Sarah Nunberg has a problem.

In the Chapel of the Good Shepherd at Saint Peter's Church, large, abstract wooden sculptures seem to move along much of the 28-foot walls. The figures are strange yet somehow calming, giving the impression of quiet and space juxtaposed against the commotion of midtown Manhattan just outside. Nearby, three five-foot-long rectangular columns, representing the Holy Trinity, hang suspended from the ceiling next to the altar, covered in a gleaming gold leaf.

The odd shapes are actually abstract representations of biblical scenes. One sculpture is called the Frieze of the Apostles, another the Cross of the Resurrection.

Upon closer inspection, however, the pieces don't look so good. "The sculptures are in really bad shape," Nunberg says. She's an objects conservator with the Objects Conservation Studio, and it's her job to restore these sculptures to their former glory.

The sculptures, designed by famed artist Louise Nevelson, were once painted white. That was 40 years ago, and they have since faded to a yellowish hue. You can't see that yellow. Over the decades, the sculptures have been repainted in a slapdash manner with different kinds of water-based paint. Now the wood has expanded and contracted in a different way from the paint, causing it to blister and chip , revealing unsightly patches of dark wood underneath. Trying to paint over it again, as has been done for decades, wouldn't work. The paint wouldn't even stay on.

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Nunberg has put together a team to figure out how to best preserve these sculptures. They are testing different solutions that will strip away the added layers of paint to reveal Nevelson's slightly yellowed original. "First we will glue everything down, and then we'll peel off the restoration layers like an onion," she says.

People visit museums to learn, but more than that, to be transported to the past. They might ooh and ahh over artifacts masterfully crafted with simple tools or paintings that depict a lifestyle long disappeared. We often assume these objects are immortal, that they will always be there to remind us of who we are as a culture. (Who wants to imagine a world in which The Last Supper or The Scream don't exist anymore?) Yet few of us think about how much had to be done to those objects simply to stop them from falling apart. Urns that gleam on museum shelves were once pulled from the ground as corroded hunks of metal. An earthenware pot might have been a million fragments before someone painstakingly pieced it back together. A dedicated scholar and craftsman has returned a faded painting to its former glory.

Art conservators fight a constant, never-ending battle against time, an unwinnable war against entropy to bring works of art back to nearly immaculate condition and keep them there. Of course, things don't always go according to plan. (Remember the Jesus fresco in Spain ruined by a bizarrely botched restoration?) Good intentions aside, old, delicate objects can be damaged by overzealous cleaning or by storing them under the wrong conditions. There have been shards of Mayan ceramics that were whittled down to fit together when the reassembly is incorrect, like snipping the knobs off jigsaw puzzle pieces when you can't figure out where they go. Prehistoric flints in storage in Verona, Italy, turned a pearly blue because of previously unknown molecules present in the plastic drawer liners. One conservator I talked to recalls an incident in London from the late 1970s in which the inlay in several intricate Fabergé eggs melted while on display.

The lights were too bright.

Centro de Estudios Borjanos/AP

Conservators are craftsmen. Restoration treatments need to be made of the right chemicals to prevent damage, but applying them requires a delicate hand and a light touch. Conservators rely on science to aid their efforts. They scrutinize the tiniest details of priceless artifacts. They conduct chemical analyses of an object to determine its molecular makeup and decide how best to clean or repair it. They place a sculpture or painting in storage or on display under environmental conditions that will delay its slide into destruction.

And yet, art conservation is not quite a science. Saving the past means navigating a sea of unanswered or unanswerable questions about what the artist intended an object to look like or how efforts to fix a piece of art could damage it. Sometimes conservators make mistakes—mistakes that destroy irreplaceable objects. Sometimes intuition fails. And though the field has become much more scientific in the past few decades, conservators still have some big questions about how to preserve the past without destroying it. "We have to take the long view of the history of these objects," says Pamela Hatchfield , the head of objects conservation at the Museum of Fine Arts in Boston.

First, Do No Harm

Bruno Pouliot was working on a number of pieces from President George Washington's Mount Vernon estate in Virginia when he received a new artifact.It had been reduced to a mess of wood and leather, but Pouliot, an art conservation professor at the University of Delaware, worked out its original identity. It was a canteen, a container that would have stored the spices and kitchen gadgets needed to prepare a nice meal while soldiers were out on a campaign. The bag didn't look much like a bag anymore, though. "It was missing big segments," he says. "It just looked terrible, like an old rotten piece of leather."

It just looked terrible, like an old rotten piece of leather.

Pouliot wanted to preserve the canteen and put it back on display at Mount Vernon if possible, but this wasn't an object that had been designed to last 250 years. The leather was treated with some waterproofing chemicals (Pouliot wasn't sure which) and the canteen had carried salts and herbs that would have degraded the materials if it weren't carefully cleaned. Before they could start to take action, Pouliot's team had to figure out what animal was the source of the leather, what kinds of waterproofing treatments were made on leather in the Revolutionary era, and what kind of degradation happened to the canteen during its centuries in storage. It's a daunting task to save the past.

From the moment it is created, an object starts to decompose. Materials release gases as they break down, which can interact with one another to accelerate decomposition. The most famous example of this is cellulose nitrate , a material that made up early film. It would release gases such as nitric oxide and nitrogen dioxide and, if unventilated, would quickly discolor and turn brittle or even burst into flames. The other things enclosed in a case with an object, or the foam padding underneath it, or even the container it's in affect how it breaks down.

Courtesy George Eastman Museum

To start, you've got to know what you're dealing with, so conservators subject an artifact to a barrage of tests that tell them the chemicals it contains. In the past, Hatchfield says, those tests would sometimes be fairly destructive. Conservators might shave off part of a bronze bowl to check for characteristic corrosion between its grains, or rip a tooth from a mummy's mouth to test its DNA. Today, they try to alter an object as little as possible, and so they test microscopic samples of a piece.

Ric Francis/AP

They use X-ray fluorescence machines that work by shooting a beam of high-energy rays at a sample, then detect how much energy is released when an outer electron becomes dislodged. They use gas chromatography mass spectrometry , in which a sample is turned into a vapor and mixed with a light gas like helium—the components in the sample separate based on their chemical properties and are then heated to become ions, which enables the device to identify the different elements based on their structure. Heating up pottery reveals the low levels of radiation trapped inside it, so conservators can approximate the age of the material via a process called thermoluminescence . CAT scans and MRIs show conservators what's under a mummy's wraps without having to physically peel them off.

"We use many different analytical techniques, but there's a limit to what these techniques can do," Pouliot says. Many still can't distinguish well between organic materials, Pouliot says, and that can make a difference in the long run, say, if you're treating cowhide versus a sheepskin. Or if the surface of a ceramic pot has been exposed to light for a long time, conservators can't figure out when it was made without drilling deeper into it.

Pouliot and his collaborators were able to restore the canteen enough so that it could be put on display at Mount Vernon, but there were "large losses" in the leather. They had to replace it with modern leather, doctored to look centuries old, that they glued to the original. "In the future, we will likely lose more of the [original] hide because we don't fully know the longevity of these adhesives that are in contact with the material," Pouliot says.

Even with the most sophisticated chemical analysis, some questions—about the artifact's past use, where it's been since it was created—just can't be answered without a time machine.

To Fix or Not to Fix

Hitler, Mussolini, Hirohito, and a skeleton. Each of the four rides a dark, maniacal horse with legs ablaze as it gallops over the planet; each clutches a dagger or a bomb. It's called Apocalypse '42. The piece, by American artist Viktor Schreckengost, has been shown a lot in the past 70 years, says Helen Ingalls, an objects conservator at the Lunder Conservation Center at the Smithsonian American Art Museum in Washington, D.C. (though there's now some debate about whether to show this piece at all because of its stereotypical depiction of Japanese people). In November the jarring sculpture was to go on display at the newly renovated Renwick Gallery. But first, something weird caught the eye of restorers when they took the piece out of storage—a red glaze from the horse's hooves seemed to have bled onto the base.

Smithsonian American Art Museum

"This is a fine art museum and we have a minimal tolerance for distracting anomalies," Ingalls says. "We want people to get sucked into the content of the work, we don't want them to think, what is that thing?" It was pretty clear that the glaze belonged there, at least according to Ingalls. "Whether or not the artist intended for it to run like that and look bloody, it definitely enhances the context and content of the work," she says. "And the artist decided to keep it. I think the artist was both horrified and delighted."

Bringing a work of art back to its former visual glory can be as simple as cleaning or stabilizing it, as in removing damaging salts that will cause cracks eventually. But other times conservators need to actively repair an artifact. To do that, conservators need to put themselves in the artist's shoes. "The original intent is something we think about a lot," Hatchfield says. This attention to detail might seem obsessive, but it's necessary.

Let's take the Nevelson sculptures, for example. Removing that yellowish original paint requires scraping and sanding, which can affect the original wood. That's simply something conservators don't do, Nunberg says—it's too damaging. "Removing original and replacing it with new paint removes important original history and is not reversible," she says. "Future generations would not be able to see original … surfaces, they would see a surface that would be a part of the conservator's handwork."

"The second you start introducing materials into something that's lasted 2,000 years, you're messing with it."

In the early days of art conservation, in the 1960s and '70s, it was in fashion to repair as much of an artifact as possible. Restorers would use glue or epoxies to fix tears, metal pins to reattach broken parts of a sculpture, fill in parts of a painting that had faded—even if their modern paint had a totally different chemical composition. "The second you start introducing materials into something that's lasted 2,000 years, you're messing with it," Nunberg says.

Over-cleaning can be a problem too. To make artifacts look nice on display, people might remove some of the chemical layers that were probably on the object during its original use—think of the oxidized layer that gives the Statue of Liberty her distinct green hue. If the statue were restored to its original gleaming copper, it would just look wrong. "I prefer to be the first one to work on any given object—there's often been too much cleaning, or things were put together incorrectly," Hatchfield says.

David L. Ryan/The Boston Globe/Getty Images

Things have changed. Today conservators are less likely to clean objects, and there's a push to make their treatments more reversible and use them more sparingly. That's partially because of a newfound value in keeping artifacts as intact as possible. But it's also because of a larger cultural shift, an understanding that objects in museums don't show up looking just as they did when they were created, and the evidence of that doesn't need to be totally erased. "These days we allow objects their own history, to have a trace of what they've been through," Hatchfield says. "They have a story to tell beyond just their manufacture and ancient use."

You need only look at the new and creative ways to save priceless art while still showing it off. When a painting by famed abstract artist Mark Rothko was faded beyond repair, conservators at Harvard projected light onto the canvas , adding color to make the painting look like its former self. Large institutions like the Smithsonian are digitizing their collections to preserve them better, which also allows them to render objects as they might have been before they were damaged. A team of Tokyo-based scientists have spent years capturing images of the collapsing Bayon temple at the center of Cambodia's Angkor Wat, which has allowed them to discover and restore new faces carved into the stone.

But even after the painstaking restoration process, it remains a challenge to put them on display to the world without letting them fall apart.

An Oddball Test

A stainless-steel oven, about the size of a dorm-style mini-fridge, sits inconspicuously on a counter in the lab. Inside sits a rack filled with about two dozen vials, each with strips of metal stuck in the top and a different colored material resting at the bottom. The vials had been in there for nearly a month, baking at a relentless 140 degrees Fahrenheit with 100 percent humidity.

This is the Oddy test. Developed in 1973 by conservator Andrew Oddy of the British Museum, it's a way to test materials that would go in a museum's display case. Mats, paints, mounts, they're all tested this way. It's meant to speed up the rate at which the materials in question release the gases and chemicals trapped inside. If the material contains any acids that would be corrosive to an object, then the three metal strips in the vial—made of copper, lead, and silver— would react. Depending on the chemical makeup of object in question and how much the metals reacted, the Oddy could show that a particular material would be too reactive to be used.

"I don't mean 99.9 percent, I mean perfect."

Sounds pretty scientific, right? It's not. Every institution has a slight variation on the test, which makes it difficult to compare the results across multiple trials. Many of the materials evaluated in the Oddy test are from a supplier and most are proprietary, which means that conservators don't actually know what chemicals are in them before they start. The manufacturers can change the recipe with no warning, so the same foam board tested a year ago needs to be tested again, just in case.

What's more, there are simply too many variables at play in this test to know what's causing a corrosive reaction. "Basically what you need for an Oddy test is a perfect environment," Ingalls says "You need everything to be perfectly clean, you need your metals to be perfect. I don't mean 99.9 percent, I mean perfect."

Conservators are aware of the failings of the Oddy test. "[At the Museum of Fine Arts] we are kind of dismissive of the Oddy test, though we do it," Hatchfield says. "It's a test that is fraught with imperfections," Ingalls says, describing it as kitchen chemistry. "Nevertheless, it can identify the worst offenders in case materials. And that's extremely valuable, because otherwise you're flying completely blind."

Art conservators are constantly negotiating with exhibition designers and the construction team to find materials that fit the museum's budget and the exhibit schedule. That means compromise. Sometimes you just need a test that basically works. "It's sort of like a canary in a coal mine situation—it can really identify the worst offenders. That's hugely important," says Ariel O'Connor, also an objects conservator at the Lunder Conservation Center.

"It can identify the worst offenders… That's extremely valuable, because otherwise you're flying completely blind."

Lastly, they call in some backup. Museums will use another type of material to absorb some of the gases that get loose in a display. These are called sorbents or scavengers, and what they're made of depends on what the object is made of. But you can think of them like the silica gel packets placed in a box of new shoes to absorb moisture. Silver cloth or copper can absorb sulfur, for example, preventing it from reaching the artifact. For most materials, though, there no formula to calculate how much sorbent is needed. It takes a lot of trial and error. "You have to think of it in terms of the whole case, not just one material," Nunberg says.

Over the next few decades, new types of analysis will help conservators noninvasively answer more questions about the chemical composition of artifacts and the materials that accompany them. At Harvard, archaeologists developed a test called peptide mass fingerprinting that can better distinguish organic materials or even different types of resins, something that's never before been possible, and precise enough that it may replace the Oddy test. Lasers and carbon dioxide treatments might provide gentle cleaning methods—soon, we could be brushing gunk off artifacts without even touching them.

The Future Is Scientific(ish)

In the mid-1980s, workers at the Hirshhorn gallery in Washington, D.C., found a sculpture "dripping" in its case, according to a 1985 Washington Post article . The 1920s sculpture by Russian-born artist Antoine Pevsner, called Head of a Woman, was made of dozens of pieces of cellulose nitrate, the same material as early film. The material started out clear and shiny. But as the material was exposed to light and released gases, it shrank and turned brown . At a conservation workshop at the Tate Gallery in London in 2007, another of Pevsner's cellulose sculptures drew collective gasps because it looked like a "plate of Doritos," according to a 2010 blog post .

ARS-ADAGP/Yale University Art Gallery

Art itself does not stand still. Conservators have spent decades learning to clean and protect paintings and pieces of wood or leather, only to find themselves confronted by the challenge of saving newer works made of newer stuff. "The big question now is what we do with the preservation of early synthetic materials," Pouliot says. That applies to sculptures like Pevsner's and those of some of his contemporaries, but it also holds true for objects made of rubber, early photographs, film, and even ladies' combs used in the 19th century. For now, that means making replicas and putting those on display, keeping the originals in cold storage to slow their breakdown. But in the future, these new analytic techniques might help conservators find an alternative way to preserve objects made of these synthetic materials.

Back at the Chapel of the Good Shepherd, Nunberg and her collaborators have a plan to strip away the layers of restoration paint and save Louise Nevelson's sculptures. They just need to figure out how to do it. On a blustery March day, Nunberg and her colleague Soraya Alcala met to test methods for removing the paint. Nunberg used a paintbrush to apply a thin gel to the painted wood, while Alcala held a thicker substance in plastic wrap and held it against the sculpture until it stuck. The thicker substance is easier to remove than the thin, clear gel.

It's taken some in-depth analysis to get here, Nunberg says. Her team has used X-ray fluorescence, Fourier transform infrared spectroscopy (to understand the emissions from a sample), and microscopic cross sections. Before applying the nanogel to remove the paint from the 586 square feet of sculpture in the chapel (a process that will take four conservators about eight months), the team will try it out on mock-ups and perform a large-scale environmental analysis to determine the ecologic impact of whichever solution they choose.

Despite all this, there's a chance that the original paint underneath all the restoration layers won't be as pristine as the team hopes. Nothing lasts forever. Nunberg just wants to make it last a little longer.