National Gallery of Art Conservation: Scientific Research
Glossary of Terms and Techniques
Glossary of Instrumental Methods
Instrumentation in the Scientific Research Department falls into the three broad classes of chromatography, spectroscopy, and microscopy. Chromatography involves the separation of a mixture of materials into individual components. Separation is achieved by the partitioning of a component via selective adsorption and desorption as the mixture passes over a stationary phase. Spectroscopy involves the interaction of "light" (electromagnetic radiation) with a sample. The instrumentation is designed to examine specific wavelength regions: x-ray, visible, infrared, or ultraviolet wavelengths. Optical microscopes use a system of lenses to magnify a sample, while an electron microscope uses a series of magnetic fields as lenses to focus a beam of electrons on a sample.
A. Gas Chromatography (GC)
Gas chromatography (GC) is a separation technique that can be used for both the qualitative and quantitative identification of materials. It relies on the selective adsorption and desorption of volatile components on a stationary phase. The components are carried through the column by an inert gas to a detector. Common detectors for gas chromatography include flame ionization (FID), thermal conductivity (TCD), and mass spectrometry (MS). Components are identified based on retention time, and, where available, mass spectrum.
Gas chromatography is used at the National Gallery of Art for the identification of protein-containing binders, as well as for the identification of oil, waxes, and low-molecular-weight natural resins containing di- and triterpenes. Higher-molecular-weight polymeric materials can be identified by pyrolysis-gas chromatography (Py-GC).
B. Pyrolysis Gas Chromatography (Py-GC)
Pyrolysis is the thermal dissociation of materials in an inert atmosphere or a vacuum. The sample is put into direct contact with a platinum wire, or placed in a quartz boat inside a platinum coil, and rapidly heated to 600o – 800oC. Large molecules cleave at their weakest points and produce smaller, more volatile fragments. Various methylating reagents, which increase the volatility of polar fragments, can be added to a sample before pyrolysis. These fragments can then be separated on a gas chromatograph (GC). Pyrolysis gas chromatography is very useful for the identification of synthetic polymeric media, such as acrylics or alkyds, and synthetic varnishes.
C. High-performance Liquid Chromatography (HPLC)
High-performance liquid chromatography uses a liquid mobile phase to transport and help separate the components of a mixture. The mixture is injected into a column packed with a stationary phase under high pressure. In the column, the mixture is resolved, via absorption and desorption, into its constituents. The interaction of the solute with mobile and stationary phases is dependent upon the solvent gradient and stationary phase. High-performance liquid chromatography is useful in the separation of compounds that are too nonvolatile to be separated by gas chromatography. At the National Gallery of Art, high-performance liquid chromatography is used for the identification of organic dyes.
D. Size-exclusion Chromatography (SEC)
Size-exclusion chromatography is a liquid chromatographic technique in which molecules are sorted according to their size. The sample is introduced to the column(s) containing closely packed porous particles and carried along by the solvent. The separation is achieved by exchanges between the solvent and the pores of the packing. Analysis of standards of known molecular-weight distribution allows for the determination of molecular-weight distributions of polymeric materials. At the National Gallery of Art, size-exclusion chromatography is used to examine the molecular-weight distribution of conservation materials, which has a direct relationship with their optical and handling properties.
A. Fourier-transform Infrared (Micro)spectroscopy (FTIR)
The technique of infrared spectroscopy uses infrared radiation in the range from 4000 to 600 cm-1, the mid-infrared region, to observe the vibrational changes in chemical bonds. The presence and intensity of specific vibrational frequencies allows for determination of functional groups in organic molecules. The class of material (proteinaceous, cellulosic, and so forth) can then be identified from these functional groups. Infrared spectroscopy serves as a first step to more complete identification using other techniques. The system at the National Gallery of Art is equipped with a microscope, and samples are examined in transmitted or reflected light modes. A micro-attenuated total reflectance (ATR) attachment for the microscope is also available for studying the composition of thin coatings, such as the varnish on a painting or photograph.
B. Ultraviolet-visible Spectroscopy (UV-vis)
The ultraviolet-visible portion of the electromagnetic spectrum that is most commonly studied covers the wavelength range from 200 to 700 nm. The energy of this wavelength range of light causes electronic transitions in molecules and depends on whether transitions involve nonbonding or bonding (π or π*) electrons. The position of absorption of the radiation depends on the degree of conjugation and the nature of substituents on the molecules. In conservation science, this technique is most useful for the identification of dyestuffs through solution spectroscopy.
C. X-ray Fluorescence Spectrometry (XRF)
X-rays, when striking an object, can cause ejection of inner-shell electrons in atoms. The vacancy is filled by an outer-shell electron, which causes secondary x-rays to be emitted. These x-rays are of characteristic energies for each element present as they represent the binding energy of the two electronic states. Therefore, a spectrum can be obtained of energy versus intensity. The open architecture x-ray fluorescence spectrometer at the National Gallery of Art allows a whole object to be analyzed without removing a sample.
X-ray fluorescence spectrometry can be used to determine the alloy composition of a metal object and the elemental composition of paintings (from which pigments may be inferred), drawings, and photographs. This technique is sometimes used in conjunction with other techniques such as x-ray diffraction or optical microscopy for pigment analysis.
D. Energy-dispersive Spectrometry (EDS)
Energy-dispersive spectrometry, similar to x-ray fluorescence spectrometry, provides a means by which characteristic x-rays emitted by elements present in a sample can be detected and processed. The analyst can use the x-ray data for either qualitative or quantitative assessment of the sample. EDS is frequently combined with scanning electron microscopy (SEM). The combination of these two analytical methods makes possible the analysis of much smaller areas of the sample, and it provides for the detection of lighter elements when the analysis is carried out in a vacuum. SEM/EDS is routinely used at the National Gallery of Art to provide elemental data along with visual information on inorganic materials encountered in samples taken from art objects.
E. X-ray Diffraction (XRD)
X-rays are diffracted when directed at a crystalline material according to its lattice structure. This is accomplished through the use of a micro x-ray diffractometer that is capable of focusing a collimated x-ray beam (20 to 800 micron diameter range) onto areas of interest within the sample. X-rays diffracted by the sample strike a detector and are converted to an electronic signal that is then further processed using proprietary software. The generation of an x-ray diffraction pattern that is characteristic for the crystalline phases contained within the sample is the result of the data collection process. Search-match software is then used to match the unknown diffraction pattern to a database of diffraction patterns collected from reference compounds. At the National Gallery of Art, x-ray diffraction analysis is used for the identification of pigments, corrosion products on metal objects, or efflorescing materials. Analysis of materials using the micro x-ray diffractometer may be carried out on small samples removed from an object, or the analysis can be conducted in situ on objects small enough to fit within the analysis chamber. Such analyses can provide precise pigment identification, rather than pigment inferences obtained by x-ray fluorescence spectrometry (XRF) or energy-dispersive spectrometry (EDS).
F. Color Spectroscopy
A spectral reflectance curve represents the fraction of incident light reflected at each wavelength from a material. From the reflectance curve, tristimulus values can be calculated by combining the reflectance curve with the three standard color matching curves as defined by the International Commission on Illumination (CIE) and the power spectrum of the incident light source. The three tristimulus values roughly correspond to the amounts of red, green, and blue light required to match the observed color. Changes in color can then be determined by calculation of the CIE deltaE*94 values using a standard equation. In conservation science, measurement of color is used for evaluation of products, monitoring of fading, and pigment identification. At the National Gallery of Art, reflectance spectra are measured using a dual beam xenon flash spectrometer (wavelength range 360 to 750 nm) with a six-inch diameter integrating sphere.
G. Mass Spectrometry (MS)
In mass spectrometry, an instrument is used to produce ions from molecules. Two common ways of doing this are bombarding the sample either with a beam of electrons (electron ionization) or with small molecules such as methane (chemical ionization). The initial ion, called the molecular ion, often undergoes fragmentation into a pattern of smaller molecular weight ions. The ions are separated according to their mass-to-charge ratio and their relative intensities, producing a mass spectrum. In a quadrupole instrument, the ions produced in the source enter a high-vacuum area between rod electrodes maintained at opposite polarity. The application of varying radio frequencies and DC and AC voltages allows only ions with specific mass-to-charge ratios to be ejected to the detector. In ion trap mass spectrometers, the ionization and storage of ions occur in the same location. The ion trap electrodes create a three-dimensional electric field that holds the ions. Application of an appropriate radio frequency voltage is then used to eject ions with specific mass-to-charge ratios. Mass spectrometers frequently serve as detectors for gas or liquid chromatographs.
A. Optical Microscopy
Optical microscopy (also called light microscopy) is a technique for the visual characterization of materials. It is possible to evaluate both physical and chemical properties of materials based on light interactions with a sample while observing it at magnification. Various reagents, optical filters, and preparation methods are available that may either enhance or provide additional visual cues, facilitating characterization. The basic configuration consists of two lens systems, in which the objective lens forms a magnified image of the object, which is further magnified by the eyepieces. Transparent samples are typically viewed with transmitted light, while opaque samples are observed with reflected light. Both lighting configurations may be necessary to characterize a sample.
Broad ranges of artists' materials lend themselves to light microscopic analysis. Materials such as pigments, textile fibers, paper fibers, woods, and corrosion products are examined using high-magnification techniques that require a prepared sample. Information about gross surface features can be gleaned using low-magnification microscopes and do not require sampling the object. In the case of multilayered objects (for example an easel painting or polychrome sculpture), samples prepared as cross sections allow the analyst to address stratigraphy issues and questions about artist's technique. A cross section is an excised sample that has been mounted in a resin block, which is cut, ground, and polished perpendicular to the layering. The cross section is then viewed in reflected light. Details of the number of layers and their ordering, thickness, condition, and composition can be assessed. The analysis of cross sections is one of the more widely used optical microscopy techniques in conservation.
B. Scanning Electron Microscopy (SEM)
Coexistent with optical microscopy in our laboratory is scanning electron microscopy (SEM). SEM is also used to examine a diverse range of artists' materials and their associated degradation products; however, the sample is visualized by electrons rather than light. A beam of electrons is scanned across the sample, and the interaction between the electron beam and the sample allows the analyst to image and analyze the material in question. The sample can be imaged by detecting electrons from the beam that the sample scatters, or by detecting electrons ejected by the sample itself. X-rays emitted in this interaction can be analyzed to characterize the elements present in the sample. The technique of x-ray characterization used in conjunction with our SEM is known as energy-dispersive spectrometry (EDS). Although capable of very high magnifications (hundreds of thousands of times), much of the SEM work in conservation science is carried out at the lower magnification ranges (that is, from a few hundred times to a few thousand times).
Scanning electron microscopy is typically used at the National Gallery of Art to examine samples that have been taken from easel paintings. These may be either pigment scrapings (dispersed particles) or cross sections. The latter reveal the layered structure of a painting from the lowermost to the uppermost layers. Other types of materials examined using SEM include fibers, metals, corrosion products, glass, and ceramics.
4. Miscellaneous Instruments
The Weather-Ometer®, a chamber used to carry out accelerated aging of materials, employs rigorous conditions to simulate the acceleration of natural degradation processes. The system at the National Gallery of Art is equipped with a xenon arc lamp that, with appropriate filters, can simulate daylight with or without an ultraviolet light component. Testing of materials to be used in conservation treatments, including clear coatings and paints, can be carried out in the Weather-Ometer®. In addition, the degradation processes occurring in artists' and conservation materials can be studied.
A rheometer measures both the viscosity and the elasticity of resin solutions. A resin solution of interest is injected between the fixed and the moveable plates of the rheometer, completely filling the gap. When stress is applied to the moveable plate, the deformation of the resin solution and its rate of shear are measured. The handling properties of synthetic resin varnishes developed at the National Gallery will be greatly improved through characterization of the rheological properties of natural resin varnishes.
C. Differential Scanning Calorimetry (DSC)
The thermal behavior of materials can be characterized by differential scanning calorimetry. In this technique, separate chambers for the sample and reference are heated equally. Transformations taking place in the sample are detected by the instrument, which compensates by changing the heat input so that there is a zero temperature difference between the reference and sample. The amount of electrical energy supplied to the heating elements is then proportional to the heat released by the sample. Differential scanning calorimetry is used to study the thermal behavior of polymers.
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