Conservation of Historic Stone Buildings and Monuments (1982) / Chapter Skim
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Analytical Methods Related to Building and Monument Preservation
Analytical Methods Related to Building and Monument Preservation
Pages 163-182
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From page 163... ...
Other techniques applied in the laboratory and that also require minimal sampling are electron spectroscopy, electron microprobe analysis, electron microscopy, and X-ray diffraction analysis. This paper examines the use of a number of these techniques, pointing out where a given method or combination of methods is most applicable and the way in which the results may be related to the weathering processes that are occurnng.
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From page 164... ...
These actions, coupled with ion exchange and physical and biological alteration, produce a marked change immediately below the stone-atmosphere interface. Although the differential stability of components in stone materials depends on the complex interaction of various ambient materials with the primary and subsequently formed substances, general understanding of mineral degradation may be derived from a study of relative bond strengths.
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From page 165... ...
determining spatial changes in composition, i.e., chemical analyses for materials heterogeneous on a micrometer level; and jU) detailing the relative bond strengths as a function of physical and chemical alteration.
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From page 166... ...
The Auger electrons also have characteristic energies, as we will see in the section Electron Spectroscopy-Chemical Analysis. Further, the photoelectron ejected initially also carries chemical information, since its maximum energy is equal to the difference between the energy of the exciting X-rays and the binding energy.
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From page 167... ...
Of particular significance is the way in which this latter mode lends itself to in situ devices. There are in fact portable instruments commercially available for in situ analysis; they use radioactive sources to produce the X-rays.
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From page 168... ...
X-RAY DIFFRACTION The power of X-ray fluorescence lies in its use for elemental analysis, whereas the analysis of crystalline phases falls within the province of X-ray diffraction. If one refers again to the Bragg expression for diffraction, no = 2d sin 0, the difference between the two techniques becomes clear.
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From page 169... ...
Such patterns are used for qualitative analysis, leading to the identification of the phase or compound. Specific identifications are usually made by reference to data in the Powder Diffraction File maintained by the American Society for Testing and Materials (ASTM)
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From page 170... ...
.5 6 Its primary function is to utilize the variation in secondary electron emission (electrons scattered Incident Electron Probe X rays Cathode-luminescence >\ Secondary ~ Electrons / Backscattered / r Electrons Electromotive At\ /: Auger Electrons r 1 1 Transmitted Electrons FIGURE 4 A beam of primary electrons, focused on a small cross-sectional area of a sample, produces a variety of signals related to the elemental composition of the sample.
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From page 171... ...
A modern microprobe usually utilizes a crystal or wavelength spectrometer for X-ray identification, while an SEM utilizes an energy-dispersive {solid state) ~nalyzer for X-ray identification.
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From page 172... ...
Auger electrons typically are obtained from dimensions of tens of A, secondary electrons from 50 to 250 A, and X-rays from 1000 A to a micron or more. This range in spatial resolution results in some signals Secondary electron and Auger)
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From page 173... ...
The added attraction of performing energy-dispersive X-ray analyses on the rough sample is that qualitative elemental composition can then be used as an aid in detailing the characterization. The electron microprobe is best suited to detailed quantitative analyses of flat surfaces, where no surface irregularities exist.
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From page 174... ...
.7 The method is built on the study of the energy distribution among the electrons ejected from a target material that is being irradiated by X-rays, ultraviolet radiation, or electrons. A convenient method for distinguishing the various kinds of electron spectroscopies is by the mode of excitation.
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From page 175... ...
The necessary components include an X-ray excitation source (usually a X-ray tube containing a magnesium or aluniinum target) , the sample, an electron energy analyzer, and the appropriate electronics for pulse counting.
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From page 176... ...
IN SITU ANALYSES BY VISIBLE AND NEAR-INFRARED REFLECTANCE RADIOMETRY The use of reflectance spectroscopy to study geological materials is well established.7 This technique utilizes radiation reflected from a surface illuminated by the sun or an artificial source to record electronic (atomic) and vibrational Molecular)
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From page 177... ...
group, which is a major component of clay minerals and so may be used to identify various clays as well as some sulfate minerals. Recent developments in the application of reflectance spectra to the weathering once alteration of rocks and minerals indicate that the ratios of various band intensities in the visible (electronic)
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From page 178... ...
MICROMETERS FIGURE 9 Spectra of clay minerals determined by reflectance radiometry. Spectra are superimposed in this figure with indicated spacings of 10 percent reflectance to MgO.
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From page 179... ...
2 3 O 5 7 8 9 10 8 16 20 22 25 37 45 60 60 o 1.0 WAVELENGTH IN MICROMETERS 1.5 FIGURE 10 Reflectance spectra of 25 percent goethite and 75 percent kaolinite as a function of the thickness of the absorbing layer. 179 emission of characteristic gamma rays.
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From page 180... ...
180 Neutron: Neutron Nucleus 0~ CONSERVATION OF HISTORIC STONE BUILDINGS Gamma Ray ~ ~_~L o Jo De-excitea Excited Nucleus Nucleus BEFORE INTERMEDIATE AFTER I N E LAST I C SCATTE R I N G Neutron N ucleus O TO ~ O New Isotope (Excited) New Isotope BEFORE INTERMEDIATE AFTER RADIATIVE CAPTURE Capture Gamma Ray Gamma Ray N uclear Particle ,' Neutron Nucleus ,: ~ 0 ~0 ~~~ ORadioisotope Stable Isotope ACTIVATION FIGURE 11 Neutron-gamma methods of analysis involve detection of characteristic gamma rays emitted as a result of inelastic scattering of fast neutrons or radiative capture of thermal neutrons.
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From page 181... ...
While this method has not been applied specifically to the problems being discussed at this conference, it appears to hold promise. The major constraints to be considered are that the methods provide strictly elAlbuquerque Spectrum Anti~oincidence Mode Wet Soil Go N S — · - — (D tO — i~ — ~533 - ~11 ., _ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0~11~1 1 1000 1400 1800 2200 2600 3000 3400 3800 200 600 FIGURE 13 The extensive spectral data shown in the figure were obtained in about an hour using the neutron-gamma method on a sample of wet soil.
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From page 182... ...
{ 1975~ Practical Scanning Electron Microscopy, Plenum Press, N.Y.
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