Museum uses micro explosions to save fine art from bird poop

(Washington Post) - Science, Labour and Art had been living outdoors for more than a decade. They endured autumn rains and winter winds. In the springtime, pollen clung to their bodies and clothes. In the summer, beads of condensation streamed down the layer of wax that covers their motionless forms. Regardless of the season, uncouth humans touched them without permission. Occasionally, a bird pooped on one of their heads.

Such indignities are an occupational hazard of being a sculpture on display in a museum garden. The allegorical trio once graced the outdoor courtyard of the Smithsonian's Freer Gallery of Art; a companion trio shows the embodiments of Love and Justice supporting a figure that represents Law. They were carved by Augustus Saint-Gaudens, the celebrated American artist, and the figures all wear slightly disdainful expressions, as though they know they were made by a master sculptor and are too good for these reduced circumstances.

The Freer's exhibits conservator Jenifer Bosworth wasn't too concerned about the figures' bad attitudes. "That's just nature," she said cheerfully, pointing to the bird droppings in the hair of the figure representing Art. "(Works of art) could stay in storage in the dark locked away, and then they could last for much longer," she acknowledged. "But that's not the objective of a museum."

Still, it was undeniable that the bronze statues were due for a cleaning. And since the Freer is approaching the end of a two-year renovation, now seemed an ideal time to do so. On a sweltering morning last week, the Saint-Gaudens sculptures were carted out in front of the museum for a surprisingly hardcore kind of bath.

"We're going to be blasting them with dry ice," Bosworth said with a laugh. "Which sounds harsh, but...it's a comparatively delicate method." Yes, the most "delicate" way to spruce up century-old sculptures is to hit them with pellets of dry ice at supersonic speeds. That's the science of fine art conservation for you.

The formal name for the technique is "carbon dioxide cleaning", and it actually has all kinds of applications: removing particles from electric circuits, cleaning telescope mirrors, preparing laboratory samples, manufacturing metal parts. But at the Freer, the objective was to scour away the protective wax layer that coats the Saint-Gaudens statues. This would allow conservators to clean and polish the bronze underneath before applying a fresh wax coating and placing the figures back on their pedestals.

This was the first time the Freer had attempted to clean a work of art with carbon dioxide ice, so the museum brought in a pair of experts. Mr Thomas Podnar, senior conservator of sculpture at Ohio's McKay Lodge Fine Art Conservation Laboratory, and Ms Christina Simms, the company's conservator of sculpture and objects, drove up early with a huge cooler of dry ice and a powerful blasting system. They conferred with Ms Bosworth and then everyone donned earplugs.

The minute Mr Podnar switched on the blaster, it was evident why. The blasting machine roared to life with a noise like an airplane taking off. The proceedings were so loud that someone working inside the nearby Smithsonian Castle came out to inquire about what was going on.

The cacophony came from the blaster's engine, which sent pellets of carbon dioxide rocketing through the attached hose and then shooting out the hose's nozzle. They struck the bronze forms of Science, Labour and Art with enough force to crack the wax coating and rub off some of the dirt and dust.

But that was just the first part of the carbon dioxide cleaning process. Part two came from the heat transfer that took place as the ice pellets made contact with the sculpture's hard wax coating, which was warm from the July sun. The shock of cold made the wax layer quickly contract, while the bronze underneath chilled more slowly. "That differential heating causes a loosening of the layers," Ms Bosworth said. As wax separated from the rest of the sculpture, it became easier to remove.

Now it was time for part three: micro explosions.

As the frozen carbon dioxide heated up, it sublimated, or transitioned straight from a solid into a gas. This phase transition was instantaneous and intense - in just a fraction of a second, the carbon dioxide expanded to 800 times its former volume. "This small micro explosion...helps to blow away the surface layer that has already been cracked by the impact," Ms Bosworth said.

All of this happened quickly and cleanly, which partly explains why carbon dioxide cleaning has become popular with art conservators in recent years. Older techniques relied on toxic solvents, which posed a hazard to conservators and the environment.

But the technique is not without risk. The friction of blasting generates static electricity, which accumulates in the object being cleaned. Mr Podnar used metal clips and a wire to ground the sculpture as he cleaned it, so the built-up energy would flow out of it into the earth. But if a conservator forgot to take this precaution, "you could get a really big shock", Ms Bosworth said.

No one was shocked during the cleaning effort at Freer - except, perhaps, the gaggle of passing Smithsonian summer campers, who stood transfixed by the process until their counselors herded them away.

By the end of the week, the Saint-Gaudens statues had been scoured, cleaned and re-waxed. Soon they'll be reinstalled in the Freer courtyard, where they can be found when the gallery reopens on Oct 14.

"I'm really happy with how it turned out," Ms Bosworth said on Monday. "They're looking much more cleaned-up and happy and much better than they were." Their world-weary expressions don't look any brighter, she noted. There are some things even carbon dioxide micro explosions can't fix.