New York City is home to some of the world's best architectureboth ultra-modern and historical. And while newer buildings show off sleek glass-and-steel facades, the hallmark of many of the city's historic buildings is their stonework. Take a gander around the city and you will marvel at just how prominent stone is in so many buildings decked out with gargoyles, cornices, ornately carved facades, and other spectacular stonework ornaments.

The value stone can add to a building is immeasurable in some real estate circles, so it's important that buildings do what they can to restore the look of what they have.

Yet keeping stone looking sharp isn't easy. The city can be a harsh place, and the ravages of time—along with weather and human factors—can change what was once a beautiful piece of stone architecture into something that looks dull, or dirty and corroded.

"New York City is a depository of not just this country's finest stonework, but some of the finest in the world," says Nathan Hafler of Landmark Restoration & Construction Corp. in Long Island City. "Unfortunately, we tend to abuse it and ignore it, and we don't often restore it very well."

In New York City, there are many different types of stone ornamentation, but some are more common than others when looking at the overall landscape.

"The most common building stone here is domestic American limestone," says Daniel Sinclair of DMS Studios in Long Island City, who feels that the best stonework in New York dates back to the period between 1860 and 1914. "Just look at The U.S. Customs House, or the New York Public Library," he says. "Both the interior and exterior have magnificent stonework. As does the Metropolitan Museum of Art."

As for the material these grand edifices were constructed with, Sinclair says it's limestone. "Limestone appears to be the most durable material to withstand the elements here in the New York area—even more so than marble, which is much harder. But for whatever chemical reason, American limestone can withstand the harsh environment and the acid rain better."

Lina Gottesman of the Manhattan-based Altus Metal & Marble has her own opinion on New York's best-loved building stone. "The most common stone is granite, I think," she says. "Granite is the hardest of all the stones we use in building materials. It's long been the choice for a lot of building exteriors, and certainly a lot of the flooring as well. Between 1910 and 1925 there were a lot of granite facades put up," she says. "A lot of steel-front buildings had bases constructed of granite. If you walk around, you'll see that today.You also see a lot of granite used in entryways within the last 10 years as well."

Along with limestone and granite, brownstone used to be very popular—particularly in parts of Brooklyn and Upper Manhattan—but it's not used so much anymore because it has been proven to be a high-maintenance, less-than-ideal material that doesn't necessarily withstand the test of time.

According to columnist Al Heavens of the website RealtyTimes.com, "Brownstone was a popular construction material in post-Civil War America, especially in New York City, and quarries in Pennsylvania, Connecticut, New York and Massachusetts provided just about all of the material.Tastes changed at the end of the 19

Brownstone decay is caused primarily by water, Heavens continues, and by human factors, such as improper sealing and masonry techniques, using the wrong materials for repair work, and painting the stone, which traps moisture and can lead to serious deterioration. Given all that, it's no wonder most stoneworkers and architects favor limestone and granite for their modern projects—and that a robust cottage industry has grown up around repairing and restoring brownstone facades.

Despite their seeming invincibility, there are many things that can hurt building stone like granite and limestone and cause them to crumble, crack and just fall apart.

"There are three primary damaging elements," Sinclair says. "The most damaging of all the elements is acid rain. Think of a piece of stone like a cube of sugar. If you put that cube in hot water, it dissolves. If the rain is acidic, it acts like that hot water, chemically affecting the binding element in the stone."

"More damage has been done to great architecture [like Notre Dame, the Vatican, and so forth] within the last 75 years than the last 1,500 years combined," Sinclair says. "Without intervention, it's very unlikely that all these great buildings will survive. We won't be able to pass these buildings down to our grandchildren—they will melt."

The other two most harmful elements are natural erosion—the steady abrasion of wind and water over time—and human abuse.

Cold weather contributes to the erosion process when it comes to older stone ornamentation on buildings, says Hafler.

"If there's a small crack in it, every time you have a freeze-and-thaw cycle, water gets into the crack and then freezes into ice. The ice pops the crack and makes it wider, because ice expands," he continues. "Whatever bad conditions you have simply get worse."

In winter months, salt and chemicals from snow and ice can be brought into the building and deteriorate all sorts of stone. Gottesman suggests that buildings put mats down during these months to protect their stone flooring.

It's also important for building staff and management to know what kind of stonework their buildng has, and what kind of care it needs. For example, marble flooring and ornamentation should never be cleaned with abrasives or acidic solvents because such chemicals will pit the stone, creating craters that only get worse over time. Make a point to know what type of on-site maintenance your building staff should be carrying out on the building's stonework, and it will last much longer.

As you can see, a lot can go wrong with stone—and that's where the maintenance and restoration professionals come in. It takes more than some spackle and a putty knife to repair something as tough as a rock. Cracks, for example can be fixed, but the process involves something like orthopaedic surgery. "You can repair a crack by drilling a hole, filling it with epoxy and putting in a stainless steel pin on both sides of the crack," Hafler says. "That will stabilize it."

There's a great deal of technology that's been developed to restore stone, and new tools and methods are still coming online. Where once stonemasons and craftsmen had to work with chisels and sandpaper, now they use diamond-tipped drills and sawblades, and use lasers to mark lines. Modern technology has created a semi-permeable sealing agent that can salvage stone that is crumbling and allows water to enter and go out without further eroding the material.

"Prior to ten or 15 years ago, we didn't have diamond-blade capabilities with on-site grinding machines," says Gottesman. "Repair crews would have to work on [the stone pieces] in factories, where they could polish them. The granite would be outrageously expensive, but to do it on-site was virtually impossible until these [new tools] were developed. With the advent of these, it has made it possible to polish granite on floors in the field."

"But," adds Gottesman, "you still can't polish granite walls."

Some organizations specialize in this new restoration technology and can advise on both the mechanical and chemical aspects of restoring. Others simply have artists who can fix existing stonework and craft new pieces from scratch when necessary.

"There is a lot of repair work being done in the city—some well done, some not-so-well done," Sinclair says. "Some repairs, particularly on sculptural elements, don't do justice to distinguishing features, like faces or figures."

His company specializes in both restoring and replicating vintage stone carvings."If someone wants ornamentation on the exterior of their building, we specialize in elaborate stone carvings," Sinclair says. "We can duplicate pretty much the whole realm of sculpture and stone."

DMS recently restored a building on West 86

"The building was built in the early 1900s with a gracious and beautiful entryway," Sinclair says. "As the real estate cycle progressed, there was a period of time when West 86

When it comes to historic buildings, there are different steps that have to be followed for restoring existing stone architecture.

"The problem is, if a building is landmarked through the NYC Landmarks Commission, you're very limited with what you can do," says Hafler. "If you have bad stone, you really have to replace with the same or use an approved application. There are a number of approved products that will mimic the color and texture of the stone, and you are obligated to use those and those only because you want to maintain the architectural integrity of the facade. You have to do it right - and it becomes much more expensive."

Unlike siding or shingles, stone exteriors require a whole range of specific tools and methods for maintenance and repair—items not usually found in the typical superintendent's or general contractor's everyday kit. When it comes to stonework repair and maintenance, it's best to work with a professional versed in the trade who knows the nature and properties of the stone inside and outside your building, who has the tools and experience to do the best job possible.

According to the the National Training Center for Stone & Masonry Trades (NTC) in Asheville, North Carolina, hiring a professional stone contractor—like any contractor brought in for a major project—can be difficult if you don't know what to look for. The NTC offers some points that any management/board team should consider before arriving at any decision:

• Ask around.
Have any neighboring buildings had their marble/tile done recently? Who did the job, and were the board and residents happy with the result? Some local stone/tile trade associations may provide recommendations.
• Gather estimates.
Almost all contractors will perform a free estimate.  Be sure you are there for the scheduled time; if the contractor fails to show for the scheduled appointment without at least calling, he obviously isn't interested in your project.
• Communicate.
Explain to the contractor your concerns and what you are trying to achieve. Give the contractor as much information as possible about what's used to clean a stone entryway or exterior. If it's a new installation, the contractor will also need to know what materials are on the floor or exterior now. Any information will help him decide how to fix the problem.
• Ask questions.
Once the contractor has determined what is needed, ask him to explain the procedure he intends to use. Are there other options? A competent contractor should be more than happy to answer any question you may have.
• Negotiate.
Some contractors will negotiate; others will stick to their guns, although if you mention that you are getting two additional estimates, even a stubborn contractor may sharpen his pencil. Above all, make sure you're comparing apples to apples. If one contractor is only going to polish and the other is going to grind, the difference in price will be considerable.
• Get samples. If possible, obtain a demo or sample of the contractor's work. Ask if a free demo can be performed; have it performed in a representative area. This will indicate what the final job will be like. Be reasonable, however; don't expect a contractor to perform a demo if the job is too small.
• Get references.
Ask for references, and check them. Many contractors in all fields have references, but you'd be surprised how rarely they are actually checked. Call at least three and ask if the contractor did a good job. Were there any problems and did he correct them? Were his employees professional?
• Verify insurance.
Ask for proof of coverage. Have your prospective contractor show you a certificate of insurance, or, if the job is large enough, have his insurance company send you one. Be sure he carries liability and workers' compensation insurance -- any reputable company will carry both.
• Be patient.
Once you choose a contractor, schedule the job —but don't be surprised if the contractor is booked for several weeks. Be patient; a good contractor will be busy, and you will have to wait your turn. At the same time, be sure that both parties agree on a work schedule for the project, and that it's put down in writing. That way, the contractor has a set schedule to go by, and your residents and board don't have to live with an open-ended project.
• Go with your gut.
Are you comfortable with the contractor? This is much more important than you might think. Even the best contractors can make mistakes. The difference between a good contractor and a bad one is the willingness to correct those mistakes.

According to Sinclair, ornamental stonework is dying out in New York City. He blames what he calls "vanilla structures," which are buildings that have been reduced to what amounts to a box with some holes in it.

"Before 1914 and the first World War," says Sinclair, "the cost of material was everything—cost of labor was nothing. After 1914 and the rise of labor unions and workers rights, the formula reversed itself. Now cost of labor is everything. So in conjunction with that, or resulting from that, architectural styles changed. People became unwilling to pay a living wage to a craftsman, so buildings were designed without the charm and beauty that we used to have."

According to the NTC, the expense of hiring skilled craftspeople resulted in fewer and fewer people going into stonework, and now such artisans are in short supply—with little evidence of the formal training required for historical reproduction and restoration projects. Also, the reduced use of stone in building construction in the last 50 years has caused quarries to close because of the lack of orders. This has made once common building stone either unavailable or too costly for restoration projects.

Gottesman doesn't necessarily agree—at least about the use of stone as a building material in new construction and design schemes.

"I think we are seeing more and more [stone] than we have in years," she says. "All the lobbies are going back to the limestone and sandstone, which creates a more rustic look. This has been very common in the past five years."

So whether you live in a rustic brownstone or a towering granite-clad fortress, it's important to remember that even something as strong as stone still needs some care from time to time. Keeping your building's stonework in good repair is every bit as important as other regular maintenance; it maintains safety, preserves beauty, and enhances your residents' investments. And you could say those three commandments—at least as far as boards are concerned—are set in stone.