Burbank (United States) (AFP) - Reels of film and the Hollywood stars who fill them share one common enemy: aging.

But while an actor can go under the knife or get a bit of filler in an effort to stay young, it's a one-way street for film, which eventually starts to break down into its original -- rather prosaic -- ingredients. 

"Film base is actually wood pulp and acetic acid in its simplest form," says Tim Knapp of California-based film preservation specialists Pro-Tek Vaults.

"Acetic acid over time produces what is called 'vinegar syndrome' which degrades the base of the film... and prevents it from being used."

And no film star wants to end up like that.

Movie-making has gone through a number of evolutions as directors sought a way to immortalize their leading men and women.

When the industry was born at the start of the 20th century, pioneers like Buster Keaton and Charlie Chaplin were captured on nitrate film, a medium capable of capturing deep blacks, infinite shades and sharp lines.

But studios quickly noticed a significant drawback: nitrate is highly flammable.

Projection rooms had to be fireproofed in an effort to avoid the kind of blazes that killed dozens of cinema-goers in the 1920s.

Even when not in use, nitrate film was not safe -- with a relatively low flashpoint, it could ignite if the room it was stored in became too hot. Huge fires at film storage sites in 1914 incinerated much of America's early cinematic history.

Acetate

The introduction of acetate film in the 1950s was a cause for celebration among movie executives and cinemas alike; a material that allowed directors to capture images in life-like resolution without the danger of it catching fire.

The problem is that it doesn't age well, and -- if not looked after properly -- in as little as 15 years it can turn into an unusable reel of plastic that reeks of vinegar.

For a movie company that has spent tens, or even hundreds of millions of dollars on a film, that's bad news.

"Keeping film in the proper environment ensures its longevity," said Doug Sylvester, CEO of Pro-Tek Vaults.

"That allows you to have a pristine, often original copy that can be used to make additional prints and digital copies over time."

TV and movie companies are increasingly looking to their back catalogues for sources of revenue, whether that is licensing clips for commercials, a reformatted re-release -- think of the number of times "Star Wars" has come out -- or the wholesale resale of titles to a streaming service.

While many movies and TV shows are now recorded digitally, a number of top-flight directors like Christopher Nolan and Quentin Tarantino still insist on using film, whose 12K resolution still trumps even the very best digital reproduction.

Old and new films all have to be stored -- with the utmost care and under tight security.

High security

Around a million reels of Hollywood history sit coiled in metal cans in top-secret temperature- and humidity-controlled units in Burbank and Thousand Oaks, just outside Los Angeles.

Huge moveable shelves are filled floor to ceiling with tens of thousands of hours of movie magic -- alongside legendary television shows, footage from presidential libraries and music videos.

Closed circuit cameras watch over the approximately 1.5 billion feet (almost half a million kilometers) of film to ensure that no one makes off with the original negatives from an Oscar winner.

Sylvester's company is cagey about what titles they have in their care, but promotional posters from films including the original "West Side Story," "Back to the Future II" and Tim Burton's "Nightmare Before Christmas" cover the walls.

Silvester said his customers are "very particular about mentioning the titles that we hold."

"But I can say that there are some classics, if you were to look at.... the American Film Institute's 100 greatest films of all time, you would see many of those here in our inventory."

The company is also involved in the cataloging and digitization of material that production houses might not even be aware they have in their own storage units.

That has included a project with record label Universal Music Group that unearthed never-before-seen footage of a Guns N' Roses concert, as well as restoring classic videos from the likes of Johnny Cash, Bon Jovi and The Cranberries.

Sylvester says uncovering hidden gems like these and then working to keep them safe is a rewarding task.

"It's part of our cultural history, and (we) love to play a part in preserving it for the future."