It took a crew of 20 roughly 7½ hours a day over 10 weeks, but the mold is gone.
The Boston Public Library's Rare Books Department reopened Tuesday after an intense cleaning to eradicate fuzzy white spores that speckled a medieval manuscript and other irreplaceable artifacts.
The mold was discovered on a reference book by a staff member in September. The collection includes 500,000 books and 1 million manuscripts, none of which sustained significant damage.
"Every book in the collection was cleaned," said Laura Irmscher, the library's chief of collections strategy. "It was comprehensive."
The crew used a special HEPA vacuum at low speed with a gentle brush. The outside of each book was vacuumed, rubbed with a dry sponge, and vacuumed again. Workers also cleaned the outside of archival boxes and scoured all shelves, walls, floors, and vents.
The cleaning has already cost roughly $325,000, Irmscher said, and the library still plans to install new carpet. Officials also replaced upholstered seats with wooden chairs.
The Rare Books Department, which has limited public access, includes two floors of stacks, a conservation lab, staff offices, and a reading room where researchers can request items for viewing. It is regarded as one of the world's best such collections.
Treasures include a Latin dictionary, circa 1460, that is believed to have been printed by Johannes Gutenberg, and five inscribed Babylonian clay tablets dating from 2350 BC.
Mold outbreaks are driven by humidity, which can be hard to control in older structures such as the library's McKim Building, which was completed in 1895. The Rare Books Department relies on the building's central air conditioning system.
The Copley Square library is in the midst of a major renovation, which made it harder to control the environment. Construction may have been a factor in the mold outbreak, Irmscher said, but ultimately it was the cumulative effect of a long, humid summer.
"The books are like sponges; they absorb moisture," Irmscher said. "Under the right conditions, mold, it can move very fast, which is what we experienced in September."
The library consulted specialists at Harvard University, Yale University, Boston College, and the Northeast Document Conservation Center to determine how to prevent future mold outbreaks. In the short term, officials added temperature and humidity monitors that send alerts when temperature and humidity spike. The data will be logged so officials can identify trends and take preventive measures.
The library also anticipates overhauling the air conditioning system to protect materials in the Rare Books Department.
It was a welcome relief Tuesday when the department reopened.
"There was not a crowd waiting," Irmscher said, "but we did have an appointment first thing."
The first patron arrived about 10:30 a.m., officials said. He examined a handwritten manuscript containing genealogical information about the Clap family from 1634 to 1695.