Salt prints are one of the earliest photographic processes. The good news is you can still make them at home with a few chemicals. The bad news is they won’t be around for that long.
People have been able to make “photos” for a surprisingly long time. All the way back in 1727, physicist Johann Heinrich Schulze was developing images of basic geometric shapes in glass bottles filled with the right combination of chemicals. By the end of the 1700s, people were well aware of a number of combinations of chemicals that darkened when exposed to light, and by 1802, people were actively developing images of still objects like leaves and landscapes.
The reason we don’t have many of these images is no one could figure out a way to “fix” the image on the page. Photos developed, and then overdeveloped, and then darkened into obscurity. One of the earliest successes in making these ephemeral images permanent was the salt print. Technically this was only the end-stage of the process. Photographers made negatives, then laid them against the salt print to make the photo—the important thing was they were the right shade and permanent. Silver chloride reacts to sunlight, but chemists weren’t able to use it to cover a piece of paper.
William Fox Talbot came up with an early solution. He dipped pieces of paper in salt water, then silver nitrate. The two together would combine into silver chloride. Talbot would but the negative against them, develop them, and then wash them with sodium thiosulfate to dissolve the unreacted silver and stop the developing process.
Only that didn’t entirely stop the process. Later salt prints were often treated with gold chloride, which fixed them and gave them a warmer tone, but early salt prints, still developed whenever they were exposed to light. Because their negatives no longer protect them from uniform light, they develop all over, darkening and losing contrast with each exposure. Many are no longer visible, and we can only guess at what they were. They replaced ephemeral images—but only with slightly longer-lasting ephemeral images.
Salt prints are still popular. Home photograph developers like the hands-on aspect of them. Even older prints are getting a new life—as a new kind of art. The Smart Museum of Art, for example, has a salt print covered by a red curtain. To see the picture, visitors have to raise the curtain and exposed the picture to the light which is slowly destroying. Which is better—destroying a piece of art to see it, or preserving it by never looking at it?