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December 2000
Vol. 9, No. 12, pp. 19–20, 22.
Instruments & Applications
Spectroscopy Ranges Far Afield

Portable FTIR helps scientists preserve art, including ancient handprints.

Nonscientists who “discover” science often express amazement at its nonquantitative side. Even in the exact sciences—chemistry, mathematics, physics, and increasingly, the biosciences—qualitative elements play a big part. Gerhard Herzberg, the German–Canadian spectroscopist who died earlier this year, received the Nobel Prize in Chemistry for his discovery of the emission spectrum of triatomic hydrogen. He was fond of saying that the best scientists could smell key data—detecting them against a welter of false signals and background noise, and even before the last experimental results were in. (Herzberg practiced what he preached. Not only was he a consummate spectroscopist who advanced his discipline, but in his spare time he gave recitals of Schubert lieder in what, by all reports, was an excellent baritone.)

Art and science have much in common even beyond their union in a cultured chemical spectroscopist such as Herzberg. Until Newton’s time, both areas were lumped under the general heading of philosophy. This common origin not only indicates that science has a qualitative side: It hints—correctly—that art has its quantitative element. Nowhere is this more apparent than in the use of exact analytic and diagnostic techniques to identify, preserve, and restore art. (See “Using LC/MS To Preserve A National Treasure”, TCAW 1998, 2, 42–46 and “Deceit, Deception, and Discovery”, TCAW 1998, 2, 50–60).

The Hardware
Whereas most identification work occurs in a laboratory under controlled conditions, portable spectroscopic instrumentation has opened up a whole new subgenre in the quantitative analysis of art. A visiting professor at a U.S. university’s archives, seeing such instruments and their capabilities for the first time, called the portable technology “a curator’s dream”.

In the area of fast Fourier transform infrared (FTIR) spectroscopy, one of the major artistic impacts of portable units has been to extend art analysis beyond its traditional subject matter of painting and sculpture found in museums. For example, a 1998 project by the Canadian Conservation Institute (CCI) established that FTIR spectroscopy could positively identify contemporary thermoplastic art materials via nondestructive in situ analysis. Objects so tested did not even have to leave their display cases or storage drawers.

In the course of one such project, CCI scientists found that portable FTIR technology could also readily characterize other organic compounds (such as varnishes and lacquers) and inorganic materials (including minerals and corrosion products). The CCI’s portable unit, a MIDAC illuminator with an IR source and interferometer, and a REMSPEC MCT detector connected to a fiber-optic probe, easily collected over 100 spectra per day.

The Proof is in the Spectrum
The ability to identify and verify relics is critical to the appraisal process. In some cases, this is easy to do. A collector could save thousands of dollars just by paying enough attention to realize that his recently purchased love letter from Napoleon to Josephine was written in English. In other cases, verification can be more difficult and long in coming. A painting of four Plains Indian chiefs, signed by the artist and dated 1875, was recently shown to contain a synthetic pigment patented in 1905—thus invalidating its supposed date. (Whether or not the artist was involved in an attempted fraud is a matter that historians can now debate.)

Paul Kane (1810–1871) was born in Ireland and came to the United States in his mid-20s. From 1836 through 1869, he traveled extensively throughout the North American frontier, using his paints to document the rapidly vanishing life of the American Indian. Kane also painted portraits of leading U.S. citizens, among them Freeman Schermerhorn Clench and his wife Eliza Cory Clench. Fast FTIR analysis of a paint box that tradition ascribes to Kane recently revealed that its silicate drying oils and iron-oxide pigments precisely matched corresponding components in the two Clench portraits. IR spectroscopy thus verifies what had been a debated hypothesis: Kane used this particular paint box when he was painting these portraits.

“IR spectroscopy has been used extensively for many years to identify a wide variety of materials associated with artifacts and works of art,” offers Elizabeth Moffatt of CCI. “It is, in fact, the first step in many analyses, because it provides an overview of the constituents of a sample.”

Organic materials such as varnishes, paint media, adhesives, and plastics can be identified by IR spectroscopy, says Moffatt. So can inorganic compounds, such as pigments, minerals, and clays. Even more importantly, she suggests, “changes in the composition of a material as the result of natural or accelerated aging can . . . be detected by IR spectroscopy.”

She goes on to explain that, as recently as the 1970s, scientists tended to use dispersive instruments, such as the Beckman 20 and PerkinElmer 283 grating spectrometers. Shortly thereafter, however, IR spectroscopy was revolutionized by the development of FTIR spectroscopy.

The major components of an FTIR device, the Canadian researcher explains, are “an IR source, a Michelson interferometer, a laser, a detector, and a computer. [This] has led to many improvements, including better-quality spectra, [better] data-processing techniques [and] search software, and reduced analysis time.”

Moffatt has special praise for the newest FTIR technique, infrared microspectroscopy (see “Expanding the Scope of Forensic Science”, TCAW 2000, 10, 44–51). “These micro techniques were not practical using dispersive instruments.” Nonetheless, they are “a great advantage in the analysis of artifacts [since] the amount of sample that must be removed from an object is very small.” She adds, “The microscope accessory has allowed more sophisticated analyses to be undertaken. A specific portion of a sample, such as a layer or an inclusion, can be analyzed by masking the rest of the sample.”

The Soft Touch
Portable analytical equipment has recently been used successfully on some unique artifacts by Diana Rolandi de Perrot, director of the Institutio Nacional de Antropologia y Pensamiento Latinamericano de Argentina. The artifacts that interest Rolandi cannot be found in exhibit cases; they are rock paintings in La Cueva de las Manos (The Cave of Hands) in the Argentine province of Santa Cruz, in Patagonia. These paintings, including hundreds of stencils of human hands, were made by a hunter-gatherer culture that may go back as far as nine millennia before the present era. An ancillary site nearby is called Cerro de los Indios (Indian Hill).

Both finds provide scientific data far more impressive than from any other local sites. They belong to a vast family of rock art (petroglyph) sites that stretch throughout the Americas—from Alaska and the Yukon in the north to Patagonia and Tierra del Fuego in the south. The Argentine secretary of culture, augmented by the local Fondacion Antorchas, funded the investigation of these two crucial sites.

La Cueva de las Manos, located several hours’ drive south of the sleepy town of Perito Moreno, was recently designated a national historic monument by the Argentine government. Not only handprints adorn the cave, but also depictions of los guanacos, the native llama, painted in natural mineral tones. Portable FTIR technology and other analytical techniques are being used to investigate and characterize the pigments, binding media, natural accretion of minerals on and near the artwork, and, sadly, deterioration mechanisms for these arresting, artistically beautiful, and anthropologically significant renderings.

Did You Know?
All You Need Are Spectra
FTIR analysis of artwork need not even restrict itself to the archival. The surface coatings of a 1965 Rolls Royce automobile, owned at one time by ex-Beatle John Lennon and brightly decorated with a “Flower Power” theme, were recently analyzed in British Columbia, Canada. The goal: discovering how best to stabilize and preserve the car’s decor.
The Science of Color
The FTIR spectrometer cannot detect absorption below 400 cm–1, and the microscope accessory cannot detect absorption below 700 cm–1. This means that the spectrometer cannot detect sulfides such as those in vermilion and cadmium yellow pigments, and the microscope accessory cannot detect the oxides in zinc white and hematite. These pigments would normally be identified by X-ray diffraction. Common colors such as the beautiful deep azure called Prussian blue are readily detected by FTIR, even at low concentrations of the pigment, because there is no interfering absorption in this region of the IR spectrum.
Such important pre-Hispanic objets d’art are, in a sense, the victims of their own excellence. Fame leads to visitors—and, regrettably but inescapably, to vandalism. Tourists cannot refrain from adding their own marks to those of their long-dead fellows; modern graffiti overwrite the ancient art. Analytical science techniques offer the hope that we may at least understand how these amazing artistic statements were made before they are changed or destroyed.

Spectroscopic data are vital here. But techniques being used to analyze the ancient designs extend beyond FTIR to photography, chemical analysis, X-ray diffraction and microanalysis, surveying, and on-site geology. These techniques investigate methods of recording, dating, treating, and managing the sites, as well as aspects of natural weathering. La Cueva de las Manos constitutes one of the first projects of its kind in Latin America.

The site is composed of a series of subsites on either side of and within a cave set high in a cliff face above an arroyo seco—a dry canyon floor. Artists and conservation scientists from North and South America are looking for ways to protect the sites. Their solutions will determine future policies on everything from how to manage visitor flow to raising public awareness of the fragility and cultural value of the petroglyph sites.

Though the work has just begun, it is already uncovering surprises. It is now apparent, for example, that the anonymous ancient artists prepared cave rock to receive their work by laying down a preparation layer of an as-yet-uncharacterized substance. There may also prove to be a material connection between the pigments used at Cerro de los Indios and Cueva de las Manos. But whatever data come to light, exact analytical techniques originally developed for the laboratory are proving of inestimable importance in protecting Earth’s threatened patrimony of ancient archaeological sites.

William Illsey Atkinson is a Seattle-born science writer who lives in Vancouver, B.C. Comments and questions for the author can be addressed to the Editorial Office by e-mail at tcaw@acs.org, by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.

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