It was a hair-raising historical discovery.

A shabby, leather-bound almanac from 1793 sat long forgotten on a shelf at Union College’s library in upstate Schenectady — until an ­archivist surveying some of the school’s collections plucked it from obscurity in December.

The book was noteworthy in itself, as it belonged to Philip J. Schuyler, the son of Gen. Philip Schuyler, a wealthy New York senator who served in the Revolutionary War and was the father-in-law of Treasury Secretary Alexander Hamilton.

But there was something else hidden away inside an envelope tucked between the book’s pages: a lock of hair belonging to George Washington.

“You had to actually open the book and see it there,” marveled India Spartz, head of Special Collections and Archives at Union’s Schaffer ­Library.

“I just think it’s a testament to the deep history of Union College and its connection to the earliest founders of this country. It’s a real honor to have these kinds of things and be able to share them.”

Washington’s iconic hairdo is plastered on every $1 bill and quarter — but contrary to popular belief, he never wore a wig.

He was a redhead growing up and powdered his hair white, a fashionable color in the 18th century.

By the time he became president in 1789, Washington’s locks had faded to a grayish white.

Inside the 1793 almanac, researchers found several strands of the Founding Father’s white hair, which had been held together by a delicate string.

“Washington’s hair, L.S.S. & (scratched out) GBS from James A. Hamilton given him by his mother, Aug. 10, 1871,” the envelope reads.

James A. Hamilton is the third son of Alexander Hamilton and Elizabeth Schuyler.

“As an archivist, we come across interesting material all of the time,” Spartz said.

“But this is such a treasure for the campus.”

So how did this prominent family come to possess a lock of Washington’s hair?

It was likely a gift. “This is something that people in that time period did,” Spartz ­explained.

“He was President Washington, so it wasn’t uncommon for his colleagues and close friends to be given [locks of hair] as a remembrance.”

Union College enlisted the help of a experts to help determine from where the lock might have come.

The family’s connections to Washington were plentiful.

Philip J. Schuyler — whose father served under Washington during the war — owned the almanac itself, titled “Gaines Universial Register or American and British Kalendar for the year 1793.”

His sister, Eliza Schuyler, married Hamilton, who was Washington’s lieutenant colonel before joining the cabinet.

Washington and his wife, Martha, were close to the younger couple.

“In an era when people frequently exchanged hair as a keepsake, it’s quite probable that Martha had given Eliza some of George’s hair, which in turn was given to their son, James, who later distributed it, strand by strand, as a precious memento to close friends and family members,” said scholar Susan Holloway Scott, according to a press release provided by the college.

John Reznikoff, a manuscripts and documents dealer in Connecticut who’s listed in Guinness World Records for having the “Largest Collection of Hair from Historical Figures,” looked at photos of the college’s find — and said he believes the strands are “100 percent authentic.”

He has collected locks from Washington, Abraham Lincoln, Beethoven and Napoleon.

“It’s not hugely valuable, maybe two to three thousand dollars for the strands you have, but it’s undoubtedly George Washington’s,” Reznikoff told college officials.

In 2009, two of Washington’s locks went for a few thousand dollars at auction, according to The New York Times.

Hair from Lincoln’s head sold for $38,837 at a 2012 auction in Dallas.

One remaining question is just how the old almanac hiding Washington’s hair ended up in Union College’s collection.

“We don’t have the true piece of paper that says ‘I’m giving you this book,’ ” Spartz said. “That’s not uncommon.”

That’s one of the reasons why the college has funded a survey of the library’s archival collections — to uncover “hidden treasures” that may be collecting dust, she said.

College officials believe that a member of the Schuyler family probably donated the book at some point, given their close connections to the college.

The elder Philip Schuyler was one of Union’s founders and advocated for establishing the school in Schenectady instead of Albany. His portrait hangs in a campus dining hall, according to school officials.

“We think there could’ve been a Schuyler relative who gave this along the way,” Spartz said.

The almanac was inscribed “Philip Schuyler’s a present from his friend Mr. Philip Ten Eycke New York April 20, 1793.” It includes handwritten notes from Philip J. Schuyler, such as a description of how to “preserve beef for summer’s use.”

Heidi Hill, site manager of the Schuyler Mansion in Albany, told the Times Union newspaper that the book may have been donated for the college’s first Founders Day in 1937.

“A lot of these objects were donated around that time because the families of the Founding Fathers were feeling squeezed out by nouveau riche American society,” Hill told the newspaper, which first reported the college’s discovery.

Spartz is working with a local conservator to figure out how best to preserve the precious lock of hair — and then she wants to display it with the almanac for the public to enjoy.

“Our goal is to make it accessible for people to look at,” she said. “There’s a balance between how we preserve things and how we display them.”