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Painting Reconstruction Examination Methods and Scientific Terms

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Other Technical TermsOther Technical TermsFourier Transform-Infrared Spectroscopy (FT-IR) Raman SpectroscopyGas chromatography-mass spectrometry (GC-MS)Pyrolysis-mass spectrometry (Py-MS) High performance liquid chromatography (HPLC)X-Ray Diffraction (XRD)Synchrotron RadiationTime of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS)Related Links and ResourcesReferences<p>​FT-IR examines the vibrational, stretching, and bending energies that are induced in compounds when they absorb light within the infrared region.  Each peak in an infrared spectrum is located at the particular frequency of a bond vibration.  For example, a peak at  2850 cm-1  is typical of an alkyl (non-aromatic, saturated) carbon-hydrogen bond stretching phenomenon.  Because infrared radiation is fairly low energy, it is very good at detecting organic compounds (such as resins, oils, etc.) and inorganic materials with components that absorb in the mid to far infrared region such as carbonates and silicates (e.g. calcium carbonate or ultramarine).  FT-IR can be used to beam infrared light through a dispersed sample (microscopic particles spread out across a transparent surface) or it can be used in conjunction with a cross-sectional sample (see Cross-section Microscopy), reflecting infrared light off of the surface of the sample and back towards the detector. Today conservators and scientists can compare their results with a comprehensive database that consists of hundreds of reference spectra of pigments, fillers, and other materials (<a href="http://www.irug.org/">IRUG Database</a>).  While portable units do exist, most often a small sample is required to perform FT-IR analysis.  FT-IR is commonly used as a “first step” before using other methods, however, the technique is often unable to identify individual components within complex mixtures and cannot distinguish between sources (proteins from animal glue vs. proteins from egg yolk).</p><p>​Raman spectroscopy is generally seen as a sister technique to FT-IR because, for the most part, if a molecule is “IR invisible” it is typically Raman visible (e.g. vermillion).  This is because Raman detects molecules based on their polarizability, or the way that they scatter incoming radiation.  Raman irradiates a sample with a laser with a known wavelength, typically in the ultraviolet, visible, or near-infrared spectrum, and detects how the sample scatters that light.  If the scattered light is of a different energy from the incoming light, it is called Raman scattering.  The energy difference is what allows for molecular identification and occasionally minor differences in crystal lattice structure (e.g. anatase vs. rutile form of titanium dioxide).  Typically Raman is not useful in the identification of organic pigments like dyestuffs, however recent advancements in technology now allow scientists to augment their existing instrumentation to overcome this problem (SERS or Secondary Enhanced Raman Spectroscopy).  While portable units equipped with fiber optics do exist, samples are only necessary if the object is too large to fit over the stage of the instrument.  It is extremely important to be aware of the laser power when using Raman spectroscopy. If the laser power is too high, it can cause a small amount of damage to fragile objects (e.g. paper and textiles) or even photo-bleach certain pigments.</p><p>​GC-MS is both a separation and identification method for organic molecules.  Gas chromatography separates molecules based on the temperature it takes to vaporize them.  Once separated, each compound travels through a long chromatography column and is detected by a mass spectrometer upon exit.  The mass spectrometer ionizes and fragments each compound, usually with a high energy electron beam, producing a spectrum for each compound with the mass to charge ratio of each fragment as the x axis.  Mass spectra collected with electron impact ionization can serve as a fingerprint for molecules and allow for easy library searches for molecular identification.  This technique requires that a small sample be collected from the object which in turn will be exhausted once it is injected through the GC column.  When dealing with complex mixtures, it is up to the analyst to decide how best to prepare the sample prior to injection as this can greatly impact what the instrument is able to detect.  Special techniques (referred to as derivatization or silylation) are used to “tag” certain classes of molecules like fatty acids in oils and amino acids in proteins after the sample is broken down using both heat and solvent extraction methods.  Problems can arise from insufficient sample size, contamination during sample preparation, and interference from certain reactive pigments (e.g. copper or lead containing).  These issues, couple with recent advancements in instrumentation, have caused scientists to reconsider certain notions like using ratios of particular species (e.g. Palmitic fatty acid/Stearic fatty acid, etc.) for the identification of materials (e.g. partially heat-bodied walnut oil).  This technique continues to provide crucial information relating to organic materials present in artworks, however work is ongoing to improve databases that are specifically relevant to the field of cultural heritage.</p><p>​Pyrolysis-MS is a technique that can monitor the presence of volatile material present within a sample. The sample is gradually heated and the emitted vapors from the surface are then collected and detected in a mass spectrometer.  This technique is also often paired with gas chromatography (Py-GC-MS) so that the molecules present in the vapors can be separated more extensively before analysis.  In the analysis of artworks, this technique has been incredibly useful for identifying larger organic compounds and polymers that are too difficult to detect with GC-MS such as modern synthetic polymers and resins.  This technique requires that a small sample be collected from the object which in turn will be exhausted once it is injected into the instrument.</p><p>​HPLC is a separation technique similar to gas chromatography; however, it performs separation with the sample present in a liquid phase and the solution is separated based on polarity.  The compounds come off the column at different times, depending on the polarity of the mobile phase solvent and the polarity of the compounds being studied, and can be detected either by their UV-vis spectrum (LC-DAD) or by a mass spectrometer (LC-MS).  Although sampling is required, this technique has proven useful in the study of dye components and organic substituents on lake pigments.   One downside, however, is that it is often up to the analyst to build their own individual reference library, a complex process that requires familiarity with the instrument and access to hundreds of references samples.</p><p>​In progress.</p><p>​Synchrotrons are particle accelerators that are used in a variety of different capabilities.  Within a synchrotron, electrons or protons are accelerated to very high speeds in a ring.  When these subatomic particles reach velocities near that of the speed of light, they emit radiation known as synchrotron radiation.  Synchrotron radiation is broad field white light source, meaning that radiation covers the majority of the electromagnetic spectrum.  What makes it a powerful analytical tool is that the radiation is highly collimated, high in brilliance, highly polarized, and can be scaled down to a beam only 0.25 µm in diameter, allowing for extremely small areas to be analyzed with precision.  This can give conservation scientists an even more detailed description of the inorganic material present in paint cross-sections This radiation is generally used in the form of high powered x-rays and can be used for instrumentation only available at a certain synchrotron facilities equipped with micro-XRF or X-ray absorption at near edge spectroscopy (XANES).  More recently, scientists have been able to apply this technique to objects such as an entire painting, revealing information relating to the distribution of pigments in each layer of paint both at and beneath the visible surface.  A downside to this technique is the incredible complexity involved with data interpretation as well as the time that is often required to collect the data (which can take up to several days).</p><p>​A new form of mass spectrometry that continues to show great promise in the analysis of paint cross sections is time of flight-secondary ion mass spectrometry (ToF-SIMS).  With this instrument, an ion beam irradiates a sample, causing molecules in the sample to ionize, partially fragment, and be ejected towards the mass analyzer.  These secondary ions are then monitored by the mass analyzer based on the time it takes each ion to reach the detector (smaller ions reach the detector first while larger mass ions take longer).  ToF-SIMS is one of the few mass spectrometric techniques that can be used to spatially map both organic and inorganic components within a cross-sectional sample.  However, the instrumentation is costly and data interpretation can be a complex process.</p><p><a href="https://www.nga.gov/resources/scienceresearch/glossary.shtm">National Gallery of Art (Scientific Research) – Glossary of Terms and Techniques</a></p><p><a href="http://www.getty.edu/conservation/publications_resources/pdf_publications/pdf/infrared_spectroscopy.pdf">Getty Publications –Infrared Spectroscopy in Conservation Science</a></p><p><a href="http://www.irug.org/">Infrared & Raman Users Group (IRUG)-Spectral Database</a></p><p><a href="http://www.webexhibits.org/pigments/intro/look.html">Pigments through the Ages-Looking Closer</a>​</p><p>Colombini, Maria P., and Francesca Modugno, eds. <em>Organic Mass Spectrometry in Art and Archaeology</em>. New York: Wiley Publications, 2009.</p><p>Cotte, Marine et. al. “Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and related methods.” <em>Analytical Chemistry</em>. 83 (2011): 1214-1223</p><p>Keune, Katrien, and Jaap J. Boon, “Imaging Secondary Ion Mass Spectrometry of a Paint Cross Section taken from an Early Netherlandish Painting by Rogier van der Weyden.” <em>Analytical Chemistry.</em> 76 (2004): 1374–85.</p><p>Mass, Jennifer L., et al. “Deconstructing a 17th c. Collaboration by David Teniers the Younger and Jan Brueghel the Younger Using Confocal X-Ray Fluorescence  Microscopy.” <em>Materials Issues in Art and Archaeology VIII</em> (2008): 3-18.</p><p>Mills, John, and Raymond White. <em>The Organic Chemistry of Museum Objects</em>.  2nd ed. London: Routledge, 1989.</p><p>Pozzi, Frederica, John R. Lombardi, and Marco Leona. "Winsor & Newton original handbooks: a surface-enhanced Raman scattering (SERS) and Raman spectral database of dyes from modern watercolor pigments." <em>Heritage Science</em> (2013): 1-23. </p><p>Sanyova, Jana, et al. “Unexpected Materials in a Rembrandt Painting Characterized by High Spatial Resolution Cluster-TOF-SIMS Imaging.” <em>Analytical Chemistry</em>. 83 (2011): 753-760</p><p>Stuart, Barbara.  <em>Analytical Techniques in Materials Conservation</em>. England: John Wiley & Sons, 2008.</p><p>Taft, W. Stanley Jr., and James Mayer. <em>The Science of Paintings</em>. New York: Springer, 2001.</p><p>Townsend, Joyce, and Jaap Boon. “Research and Instrumental Analysis in the Materials of Easel Paintings.” In <em>The Conservation of Easel Paintings,</em> 341-366. Abingdon, England: Routledge, 2012.​</p>Various techniques used in the analysis and examination of easel paintings including Fourier Transform-Infrared Spectroscopy (FT-IR), Raman Spectoscopy, Gas chromatography-mass spectrometry (GC-MS), Pyrolysis-mass spectrometry (Py-MS or Py-GC-MS), High Performance Liquid Chromatography (HPLC), X-Ray Diffraction (XRD), and Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS).

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