Paul Whitmore was barely a week into his job at Yale’s Institute for the Preservation of Cultural Heritage (IPCH) when he encountered a sticky dilemma. A type of glue used to bind and preserve fossils housed at the Peabody Museum of Natural History was reacting strangely with certain specimens.
The adhesive, polyvinyl butyral, typically dissolves in alcohol. It should remain soluble as long as it has little exposure to ultraviolet light, but on some fossils the polymer quickly becomes insoluble with little or no UV exposure. Instead of dissolving the glue, the alcohol transforms it into a thick jelly that must be scraped off, which is time-consuming and could damage the fossilized material underneath.
“This was the puzzle posed to me when I first arrived two-and-half years ago,” says Whitmore, director of the institute’s Aging Diagnostics Lab at West Campus. “Since the fossils in question aren’t likely to get much UV exposure at the Peabody, I am trying to figure out why the polymer is reacting this way and the circumstances in which such chemistry might occur.”
Solving the problem is painstaking work that involves trying to reproduce the chemical environment on the fossils, which are complex composites that can contain many different minerals from one end of a bone to the other.
Whitmore says it could turn out that something in the fossils’ composition, a salt or other mineral, is causing the resin to react in an unusual manner.
“That’s the challenge: to understand whether this is a peculiar event or more widespread and you’re just better off shifting to another substance,” he says.
These sorts of challenges are routine at the Aging Diagnostics Lab, which studies how materials in Yale’s collections age and develops analytical techniques to measure and monitor aging and other changes wrought by time and environmental conditions. Studying the kinetics of decay and possibly identifying ways to slow down such processes is among the institute’s key research priorities.
Stefan Simon, director of the IPCH, says that the aging properties of polyvinyl butyral have concerned scientists for more than 30 years. He encountered similar issues when he worked at the National Museum in Berlin, where the polymer was used to consolidate the blue-glazed tiles of the famed Ishtar Gate, which has been on the museum island since the 1970s.
“Conservators generally aim at reversibility or, more realistically, retreatability for their interventions,” Simon says. “Studies of the long-term behaviors of polymers, like this one being carried out by Paul, provide important contributions for the conservation community, reaching from the stability of a glue applied on fossils to the enormous challenge of conserving 20th-century art objects and time-based media.”
Whitmore is helping colleagues at the Smithsonian Institution to tackle another fossil-related conundrum — one associated with the polyvinyl butyral puzzle.
Over the 150 years or so that paleontologists have been removing fossils from the ground, they have applied various substances to the specimens to strengthen and preserve brittle and irregularly shaped specimens or to knit together fractured and splintered fragments.
In the late-19th and early-20th centuries, shellac was one of the most commonly used coatings. Other substances used over the years include liquid nylon and Duco cement.
Over time these substances begin to fail. Shellac becomes dark and shrinks, which can cause stress fractures and damage the surface of the fossilized bone. The coatings also conceal material that scientists want to study.
“The needs of scholars evolve,” says Whitmore. “The things scholars studied when these fossils were discovered were different than what researchers are studying now. Scholars have more sophisticated tools. They want to do DNA analysis, but 100 years ago they didn’t know about DNA.”
Marilyn Fox, a preparator in the Peabody’s Vertebrate Paleontology Preparation Lab, says she is often asked to remove a coating from a fossil specimen to enable research.
Fox provided an example of a fossilized crocodile jaw that was covered in a dark brown, shiny coating of shellac. “This coating is very thick and it covers details that somebody here wants to research,” she says. “We spend a lot of time trying to remove these coatings.”
Fox says it can take several days of work to clean a specimen using different combinations of solvents and tools like scalpels and mini-sandblasters.
Rebecca Kaczkowski, a conservator at the Museum Conservation Institute at the Smithsonian, has come to the Yale lab as part of a project to figure out new ways to clean fossils that will protect the scientifically valuable material underneath the coatings. Kaczkowski started the project while she was an intern at the National Museum of Natural History. The Smithsonian’s dinosaur halls are under renovation, providing an opportunity to conserve and clean the specimens displayed there. It opened the door to finding new ways to remove shellac and other adhesives.
The fossils exhibited at the Smithsonian are important scientific specimens and inappropriate for experimentation, notes Kaczkowski. Instead, she borrowed a few marine-mammal specimens from the museum’s teaching collection. High-resolution photographs were taken of the untreated specimens, and electron scanning microscopy was performed on them.
Kaczkowski brought the specimens to Whitmore’s West Campus lab last winter. They painted the specimens with the same adhesives used on fossils 100 years ago. Then Whitmore used applications of heat and light to accelerate the aging process so that the mock-ups simulated the actual specimens displayed at the museum.
Kaczkowski will experiment with various cleaning techniques, including two types of lasers and various gelling media to thicken solvents, and allow them to sit longer on the surface of the adhesive without penetrating to the fossilized bone.
The cleaned mock-ups will be photographed and analyzed to determine whether any fossil material is damaged.
“First and foremost, we need to preserve the scientific utility of the specimen,” she says.
Simon says he welcomes the collaboration.
“We’re very pleased to work with our colleagues at the Smithsonian’s Museum Conservation Institute on solving this problem,” he says. “This kind of partnership benefits both institutions as well the wider scientific community.”
Whitmore says that studying specimens in the fossil collection informs his lab experiments, which are aimed at accelerating the aging process so that he can predict how materials will change and degrade over hundreds of years.
“These examples of 100- or 150-year-old materials that have been aged under well-documented circumstances provide a glimpse of what that looks like and whether my lab tests have hit the target and are predicting accurately,” he says. “It teaches me a lot.”
A discovery made while working with the Peabody’s fossils can be applied to other aspects of the university’s collections. For instance, he says, work with polyvinyl butyral raises questions about how the substance reacts with stone artifacts removed from the ground.
“Am I going to start seeing it on architectural conservation or archaeological material where those substances are used?” he says. “Is it a more general phenomenon that goes beyond the fossil problem? It’s those kind of chemical problems that have wider application that intrigue me and have great importance.”
Source: UC Berkeley