How long do
photos last?

A very long time, if they were processed well and are kept cool and dry. The Canadian Conservation Institute (CCI) says well-processed silver gelatin prints on fibre-based paper are essentially stable to light, and that there is no published evidence that light alone makes black-and-white images fade or discolour.

What harms them is chemistry. CCI says leftover processing chemicals contribute to the decay of silver images, and that pollutants such as hydrogen sulphide, peroxides, ozone and sulphur dioxide discolour the image when combined with high humidity or heat.

There are two exceptions to watch for.

Because the picture is made of dyes, and once a dye breaks down the loss is permanent. CCI says the dyes in colour photographs appear to be the only visual art materials that fade appreciably in the dark, and that colour photos can build up stains in the dark too, such as white areas turning yellow. Colour materials are also more sensitive to humidity and heat than black-and-white ones.

The US National Archives says colour negatives, slides and prints are vulnerable to fading and deterioration within decades at room temperature. The Library of Virginia notes that chromogenic colour prints, made from about the 1940s to the present, may have a yellowish or reddish cast once faded.

Cold slows it down. The Library of Congress recommends 4°C (40°F) or below for colour photographs and film negatives, and CCI calls cold storage the most effective preservation measure for large collections of colour photographs. At home, the practical version is the coolest steady room you have; see how to store old photos.

Film has two clocks. In colour film the dyes fade, and in most film the base itself slowly decays. A great many films made since the 1920s are cellulose acetate, and the Image Permanence Institute (IPI) says amateur roll films marked “Safety” are likely to be acetate, apart from a few recent ones. Acetate breaks down in a reaction called vinegar syndrome: a vinegar smell first, then buckling, shrinking and brittleness. IPI’s predictions for fresh acetate film show how much the room matters:

Those figures mark the start of the problem, not the end of the film. IPI says that at the onset the film smells of vinegar but is still usable, and that decay then speeds up. The National Archives (UK) says film negatives can give off acidic gases as they age, so keep them away from prints, and CCI advises duplicating valuable slides and projecting the copies. Our guide to digitising slides and negatives explains how to rescue them.

Not as long as you might hope. The New York State Archives says prints made at home are not permanent, even on a laser printer with “photo quality” paper. Inkjet inks are more stable than they used to be, it says, but inkjet printouts will probably fade or change colour over time, and only black-and-white laser printouts will likely last for decades.

It also warns that some photo lab prints won’t stand the test of time. For prints meant to last decades, it suggests a lab that prints on an archival medium, PVC-free enclosures and a dark, cool place, with home prints kept for frames and display.

As long as someone keeps copying it. A file doesn’t fade like a print, but the drive, disc or online account it lives on can fail, wear out or close, and the New York State Archives says there is no certain way to predict how long any one drive, disc or tape will last.

Our guide to backing up family photos turns this into a routine.

Ancestorii isn’t a backup service, so keep your own copies as above. What it adds is a home the whole family uses. Scanned photos go into your family’s private Library and albums alongside the names, dates, stories and voice notes that explain them. Everyone you invite can see them on the website, and albums open in the free iPhone and Android app too. The knowledge stops depending on one shoebox, one drive or one person’s memory.

When you want something on paper, a Memory Book is an A4 landscape hardcover, from £44.99 plus delivery, made from your photos and written words. Books ship to 20 countries.