PHILADELPHIA — Early in its storied past, the Liberty Bell became a proud symbol of resistance against the British monarchy.

These days, the big hunk of bronze has to contend with more prosaic enemies: airborne pollution and grubby tourist fingers.

So it was that museum tech Jonathan Miller was charged with applying a protective coat of wax to the massive bell's insides last week, working with a cotton cloth and a pair of white gloves after the crowds had gone home.

"It's great," said Miller, 43, of Upper Darby, Pa. "I can tell people that I preserve our national treasure."

The bell's exterior is covered by a motley assortment of unidentified coatings, which seem to offer adequate protection from dirt and any airborne chemicals, said Andrew Lins, chairman of the conservation department at the Philadelphia Museum of Art.

But inside the bell's dark cavity is another matter.

In the early 1980s, Park Service officials started to notice an odd, greenish-white powder developing inside the bell, said Lins, who was called in to study the problem along with a specialist from the DuPont Co.

A chemical analysis identified the powder as ammonium sulfate, which was discovered to have been used as fertilizer on a flower bed outside the old Liberty Bell Pavilion, Lins said. The theory was that fertilizer particles became airborne and became trapped in the relatively stagnant air underneath the bell, where they formed a salty powder on the coppery surface.

The Park Service moved the flowers farther from the entrance, and for good measure decided to ban any ammonia-based cleaners inside the pavilion as well. The same precautions were taken in the new Liberty Bell Center, on Market Street between Fifth and Sixth Streets.

And to guard against any other type of contaminant, the Park Service now regularly coats the interior with wax. This substance was chosen because, while it must be renewed periodically, it is a neutral material that can be applied and removed without harming the surface, Lins said.

Wednesday night Miller waxed only the bottom eight inches of the bell's inside, where visitors sometimes reach in for a close encounter with history.

The Park Service discourages such touching, both inside and outside of the bell, but it happens, said Bob Giannini, Park Service curator.

Miller used a special micro-crystalline wax, made in England to the specifications of the British Museum.

Working carefully but quickly, he had it spiffed up in 15 minutes.

History preserved.