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Black halo; green granite
PROSOCO and the Nasher Sculpture Center

There was trouble on the terrace.

Seven thousand square feet of South African "Verde Fontaine" green granite had suddenly developed ugly black "halos" around the grout joints.

Until they were clean, the protective treatment couldn't be applied.

But the protective treatment had to be applied before the imminent, un-delayable arrival of several million dollars worth of modern sculpture, set for exhibition on the terrace.

"We were in a terrible time crunch," said Randy Prescott, project superintendent, recalling the complication posed by the unexpected stains. Mr. Prescott's company, the Dallas-based Beck Group, served as general contractor for the $70 million Nasher Sculpture Center.

A downtown Dallas parking lot just a few years ago, the center showcases the multi-million dollar Raymond and Patsy Nasher Collection. It opened to the public Oct. 20.

Mr. Nasher personally financed the center. It occupies a full city block in the Dallas arts district.

Parallel white travertine walls divide architect Renzo Piano's 55,000 square-foot museum into five equally sized pavilions. A glass overhead supported by thin steel rods lets in natural light, controlled by an overlaid custom aluminum sunscreen.

Outside, along with the granite terraces, cobble walkways of the same stone wind through landscape architect Peter Walker's 1.5-acre tree-filled sculpture garden.

"The Verde Fontaine is a nice-looking stone," commented Mr. Prescott. It's a beautiful medium-to-dark green, with black flecks, and light-green highlights."

Masonry contractor Dee Brown Inc., Dallas, had previously cleaned the 50,000 square feet of walkways and terraces with Sure Klean® Vana Trol®.

It needed cleaning, according to Dee Brown's project manager, Rick Troutte.

Along with the excess grout from the installation, workers had tracked dirt all over the terrace and sidewalks.

"They really ground it in," he said.

Dee Brown workers cleaned with Vana Trol® diluted one to 10 with fresh water.

VanaTrol is a good choice for cleaning stone. Its small acid component is strong enough to dissolve mortar and grout smears, but combined with wetting agents, inhibitors and buffers, won't react with the stone's metallic elements.

To maintain the cleaned stone's appearance, specifications called for Stand Off® SLX100, a penetrating, breathable silane water and oil repellent.

"The architect specified SLX because of its ability to weather- and stainproof the stone -- without affecting surface texture or appearance," Mr. Prescott said.

"The stone needs protection from food and wine stains," he pointed out. Mr. Prescott noted that in addition to the terrace seating for the center's small cafe, upcoming social events at the center featuring food and drink also posed a threat to the stone's appearance.

But the SLX application had to be delayed until workers could rid the stone of the unidentified black halos around the joints.

PROSOCO manufacturer's representative Bob Holmes of R.K. Holmes Company, Waxahacie, Texas, tried several stain removers that were effective, Mr. Prescott recalled. But with just one day before the sculptures were due, there wasn't time to order the quantity needed.

"We had a 55-gallon drum of Vana Trol® left over from Dee Brown's cleaning," Mr. Prescott said. "We tried that, diluted one part cleaner to four parts water. It worked perfectly."

With the help of local distributor Silicone Specialties Inc. (SSI), the workers came up with just enough Vana Trol® to re-clean the terrace.

At 6 a.m., two-and-a-half hours before the Saturday morning arrival of the sculptures, the terrace was ready for the SLX treatment.

A crew from Dallas-Fort Worth-based Andreolia Waterproofing applied the treatment with pump-up sprayers. They wiped down the stone with lamb's wool brushes.

"It penetrated fast," Mr. Prescott said. "The application couldn't have been easier." Since the sculptures were due in one at a time, the crew treated the exhibit areas in order of arrival. When the first sculpture showed up at 8:30 a.m., the terrace was ready.

"Rain, snow, wine or hors d'oeuvres," Mr. Prescott said, "now the terrace is ready for them too."