For a national icon, the Liberty Bell in Philadelphia has been through some tough times.
No sooner was the new bell hoisted into its tower in the State House of the Province of Pennsylvania (now Independence Hall) than it cracked. Probably the unusually high percentage of tin in the bronze alloy made the bell brittle.
The bell was melted down and recast with extra copper in the mix, but the result was a dud, a bell that sounded awful. So it was melted down again. The second recasting proved satisfactory, and in June of 1753 the bell was back in business.
But then it cracked again -- this is the famous fracture that gives the Liberty Bell its distinctive profile. No one knows exactly when this happened, although it was certainly in the early 19th century. We do know that the bell rang for the last time on Feb. 23, 1846, to celebrate Washington's birthday.
Since then, the bell has been displayed at various locations in and near Independence Hall as a tourist attraction. On Jan. 1, 1976, it was moved to a glass-walled pavilion on Market Street, where it's visible to all comers 24 hours a day.
You might think that housing the bell in a climate-controlled building would solve all its problems. Not so. A few years after the pavilion opened, curators at Independence National Historical Park, who inspect the bell every two weeks, noticed something curious. A white powder was forming on the bell's inner surface, mainly up under its cap.
Over the next several years, the powdery deposit became more extensive. Finally, in 1983 the Park Service called in some of its own conservation experts and consultants from DuPont and the Philadelphia Museum of Art. They were given the task of figuring out what the substance was, how it was being formed, how to remove it and whether it was damaging the metal permanently.
They were only partially successful. The powder was identified as ammonium sulfate, but the experts couldn't say why it was building up on the bell, although they theorized that temperature fluctuations created by direct sunlight were a factor. They didn't find any evidence that the chemical was damaging the bell, though.
Nevertheless, the Park Service eliminated all cleaning products containing ammonia from the Liberty Bell Pavilion, and even checked the fertilizer used on the lawn outside.
Andrew Lins, a conservation specialist at the art museum, cleaned the inside of the bell thoroughly and sealed its surface with two coats of wax, soft over hard. In 1986, the Park Service installed a fiberglass screen on the south side of the pavilion to prevent direct sunlight from heating up the metal.
So far, those measures have worked just fine. The bell has remained powder-free. But that's not the end of our story.
Every year the Liberty Bell receives about 1.5 million visitors, many of whom want to lay their hands on it. The Park Service discourages touching the bell because every fingerprint contains moisture and corrosive salts. But touching isn't prohibited, and many visitors lean over the ropes and reach up under the rim.
The Park Service doesn't prohibit touching because it realizes that the bell is such a powerful symbol of American ideals that some visitors simply must put their hands on it. "We want to provide as much access as possible," said Karie Diethorn, chief of the park's museum branch.
Yet thousands of such touches every week soon wear off the interior's protective wax. So every year, the curators have given the bell a thorough cleaning and have renewed the wax on the bottom eight inches of the inner surface.
They don't wax the outside, according to Diethorn, because many more hands are laid on the outside, and the wax would wear off in a week. And the powder buildup, which the wax prevents, never appeared on the outer surface.
Beginning this year, the bell will be treated twice a year, to ensure that the wax barrier remains effective.
This year's first cleaning took place a few weeks ago. The curators washed away accumulated dirt, dust and the old wax with a mild detergent and tepid water, then dried the surface with a hair dryer set on cool before applying the wax. The whole process, done after visiting hours, takes about two hours. Other than a weekly dusting, that's all the care the bell needs. If visitors to the bell could only resist the urge to fondle it, America's premier symbol of liberty would never need waxing again.