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PRESERVING M
delicate document (painting, sculpture, dress)?" without taking any sample, if possible, or by taking only a minuscule sample from a noncritical region. They are governed by a code of ethics that dictates when and where they can remove material. Depending on the need, samples might be taken from restoration areas, already damaged areas, or edges. Conservation scientists view sampling almost as a surgical procedure in fact, some samples are removed with surgical scalpels. Suzanne Lomax, an organic chemist at the conservation laboratory at the NaArt conservation relies on three groups tional Gallery of Art in Washington, DC, of people for optimal care and treatment of describes the procedure for procuring a sample from a painting: "The painting is objects: the curators who deal with the laidflaton a table, and the microscope is collections on a daily basis, the conservators who provide restoration and preven- wheeled over. We look for an area where there is already a crack or loss. Using scaltive care services, and the conservation pels some are used for eye surgery we scientists who bring chemical and physical expertise to the mix. For effective care take cross sections through the layers of of collections, the three groups must paint smaller than could be visibly noticework together, combining their respective able. You would need a microscope to be knowledge of art history, artistic methable to see where we had taken the samods and chemical data to provide approple " priate treatment The differences in backAlthough their innate scientific curiosgrounds most noticeably between the cu- ity may tempt them to take samples purely rators and the scientists can complicate for research purposes, the scientists realcommi mication between the three groups ize the dangers of indiscriminate sam(Some academically trained conservators pling. Lomax says, "Sampling shouldn't have be done for frivolous reasons. If [a curaa formal tor] wants to take a sample, I try to take it science from the edge, so no one can see." LoViackcrrnnnH at the max sees questions of binding materials, undergraduate or graduate level ) original layer versus overpaint (something Conservation techniques can be compared with forensics techniques, which dif- that would reveal a restored area), or egg tempera versus oil paint (a matter of art fer primarily in the types of questions historical significance) as matters that asked. Just as forensic scientists want to justify the removal of a sample Other valid answer "whodunit" without consuming all Courtesy of the Freer Gallery of Art, Smithsonian Institution, reasons for sampling may include pigment Washington. DC the evidence, conservation scientists want to answer "Is this an original? What causedJunJ this crack? How can we protect this 364 A Analytical Chemistry News & Features, 1, 1996
Instrumental methods allow conservators to preserve museum collections
useum collections depend on art and research materials that are subjected to the punishment of handling and environmental conditions. Conservation scientists attempt to understand the materials comprising objects and the effects of the environment on those objects so that they may postpone the inevitable degradation process. In this quest conservators have turned to analytical chemistry to answer their questions about how materials age and what preveiitive measures or treatments might tard that aging
THE PAST identification or cross-sectional analysis to answer questions about artists' working methods. However, the decision to remove even a minute sample must be made by the curators and conservators. Adding to the potential for misunderstanding between chemists and conservators is the fact that they define "destructive" methods differently. For a chemist, a nondestructive method does not consume the sample during analysis, but in the conservator's eye, "the minute you take a sample, it's considered destructive," says Judith J. Bischoff, assistant professor of art conservation at the State University of New York College at Buffalo. "That's often where the line is drawn between what we can or can't do." Instrumental methods
Conservators have a variety of analytical methods at their disposal. Starting with microscopy of all sorts, the list of techniques reads like the introduction to an instrumental analysis textbook: polarizing light microscopy, UV-fluorescence microscopy, scanning electron microscopy, Xray diffraction, X-ray fluorescence, FT-IR, GC, HPLC, MS, GC/MS. This list doesn't even include the classical wet chemistry techniques used by conservators. The foundation of all conservation laboratories is the microscopy facilities. The Conservation Analytical Laboratory (CAL) at the Smithsonian Institution is currently increasing its optical microscopy facility. Melvin J. Wachowiak, furniture conservator and microscopist, says that they are "trying to create a bridge between the physical world as we see it and the instrumental [results]." Scientific anal-
ysis can involve "going from a gross-level evaluation of the object where you can see something is wrong to taking samples and sending them through a GC," Wachowiak says. "But the answers may be found at a different level. What you might miss is the relationship of the parts to the whole. You may know what ts present but not how it is arranged." Conservators use instrumental methods to characterize objects, sometimes by using chemical composition combined with art history as a dating method. Painting conservators might try to identify by
15 years, the lab has expanded its instrumental base to include a variety of additional microscopess X-ray diffraction, FTIR, and GC-MS. "We're much better situated [now] to analyze the microsamples that are available to us," Lins says. Conservation projects
Conservators use scientific methods to analyze a wide variety of objects, from paintings and sculptures to videotapes and ball gowns. "The Court of Death." Conservation analytical methodsfirsttook hold and are the most mature in painting conservation. Bischoff works in the area of conservation known as technical studies, in which the application of a wide variety of techniques allows a deeper understanding of a work of art and the artist's methods. Her field is controversial, Bischoff says; many people in the conservation profession question the appropriateness of technical studies and feel that only projects with immediate conservation implications Jiff1
justifiable Bischoff disagrees chemical composition whether a layer is ins? tfiat a better understanding of materioriginal or restored. The correct identifials and methods can lead to improved cation of pigments and binding media— preservation methods and can assist in gums, resins, waxes—can guide the conmatters of attribution servator to make appropriate decisions At the Detroit Institute of Arts, she about cleaning and restoration efforts. Only in the last 10 years have conserva- studied Rembrandt Peale's "The Court xof Death," an unusual and very large (11 k x tion laboratories significantly increased 24 lk ft) painting for the American portheir instrumental capabilities. For example, when Andrew Lins, senior conserva- trait artist. Through collaborations with facilities such as Wayne State University, tor for sculpture and decorative arts at Miami University of Ohio, and BASF Corthe Philadelphia Museum of Art, joined the museum's staff, the conservation labo- poration, she was able to use GC, scanratory had one microscope: a Zeiss polar- ning electron microscopy, optical microsizing light microscope. In the subsequent copy (visible and UV-fluorescence), and FT-IR to analyze the pigments and bindAnalytical Chemistry News & Features, June 1, 1996 3 6 5 A
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Figure 1 . Micrographs of a totally corroded cast bronze horse fitting t a k e n from the Qu Cun excavation site. (left) Cross section image (~ 18x) obtained using a metallographic microscope. The original metal structure is preserved in corrosion; the outside of the piece is on the left. The olive green portion is primarily Sn0 2 and provides a protective coating. Bright field (center) and dark field (right) images of the corroded center are also shown at ~ 218x. The banding in the high- magnification images is believed to be a result of the corrosion process and not the original fabrication. (Photographs courtesy yf the Department of Conservation ona Scientific Research, Freer Gallery of Art/Arthur M. Sackler Gallery, Smithsonian Institution.)
ers in the paint. She scientifically substantiated the artist's unpublished manuscripts and letters concerning his methods and will continue studying Peak's work this summer at the National Gallery of Art in Washington, DC, where she will be examining portraits that he painted. She hopes that the project will culminate in the publication of Peak's painting manual, "Notes of the Painting Room." The First Ladies' gown.. When the exhibit of First Ladies' gowns at the National Museum of American History closed for renovation, Mary Becker, then a graduate student at The Johns Hopkins University, and Noreen C. Tuross, senior research biochemist at CAL, were given the opportunity to analyze the fabrics to determine a quantitative biochemical indicator for the degradation of the gowns. "The challenge," says Tuross, "is that the dresses have different histories both before and after they get to the museum." To establish a baseline, Becker and Tuross analyzed the amino acid content of modern fresh silk before and after it had been exposed to light. Natural silk, usually made from the cocoon of the domesticated silkworm, consists of a proteinaceous polymer called fibroin coated by sericin, a gumlike protein. Fibroin is generally insoluble in aqueous solutions. It also has proportionally large glycine (> 40 mol %), alanine (> 29 mol %), and serine (> 10 mol %) )omppsitions. Mooern silk processing removes most of the tective sericin coating bv one of three "degumming" methods high-temperature water extraction dilute aqueous alkali extraction or enzymatic proteolysis 366 A
Silk was artificially light-aged with a xenon arc lamp filtered through three different glass filter combinations: borosilicate-soda lime (300-nm cutoff) to simulate exposure to sunlight through glass; borosilicate-borosilicate (285 nm) to simulate artificial sunlight; and quartz-quartz (230 nm) for nearly direct output from the xenon lamp. Tuross then determined the amino acid content of the new silk the artificially light-aged silk, and 19 First Ladies' gowns using ion-exchange HPLC and postcolumn derivatization She verified that low tyrosine levels in silk are a good indicator of photolytic damage. Sericin-rich silk tends to have a larger amount of tyrosine, which seems to indicate that the protective properties of sericin reduce the damage to the fabrics caused by light. The more recent First Ladies' gowns are possibly in greater danger because extensive modern processing techniques have stripped them of their sericin protection. Chemical analysis can warn conservators that for sericin- depleted fabrics "every precaution about light levels is warranted" Tuross says "All other things being equal you can't make the simple assumption that age is a higher risk factor" Under the SEM, the fabric samples showed signs of damage, but Tuross believes that the micrographs are not a particularly good indicator of the overall condition of the gowns and that some of the damage was artifactual, caused by sampling a region of stitching. She says, "The material that's really damaged you can usually tell just by looking, but chemical analysis can tell you if there's a problem before the damage is visible."
Analytical Chemistry News & Features, June 1, 1996
Metal alloy objects. W. T Chase, head conservator at the Smithsonian's Freer Gallery of Art and Arthur M. Sackler Gallery in Washington, DC, and coworker Wang Quanyu are using the microscopes at CAL to study severely corroded Chinese bronzes taken from tombs at the Qu Cun archaeological site excavated by Peking University in the Chinese province of Shanxi (Figure 1). They date from approximately 1000 B.C. to 450 B.C. The bronzes were unearthed from a damp environment (with 100% relative humidity), which contributed to their corrosion. The alloy used in Chinese bronze has two phases: a, with a higher Cu concentration; and S, with a higher Sn concentration. Although the 8 phase of Chinese bronze usually corrodes first, the a phase in some Qu Cun bronzes is corroding first; some areas of the bronzes have no a phase remaining. Chase believes that this may be due to an original corrosionresistant film on the surface but he has not yet verified his hypothesis Chase and Wang are using SEM to determine how the corrosion has taken place. Wang has determined many of the corrosion products, including the minerals tenorite (CuO) and malachite (Cu2(OH)2C03), with X-ray diffraction. Chlorine is often associated with the corrosion of archaeological materials; although some chlorine has been detected, they have not found common chlorinecontaining copper minerals such as nantokite (CuCl) oratacamite (Cu (OH) CD Elemental analysis with SEM reveals the of chlorine but not at the corrosion front where the conservators thought
it should be. "When looking at corrosion, we usually think that when we get to the metal, we're seeing the first thing that happens," Chase says. "I'm not so sure anymore. We need a better understanding of the structure of these pieces and what it's telling us." In some pieces the corrosion has completely penetrated the alloy—a small hole or hairline crack goes through all the layers. The work with these bronzes indicates that the stance taken by many conservators, that similar artifacts should not be cleaned with aqueous solutions, is probably advisable, says Chase. Lins has studied corrosion products, even out of the typical museum setting. Working with Norm Neilsen, a metallurgist from DuPont, Lins was asked to determine the source of ammoniated CuS04 that was appearing on the Liberty Bell in Philadelphia. According to Lins, cracking caused by exposure to ammonia (stress corrosion cracking) is a common problem with older, highly stressed copper alloys. 'The cause of [the ammonia deposits] seemed to be either cleaning fluids used in the Liberty Bell Pavilion or fertilizused on riants the Bell" Lins says His findings resulted in adapted cleaning and fertilizing regimens to reduce the level of vapor-phase ammonia near the Lihprty
ences in videotapes between manufacturers and between years of production. The open-stage architecture other IR microscope allows her to spool out the tape and analyze videos from the Smithsonian's archival collection in a truly nondestructive technique, without removing a sample or damaging the tape. The ultimate goal is to reduce copying costs by eliminating the need for superfluous duplication of videos. "We really hope it will be a tool for collections management," she S3VS Lack of funding
The conservation scientists conducting research in the application of analytical techniques to art conservation have difficulty financing their research, even if they have advanced science degrees, as Baker, Bischoff, Lomax, and Tuross all do. "Conservation science doesn't have the funding that chemistry does," says Bischoff. "It's too much science for NEA
Conservatton scientists continue to search for efficient nondestructtve methods
to find donors or companies that are downsizing or upgrading their equipment and would be willing to donate to a not-forprofit organization. Even as one of the five largest museums [in the country], we have a low budget for this sort of acquisition." The future of chemistry and art conservation
The consensus seems to be that the future of art conservation and analytical chemistry is more about the people than the techniques. Bischoff says, "Collaboration between conservators and scientists has shown that these collaborations are important—[individuals] working together and recognizing that everyone has a contribution to make to the understanding of a painter or a work of art." Without such collaborations she would not have been able to pursue her study of Rembrandt Peale's work. "There has been for the scientists and conservators a problem of communication; agreeing on what is possible and available has been the biggest problem," Lins says. "Conservators have often expected more than science could actually deliver with the sampling techniques available." However, he continues, "As curators become more sophisticated about scientific data, communication will improve." Programs in art conservation, such as those at the State University of New York College at Buffalo New York University the University of Delaware and CAL's Furniture Conservation Training Program are teaching scientific acproaches to conservation and continuing education for museum professionals inscientific techniques
Bell "This tvoical problem addressed to conservation scientists: What is the effect of the envior NEH, but not enough science for NSF." ronment on artwork?" Lins says In other words, conservation science ofVideotape and other magnetic ten falls through the cracks between grantmedia. Mary Baker, a polymer chemist funding agencies. at CAL, studies a variety of polymers found Lins agrees with Bischoff: "Unfortuin modern materials such as videotapes. nately, it is difficult to find funding beShe is investigating the aging properties of cause of the intellectual divisions in Washvideotape and hopes that it will provide As for instrumental methods, conserington. The NSF, for example, has been insights into similar magnetic media. vators hope for more techniques that fulvery reluctant to acknowledge this area. 'This is an aging phenomenon," Baker fill their criieria for nondestructiveness— This is definitely a dilemma that has not says. "Modern videotape is good for 10 been very well resolved for museums that X-ray fluorescence, radiography, and some FT-IR techniques are currently the years. That may be good for consumers don't have huge endowments or federal but it's short for museums " whose collec- funding for the purchase of equipment for only nondestructive techniques in wide use. Conservation scientists can imagine tions include film and audio biographies. their laboratories." According to its 1994 usingfiber-opticRaman spectroscopy, but Baker is combining data obtained from annual report, CAL spent more than videotape manufacturers with the data $80,000 on upgrading and purchasing new Lins says that even that technique would not be useful without a spectral library. In generated by using the Smithsonian's arresearch equipment that year. the meantime conservation scientists chival video collection to construct a caliIn a typical year, Lins' laboratory might will continue to look for efficient methods bration curve that will accurately predict have $10,000 for supplies and instrumentathat give them all the information they when a video should be copied based on tion, which is not enough to purchase need with as little sample as possible its IR spectrum, which indicates signs of new instruments. "We're looking for oxidation. Using attenuated total-reflecsources for an SEM," he says. "We have Celia Henry tance IR, Baker has seen spectral differAnalytical Chemistry News & Features, June 1, 1996 367 A
