Protective Material Coatings for Preserving Cultural Heritage Monuments and Artwork E-Book
Amir Ershad Langroudi
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- Herausgeber: Bentham Science Publishers
- Kategorie: Geisteswissenschaft
- Sprache: Englisch
- Veröffentlichungsjahr: 2001
Long-standing artworks and monuments show the wisdom and cultural identity of an ancient society along with the educational, material and spiritual merits of the people of that time. However, many historical artifacts and cultural monuments have been eroded over time and are in danger of deterioration beyond repair. There is a need to protect and conserve these artifacts. Restoration and preservation requires a multidisciplinary understanding of the inherent properties of these works based on the type of material and sufficient information in the properties of protective and conservation materials and research methods.
Protective Material Coatings for Preserving Cultural Heritage Monuments and Artwork aims to familiarize students with the recent practices in conservation and restoration science in recent years by presenting a modern orientation on the subject focused on material coatings. Readers will be able to understand the properties of different materials in antique objects and how to adopt appropriate treatment methods based on these properties.
This book consists of 5 chapters. In the first chapter, materials analysis techniques are described for historical monuments along with coatings used to preserve them. The second chapter introduces the properties of metals, alloys, and their common corrosion and explains protection strategies for metal monuments. An emphasis is given to nanocomposite coatings to prevent decay, especially through electrochemical corrosion. Chapter 3 is devoted to studying natural leather and parchments and their conservation from damage by environmental factors such as UV radiation, temperature, and humidity. The fourth chapter deals with stone works, which are in many historical objects. Chapter 5 introduces the reader to additional preservation materials and innovative methods employed to protect historical monuments and cultural heritage sites. Information about the removal of materials, cleaning of improper prior repairs is also given.
Protective Material Coatings for Preserving Cultural Heritage Monuments and Artwork is an ideal book for students of archeology, architecture, materials science and contemporary arts courses who are required to learn about the techniques of preserving antique buildings and works of art. It also serves as a handy reference for professionals and general readers interested in the curation of museums and the conservation of buildings, and cultural heritage sites.
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Preface
Amir Ershad-LangroudiHistorical and cultural monuments are a bridge between past and present generations. These works have irreplaceable material and spiritual value, which indicates the need to preserve and maintain them in the best way for transmission to future generations. However, in many cases, the aging deterioration of these artworks can not be completely prevented, but appropriate methods of maintenance and protection can significantly reduce the rate of their decline and erosion. However, in many cases, monuments and artworks are in open spaces without care and protection guards, exposed to vandalism and weathering degradation, or air pollution sources.
Another challenge with historical artifacts is that many of them have been made by unknown individuals, artists, and artisans over decades and even centuries.
In many cases, in addition to proper maintenance, serious interventions are required to use appropriate coatings to maintain these works with long-term efficiency. Various processes are used to produce nanocomposite coatings, one of which is the wet chemical process by the sol-gel way.
Given that historical and cultural works are made of different materials, it isn't easy to address them in one book. Hence, the protective coatings for some of the most influential ones in human civilization, such as leather, parchment, paper, metals, historical artworks, and monuments made of brick, and stone are addressed in the form of 5 chapters in this book. In the first chapter, various analysis techniques are used to describe the materials used in historical monuments or the coatings used to preserve them. The second chapter introduces the properties of metals, alloys, and their common corrosion and proposes protection strategies for metal monuments. Also, various protective coatings, emphasizing nanocomposite coatings to prevent corrosion, especially electrochemical corrosion, are examined.
Chapter 3 is devoted to studying natural leather and parchments, which, as natural polymers, must be protected from damage by environmental factors such as UV, temperature, and humidity. There are various coatings for surface treatment of leather and parchment, each of which has advantages and disadvantages that are discussed in this chapter.
The fourth chapter deals with stone works, which contain many historical sources. Having sufficient knowledge about erosion processes and the mechanisms governing these processes, and the impact of environmental factors can improve the preservation of these works in the open space. In recent years, nanocomposite coatings have been considered for their protection. In addition, it is possible to add antimicrobial agents and nanoparticles to the base composition of the coating to create additional functions such as self-cleaning properties and resistance of microorganisms.
Chapter 5 deals with nanotechnology. It creates more effective materials and innovative methods to protect historical monuments and cultural heritage. This technology can be used in various fields of conservation and restoration of historical and cultural monuments, production of transparent coatings with the suitable application for protecting historical monuments, removal, cleaning of improper repairs of the past, and new techniques and methods for protection and restoration.
It is hoped that this book will be a positive step towards promoting scientific research and its application in protecting and preserving cultural and historical heritage. By better preserving, these works will create a better perspective for the future.
Instrumental Techniques for Characterization of Historical Materials: Conservation and Protection
Amir Ershad-LangroudiAbstract
Conservation methods and intervention materials should be compatible with preserving historical and artistic monuments. Therefore, it is necessary to characterize the chemical compositions, microstructures, thermal and mechanical properties, the corrosion and weathering conditions of historical and cultural monuments, and the proposed coating to protect them. The environmental impacts and mechanisms of change due to their natural aging or artificial aging processes should be investigated. Various chemical methods have been developed because of the incredible variety of materials used to prepare historical and cultural monuments and artifacts. A variety of ways for analyzing elements, including atomic absorption spectroscopy, X-ray fluorescence, proton emission X-rays (PIXE), and μ-PIXE, and X-ray energy spectroscopy in scanning electron microscopy, are presented with a few practical examples. Infrared and Raman spectroscopy are the conventional methods used to characterize mineral and organic compounds. X-ray diffraction (XRD) is also a fast and inexpensive technique for distinguishing amorphous and crystalline materials and characterizing natural and synthetic crystals. Among the imaging techniques, the optical microscope is used for obtaining necessary information about various specimens in relatively small magnifications up to about 2,000 times. Scanning and transmitting electron microscopes (SEM-TEM) provide images at high resolution. The Atomic Force Microscopy (AFM) represents the three-dimensional topography of the surface. Thermal analysis is a quick and accurate method for measuring the percent of mass changes in material with temperatures such as water evaporation, solvents and volatile materials, and organic compounds decomposition. Mechanical-thermal analysis can provide practical information such as glass transition temperature and elastic modulus. A variety of methods for determining weathering resistance, corrosion resistance, and surface hydrophobicity are introduced. This review describes some of the typical applications of laboratory techniques and provides scientific information for right choice of materials and valuable coatings for their protection.
INTRODUCTION
There are several ways to determine the elements that make up a material, its crystal structure, chemical composition, and physical and mechanical properties.
One of the most practical methods can be based on the nature of identification. In most detection methods, a focused photon, electron, or ion beam is emitted into the material. The collision of this beam with a matter or its reflections provides different information about the case. Special equipment records and analyzes the amount and manner of changes such as energy, intensity, or radiation distribution. Finally, by analyzing the data, the type and amount of constituent elements, chemical compounds in the material, variety of phases, and crystal structure can be determined [1].
Fig. (1) indicates the spectrum of electromagnetic radiation in terms of wavelength. According to Planck's formula (Eqn. 1), the relationship between energy and wavelength for photons can be obtained from the following equation:
Fig. (1)) Electromagnetic spectrum, with wavelength, frequency, and energy scale [2].E=hν=hc/λ
(1)In this formula, E is the energy, λ is the frequency, n is the wavelength, h is the Planck constant, and c is the light speed. According to this formula, low wavelength rays (i.e., γ-rays and X-rays) have high energy. They interact with a small level of atoms with relatively high energy, such as the nucleus of an atom or electrons in orbits close to the nucleus (core electrons). Medium energy bands (e.g., U.V. and Visible) interact mostly with electrons located in the outermost orbits of atoms (valance electrons). Microwave and radio waves have long wavelengths and low frequencies. They interact with shallow energy levels, such as electrons located between the atoms that make up chemical bonds or stimulate atomic and molecular motions.
Table 1 shows several types of techniques designed based on the physical process for detection and measurement. They include the nature of the incident beam and the type of radiation or particle detected. It should be noted that each instrument uses a specific type of electromagnetic or particle beam to investigate the subject. Therefore, their applications are limited to particular experiments.
The radiation will be scattered and elastic in the absence of energy exchange between the probe beam and the material. The process will be considered as scattering (e.g., wave interference). Considering the interaction involves exchan-ging energy between the incident radiation and the atom in the material, the process is anelastic and is treated as spectroscopy. When particles are irradiated with atoms in the sample, the absorbed energy can stimulate specific energy levels, ranging from much energy to stimulate the nucleus to an electronic and molecular search for lower-energy molecules. Therefore, spectrometry techniques can also be classified based on energy levels. In general, any small measurement of the relationship between intensity and energy in any part of the electromagnetic spectrum is called spectroscopy. These measurements are called colorimeters in the visible part of the spectrum.
Both scattering and spectroscopy processes can involve imaging techniques by a visual image or a distribution map. Scanning tunneling microscopy or atomic force microscope deal with the altitude distribution of matter at the atomic level work based on direct measurement of interatomic forces or tunneling energy.
In general, the classification of characterization methods can be as following (see Scheme 1):
1. The chemical nature of materials includes analyzing chemical elements in the sample and the chemical composition and structural nature of crystalline materials in amorphous and crystalline.
2. Studying texture and morphology of the material with a microscope in different magnifications (i.e., imaging techniques).
3. Investigation of thermal properties of materials such as changes in humidity and volatile materials, as well as the stability of its various phases.
4. Investigation of mechanical properties in both static and dynamic modes (DMA).
5. Investigation of material stability in corrosive environments as well as aging and weathering.
6. Wettability and hydrophobicity of materials or coatings designed for interventional protection.
Scheme 1) Categorize experimental techniques for characterizing historical materials and their protective coatings [4-6].These studies can provide essential data about the materials used in historical and ancient artifacts, as well as their manufacturing processes and the state of their change in severe conditions. Furthermore, these experiments can be directed to adapt to an appropriate treatment if intervention protective coatings are needed.
Methods of Identifying the Type of Element
Chemical analysis or elemental analysis is one of the oldest methods of material analysis. This analysis determines the type of element in the sample. In this case, the kind of element is determined regardless of the crystal structure and chemical composition. For example, in a clay sample, the element silicon may be in three different forms, including silica (SiO2), talc (3MgO 4SiO2.H2O), and kaolinite (Al2O3.2SiO2.2H2O), which differ in composition and structure, but the elemental analysis method measured the amount of silicon in this soil regardless of its structure. Furthermore, due to the wide variety of analyzing techniques available for historical materials' characterization, only some conventional methods are mentioned in this chapter.
Atomic Absorption Spectroscopy
In the spectroscopic method, qualitative and quantitative analysis of the material is performed by examining the absorption or scattering of visible light from the material or other radiation with different wavelengths. Atomic absorption spectroscopy (AAS) is one of the spectroscopy methods. The concentration of about 70 chemical elements in the sample can be determined with high accuracy using the absorption of optical radiation by the atom in the plasma (gaseous) state. The basic principle on which this method works is that all atoms can absorb light at a specific wavelength different for each element. The amount of light absorbed depends on the atoms' concentration or the concentration of the component in the sample. Before measuring the ingredients in the piece, the model must be dissolved. Therefore, by performing the dissolution and digestion of instances, there is no restriction on the type of sample, e.g., liquid, soil, food, alloys, ore, and metal, which can be analyzed by this method.
In atomic spectroscopy, the matter is first dissolved in a suitable solvent and converted to vapor in a flame, furnace, or plasma, where vaporized atoms are measured by absorbing or emitting ultraviolet or visible rays. One of the advantages of atomic spectroscopy techniques is identifying complex samples and their high sensitivity (e.g., small amounts of elements can be determined in ppm). Atomic spectroscopy techniques are analyzed in three ways: absorption, diffusion, and fluorescence.
The exact sample is weighed in the atomic absorption spectroscopy (AAS), and about 10 mg of the piece is first dissolved in strong acids. Sample solutions should then be diluted to produce concentrations within the detection range of the device. The sample is mixed with an oxidizing compound (usually acetylene and air) before entering the flame and absorbed by the flame at high temperature (2400-2700 K), and atomized by this process [4, 7].
Also, the wavelengths and absorption are proportional to the type of element and its concentration in the solution. Therefore, the standard type and concentration of the unknown component can be measured using standard samples with a specific concentration of each element. To minimize the effect of matrix on measurements, the composition of standard samples should be close to the unknown sample. Another limitation of this method is that the measurements must be done separately for each element to analyze the whole piece, slowing down the sample's analysis speed. Replacing the flames with a graphite furnace is more sensitive than fire flames to detect the elements and requires fewer samples (approximately 1-100 microliters, about 0.1 to 0.01 samples necessary for flame analysis). Besides bypassing argon gas over the furnace, the sample's oxidation is prevented, and the solid samples can be analyzed. Another limitation of this method is that a separate lamp source is required to analyze each element. Although using a continuous light source and passing the desired wavelength by a filter can be somewhat helpful, the results are not exactly a separate lamp source for each element [4, 8, 9].
AAS has been used to identify pigments on the surface of a painting [10, 11]. Considering that metal detection in samples can be associated with specific pigments.,the AAS method indicated copper in the pigment, and by using the Raman spectra, it was identified as green phthalocyanine. Also, a combination of these two methods identified zinc oxide as a white pigment [12].
Fluorescence of X-Rays
X-ray fluorescence or X-ray spectroscopy (XRF) is an elemental analysis method widely used in identifying elements. In this method, the X-ray is irradiated to the sample and causes secondary X-rays generated by inducing atoms. By determining the wavelength or energy of the secondary X-ray, the desired elements were identified. In most cases, internal K and L shells are involved in XRF [4-6, 13].
The sample preparations are in different ways. A homogeneous powder is gathered from an unknown sample in one of these ways, and then it is mixed alone or with an organic adhesive and compressed into a plate. Alternatively, an unknown specimen with borax is melted and poured into a platinum mold.
XRF devices are of the Wavelength Separation (WDS) or Energy Separation (EDS) type. In WDS Separation (WDS), the X-ray output of the unknown sample is separated by a crystal before entering the detector. In the EDS type, the output beam enters the detector without being separated by the analyzer crystal. The EDS device's speed of analysis is much higher than the WDS device, but its sensitivity is lower than the WDS. XRF has replaced more chemical analysis methods in many applications due to its high speed and high accuracy. Energy-dispersive XRF spectrometers (EDXRF) have been developed to produce portable, non-destructive, and inexpensive spectrometers [4, 5, 14-17].
When a quantitative sample analysis is desired, the percentage of that element in the sample can be determined using the measured intensity for a wavelength and its comparison with the intensity obtained from a standard sample (control). For this purpose, a calibration curve should be drawn first using standard samples with a certain percentage of the desired element. The value of that element should be determined by measuring the beam intensity in the unknown sample and comparing it with this curve. It should be noted that the effect (background), which includes the other elements in the piece, can be helpful in the measured value [4, 18, 19].
Recently, various methods of X-ray fluorescence analysis have been used to study the pigments of historical paintings. The Kα / Kβ technique predicted the depth of the layers. Conventional XRF created depth specifications based on differences in X-ray absorption in the paint layers [4, 20, 21].
Proton Induced X-Ray Emission (PIXE) and μ−PIXE Method
Pixe X-ray emission, also known as proton scanning microscopy, is a robust, non-destructive, rapid, multi-element analysis of various samples. The studied sample is subjected to proton radiation with 2-3 megawatts of electron volts in this analysis method.
Proton collision with electrons emits a characteristic x-ray. The x-rays energy determines the elements' type and concentration by using x-rays with a specific energy.
The beam spot size used in a conventional pixel is about 2×2 mm2. However, with electromagnetic lenses, the diameter of the range used in the analysis can be reduced to less than a few microns. The pixe method's abilities and analytical capabilities can significantly increase by the beam spot's micron range.
The analysis of materials using a microbeam spot of ions is called the μ-pixe method. Due to the small size of the proton beam, only one point of the sample analyzes. It is also possible to sweep the sample's surface by micron proton beam and obtain the distribution of elements related to each point. In this way, an image of each element's distribution in the sample is obtained [22, 23].
A recent study to identify Parthian glass objects' chemical composition and constituent elements using the Micro-PIXE technique showed that the samples' constituent elements were sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and potassium. It specified that the composition of these samples consists mainly of SiO2 (63-65% by weight), Na2O (13-18% by weight), and CaO (6-8% by weight). As a result, all of these specimens are made of silica-soda glass [24].
The X-ray diffraction (PIXE) and X-ray diffraction (XRF) techniques are commonly used to analyze historical coins. However, the presence of surface corrosion can make it challenging to study. A recent XRF study to complete PIXE data on two historical Portuguese copper coins determined the sample elements' main composition [25].
Energy-Dispersive X-ray Spectroscopy (EDS)
Dispersing is one of the electromagnetic radiation attributes, making the electromagnetic beam diverted through a hole or edge. Using the dispersing of X-rays, electrons, or protons and their effect on the issue, the elements that make up the material can be recognized and analyzed. Electrons and protons also have wave properties, the wavelength of which depends on their energy. Likewise, each of these techniques has various attributes; for instance, the penetration depth of these methods is different in the material. Thus, X-rays and protons penetrate deeper into matter than electron beams, respectively. Electron beam excitation is utilized in electron microscopes, especially in their scanning type. At the same time, X-ray excitation is used in X-ray fluorescence spectroscopy (XRF). In this way, the sample's elements are measured qualitatively and quantitatively, as well as the distribution of elements indicates in the sample. The basis of this method also relies on the interaction between a source of X-ray excitation and a sample.
For example, two types of commercial siloxane-based reinforcing materials called RC90 and RC80 were studied to consolidate a marble specimen from a church in Italy. Microcracks were observed at the surface of the specimens. The white color on the sample's surface with the EDS method showed that this material is tin, used as the initiator of the curing reaction of siloxane coating. However, its non-uniform distribution in siloxane coating can cause micro-cracks in the protective coating [26].
Methods for Determining the Structure and Chemical Composition of Materials
In this section, some analyzing methods are mentioned to characterize materials' structure and chemical composition with protection, restoration, and architecture applications.
Infrared and Raman Spectroscopy
Vibration methods, including IR, FTIR, and ATR spectroscopy, study the molecules' vibrational modes. They were used to analyze and identify polymers and some additives and pigments. Electromagnetic radiation frequency in the infrared region is under the atoms of a bond's natural vibrational frequency. After absorbing infrared waves in a molecule, it creates a series of vibrational motions that form the basis of the infrared radiation spectrum. The simplest type of vibrational motion in a molecule is bending and stretching movements. The FTIR device uses mathematical conversion and has many advantages over conventional IR devices, such as the high speed of data collection and the better signal-to-noise ratio.
Almost all compounds with covalent bonds, whether organic or mineral, absorb different electromagnetic radiation frequencies in the infrared region. The infrared region is an electromagnetic field with a wavelength longer than visible light (400 to 800 nm) and shorter than microwaves (wavelengths longer than one millimeter). Infrared spectroscopy has several uses, some of which are:
1- Identification of the molecular structure of materials [27, 28].
2- Quantitative determination of molecular components in mixtures [29, 30].
3- Determining the molecular species absorbed on the surface [31, 32].
4- Determining the number of elements in complex fields [4, 33].
5- Determining the molecular structure and the direction of the thin sedimentary layers on the metal substrates [34, 35].
6- Identifying and introducing different phases in solids or liquids [36, 37].
A specific part of infrared energy is absorbed in a molecule section and vibrated at a particular frequency. For a molecule to be infrared, the exciting polar moment of the molecule must change during vibration. Vibrations can be in the form of a change in bond length (stretching) or a bond angle (bending). Bonds can also be stretched in-phase (symmetrical stretching) or out of phase (asymmetric stretch-ing). Even for relatively simple molecules, there will be different vibrations. Fourier-transform infrared (FTIR) spectroscopy is the most common IR technique based on radiation interference between two beams. The domains of distance and frequencies convert by the mathematical method [38, 39]. The sample must be transparent to infrared radiation, and alkaline halides such as potassium bromide and calcium fluoride are commonly used as solvents in transmission methods. The solvent must dissolve the sample and be as non-polar as possible to minimize the solvent-soluble interactions. It should also not absorb too much-infrared radiation [40, 41].
Using the FTIR technique, the functional groups are identified in a material. Since each functional group can only absorb infrared radiation with a particular frequency, Infrared spectrum databases are available for different chemical performance groups. The functional groups, as well as an unknown sample, can be detected using the spectrum of the intensity of the radiation (transmission or absorption) against the frequency (wavenumber) [42].
The infrared spectrum can be divided into three main regions: far-infrared (<400 cm-1), medium-infrared (4000-400 cm-1), and near-infrared (13000-4000 cm-1). Many infrared analyzes are performed in the medium-infrared region. Table 2 shows the infrared wave for some of the functional groups.
FTIR reflection techniques (ATR) are also used for non-destructive testing or surface analysis [43-45]. The results showed that the combination of far-infrared IR spectroscopy with ATR was applied to identify compounds in the medium-infrared technique, such as murals and metal corrosion [44, 46-49].
Molecular vibrations of single mineral pigments, such as metal oxides or sulfides, occur in the far-infrared region. However, mineral pigments with more complex structures of anions may vibrate in the middle infrared, which can characterize them.
Furthermore, the FTIR method was used to investigate commercial polysiloxane coatings to consolidate the church's marble after exposure to environmental and aging conditions [26].
The study of spectra by the µ-FTIR method indicated methyl bands in 1267cm-1 and phenyl bands in 3068, 1464-1429, 706cm-1, polysiloxane bands in 1030, and 1082 cm-1. Moreover, calcium carbonate peaks (i.e., Gypsum), representing a marble component at 3400-3550, 1146-1117, 1620, and 1684cm-1, have been converted into the sulfated group. In the old polysiloxane coatings (i.e., XR893 sample), the peaks of 1600 to 2000 cm-1 represent the alkyl groups in this sample. While in the new polysiloxane (RC90) samples, these peaks did not designate, which indicates the difference between the composition of old and new commercial coatings. This evidence suggests that the old polysiloxane XR893 contained methyl-phenyl compounds in siloxane resin, dissolved in the toluene solvent.
In contrast, the new resin compound contained a mixture of ethyl silicate with methyl and phenyl resin. Thus, alkyl groups were more abundant in the old XR893 resin than in the new sample. The use of benzene solvents in the new sample may be the reason for cracking in the protective coating layer, which had converted carbonate to sulfate in the marble sample.
As an analytical approach, FTIR imaging has been able to identify the characteristics of different layers of paint. In one study, different paint layers were identified: silicate in one layer of a red background, lead white and lead carboxylates in a white coating over the background, and calcium carbonate (CaCO3) in one layer of paint. Different colors and groups of organic agents were identified in the lacquer layer [50, 51].
Because vibration mechanisms and modes differ in Raman spectroscopy and infrared spectrometer, they provide additional information on historical material analysis. It is also a reliable, fast, sensitive, non-destructive, and valuable way to describe ancient materials. Raman's scattering mechanism is that the incident radiation changes the atomic charge, inducing more instantaneous polarity in the molecule. Depending on the molecule's polarization, if the electric charge's deformation corresponds to a possible state of molecule vibration, then the peak of Raman scattering is observed. Although Raman spectroscopy is used to analyze various historical materials [52-55], it helps analyze pigments [54, 56] due to its access to the low-wavelength region (<500 cm-1). Extensive databases of the Raman spectrum have been developed to identify the pigment stage. Table 3 shows some of the conventional pigments characterized in the medium infrared region and Raman spectroscopy.
Ultraviolet-Visible Spectroscopy
This technique involves irradiation in ultraviolet and visible areas, which causes the transitions of electrons between the outermost energy levels of atoms.
Ultraviolet-visible spectrophotometers are used to study absorbing electron transitions in UV (200-400 nm) and visible (400-800 nm) electromagnetic spectra [4, 57].
In this technique, the samples are examined as diluted solutions (or gases). The transmitted light intensity is proportional to the molecule's concentration absorbed by the Beer-Lambert law. Typically, the measurement in this method is similar to the IR techniques. The visible UV spectrum bands are usually broad, and the maximum wavelength (λmax) is assigned. This technique applies to identify mineral pigments and dyes in fabrics and paintings, manuscripts, gemstones, and glass are containing specific mineral pigments [5]. It is essential to be aware that visible UV spectra are sensitive to solvent, pH, and compound differences. They may cause the maximum wavelength shifts to longer or shorter wavelengths [5, 58, 59]. When the change in λmax shifts to longer wavelengths, it is known as a redshift. Conversely, the change is known as a blue shift [60, 61].
X-ray Diffraction Method
Roentgen discovered the x-ray in 1895, and it was called X-ray because it was unknown at the time. The beam is invisible, and its direction is direct, and it also affects the photographic film. Unlike natural light, X-rays can penetrate materials. One way to identify historical materials is to use X-ray diffraction methods. X-ray diffraction (XRD) applies to determine the crystal structure in solids [55, 62-65]. Fig. (2) shows the diffraction of X-rays. A few waves at angle θ are diffracted by adjacent layers at the same angle when waves are completely below each other; they are in phase. In this method, a beam from the x-ray generating lamp strikes the surface of the sample atoms, and a detector analyzes the reflected beam. The wave rays are in phase at an angle θ if the difference in path length is equal to the total number of wavelengths, nλ, where n is the whole number, and λ is the wavelength. The fundamental relationship in XRD is the Bragg equation (Eqn. 2):
Fig. (2)) X-ray diffraction with crystalline material.nλ=2dsinθ
(2)The most common XRD technique uses a monochromatic beam strike on a powdered sample. The sample holder rotated, and the intensities of the diffracted beams were measured in terms of 2θ. The characteristic material pattern is compared with the XRD patterns database to identify the unknown sample's crystal structure.
X-ray tests were performed in two ranges: wide-angle X-ray scattering (WAXS) and small-angle scattering (SAXS). The WAXS technique measures at 5° to 120°, and the SAXS method measures 1° to 5°. While the first method helps obtain structural information in the range of 1 to 50 angstroms, the second method gets information ranging from 50 to 700°A [4].
The X-ray method has many uses, the most important of which are:
1- Quantitative and qualitative analysis of materials
2- Determining the amount of crystallization in polymeric materials
3- Orientation of single crystals
Besides, the XRD method has the following advantages over other methods:
1. The method is not entirely destructive. Although the powdered sample needs, in most cases, the sample is not destroyed during the analysis.
2. Determining and identifying compounds that have different crystal structures despite the same chemical composition. Like titanium dioxides, which can have various crystalline forms of rutile and anatase.
3. Low amount of sample required for analysis
Therefore, the X-ray spectrum of a material has a crystalline and regular structure when it has specific peaks. Therefore, X-rays apply to determine the content is amorphous or crystalline. Various lamps produce different wavelengths in the X-ray diffraction method, such as nickel and iron, but copper lamps are routine.
X-ray is a way to estimate the distance between the plates in the crystalline material and particles' size. The powdered materials place in the direction of the X-ray. Then the angle between the extension of the beam and the line of reflection is measured by setting the value of the angle in Bragg's equation, the particle size estimates.
Fig. (3) shows changes in titanium dioxide structure from irregular (amorphous) state to anatase crystalline structure (A) and then rutile crystalline (R) by heating at different temperatures (from 200 to 850°C). As seen in this figure, titanium dioxide, prepared by the sol-gel method, remains irregular (i.e., amorphous) up to 200 °C, and only after heat treatment at 450 to 650 °C, its structure change from irregular to regular (i.e., anatase crystalline).
Fig. (3)) XRD of titanium dioxide prepared at different temperatures [66].The structure of anatase and rutile has photocatalytic properties among the various titanium dioxide structures, which can apply to the glass displaying historical objects, walls, and even some artwork that removes environmental pollutants, bacteria, odors, and the same.
As the heat treatment temperature increases to more than 650°C, the titanium dioxide structure changes from anatase to rutile, decreasing its photocatalytic property. Therefore, the X-ray diffraction method uses to determine the best crystalline structure of titanium dioxide to find the best conditions for photocatalytic nanoparticles' production [66].
Microscopic Methods
Various microscopic methods handle to examine historical specimens as well as protective coatings. Fig. (4) shows a comparison of resolution by the human eye, the optical microscope (OM), the scanning electron microscope (SEM), and the transmitting electron microscope (TEM) [67-70].
Fig. (4)) The resolution in the human eye, optical and electron microscopes (SEM and TEM).Optical Microscope
Optical microscopes analyze reflected or transmitting visible light. The optical microscope resolution is about 0.2 microns, and the maximum possible image magnification with lenses is about 2,000 times (using some oils and changing the refractive index of light). In this type of microscope, the light source is a visible light passing through glass lenses to focus light. The sample's surface images are observed in the microscope's eye lens or a monitor screen based on the color and refraction of light. The sample's surface must be polished and, if necessary, etched with suitable acidic or colored chemicals to identify different phases.
The sample surface can be color etching with chemical compounds to obtain color images of the character. Also, polarized light microscopy (OM) is one of the most valuable tools in studying mineralogy and petrography of ancient mineral artworks, stone sculptures, and building materials [71, 72]. Fig. (5) shows the steps for preparing a sample, from cutting to sanding to prepare a thin section for petrographic study with a polarizing microscope. Fig. (6) also shows an example of a thin section of prepared stone, and Fig. (7) presents the petroglyphs results of ancient stone. These pictures include algae fossils, foraminifers, corals, dark iron oxides, quartz, and feldspar minerals [73].
Fig. (5)) Stages of a thin section of stone preparation for petrographic examination with light microscopy with polarized light [73]. Fig. (6)) Preparation of a thin section of the stone sample [73]. Fig. (7)) Thin microscopic images: coral fossil, dark iron oxide, quartz, and feldspar minerals [73].Optical microscopes are usually transmitted or reflective. In transmitted light microscopes (TL-OM), visible light (or polarization) passes through the sample's delicate part. Still, in reflective light microscopes, the light reflects by the polished surface of a thick specimen. Thus, the sample's shiny surface investigates optical microscopes to provide the desired information, such as phase information, grain size, defect, morphology, and amorphous and crystalline phases.
A study of salt crystallization in a monument showed that solutions containing sulfate compounds precipitated magnesium sulfate and calcium sulfate near the stone surface, leading to the dissolution of carbonate cement. Besides, the proximity of rock material with a solution containing calcium sulfate or potassium sulfate intensifies salt's crystallization, the abolition of cement, and the degree of fracture. Also, the crystallization of sodium sulfate at different relative humidity using the OM method showed that the crystallite of sodium sulfate is usually formed in fine needles in dendrites at higher RH than 40 .٪ At the same time, less than RH 13%, the crystallites have a prismatic shape. Furthermore, OM has been used to identify the significant constituents of pigments. Quartz, feldspar, phyllosilicates, and carbonate minerals associated with iron oxide or hydroxides have been investigated by OM techniques [74-76].
Scanning Electron Microscopy (SEM)
In scanning electron microscopy (SEM), the sample's image is produced using a beam of electrons rather than visible light in optical microscopy [77, 78].
Objects can be analyzed in magnification over 100,000 times Using SEM, and it is also possible to study three-dimensional samples. Besides, a small specimen is required to investigate with SEM, making it helpful in studying historical investigations without considerable damage and destruction.
Fig. (8) shows a schematic diagram of a scanning electron microscope. Current electricity passes through an electron gun consisting of a hot tungsten filament and applying a voltage difference between a cylinder and a tungsten filament; electrons are emitted from this tungsten filament. Electromagnetic lenses are used to focus the beam in the direction of the flow of electrons to the sample. The electron beam is moved to the selected part of the piece. The interaction of electrons with the specimen causes the electrons to come out of the atoms in the sample. The generated electrons are identified and amplified. The detector detects scattered X-ray and secondary electrons.
Fig. (8)) A schema of scanning electron microscopy (SEM).Its sample preparation is relatively simple compared to TEM. Usually, it requires a thin layer of the metal coating (e.g., a gold sputtering for imaging) or a thin layer of graphite (for chemical analysis) when the sample has low electrical conductivity. SEM samples are analyzed in a vacuum chamber (because air molecules scatter and absorb electrons). Although measuring samples in the vacuum chamber for solids and minerals is not generally difficult, specimens such as biofilms and biological or volatile compounds require special preparation [5, 79
