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Montana State Capitol
  Scaling, peeling, eroding, flaking, cracking--after nearly 100 years of the tough Montana climate, the American Renaissance statehouse in Helena was a glossary of stone problems.
  That's according to PROSOCO Technical Director Fran Gale, who served as a consultant during the capitol's exterior restoration cleaning in 1995.
  Designed by Helena architects Bell and Kent, and completed in 1902, the compactly massed five-story main building was built of local sandstone with a base of Montana granite, and crowned with a copper dome. Matching east and west 4-story wings of Montana granite were added in 1909.
  Ms. Gale, and the project's historical architect, James McDonald of A & E Architects, Billings, Montana, inspected the statehouse exterior in 1994. They found severe water-related deterioration on and beneath the cornices and in the sandstone just above the base. Lab testing showed high levels of clay in the stone.
  "That's a problem for sandstone," Mr. McDonald commented. "Water gets in and swells the clay. It can literally explode the stone."
  Water also corroded iron-containing minerals in the sandstone, causing more spalling.
  In addition to the damage, there were copper stains from the dome, decades of pigeon waste, and efflorescence from naturally occurring salts in the sandstone, Ms. Gale said.
  Although the wings showed less deterioration than the original building, according to Mr. McDonald, they had their own stain problems. Years of water penetration reacted with the granite's high iron content to mobilize reddish stains.
  "It all cleaned up very nicely, though," Mr. McDonald recalled.
  Workers used Sure Klean® Restoration Cleaner. They brushed it on after prewetting the stone with clean water. During a brief dwell time, the cleaner broke the bond between soiling and stone. The decades of contaminants were then pressure-rinsed away.
  Decayed sandstone on and under cornices and above the base was strengthened with Conservare® OH Consolidation Treatment. The treatment stabilizes masonry by replacing the natural binder the stone lost to time and weather with similar silicon dioxide.
  Weather Seal H40, a penetrating, breathable waterrepellent with stone-strengthening characteristics was applied to the entire sandstone portion of the building.
  Although isolated sections of the granite base showed some spalling from constant water exposure, Mr. McDonald commented, it wasn't weather-related. And the solution was simple.
  "We just moved the sprinkler system away from the building," he said.