Walter Cronkite’s news broadcasts are among the videotaped programs that the Library of Congress is digitizing. (CBS PHOTO ARCHIVE/GETTY IMAGES)

At a Library of Congress facility in Culpeper, an effort is underway to convert old videotapes into digital files, ensuring the long-term survival of a host of 1950s-through-1970s TV shows, including the “CBS Evening News” with Walter Cronkite and “Rowan & Martin’s Laugh-In.”

Preserving these shows turns out to be a challenging and time-consuming task. But unless the videotapes are transformed, experts say, future generations will have a diminished appreciation of the era of JFK, flower power and Watergate.

Two-inch-wide quadruplex (or quad) videotape, which was the TV-industry standard from 1956 through the late 1970s, was never meant for long-term storage of sound and images. Developed by Ampex, a company based in California, it allowed network shows to be recorded while being broadcast in New York and then played back later the same evening for West Coast audiences.

Thrifty producers were grateful that videotape could easily be erased, then reused.They were slow to realize that the initial recordings might have value in the distant future.

The videotapes have delicate coatings — essentially “polyurethane paint with magnetic particles inside it,” says Jim Lindner of Media Matters, which specializes in transferring videotaped material to more stable formats. Over time, these coatings absorb moisture, grow sticky and sometimes separate from their backing. With every fleck that peels away, Lindner says, “a bit of recorded history does, too.”

At the Packard Campus of the Library of Congress’s National Audio-Visual Conservation Center, technicians often “bake the tape” in a 130-degree oven for days to resolidify these loose coatings. That is just one of the difficult steps in the tape-to-digital conversion.

Once resolidified, the tapes can be played back only on old video players as part of the conversion process. Doing so entails a risk that they will snap in two as the players’ magnetic heads whir across them at 88 mph. If the head encounters a bump, says Packard video-lab supervisor Paul Klamer, “it hits it like a Mack truck and saws directly through the tape — zing! These are scary to play back.”

Much of our video heritage is already lost to history. “A lot of things happened culturally because of TV, but in many cases we no longer have those tapes,” Lindner says. “What we have now is just what was left over” after routine erasures and discardings.

For example, the Vietnam War played out on nightly network news shows, but “we have very few [tapes of those shows] today,” according to Lindner. Did late-night humorists contribute to changing social mores in the ’60s? Hard to say, since episodes of “The Tonight Show Starring Johnny Carson” from 1962 to 1970 have almost totally disappeared. “It was only after Carson secured the rights to the show from NBC that he insisted on keeping copies for subsequent clip licensing,” says Mike Mashon, who heads the moving image section at the Packard Campus.

The archives of one major network, which Lindner declined to identify, contain fewer than 3,000 tapes of evening news broadcasts from the heyday of quad videotape; he says that is less than half of the shows that aired.

Because an hour of programming required nearly a mile of quad tape — four times more than today’s tape requires — preserving it is a major undertaking: Every inch must be carefully cleaned and inspected before playback is attempted.

And the tape can be played back only on pre-1980 equipment. “Keeping the equipment running is a big part of the problem,” Klamer says. If a part breaks in the Ampex machines that are required to play the “Laugh-In” tapes, a replacement can be almost impossible to find. Today there are barely 100 working Ampex units in the world, Klamer says, making them rarer than Edsels. Packard owns 27 of the machines (original price $100,000), only two of which are operational.

In fact, a shortage of parts may prove to be the most difficult problem for anyone interested in watch early videotapes. In storerooms heaped with obsolete technology, Packard staff members cannibalize parts from old video players. Only one company that refurbishes magnetic heads remains in business, charging about $5,000 per head.

The Library of Congress is hoping eventually to convert all of its 700,000 tapes to a digital format. Most of its holdings were deposits required by law: Any movie or TV show that is copyrighted must have a copy donated to the library. Once converted, the original tapes are kept for posterity.

Others are also trying to save tapes found in garages and basements, where conditions hasten their decomposition.

Working as a consultant to museums, corporations and other clients, Lindner often encounters discouraging masses of unlabeled tapes. In the 1990s, he came upon a tape belonging to a news company that bore the cryptic label “Resignation/Disneyland.” Was this tape worth spending $400 to convert? He nearly passed it over — but it turned out to be a speech that Ronald Reagan, then the governor of California, gave at Disneyland in 1974, in which he commented on Richard Nixon’s resignation that year as president.

Chris Lewis, a son of the entertainer Jerry Lewis, is working to preserve his father’s extensive quad tape collection, including a 1959 NBC show in which his father starred. Unseen since the Eisenhower era, the tape was “in a very delicate state,” according to David Crosthwait of DC Video, which is handling the digitization.

Chris Lewis calls “The Jazz Singer” — a TV remake of the famous 1927 film that ushered in the “talkie” era — especially rare for having been broadcast in color and for showing Jerry Lewis “in his first dramatic role.”

These efforts notwithstanding, quad tape “is a very endangered species,” says Ken Weissman, supervisor of the film preservation laboratory at Packard. “And things are absolutely going to disappear.”

Maynard is a lecturer at Princeton University and the author of five books on American history and architecture, most recently “Princeton: America’s Campus.”