The New York Times

At the intersection of Flatbush and Atlantic Avenues in Brooklyn, dozens of men and women in hard hats are scurrying about each day, hanging this and attaching that, preparing the Barclays Center for its opening, which is now just a few weeks away. The lights have come on inside, some treetops are visible along the arena’s edge, and some bright blue signage has gone up along the building’s facade, an undulating shell of brownish steel.

After years of building, and even more years of bickering, the arena is almost finished — but this is not immediately obvious to all those who wander by.

“Is it meant to be that way, with the rust?” one woman asked, squinting at the steel.

“I thought they were going to paint it,” said a man who stopped to stare.

No, they are not. For the facade of the Barclays Center, more traditional materials were rejected in favor of 12,000 separate pieces of what is called “weathering steel,” and that leathery brown hue, which is the arena’s final finish, is not paint but an intended layer of rust.

Weathering steel — often known by its old brand name, Cor-Ten — develops a fine layer of rust, which then acts as a protective coating against moisture, slowing its own corrosion process almost to a stop. While it can look suspiciously unfinished to the casual observer, it has many fans in the world of art and architecture.

This industrial, raw-looking material can be seen on a smattering of homes in and around New York City, and though they may be vastly different in design, scale and method of construction, they all have one thing in common: a fiery apron of orange on patches of the pavement below. That is because especially in its early life, weathering steel drips.

“When the material gets wet, there is a rusty wash that goes down onto adjacent areas of concrete,” said Michael Devonshire, a materials expert at Jan Hird Pokorny Associates, an architecture firm. “It can get really funky looking.”

The East of East condominium building in Long Island City, Queens, for example, uses thin slabs of corrugated weathering steel, applied as a scrim over the building’s envelope. The slabs have rusted to a deep burnt orange, staining sections of the surrounding sidewalk.

But Andrea Salerno, who recently rented an unsold unit with views of the Manhattan skyline, pointed out that the rusty stains on the sidewalk have plenty of competition from less pedigreed grime.

“This is New York,” Ms. Salerno said. “It’s not exactly pristine.”

Property owners are responsible for maintaining their sidewalks, but a spokesman for the Department of Transportation said it would not generally issue a notice of violation for a big orange splotch. If those sunset hues are not to your liking, however, Jeremy Stitt, a project manager at Dissimilar Metal Design, which has worked on the Barclays Center project, said afflicted sidewalks could be cleaned with typical rust removers.

Amol Sarva, the developer of East of East, said he planned on tidying up the sidewalks by pouring new ones. (His architect said that a gutter system was designed to catch the rusty drips, but they were not properly installed). There are those, however, who prefer to leave the drips where they drop.

“I kind of like the lines it makes and the marks, that russet red,” the architect Matthew Baird said of the colors left by weathering steel just outside a town house he designed in Manhattan’s meatpacking district. “It looks good.”

The town house, which stands on Greenwich Street just south of Gansevoort Street, has a solid piece of weathering steel stretching from the base of the second floor to the top of the four-story facade. It is 40 feet tall, 14 feet wide and an inch and a half thick.

The home, which is surprisingly bright inside, thanks to a rear wall of glass, was built about six years ago, and Dr. Neal M. Blitz, who lives across the street and looks directly at the steel plate from his living room, said passers-by still photograph it daily. Every one of them, he added, seems to have an opinion.

“People either say, ‘This is really amazing,’ or ‘Why would they do that?’ ” Dr. Blitz said.

Though there is plenty of weathering steel on display in the New York area, from highway supports to the Ford Foundation headquarters, residential examples are not plentiful. Architects suggest that the reason might be that the material can be tricky to work with and the detailing must be done precisely. For example, if the design allows water to pool in a given spot, the building is likely to end up with holes in its facade where the rust has eaten all the way through.

Brian Messana, a partner at the architecture firm Messana O’Rorke, who built a weathering steel extension on his own country house in Columbia County, N.Y., mentioned another big drawback.

“It rusts,” he said. “In the city, you don’t want it to rust onto somebody else’s property.”

Mr. Messana said he considered the material to be quite beautiful, describing it with words like modern, warm and tactile. But he has reservations about it, even in his own country home.

“It stained some of the glass,” he said. “Just a couple weekends ago I was thinking, ‘I should really figure out how to get that off.’ “

To fend off some of the headaches, the steel on the Barclays Center was weathered before it ever made it to Brooklyn. Gregg Pasquarelli, a principal at SHoP Architects, which designed the arena, said the steel components spent about four months at an Indianapolis plant where they were put through more than a dozen wet-and-dry cycles a day. (Mr. Pasquarelli said the arena looked to him like what would happen if “Richard Serra and Chanel created a U.F.O. together.")

The process put about six years of weathering onto the steel, according to Robert Sanna, an executive vice president and director at Forest City Ratner, the developer of the Barclays Center. So while there probably will be some rusty dripping, Ms. Sanna said, “this should keep it to a minimum, and you won’t have to worry that it will stain your sweater as you walk by.”

In addition to its texture, its dynamism and its drippings, weathering steel has another dimension that was richly on display at the Greenwich Street town house last week. Against the steel’s shimmering grays and giant streaks of reddish rust, there were a dozen or so tiny black circles, squares and triangles, each no more than an inch across, clinging to the lower portion of the slab.

They were magnets, presumably tossed onto the facade by a mischievous passer-by.