Built in 1893, the Congress Plaza Hotel in Chicago, IL, withstood years of torment from the elements -- mainly carbon and bird waste. Originally built to accommodate visitors to the Colombian World's Exposition, the luxury hotel's limestone facade on the east elevations of the towers had become blotched and darkened over the course of the century.

"The building had never been restored," said Jerry Carli of Pullman Building Restoration in Chicago. "It was really dark, dingy and dirty."

Carli and four workers set to work on the stains on the east face of the 14-story building, which took them about five weeks to complete.

"The east side of the building is the biggest representation of the building," said Carli. "It faces toward a lake and Michigan Avenue."

The workers began from the top of the building and worked their way down, using all ProSoCo products. They used Sure Klean 766 Limestone & Masonry Prewash, followed by a neutralizing application of Limestone & Masonry Afterwash.

"The alkaline afterwash neutralizes the prewash, and is also used to clean the stone," said Carli. "Though we don't need prewash on some buildings, we used it in this case because of the heavy carbon staining."

The alkaline-based prewash dissolved the staining without affecting the limestone. The stone was pressure-rinsed to rid it of contaminants, then low-pressure sprayed to brighten the cleaned surface. Once the limestone was cleaned, Carli saw that the limestone was in fairly good condition, though it hadn't be cleaned in a century.

"The limestone did have some spalling on the corners, which needed to be repaired," said Carli. "But the color of the stone was good and when it was done, it looked great."