Analysis of historical porous building materials by the NMR-MOUSE

Nondestructive Analysis of Wall Paintings at Ostia Antica

Roman wall paintings at Ostia Antica were studied for the first time in situ in an integrated approach using nuclear magnetic resonance (NMR) depth profiling, portable X-ray fluorescence (XRF), and visible induced luminescence (VIL) in order to explore the materials used in their construction and gain insight into the evolution of the Roman painting technique over time. NMR revealed the signatures of covered wall paintings through details of the structure of the top painted mortar layers, and the loss of this information that can be encountered when paintings are detached from the wall for preservation purposes. XRF provided information about the pigment composition of the paintings, and VIL was used to identify Egyptian Blue. Egyptian Blue was only found in the earlier wall paintings studied dating from 1st century B.C.E. to the 1st century C.E. The pigment palette seems to become limited to iron-based pigments in the later paintings, whereas the palette of the earlier paintings appears to be more varied including mercury, lead, and copper-based pigments.

An Ontology-Based Approach to Enable Data-Driven Research in the Field of NDT in Civil Engineering

Although measurement data from the civil engineering sector are an important basis for scientific analyses in the field of non-destructive testing (NDT), there is still no uniform representation of these data. An analysis of data sets across different test objects or test types is therefore associated with a high manual effort. Ontologies and the semantic web are technologies already used in numerous intelligent systems such as material cyberinfrastructures or research databases. This contribution demonstrates the application of these technologies to the case of the 1H nuclear magnetic resonance relaxometry, which is commonly used to characterize water content and porosity distribution in solids. The methodology implemented for this purpose was developed specifically to be applied to materials science (MS) tests. The aim of this paper is to analyze such a methodology from the perspective of data interoperability using ontologies. Three benefits are expected from this approach to the study of the implementation of interoperability in the NDT domain: First, expanding knowledge of how the intrinsic characteristics of the NDT domain determine the application of semantic technologies. Second, to determine which aspects of such an implementation can be improved and in what ways. Finally, the baselines of future research in the field of data integration for NDT are drawn.

Fourier and Laplace-like low-field NMR spectroscopy: The perspectives of multivariate and artificial neural networks analyses

Low field Nuclear Magnetic Resonance (LF-NMR) is a rich source of information for a wide range of samples types. These can be hard or soft solids, such as plastics or elastomers; bulk liquids or liquids absorbed in porous materials, and can come from biomaterials, biological tissues, archaeological artifacts, cultural heritage objects. LF-NMR instruments present a significant advance especially for in situ, ex situ and in vivo measurement of relaxation and diffusion. Moreover, high resolution 1D and 2D spectroscopy, as well as magnetic resonance (MR) imaging are available in these fields. In this work we discuss the advanced analysis of the data measured in LF-NMR from the perspectives of tertiary level that implies the analysis on principal components (PCA), and on the quaternary analysis that uses an artificial neural network (ANN). The principles of PCA and ANN are largely discussed. For the PCA analysis, a series of 52 spectra were analyzed, having been recorded in vivo by LF-NMR. Of these spectra, 38 were generated from normal uterus, 7 by uterus tissue with endometrial cancer, and another 7 were obtained from tissues of women with uterine cervical cancer. The PC1 vs PC2 plot was further analyzed using an artificial neural network, and the results are presented as 2D maps of probability. Furthermore, the perspectives of applying an ANN to solve the problem of Laplace-like inversion are discussed. An example of such ANN was presented and the performance was discussed. Finally, a model of complex ANN, capable to sequentially solve this kind of problems specific to LF-NMR is proposed and discussed.

Effect of pigment concentration on NMR relaxometry in acrylic paints

Acrylic emulsion paint is among the most common media employed by 20th century artists. Since early acrylic paintings have begun to require the attention of conservators, scientists are working to characterize the properties of these paints to facilitate conservation efforts. In this study, we report an investigation of the physical and chemical properties of acrylic emulsion paints using single-sided NMR in conjunction with gloss measurements and scanning electron microscopy-energy dispersive spectrometry. Combining the data from these techniques gives insight into pigment-base interactions and the acrylic curing process, showing that as pigment concentration is increased in paints, the amount of acrylic base adsorbed to pigment particles increases, resulting in films with differing relaxation times. This research both emphasizes and contextualizes the utility of NMR relaxometry in studying cultural heritage objects and prompts further study into the effects of pigment concentration on the curing and conservation of paint films.

Sequence for simultaneous measurement of long-limit diffusion and longitudinal relaxation in unilateral NMR

When unilateral NMR is employed with large gradients (20 T/m), measurements of T₁ using standard sequences become affected by Brownian motion of spins, particularly in samples with long spin-lattice relaxation times T₁ (>2000 ms) and a large diffusion coefficient D (2*10^-6 mm²/ms). In light of this, a modified saturation sequence which we have called GAUSS-SR is proposed that allows direct measurement of both D and T₁ to be carried out subject to certain constraints. The evolution of M_z magnetization is the main phenomenon to be modeled. The sequence is composed of three main parts: (i) a saturation train designed to render the M_z profile in Gaussian form, (ii) a main delay where by the simultaneous effects of T₁ and D on this profile has been solved analytically and (iii) a detection train to ensure a good signal-to-noise ratio. An NMR-MOUSE was used to acquire the desired measurement through this sequence. By relying on the coherence of the longitudinal rather than the transverse magnetization component, the sequence successfully provides the long-limit value of the diffusion coefficient.

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.

Topology optimization based design of unilateral NMR for generating a remote homogeneous field

This paper presents a topology optimization based design method for the design of unilateral nuclear magnetic resonance (NMR), with which a remote homogeneous field can be obtained. The topology optimization is actualized by seeking out the optimal layout of ferromagnetic materials within a given design domain. The design objective is defined as generating a sensitive magnetic field with optimal homogeneity and maximal field strength within a required region of interest (ROI). The sensitivity of the objective function with respect to the design variables is derived and the method for solving the optimization problem is presented. A design example is provided to illustrate the utility of the design method, specifically the ability to improve the quality of the magnetic field over the required ROI by determining the optimal structural topology for the ferromagnetic poles. Both in simulations and experiments, the sensitive region of the magnetic field achieves about 2 times larger than that of the reference design, validating validates the feasibility of the design method.

Spatially Resolved Monitoring of Drying of Hierarchical Porous Organic Networks

Evaporation kinetics of water confined in hierarchal polymeric porous media is studied by low field nuclear magnetic resonance (NMR). Systems synthesized with various degrees of cross-linker density render networks with similar pore sizes but different response when soaked with water. Polymeric networks with low percentage of cross-linker can undergo swelling, which affects the porosity as well as the drying kinetics. The drying process is monitored macroscopically by single-sided NMR, with spatial resolution of 100 μm, while microscopic information is obtained by measurements of spin-spin relaxation times (T2). Transition from a funicular to a pendular regime, where hydraulic connectivity is lost and the capillary flow cannot compensate for the surface evaporation, can be observed from inspection of the water content in different sample layers. Relaxation measurements indicate that even when the larger pore structures are depleted of water, capillary flow occurs through smaller voids.

Characterization of aging and solvent treatments of painted surfaces using single-sided NMR

Typical experiments conducted with single-sided NMR are incapable of unique chemical identification and, thus, often rely on comparative measurements in scientific study. However, cultural heritage objects have unique natures and histories, making a genuine 'control' sample a rarity and complicating many scientific investigations. In this paper, we present some comparative results enabled by such a rare, control sample. Two paintings, The Dinner and The Dance from the 1616 set Pipenpoyse Wedding, were made by the same artist with indistinguishable materials and techniques. However, despite their shared history, The Dinner has undergone varnishing and subsequent varnish removal multiple times, whereas The Dance has not. NMR measurements on these two paintings show the effect of organic-solvent-based treatments on the stiffness of the paintings as measured by T(2,eff), supporting visual and tactile observations that The Dinner is stiffer throughout its thickness than The Dance, probably due to ingress of natural resins and organic solvents into the paint and ground layers. In addition to a comparative analysis of these two paintings, initial experiments to compare solvent penetration with different varnish removal methods are described. Model canvas painting samples were treated with solvent in two ways--with free solvent on a swab and with cellulose gel thickened solvent in a tissue. Both treatment methods cause a measurable change in T(2,eff) ; however, the thickened-solvent method affects a narrower region of the model than does the free solvent.

Low-field permanent magnets for industrial process and quality control

In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.

Single-sided magnetic resonance profiling in biological and materials science

Single-sided NMR was inspired by the oil industry that strived to improve the performance of well-logging tools to measure the properties of fluids confined downhole. This unconventional way of implementing NMR, in which stray magnetic and radio frequency fields are used to recover information of arbitrarily large objects placed outside the magnet, motivated the development of handheld NMR sensors. These devices have moved the technique to different scientific disciplines. The current work gives a review of the most relevant magnets and methodologies developed to generate NMR information from spatially localized regions of samples placed in close proximity to the sensors. When carried out systematically, such measurements lead to 'single-sided depth profiles' or one-dimensional images. This paper presents recent and most relevant applications as well as future perspectives of this growing branch of MRI.

Unilateral NMR: a Noninvasive Tool for Monitoring In Situ the Effectiveness of Intervention to Reduce the Capillary Raise of Water in an Ancient Deteriorated Wall Painting

Portable unilateral NMR was used to quantitatively map in a fully noninvasive way the moisture distribution in an ancient deteriorated wall painting before and after an intervention to reduce the capillary raise of water through the wall. Maps obtained at a depth of 0.5 cm clearly showed the path of the capillary raise and indicated that, after the intervention, the moisture level was reduced. Maps obtained by measuring the first layers of the wall painting were affected by the critical environmental conditions of the second hypogeous level of St. Clement Basilica, Rome, and by the presence of salts efflorescence and encrustations on the surface of the wall painting. The morphology and the elemental composition of salts investigated by SEM-EDS indicated that efflorescences and encrustations were mostly constituted of gypsum and calcite. The presence of these salts is explained with the presence of high concentration of carbon dioxide and sulphur-rich particles due to pollution which, along with the high-moisture level and the extremely feeble air circulation, cause recarbonation and sulphation processes on the plaster surface.

Unilateral NMR, 13C CPMAS NMR spectroscopy and micro-analytical techniques for studying the materials and state of conservation of an ancient Egyptian wooden sarcophagus

A multi-technique approach was employed to study a decorated Egyptian wooden sarcophagus (XXV-XXVI dynasty, Third Intermediate Period), belonging to the Museo del Vicino Oriente of the Sapienza University of Rome. Portable non-invasive unilateral NMR was applied to evaluate the conservation state of the sarcophagus. Moreover, using unilateral NMR, a non-invasive analytical protocol was established to detect the presence of organic substances on the surface and/or embedded in the wooden matrix. This protocol allowed for an educated sampling campaign aimed at further investigating the state of degradation of the wood and the presence of organic substances by (13)C cross polarization magic angle spinning (CPMAS) NMR spectroscopy. The composition of the painted layer was analysed by optical microscopy (OM), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Raman and surface enhanced (resonance) Raman spectroscopy (SERS/SERRS), infrared and GC-MS techniques, evidencing original components such as clay minerals, Egyptian green, indigo, natural gums, and also highlighting restoration pigments and alteration compounds. The identification of the wood, of great value for the reconstruction of the history of the artwork, was achieved by means of optical microscopy.

In situ investigation of leaf water status by portable unilateral nuclear magnetic resonance

A portable unilateral nuclear magnetic resonance (NMR) instrument was used to detect in field conditions the water status of leaves of herbaceous crops (Zea mays, Phaseolus vulgaris), mesophyllous trees (Populus nigra), and natural Mediterranean vegetation characterized by water-spending shrubs (Cistus incanus) and water-saving sclerophyllous trees (Quercus ilex). A good relationship was observed between NMR signal, leaf relative water content, and leaf transpiration in herbaceous leaves undergoing fast dehydration or slowly developing a drought stress. A relationship was also observed between NMR signal and water potential of Populus leaves during the development of a water stress and when leaves recovered from the stress. In the natural vegetation, the relationship between NMR signal and water status was found in Cistus, the species characterized by high transpiration rates, when measured during a drought stress period and after a rainfall. In the case of the sclerophyllous Quercus, the NMR signal, the relative water content, and the transpiration rate did not change at different leaf water status, possibly because a large amount of water is compartmentalized in cellular structures and macromolecules. The good association between NMR signal and relative water content was lost in leaves exposed for 24 h to dehydration or to an osmotic stress caused by polyethylene glycol feeding. At this time, the transverse relaxation time became longer than in leaves maintained under optimal water conditions, and two indicators of membrane damage, the ion leakage and the emission of products of membrane lipoxygenation [(Z)-3-hexenal, (Z)-3-hexenol, and (E)-2-hexenol], increased. These results taken all together give information on the physiological state of a leaf under a developing stress and show the usefulness of the NMR instrumentation for screening vegetation health and fitness in natural and cultivated conditions. It is concluded that the portable unilateral NMR instrument may be usefully employed in field conditions to monitor nondestructively the water status of plants and to assist agricultural practices, such as irrigation scheduling, to minimize stomatal closure and the consequent limitation to plant production.

Unilateral NMR study of a XVI century wall painted

Wall paintings in the XVI century Serra Chapel in the "Chiesa di Nostra Signora del Sacro Cuore" Rome, have been studied using unilateral NMR. In order to map the distribution of moisture content in the wall painted, a large number of Hahn echo measurements, covering large areas of the wall painting were performed. Because the intensity of the Hahn echo is proportional to the amount of moisture in the area under study, the experimental data were transformed into 2D gradient colour maps which allowed an easy visualization of the moisture content of the wall. The state of conservation of the wall painting was monitored using T2 measurements specially with regards to outcropping salt.

A unilateral NMR magnet for sub-structure analysis in the built environment: the Surface GARField

A new, portable NMR magnet with a tailored magnetic field profile and a complementary radio frequency sensor have been designed and constructed for the purpose of probing in situ the sub-surface porosity of cement based materials in the built environment. The magnet is a one sided device akin to a large NMR-MOUSE with the additional design specification of planes of constant field strength /B0/ parallel to the surface. There is a strong gradient G in the field strength perpendicular to these planes. As with earlier GARField magnets, the ratio G//:B0/ is a system constant although the method of achieving this condition is substantially different. The new magnet as constructed is able to detect signals 50mm (1H NMR at 3.2 MHz) away from the surface of the magnet and can profile the surface layers of large samples to a depth of 35-40 mm by moving the magnet, and hence the resonant plane of the polarising field, relative to the sample surface. The matching radio frequency excitation/detector coil has been designed to complement the static magnetic field such that the polarising B0 and sensing B1 fields are, in principal, everywhere orthogonal. Preliminary spatially resolved measurements are presented of cement based materials, including two-dimensional T1-T2 relaxation correlation spectra.

Application of a portable nuclear magnetic resonance surface probe to porous media

A portable nuclear magnetic resonance (NMR) surface probe was used to determine the time-dependent self-diffusion coefficient D(t) of water molecules in two fluid-filled porous media. The measuring equipment and the inhomogeneous magnetic fields in the sensitive volume of the probe are described. It is discussed how to evaluate D(t) using a surface probe from the primary and stimulated echoes generated in three-pulse experiments. Furthermore, the evaluation of D(t) allows one to determine the geometrical structure of porous materials.

Single-sided mobile NMR with a Halbach magnet

A single-sided mobile NMR apparatus with a small Halbach magnet was constructed for the first time. It is lightweight, compact and exhibits good sensitivity. The weight of the device is only 2 kg, and the NMR signal of the pencil eraser block can be detected in one shot using the device. This study describes the characteristics of this instrument, including the profile of static magnetic flux density, B0, the sensitivity in the depth direction and its effectiveness in one-dimensional profiling. Its usefulness in differentiating soft materials and evaluating the extent of damage of a material is demonstrated based on T2 relaxation data. The moisture absorbance also can be observed from the increase of the echo amplitude of the NMR spin echo signal.

Fresco paintings studied by unilateral NMR

Unilateral NMR has been used to monitor the state of conservation of frescoes in the Vasari's house in Florence. The causes of deterioration of ancient frescoes are varied, which result in the detachment and crumbling of the painted film from the supporting plaster and in the outcropping of salts. Unilateral measurements of Hahn echo performed on such frescoes have allowed a perfect identification of the detachment of the painted film from the plaster. The presence of soluble salts on the pictorial film affects the spin-spin relaxation times, T(2). It is then possible using this technique, to characterize the effect of chemical treatments, of cleansing and consolidation procedures using the distribution of T(2) spin-spin relaxation times.

An analytical methodology for magnetic field control in unilateral NMR

Traditionally, unilateral NMR systems such as the NMR-MOUSE have used the fringe field between two bar magnets joined with a yoke in a 'U' geometry. This allows NMR signals to be acquired from a sensitive volume displaced from the magnets, permitting large samples to be investigated. The drawback of this approach is that the static field (B0) generated in this configuration is inhomogeneous, and has a large, nonlinear, gradient. As a consequence, the sensitive volume of the instrument is both small and ill defined. Empirical redesign of the permanent magnet array producing the B0 field has yielded instruments with magnetic field topologies acceptable for varying applications. The drawback of current approaches is the lack of formalism in the control of B0. Rather than tailoring the magnet geometry to NMR investigations, measurements must be tailored to the available magnet geometry. In this work, we present a design procedure whereby the size, shape, field strength, homogeneity, and gradients in the sensitive spot of a unilateral NMR sensor can be controlled. Our design uses high permeability pole pieces, shaped according to the contours of an analytical expression, to control B0, allowing unilateral NMR instruments to be designed to generate a controlled static field topology. We discuss the approach in the context of previously published design techniques, and explain the advantages inherent in our strategy as compared to other optimization methods. We detail the design, simulation, and construction of a unilateral magnet array using our approach. It is shown that the fabricated array exhibits a B0 topology consistent with the design. The utility of the design is demonstrated in a sample nondestructive testing application. Our design methodology is general, and defines a class of unilateral permanent magnet arrays in which the strength and shape of B0 within the sensitive volume can be controlled.

Advances of unilateral mobile NMR in nondestructive materials testing

Unilateral mobile NMR employs portable instrumentation with sensors, which are applied to the object from one side. Based on the principles of well-logging NMR, a hand-held sensor, the NMR-MOUSE (MObile Universal Surface Explorer) has been developed for nondestructive materials testing. In the following, a number of new applications of unilateral NMR in materials science are reviewed. They are the state assessment of polyethylene pipes, the characterization of wood, the in situ evaluation of stone conservation treatment, high-resolution profiling of rubber tubes and 2-D imaging for defect analysis in rubber products.

Surface normal imaging with a hand-held NMR device

Recently the capabilities of single sided nuclear magnetic resonance (NMR) devices have been extended towards three-dimensional imaging. This paper details the use of a magnetic field sweep coil to obtain spatial resolution in the plane normal to the surface of a hand-held NMR device-the NMR-Mobile Universal Surface Explorer (MOUSE). One-dimensional depth profiles can be recorded by varying the current in the sweep coils. Preliminary results from multi-layer rubber and glass sample phantoms demonstrate a sample penetration depth of 7 mm. Two-dimensional images were acquired via the inclusion of phase encoding coils. Non-destructive cross-sectional images of small rubber phantoms were successfully recorded.