Spectroscopy Aids the Expert Eye
Spectroscopy is frequently used by art historians to study the materials, methods and pigments used in different periods and by different artists, and to aid in restoration. The same information, however, can also be used to determine whether a piece of art is authentic – whether it comes from the time period believed, is consistent with a particular style, or if it is by a specific artist.
In recent decades, the art world has embraced a wide variety of optical techniques to provide definitive analysis of artworks, including UV fluorescence, UV/Vis/NIR reflectance, FTIR microscopy, Raman spectroscopy and LIBS (laser induced breakdown spectroscopy). Even SERS (surface-enhanced Raman scattering) is being employed for its ability to detect trace amounts of material. These techniques offer information about chemical composition of pigments, often penetrating into sublayers nondestructively.
Even when the technique alters the sample, as when LIBS burns a microscopic hole for elemental analysis, the information obtained is often well worth the compromise. A custom-configured LIBS system from Ocean Optics used in Beijing’s Antique City enables antiques appraiser Guan Haisen to identify artificially aged ceramic vases, complementing his own visual analysis1.
Seeing Colors in a New Light with FORS
Fiber optic reflectance spectroscopy (FORS) can be used not only to obtain quantitative measurements of color of pigments (380-780 nm), but also to probe their nature at wavelengths beyond the human eye’s perception. We’ve explored this previously in the application note “Searching for the Author of Vukovar Landscapes,“ in which pigments from both signed and unsigned landscapes were measured in an effort to establish authorship.
FORS can also identify pigments in ancient ceramics, like the red ochre found on ceramic pieces excavated from archaeological sites in Croatia using a USB2000 spectrometer configured for Vis-NIR reflectance from 400-1000 nm2.
FORS is also being used by collaborators at the Institute of Information Theory and Automation and the Academy of Fine Arts in Prague, Czech Republic, to better understand how NIR light penetrates different pigments from the Gothic and Renaissance periods as a potential method for underdrawing detection3. The researchers have used a modular USB-series spectrometer system to create the m3art database, which provides open access to over 6,000 Vis-NIR reflection and transmission spectra for different combinations of canvas, underdrawing and color layers.
Getting to Know Your Pigments
Given an appropriate database of pigment spectra, Vis-NIR reflectance spectroscopy can also be used to authenticate pigments used in a piece of artwork. That is exactly what a team at the University of Bergamo (Italy) did after performing a comprehensive study of blue and green phthalocyanine pigments, which became popular after 1935 for their brightness and colorfastness4. They were particularly interested to understand how much information Vis-NIR reflectance spectroscopy could provide as an alternative to other techniques. Raman spectra tends to show significant fluorescence due to binders and varnishes for paintings in situ, while XRF (X-ray fluorescence) yields low signals and FTIR spectra are muddied by signal from inorganic fillers.
The researchers began by collecting hundreds of spectra in the lab of commercially available pigments containing several specific phthalocyanines often used in painting and restorations in recent decades. The pigments were sourced from a variety of producers and applied to gesso-prepared wood panels or cardboard using varnish, tempera and acrylic colors, as well as watercolors. By analyzing the extensive library created, they were able to identify characteristic features of each particular phthalocyanine pigment, as well as shifts in behavior when mixed with yellow pigments.
The spectra showed the most distinctive information from 580-800 nm, and varied sufficiently from one phthalocyanine pigment to another to allow each to be uniquely identified. With this information in hand, the team was ready to take to the field with an Ocean Optics modular system based on an HR2000+CG spectrometer and a reflection probe measuring normal to the surface, viewing a ~ 5 mm sampling area.
The Retouched Madonna
Their first stop was at the Sagramoso Library in St. Bernardino church in Verona to study frescoes of the Holy Virgin, angels and Franciscan saints painted circa 1500 by Domenico Morone. The original blue areas painted using copper carbonate (azurite) show a reflectance band centered at 640 nm (curves 2, 3, 4 below), usually over a red ochre or hematite layer (curve 5). The hematite underlayer has its own weak absorbance band at 550 nm and a shoulder at 590 nm, affecting spectra of the sky and cloak shown in curves 3 and 4.
Spectral measurements were also able to identify the use of blue phthalocyanine pigment in some areas of the sky retouched in the 1980s (curve 1), as evidenced by the deviation in its spectra above 600 nm. These measurements were confirmed using false color IR (IRC) remote imaging, a method often used to identify repainted areas in paintings.
“EL 1920” – Real or Fake?
The real test was yet to come in the form of a painting from a private collection, signed “EL 1920.” Believed to be painted by the Russian painter El Lissitsky, an important figure in the Russian avante-garde movement in the early 20th century, the piece had not previously been authenticated by spectroscopic means. Vis-NIR reflectance measurements of blue and grey-blue areas of the painting by the team from the University of Bergamo found the presence of blue phthalocyanine pigments. Given that phthalocyanine pigments were not available until after 1935 (and later in Russia), the date of 1920 on the painting cannot be authentic, and the painting did not originate during the Russian avant-garde movement. XRF analysis performed on the pigments in question to validate the spectral analysis showed the extensive presence of titanium white (in rutile form, according to the Vis-NIR spectra), a material that would only have become available after 1920. Spectroscopy had proven “EL 1920” to be a fake. Learn more about the research conducted by Dr. Poldi’s team at the University of Bergamo.
Spectroscopy – A Mobile Art
Vis-NIR reflectance spectroscopy is a nondestructive technique capable of quickly and accurately characterizing pigments for the purpose of art authentication. The portability of modular reflectance systems like those from Ocean Optics is uniquely suited to in situ studies of large artworks like frescoes, and allows this technique to be brought directly to the private art collector.
Not only can the results of spectroscopic analysis provide guidance on date of artwork creation, but they can also identify restored areas versus original ones. Most importantly, portable spectroscopy can provide a definitive answer to the question previously reserved for only the most discriminating art expert: real or fake?
1 Wallace, John. “Ocean Optics’ LIBS Technology Spots Fake Antiques.” http://www.laserfocusworld.com/articles/2010/08/ocean-optics-libs.html. Laser Focus World, 11 Aug. 2010. Web.
2 Lukačević, Igor, and Dragana Rajković. “Non-invasive Analyses of Ancient Ceramics Colorants.” Croatica Chemica Acta 88.1 (2015): 53-58.
3 Blažek, Jan, et al. “M3art: A Database of Models of Canvas Paintings.” Digital Heritage. Progress in Cultural Heritage: Documentation, Preservation, and Protection. Springer International Publishing, 2014. 176-185.
4 Poldi, G., and S. Caglio. “Phthalocyanine identification in paintings by reflectance spectroscopy. A laboratory and in situ study.” Optics and Spectroscopy 114.6 (2013): 929-935.