The building survived the 1836 battle in which the Texans were defeated by the Mexican Army.
But the tourist attraction suffers when visitors lean on its walls or step on the base of its columns, and most of all, when they touch the walls, Alamo officials say.
The rough limestone is already eroding from moisture and salt, but the oil from people's skin further degrades the cream-colored stone and blackens it.
So, for a month, Alamo staffers and historic preservation students from the University of Texas have been applying a mix of water and cleanser with sponges or paintbrushes and then scrubbing the walls with tooth brushes or nail brushes.
The scrubbing takes the dingy walls back to creamy white.
"It's labor intensive," said Sherri Driscoll, a history interpreter at the Alamo who says she can only stand a few hours of scrubbing at a time. "We have to give our knees and back a rest."
Bruce Winders, Alamo historian and curator, said the method cleans the walls without being too aggressive.
The cost of the cleanup has only been a few thousand dollars, but Alamo officials plan to spend more this fall on a poultice of water and acid-free paper pulp to remove salts from the limestone to further preserve the building.
The Alamo, operated by the nonprofit Daughters of the Republic of Texas, is trying to raise $30 million for preservation of the site over the next several years. The estate of an Arizona woman donated $450,000 recently, and some of that money will be used to clean the shrine.
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