Looking at the history of chemical photography processes shows us how important silver is in capturing scenes. Without this metal, photographers would be at the behest of bitumen (naturally occurring tar) and vegetable juices (phytotypes) to create images.
This light-sensitive material, however, was almost too good – it would continue to work until the silver was dissolved. This is where Hypo comes in. Read on to understand what it is and why it was important.
Cover photo - Leonhard Niederwimmer on Unsplash
What is Hypo?
At its most basic, Hypo is a chemical fixer, needed in the world of alternative photography processes. It was first used in the 19th century to stop UV light from further darkening an image when reaching the desired tonal range. The name hypo comes from hyposulfite of soda, another term for sodium thiosulphate.
Before digital photography, photographers relied on light-sensitive material to capture a scene. This is both true for photography film, and older processes, such as Cyanotypes, Calotypes, and Daguerreotypes. These materials used silver salts (silver mixed with a halogen) to record a scene, which, when processed, reveals the latent image.
Why is Hypo Important?
Once the latent image is revealed, the chemical development of the image continues. Without removing the light-sensitive silver from the material, all further UV light would continue to darken the image. It would reduce the quality of the image to fogging. Fixing isn't needed if the print or image were to stay in a darkroom. What if photographers wanted to show others their work?
Silver bromide works for photography because its atoms react to UV light. In sunlight, the silver turns dark. The atoms decompose and convert to metallic silver. A stronger UV light creates a stronger reaction, and vice versa, giving us an image's tonal range. A Hypo (sodium thiosulphate) breaks down the remaining silver salt and allows them to be washed from the image.
Who Developed Hypo First?
Alchemists knew about light-sensitive salts centuries ago. Johann H. Schulze found darkened letters on a bottle of a light-sensitive mixture when left in direct sun in 1717. Yet, he didn't continue any further into the world of photography. A way to halt the exposure process became important.
Thomas Wedgwood and Humphry Davy captured images in the form of photograms in 1800, yet had no way to fix them in permanent form. Other photographers after Wedgwood didn’t continue the search for a fixer as he and Davy had, in their own words, failed.
Nicéphore Niépce was the first to fix an image, but as his images needed a minimum of 8 hours of exposure, they were very crude. The torch passed on to Frenchman Louis Daguerre through their collaboration. At the same time in England, Henry Fox Talbot and John Herschel used Hypo to dissolve the silver salts in the Salted Print and Calotype processes.