When you think of an old map or manuscript, you might picture something yellowed, tattered or even torn because of how long it's been around. But millions of historic documents, from presidential papers to personal slave journals are facing an issue apart from age: a preservation method that has backfired.

In a cold, white room on the first floor of South Carolina's state archives, a dehumidifier keeps a mass of old documents safe.

"We try to keep this to be an odor-free zone," says Eric Emerson, director of the state's archives and history department.

Past a row of 10-foot metal shelves sit the department's crown jewels.

"These are the seven constitutions of the state," Emerson says.

From left to right, the constitutions range from those written during the Revolutionary War all the way through Reconstruction.

Emerson's colleague Patrick McCawley is worried about these historic papers. He points to South Carolina's first constitution from 1776. It's stiff, stuck together, with a plastic outer coating like a restaurant place mat.

"Some of these documents like this? We really can't put it on display because it's in such poor condition," McCawley says.

The other constitutions look the same.

"You can see how brown this is coming," McCawley says. "This should not normally be this brown."

For 20 years, beginning in the 1950s, the state laminated documents like this to try to protect them from again. This discoloration is not supposed to be happening — it's caused by a chemical reaction. The natural acids from the paper mix with the degrading laminate to create a noxious vinegar. Each passing year will further degrade the document until it's gone.

"You're effectively forming an envelope where you're keeping the acids in the paper, not allowing them to migrate out," says Molly McGath, a researcher at Johns Hopkins University.

McGath has written extensively about lamination and she says South Carolina isn't the only state having this problem. She says the method was performed around the U.S., and other countries, throughout the 20th century. There are as many as 6 million laminated historical documents.

She says the method was first popularized as a cheap and easy way to preserve them.

"You put plastic on either side, and then you put it through the press and it's done," McGath says. "You had a very high through-put."

McGath says some archives laminated 20,000 pages a year. In the 1960s, approximately 10 years later, curators began to notice a problem: the scent of vinegar. After the 1970s, the method ground to a halt. Now, states are stuck with slowly-degrading documents.

"We are sort of fighting the clock," McGath says.

In Texas, archivists have started scanning their collection, like the 1836 treaty between state commissioners and the Cherokee Indians. In Virginia, the state conservator Leslie Courtois is choosing to remove the lamination entirely. Courtois has spent 20 years delaminating thousands of important, old papers.

"It's tiring, it's tedious, it's very laborious, it's messy," Courtois says.