidence that terahertz imaging can provide complementary information to that obtainable from x-ray CT scans of mummies, giving better visualisation of low density regions. In addition, we demonstrate that terahertz imaging can distinguish mineralized layers in metal artifacts