Every ten years, the bronze sculpture of William Penn on top of Philadelphia's City Hall must be cleaned and refinished. For the next few weeks, scaffolding with mask the iconic statue perched 500 feet above the city, as workers give him his regular grooming. 

From street level, the statue of William Penn may appear to be about the size of a person, but it's actually enormous. At 37 feet tall, weighing 53,000 pounds, it is the largest bronze sculpture on top of any building in the world.

Artist Alexander Milne Calder cast this sculpture in 1888 with remarkable detail. Up close, one can see the textured nap of his coat, and the basket weave design of his buttons.

"The details in his clothing, the cuffs of his sleeves, the scarf around his neck, the curls in his hair," said Margot Berg, the city's director of public art, while wearing a hardhat and standing eye-to-eye with William Penn. "For something that was going to be seen from the ground, 500 feet away, it's an incredible labor of love."

As a towering symbol of Philadelphia, Billy Penn attracts city folklore.  Was it his curse that thwarted our professional sports teams for generations? He also endurse his fair share of weather. Year after year, exposure to severe heat, cold, and air pollution damages the metal surface. Once a decade, since the 1980s, he gets a cleaning, a new coating, and a look under the hood.

Most people don't know this, but the top of Billy Penn's hat is a hatch door, allowing you to literally crawl inside his head.

"We're in the chest interior cavity of the William Penn sculpture," said Constance Bassett of Mooreland Studios, who, with her husband David Cann, was hired to conserve the statue. She crouched inside it, underneath what would be his collarbone. "Behind you is the right arm, flange-jointed together."

For more than a century, workers have scrawled their names inside the statue. The graffiti — mostly in chalk — are each dated, going back to the 1920s. Hidden inside the city's most recognizable sculpture, workers left marks solely for future generations of those doing this repair work.

From this point of view, you can see how the sculpture was constructed: 47 individual parts were cast in sand, just like the Liberty Bell, and bolted together by their flanges.

"I don't think any foundry in the world could do this today — this internal flange-bolting system," said Cann, marveling at the 19th century craftsmanship. "For a piece this big newer techniques are used, but to go back to this level, I don't think anybody could do it."

The conservators respect those original techniques, but they don't mimic them. After 125 years, metal science has improved greatly.

For example, the surface of the statue is pocked with tiny holes, where decades of acid rain eroded the zinc out of the bronze alloy. To soothe that, Bassett uses coatings developed with cutting-edge technology.

"Because it's built in sections, there is some movement, and the wind loads on the top of the tower are such that everything needs to move," she explained. "The coatings are a real issue. They are designed to be flexible. That's something we're looking at in the future, what new coatings can we apply to this project."

Because of the expense and difficulty, the city conserves William Penn only once a decade. This time it will cost about $278,000, some of which came from the National Endowment for the Arts, for the sake of protecting a national treasure.