Effects induced in marbles by water-repellent compounds: the NMR contribution

The purpose of this paper is to investigate nuclear magnetic resonance (NMR) applications to porous materials widely employed in artistic and historical monuments and largely studied in the Cultural Heritage conservation field, with the aim of studying structural variations induced in the porous matrix by the application of strengthening and water-repellent agents. Carrara marble and Candoglia marble samples, both in the native state and after treatment, were studied and data from relaxation time measurements were compared. Results collected on treated samples are reported and explained in terms of the relation between spin-lattice relaxation time and the structure of the porous materials. Traditional techniques were employed in addition to NMR analysis on marble samples for the evaluation of the preservative performances of three agents generally employed in the restoration yards. Comparative images showing untreated samples with the treated ones were obtained suggesting very useful applications for the determination of treatment effectiveness.     

Efficiency and resistance of the artificial oxalate protection treatment on marble against chemical weathering

The artificial oxalate protection method was analyzed in laboratory experiments in order to achieve an optimum treatment application and concentration giving rise to its most effective protective nature. Spectroscopic (Fourier transform infrared, Micro-Raman and UV-vis colorimetry), microscopic (scanning electron microscope) and contact-angle analyses were carried out to characterize Carrara marble samples before and after application of the treatment to validate its efficiency. The resistance effects against chemical weathering were subsequently observed in a lab-controlled weak acid rain experiment. An acid spray at pH 5.5, representative of normal rain was used to provoke degrade of natural marble, marble treated with the artificial oxalate protective at concentrations of 0.4 and 5% and marble treated with a commercial organic silicon product. Run-off solutions sampled at timely intervals were tested for any change in pH followed by ion chromatography measurements for the presence of calcium ions in solution. The chromatography results of the oxalate treatment applied at a 5% concentration are analogous to an organic commercial product indicating its validity as a method for the conservation of carbonate substrates conferring protection to stone materials against acid environments.     

Sugaring marble in the Monumental Cemetery in Bologna (Italy): characterization of naturally and artificially weathered samples and first results of consolidation by hydroxyapatite

The so-called sugaring of marble is a very common degradation phenomenon, affecting both historical monuments and modern buildings, which is originated by environmental temperature fluctuations. Thermal cycles are indeed responsible for micro-cracks formation at the boundaries between calcite grains, so that marble is subjected to granular disintegration and can be reduced to a sugar-like powder of isolated calcite grains by just the pressure of a finger. Since no effective, compatible and durable treatment for sugaring marble consolidation is currently available, in this paper a novel consolidating treatment recently proposed for limestone, based on the formation of hydroxyapatite inside the stone, was investigated for weathered marble. To test the new treatment on suitably decayed marble samples, some naturally sugaring marbles from the Monumental Cemetery in Bologna (Italy, nineteenth century) were firstly characterized by scanning electron microscope (SEM), mercury intrusion porosimetry (MIP) and ultrasonic pulse velocity (UPV) measurement. Then, artificially weathered samples were produced by heating fresh Carrara marble samples at 400 °C for 1 h. The effects of artificial weathering were characterized using the same techniques as above, and a very good agreement was found between microstructure and mechanical features of naturally and artificially weathered samples. Then, the hydroxyapatite-based treatment was tested on the so-obtained artificially weathered samples, and the treatment effects were characterized by UPV, MIP and SEM equipped with energy dispersive spectrometry and Fourier transform infrared spectroscopy. The hydroxyapatite-based treatment exhibited a remarkable ability of restoring marble cohesion and a good compatibility in terms of modifications in pore size distribution, which leads to regard this treatment as a very promising consolidant for weathered marble.     

Thermoactivation mechanism of relaxation of the mechanoelectric effects in solid dielectrics

The mechanoelectric effects under weak electric polarization and elastic deformation of glass and marble samples are investigated. The electric potentials of no relation to the piezoelectric effect are measured. The induced electric potential relaxation is studied and the activation energy of this process is estimated. The induced potential relaxation mechanism is proposed. The electric field potentials in the sample are measured by the contactless method with a specially constructed electrometer of a high input voltage.     

Water absorption kinetics in different wettability conditions studied at pore and sample scales in porous media by NMR with portable single-sided and laboratory imaging devices

NMR relaxation time distributions of water (1)H obtained by a portable single-sided surface device have been compared with MRI internal images obtained with a laboratory imaging apparatus on the same biocalcarenite (Lecce Stone) samples during capillary water uptake. The aim of this work was to check the ability of NMR methods to quantitatively follow the absorption phenomenon under different wettability conditions of the internal pore surfaces. Stone wettability changes were obtained by capillary absorption of a chloroform solution of Paraloid PB72, a hydrophobic acrylic resin frequently used to protect monuments and buildings, through one face of each sample. Both relaxation and imaging data have been found in good quantitative agreement each other and with masses of water determined by weighing the samples. In particular the Washburn model of water capillary rise applied to the imaging data allowed us to quantify the sorptivity in both treated and untreated samples. Combining relaxation and imaging data, a synergetic improvement of our understanding of the water absorption kinetics at both pore and sample scales is obtained. Since relaxation data have been taken over the course of time without interrupting the absorption process, simply by keeping the portable device on the surface opposite to the absorption, the results show that the single-sided NMR technique is a powerful tool for in situ evaluation of water-repellent treatments frequently used for consolidation and/or protection of stone artifacts.     

Enhanced diamagnetism in YBa2Cu3O7−δ by hydorgen-plasma treatment

We report that the diamagnetism of as-grown YBa₂Cu₃O_7−δ powders can be enhanced by treatment with RF-generated hydrogen plasma. The field-cooling susceptibility at 5 K is found to increase by up to 24%. The enhancement is reproducible for all five samples studied. The lattice parameter c decreases after hydrogenation. For a sample treated for the shortest period, its pinning potential,obtained from magnetic relaxation measurements, is reduced following H-treatment.     

Hydrophobizing coatings for cultural heritage. A detailed study of resin/stone surface interaction

Conservation of historical buildings is an important issue and the environmental conditions seriously affect the monument’s stones. The protection of cultural heritage buildings and monuments by surface treatment with polymers is a common practice due to their ability to form a protective layer on the monument’s surface as well as to control the transport of different fluids from the surface to the monument’s interior. In this work, three different substrates were used: Carrara marble, Botticino limestone, and Angera stone. A commercially available Si-based resin (Alpha^®SI30) was used as protective agent to improve the hydrophobicity features of the different tested materials. The surface properties of the coating and the relative interaction with the adopted stones were studied using different techniques such as contact angle measurements, electron microscope coupled with an energy dispersive spectrometer, X-ray photoelectron spectroscopy, atomic force microscopy, and attenuated total reflection infrared spectroscopy.     

Stability of atmospheric-pressure plasma induced changes on polycarbonate surfaces

Polycarbonate films are subjected to plasma treatment in a number of applications such as improving adhesion between polycarbonate and silicon alloy in protective and optical coatings. The surfaces that undergo changes in surface properties due to plasma treatment have a tendency to revert back to their original states. Thus, the stability of the plasma induced changes on polymer surfaces over a desired time period is an important issue. The objective of this study was to examine the effect of ageing on atmospheric-pressure helium-plasma treated polycarbonate (PC) sample as a function of treatment time. The ageing effects were studied over a period of 10 days. The samples were plasma treated for 0.5, 2, 5 and 10 min. Contact angle made by water droplet on polymer surfaces were measured to study surface energy changes. Modification of surface chemical structure was examined using X-ray photoelectron spectroscopy (XPS). Contact angle decreased from 93° for untreated samples to 30° for samples treated with plasma for 10 min. After 10 days the contact angle for the 10 min plasma treated sample increased to 67°, but it never reverted back to that of the untreated surface. Similarly, the oxygen-carbon (O:C) ratio increased from 0.136 for untreated samples to 0.321 for 10 min plasma-treated samples, indicating an increase in surface energy.     

Imaging inert fluorinated gases in cracks: perhaps in David's ankles

Inspired by the challenge of determining the nature of cracks on the ankles of Michelangelo's statue David, we discovered that one can image SF(6) gas in cracks in marble samples with alacrity. The imaging method produces images of gas with a signal-to-noise ratio (SNR) of 100-250, which is very high for magnetic resonance imaging (MRI) in general, let alone for an image of a gas at thermal equilibrium polarization. To put this unusual SNR in better perspective, we imaged SF(6) in a crack in a marble sample and imaged the lung tissue of a live rat (a more familiar variety of sample to many MRI scientists) using the same pulse sequence, the same size coils and the same MRI system. In both cases, we try to image subvoxel thin sheets of material that should appear bright against a darker background. By choosing imaging parameters appropriate for the different relaxation properties of SF(6) gas versus lung tissue and by choosing voxel sizes appropriate for the different goals of detecting subvoxel cracks on marble versus resolving subvoxel thin sheets of tissue, the SNR for voxels full of material was 220 and 14 for marble and lung, respectively. A major factor is that we chose large voxels to optimize SNR for detecting small cracks and we chose small voxels for resolving lung features at the expense of SNR. Imaging physics will cooperate to provide detection of small cracks on marble, but David's size poses a challenge for magnet designers. For the modest goal of imaging cracks in the left ankle, we desire a magnet with an approximately 32-cm gap and a flux density of approximately 0.36 T that weighs <500 kg.     

Effect of oxygen on the NMR relaxation properties of crude oils

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We comparedT ₁ andT ₂ relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T _1,2 ^ox to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T _1,2 ^ox for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ms, no significant oxygen effect is observed.     

The protection of different Italian marbles with two partially flourinated acrylic copolymers

Committing stone protection to polymeric materials started in the sixties but the study and knowledge of the complex and multiple interactions between stone and polymers has only been carried out recently. Its important to note that, together with the factors related to the polymeric system itself, intrinsic properties of the stone substrate, like composition, porosity, and crystalline characteristics, play a relevant role. In this paper the issues related to protection of three different Italian marbles have been investigated: Candoglia marble, employed in the building of the Milan Cathedral, Carrara marble, widely used in sculpture and historical architecture, and S. Giuliano marble, used in the building of the Pisa Cathedral and its famous leaning tower. Specimens coming from blocks of the three quarried stones have been characterized, treated with two new partially fluorinated acrylic copolymers, 2,2,2-trifluoroethyl methacrylate/methyl acrylate (TFEMA/MA), and trifluoromethyl-2,2,2-trifluorethyl methacrylate/methyl acrylate (HFIMA/MA), and tested according to UNI-Normal Italian protocol. All the measurements including capillary water absorption, static contact angles, colour variation, water vapour permeability, and SEM morphological analysis have been carried out before and after the polymeric treatment. The aim of this work is to evaluate the protective efficacy of these two new partially fluorinated acrylic copolymers on the three different marbles, and to correlate the different behaviours with the polymers properties and with the stone substrates characteristics. PACS 61.41.+e; 81.05.Lg; 68.08.de; 68.60.wm     

High-sensitivity apparatus for measuring linear thermal expansion: Preliminary results on the response of marbles to thermal cycles

Summary The aim of this paper is to present a high-sensitivity apparatus for measuring the thermal expansion of rocks, minerals and solids in general. The measurement of the magnitude of the linear expansion of a sample at any temperature of its heating-cooling cycle can be obtained with resolution equal to or better than 10 nm. Preliminary results on marble samples having different petrographic features are reported.     

Effect of flow on the apparent transverse relaxation time in a microfluidic nuclear magnetic resonance chip

The detection of samples in a microfluidic nuclear magnetic resonance chip is generally performed under flow condition. To study the effect of sample flow on the apparent transverse relaxation time in a microfluidic nuclear magnetic resonance chip, theoretical calculations were performed on three microfluidic samples (including deionized water, absolute ethanol, and copper sulfate pentahydrate) for flow velocities in the range 1.7–25?mm/s. A microfluidic nuclear magnetic resonance device with a low cost microfluidic solenoid coil was fabricated to verify the theoretical calculations by experiments. The results show that the apparent transverse relaxation time of the sample is a monoexponential decay with respect to flow velocity. In addition, it was found that the experimental values and the theoretical values of the apparent transverse relaxation time are identical when the samples are prepolarized completely; but for the samples that are not prepolarized completely, all the experimental values are smaller than the theoretical values and their difference increases with the flow velocity of the sample. After further study, it was discovered that the relative error between the experimental values and the theoretical values is a monoexponential decay to the level of the sample to be prepolarized. This discovery is very useful, because it can be used to modify the theoretical calculation model of the apparent transverse relaxation time for the samples that are prepolarized incompletely, as well as improve the application of microfluidics on nuclear magnetic resonance.     

Large surface mapping by a unilateral NMR scanner

The nuclear magnetic resonance (NMR) surface scanner, which provides images of sample surfaces larger than the probe dimension, has been realized using a single-sided device. Although conditioned by distortion effects originated by convolution between the sensitive volume of the probe and the space structures to be imaged, the scanner is able to provide images with good spatial resolution. The images obtained by the surface scanner can be made sensitive to relaxation parameters, magnetization or molecular self-diffusion; also, the dimension perpendicular to the sample surface can be scanned by varying the depth from which the probe detects the sample signal. It may scan surfaces arbitrarily large and with some degree of curvature. This aspect, together with the noninvasive characteristic of the apparatus, indicates that the surface scanner could be used profitably in the field of cultural heritage, where it could provide NMR maps of frescos, paintings on wood, marble artifacts, books and others.     

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi₄Ge₃O₁₂ doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T ₂* occurred upon doping the pure Bi₄Ge₃O₁₂ sample. Unlike T ₂*, the effective spin-lattice relaxation time T ₁* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.     

A faster way to characterize by triple-quantum-filtered (17)O NMR water molecules strongly bound to macromolecules in solution

The simultaneous use of transverse and longitudinal relaxation rates, together with a transverse triple-quantum-filtering NMR sequence, was estimated for the adequate characterization of (17)O-water relaxation behavior in protein solutions. A complementary contribution to transverse relaxation was found, which was interpreted as chemical exchange of (17)O-water between different sites of the proteins. This contribution was estimated via calibration measurements. Then, for other similar samples, faster experiments could be performed. The analysis of the results obtained in this way gave adequate values of the relaxation rate of water in fast motion, of the fraction of water in slow motion, and of its correlation time. Hence, it permitted the complete characterization of the sample in a reasonable experimental time.     

Mechanical relaxation of single crystal mats of polyethylene in a crystalline dispersion region

Stress-relaxation measurements have been conducted over the temperature range 15°–105°C on mats of single crystals of two linear polyethylene fractions. The single crystals were grown isothermally at several different temperatures. The relaxation modulus was observed to be strain dependent, indicating that the single crystal mats exhibited nonlinearity.

In spite of this appearance of nonlinearity, it was found possible, when the relaxation modulus was extrapolated to zero strain by an appropriate method, to obtain correct relaxation spectra for the mats of single crystals prepared by isothermal crystallization. This spectrum was then used to calculate the dynamic viscoelastic functions and, for the unannealed sample crystallized at 80°C, good agreement was found between experimental results and calculated ones. Two annealed samples showed multiple absorptions and, under these circumstances, strict application of the method of reduced variables for time and temperature was impractical.

The effect of molecular weight on the intensity of the relaxation spectrum was investigated. It was found that the single crystal with the higher molecular weight showed an increased spectrum intensity.

Observations were also made of the effect of increased lamellar thickness on both the relaxation spectra and the dynamic complex moduli and the results obtained on the fractions studied were compared with prior studies in whole polymer.     

Low-frequency elastic properties,domain dynamics,and spontaneous twisting of SrTiO<Subscript>3</Subscript> near the ferroelastic phase transition

The low-frequency elastic properties of strontium titanate near the ferroelastic phase transition were studied by the torsional-vibration technique. Domain wall motion was shown to contribute noticeably to the anomalies in the shear modulus and internal friction. It was established that the wall motion under varying elastic stresses is an unactivated process corresponding to viscous flow with a relaxation time inversely proportional to temperature. Spontaneous twisting of samples at the phase transition was revealed, and a model is proposed to account for the sample chirality and the spontaneous twisting effect.     

Optical Absorption Study on the Conversion and Restoration of Color Centers in Pure KCI Crystals

The conversion and restoration processes of colour centers in pure KCI crystals were studied by optical absorption measurements. When the sample is heated the aggregative centers in it begin to dissolve to F centers, and the opposite process will take place, i.e. F centers will aggregate to form F₂, F₃, F₄centers if the samples are cooled down. It was found that the conversion and restoration processes are dependent on the annealing temperature and time. In our experiment, the samples were treated at various temperatures and time. In addition the restoration process for the 350°C treated sample was studied live. We found that the higher the annealing temperature is the slower the restoration in the samples proceed, and the conversion process maintained for a few minutes even after annealing treatment.