A significant mold outbreak has forced the Boston Public Library to close its Rare Books Department for five to 10 weeks after staff found fuzzy white spores on a medieval manuscript and other prized items in the renowned repository for ancient publications and artifacts.
The library routinely battles mold, but has never faced an outbreak “to this scale,” according to Laura Irmscher, the chief of collections strategy. Mold was discovered on a reference book by a staff member Monday in the main library in Copley Square, Irmscher said.
Staff members then found mold in multiple locations in the Rare Books Department, Irmscher said, although spores were “certainly not on everything,” she added.
Library workers are assessing the more fragile items in the collection, but officials said they are hopeful the outbreak has not inflicted permanent damage on the collection of 500,000 rare books and 1 million manuscripts.
The timing is difficult for an institution that was engulfed in controversy earlier this year when the misfiling of two valuable prints led to the resignation of the library president. Irmscher said there was no connection between the misfiled prints and the mold outbreak.
“This is something that is unfortunately part of life when you are dealing with rare books and rare materials,” Irmscher said. “They have been around for several hundred years and are so susceptible to the climate. We certainly do our utmost to take care of them, but there are conditions that sometimes go beyond our control.”
Mold outbreaks are driven by humidity, which can be hard to control in an older structure such as the McKim Building, which was completed in 1895. The Rare Book Department relies on the building’s central air conditioning system. The staff also monitors humidity and deploys dehumidifiers to combat dampness.
Mold spores can grow quickly. If the relative humidity surges above 70 percent for more than two days it can be enough to cause an outbreak, according to Michele Brown, a Cornell University book conservator who studies mold in libraries.
“A lot of rare books departments have a sealed vault where the temperature and humidity are controlled somewhat separately from the rest of the building,” said Brown, who worked at the Boston Public Library from 1982 to 1985. “I’m not criticizing them. That’s hard to do, and it’s very expensive.”
The Copley Square library is in the midst of a major renovation, which could make it harder to control the environment. A library spokeswoman said the construction may be an issue.
“It was the perfect storm of a lot of conditions,” Irmscher said. “I think the time of the year and the extended humidity really played a significant part in it.”
The Rare Books Department includes two floors of stacks, a conservation lab, staff offices, and a reading room where researchers can request items for viewing. Public access is restricted.
The collection is regarded as one of the world’s best. It includes handwritten sermons published in 925 in France, 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.
There are letters from John Hancock, Samuel Adams, George Washington, and other important figures from the Revolutionary War; a trove of writings by William Lloyd Garrison and other abolitionists; and the Bay Psalm Book, which is the first book written and printed in Colonial America in 1640 in Cambridge.
Other treasures include one of the oldest copies of Shakespeare’s printed works and historic objects, such as the nation’s first Congressional Gold Medal, which was awarded to George Washington.
On Monday, the outbreak was discovered when a staff member went to retrieve the “British Museum Catalogue of Printed Books.”
“He noticed right away there was mold on it,” Irmscher said. “He looked at some others and immediately notified his superior, and we started looking at what exactly the problem was and how widespread it was.”
The library is in a constant battle against humidity, she said, and “there are, routinely, mold outbreaks that have been handled through isolation and containment.”
“It’s common, and it’s something that people are actively monitoring and fighting against, especially in a climate like New England where we have long stretches of humidity,” Irmscher said. “Right now, in September, we are having some of our highest humidity.”
The library enlisted an outside consultant, Polygon Group, to determine the extent of the problem and help with cleanup. Initial estimates put the cost at more than $250,000, Irmscher said.
Officials closed the Rare Books Department late Wednesday to try to prevent mold spores from circulating. Staff deployed 10 high-grade dehumidifiers, and they said they plan to wire electricity to provide power to another 10 dehumidifiers by the end of the weekend, a library spokeswoman said.
The initial goal is to stabilize the humidity level and then decontaminate the materials. Most of the manuscripts are protected from mold in archival boxes. The medieval manuscript tainted with mold was part of a bound collection kept on a shelf, Irmscher said.
The rare books are at greatest risk. Each of the 500,000 volumes must be inspected and potentially cleaned with a vacuum designed to capture mold spores. Officials plan to make items available to researchers once the materials have been cleared.
“It’s a constant battle to make sure that we’re combating what the environment is doing, what the climate is, and what our systems can handle,” Irmscher said. “But, unfortunately, this is part of what a lot of libraries have to deal with.”Andrew Ryan can be reached at email@example.com Follow him on Twitter @globeandrewryan.