As the years pass, the memories of the 1906 earthquake and fire that nearly destroyed San Francisco are fading — and now even some of the priceless photographs of the disaster are being lost, too.

The pictures — 158 irreplaceable records of the ruined city — were taken by Arnold Genthe, one of the most famous photographers of his time.

The collection is endangered. Genthe’s photographs were made using cellulose nitrate negatives, and the negatives are fading and the images on them will soon be gone. The process is irreversible. “They are disappearing before our eyes,” said James Ganz, curator of the Achenbach Foundation for Graphic Arts in San Francisco.

The solution is to digitize the photographs, but that’s an expensive and time-consuming proposition. However, said Karin Breuer, another curator, the photographs are “a priceless record of the earthquake” and what followed.

The cost to digitize the negatives and make prints is about $40,000, and the city’s fine arts museums have begun a campaign to raise the money.

Many of Genthe’s earthquake pictures are famous, particularly a noted shot of families standing on Sacramento Street, dressed in their best clothes, surrounded by damaged buildings and watching a huge fire approach.

A few dozen of Genthe’s best-known pictures from 1906 have been published in books, and many are in the Smithsonian Institution collection. But more than 100 others are in refrigerated vaults at the Legion of Honor museum in San Francisco.

“A majority have never been seen in public before,” said Victoria Binder, who is an associate conservator at the Legion of Honor.

The museum showed a reporter some samples this month — highly unusual pictures of the destruction and rebirth of the city and its people.

There is a rare night shot of a large building ablaze, with flames glowing in every window. There is a moody picture of the entrance to a ruined Nob Hill mansion — the so-called “Portals of the Past,” only this one is taken at dusk and the portals look like something out of an ancient Greek ruin.

There are many shots of San Francisco families living in the parks and in the Presidio Army post, and women cooking dinner on stoves set up on the street. Some pictures show small children made homeless by the earthquake playing together as if they were at summer camp in the park.

Some shots show the city rebuilding, with streetcars on new tracks, with jam-packed passengers hanging on for dear life outside the car.

Genthe, who was born in Germany, came to San Francisco in 1895 to be a language tutor. At that time, the city was a center for art and photography, and he became a self-taught photographer, fascinated by the city. His best-known work was a series of highly regarded photographs of San Francisco’s Chinatown.

He developed a successful photo business, but when the earthquake struck on the morning of April 18, 1906, it was clear to him he was in the middle of a major catastrophe.

His studio was destroyed, along with many of his glass plate negatives. But he could see that the quake was only the beginning. “He could see that his city was burning down before his eyes,” as Ganz put it.

So he went to his regular photo supply shop. The owner told him to take anything he wanted since it was all going to be destroyed by the fire, now burning out of control.

Genthe took a Kodak A3 camera, the top of the line, and stuffed his pockets with the technically advanced cellulose nitrate film. He roamed the streets, photographing the devastation in what is now called real time, never dreaming that his pictures would last more than a century.

Others took hundreds of pictures of the 1906 disaster, including Jack London, the writer. But there was only one Arnold Genthe, and his are clearly the best.

And now they are vanishing. The curators offered examples of the original negatives, and it is clear that the images are slowly fading into nothingness.

The museum has made 3-by-5-inch contact prints of some of the negatives, but the idea is to make large display prints so the photographs can be put on exhibition.

When they are digitized, they can be enlarged in high resolution to capture more detail.

“That way, we’ll be able to look inside the houses and see how the people lived” during those famous earthquake and fire days, Ganz said.

But first, money has to be raised, and the museums are offering an incentive. For $2,500, donors can pick a negative to adopt. For a gift of $250 they can adopt a picture chosen by the museum.

“Our motto,’’said Victoria Binder, “is do something positive for the negatives.”

Carl Nolte is a San Francisco Chronicle staff writer. E-mail: cnolte@sfchronicle.com Twitter: @carlnoltesf

Photo restoration

For details about the project, go to www.famsf.org or call (415) 750-3676.