Applications of nuclear magnetic resonance sensors to cultural heritage

Comprehensive characterization of waterlogged archaeological wood by NMR relaxometry, diffusometry, micro-imaging and cryoporometry

Chemical, physical, and biological decay may partially or totally hide the historical and technological information carried by waterlogged wood. Investigation of the above-mentioned decay processes is essential to assess the wood preservation state, and it is important to find new methods for the consolidation and safeguarding of wooden archaeological heritage. A conventional method for assessing the wood preservation state is light microscopy. However, the method requires sample slicing, which is destructive and challenging when dealing with fragile and spongy submerged remains of heritage wood. To this end, a promising alternative non-destructive technique is proton nuclear magnetic resonance (1H-NMR) which considers wood as a porous system. This work aimed to perform a comprehensive analysis of structures, porosity, water distribution, decay, and possible structural inclusions of three archaeological waterlogged wood fragments of the Roman age using NMR relaxometry, micro-imaging (μ-MRI), NMR diffusometry, and NMR cryoporometry. The results were compared with a similar analysis of the three contemporary wood samples of the same species. The multimodal approach presented in this study allowed us to cover all the dimensional scales of wood, from nanometers to sub-millimeters, and reconstruct the alteration of the entire archaeological wood fragment caused by degradation.

Monitoring Water Absorption and Desorption in Untreated and Consolidated Tuff by a Non-Invasive Graphene-Based Humidity Sensor

A hybrid montmorillonite (MMT)/reduced graphene oxide (rGO) film was realised and used as a non-invasive sensor for the monitoring of water absorption and desorption in pristine and consolidated tuff stones. This film was obtained by casting from a water dispersion containing graphene oxide (GO), montmorillonite and ascorbic acid; then the GO component was thermo-chemically reduced and the ascorbic acid phase was removed by washing. The hybrid film showed electrical surface conductivity that varied linearly with the relative humidity, ranging from 2.3 × 10^-3 S in dry conditions to 5.0 × 10^-3 S at 100% RH. The sensor was applied onto tuff stone samples through the use of a high amorphous polyvinyl alcohol layer (HAVOH) adhesive, which guaranteed good water diffusion from the stone to the film and was tested during water capillary absorption and drying tests. Results show that the sensor is able to monitor water content changes in the stone, being potentially useful to evaluate the water absorption and desorption behaviour of porous samples both in laboratory environments and in situ.

Multiphoton Excitation Fluorescence Microscopy and Spectroscopic Multianalytical Approach for Characterization of Historical Glass Grisailles

The preservation of the integrity of artworks and cultural heritage items during characterization and conservation operations is of high priority, therefore, the application of non-invasive techniques is commonly suggested and recommended. Nonlinear optical microscopies (NLOM), based on the use of tightly focused pulsed femtosecond lasers, are emerging techniques for structural and chemical analysis of heritage objects with micrometric lateral and axial resolution. The results obtained with a set of optical and spectroscopic techniques for the chemical and physical characterization of grisaille paint layers on historical stained glasses, from different chronologies and provenance in Spain, are presented in this work. Optical behaviour and chemical composition were investigated by NLOM, using a laboratory set-up in the modality of Multi-Photon Excitation Fluorescence (MPEF), and by a multi-analytical combination of Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectrometry (FESEM-EDS), Laser Induced Breakdown Spectrosocopy (LIBS) and Laser Induced Fluorescence (LIF). Thicknesses values of the historical grisaille paint layers measured with MPEF were compared with those retrieved through FESEM, showing significant consistency and agreement. Under proper conditions, analysis via MPEF microscopy avoids the photochemical and physical damage to the examined materials, thus ensuring their preservation. This approach paves the way for future in-situ, non-invasive stratigraphic investigations on cultural heritage objects.

Observation of iron oxide nanoparticle synthesis in magnetogels using magnetic resonance imaging

We show that magnetic resonance imaging (MRI) can be used to visualize the spatiotemporal dynamics of iron oxide nanoparticle growth within a hydrogel network during in situ coprecipitation. The synthesis creates a magnetic nanoparticle loaded polymer gel, or magnetogel. During in situ coprecipitation, iron oxide nanoparticles nucleate and grow due to diffusion of a precipitating agent throughout an iron precursor loaded polymer network. The creation of iron oxide particles changes the magnetic properties of the gel, allowing the synthesis to be monitored via magnetic measurements. Formation of iron oxide nanoparticles generates dark, or hypointense, contrast in gradient echo (GRE) images acquired by MRI, allowing nanoparticle nucleation to be tracked in both space and time. We show that the growth of iron oxide nanoparticles in the hydrogel scaffold is consistent with a simple reaction-diffusion model.

How mobile NMR can help with the conservation of paintings

The conservation of paintings is fundamental to ensure that future generations will have access to the ideas of the grand masters who created these art pieces. Many factors, such as humidity, temperature, light, and pollutants, pose a risk to the conservation of paintings. To help with painting conservation, it is essential to be able to noninvasively study how these factors affect paintings and to develop methods to investigate their effects on painting degradation. Hence, the use of mobile nuclear magnetic resonance (NMR) as a method of investigation of paintings is gaining increased attention in the world of Heritage Science. In this mini-review, we discuss how this method was used to better understand the stratigraphy of paintings and the effect different factors have on the painting integrity, to analyze the different cleaning techniques suitable for painting conservation, and to show how mobile NMR can be used to identify forgeries. It is also important to keep in mind its limitations and build upon this information to optimize it to extend its applicability to the study of paintings and other precious objects of cultural heritage.

Identification of complex structures of paintings on canvas by NMR: Correlation between NMR profile and stratigraphy

Paintings on canvas are complex structures created by superimposing layers of different composition. Investigations on the structure of these artworks can provide essential information on their state of conservation, pictorial technique, possible overpaintings, and in planning a proper conservation plan. Standard methods of investigation consist in sampling a limited number of fragments for stratigraphic analyses. Despite the recognized validity of these methods, they are affected by evident limitations. Nuclear magnetic resonance (NMR) profiling, often named NMR stratigraphy, is an NMR relaxometry technique applied by single-sided portable devices developed to overcome the disadvantages of microinvasive stratigraphic analyses. The potential of this approach on artworks, including wall paintings and a few examples of painted canvas, is described in the literature. In this study, NMR profiles of painting on canvas were examined by analyzing transverse relaxation time data by T₂ quasi-continuous distributions and the results compared with standard stratigraphic cross-sections analysis. Combining signal intensity and T₂ quasi-continuous distributions, the identification of textile, preparatory, and paint layers was enhanced. The diction "NMR stratigraphy" for these inhomogeneous layered artworks is also discussed. Indeed, unlike the stratigraphic cross-sections, NMR profiles provide information on a volume (flat slice), rather than on a surface, and the collected signal can derive from nonuniform and partially overlapping layers. This study paves the way for extensive investigations on relaxation time quasi-continuous distributions in various binder/pigment mixtures in order to improve the reliability of NMR profile as an innovative, non-invasive, and nondestructive method for analyzing paintings on canvas.

Combined photoacoustic imaging to delineate the internal structure of paintings

In this Letter, we present a combined photoacoustic imaging method, based on consecutive excitation using either the fundamental or the second-harmonic wavelength of a pulsed Nd:YAG laser for the stratigraphy of painted artworks. Near-infrared excitation was employed for the imaging of hidden underdrawings in mock-up samples, whereas visible light was used for the thickness mapping of the overlying paint through the detection of photoacoustic signal attenuation. The proposed methodology was proven effective in measuring thick and strongly absorbing layers, which would not be possible by means of other pure optical techniques, while also enabling the visualization of features underneath the painted surface. Such an implementation expands significantly the applicability of the previously presented photoacoustic technique, which was limited to point-measurements, and paves the way for novel application in historical and technical studies, as well as in documenting restoring operations.

Multianalytical non-invasive characterization of phthalocyanine acrylic paints through spectroscopic and non-linear optical techniques

The documentation and monitoring of cleaning operations on paintings benefit from the identification and determination of thickness of the materials to be selectively removed. Since in artworks diagnosis the preservation of the object's integrity is a priority, the application of non-invasive techniques is commonly preferred. In this work, we present the results obtained with a set of non-invasive optical techniques for the chemical and physical characterization of six copper-phthalocyanine (Cu-Pc) acrylic paints. Cu-Pc pigments have been extensively used by artists over the past century, thanks to their properties and low cost of manufacture. They can also be found in historical paintings in the form of overpaints/retouchings, providing evidence of recent conservation treatments. The optical behaviour and the chemical composition of Cu-Pc paints were investigated through a multi-analytical approach involving micro-Raman spectroscopy, Fibre Optics Reflectance Spectroscopy (FORS) and Laser Induced Fluorescence (LIF), enabling the differentiation among pigments and highlighting discrepancies with the composition declared by the manufacturer. The applicability of Non Linear Optical Microscopy (NLOM) for the evaluation of paint layer thickness was assessed using the modality of Multi-photon Excitation Fluorescence (MPEF). Thickness values measured with MPEF were compared with those retrieved through Optical Coherence Tomography (OCT), showing significant consistency and paving the way for further non-linear stratigraphic investigations on painting materials.

Cultural Heritage Studies with Mobile NMR

Nuclear magnetic resonance (NMR) provides in situ information about selected isotope densities in samples and objects, while also providing contrast through rotational and translational molecular dynamics. These parameters are probed not only in NMR spectroscopy and imaging but also in nondestructive materials testing by mobile stray-field NMR, the unique properties of which are valuable in cultural heritage studies. We present recent progress in the analysis of cultural heritage with mobile ¹ H NMR stray-field sensors, for which the detection zone is outside of the NMR magnet. Prominent applications include the analysis of stratigraphies in paintings and frescoes, and the assessment of material states changing under the impact of aging, conservation, and restoration.