Hypo is the common name for a photographic fixing bath made with sodium thiosulphate salts, historically known as hyposulphite of soda. It is used in film and print processing to remove unexposed silver halide crystals, making the image permanent and light-stable.
Proper use of hypo is essential for achieving archival-quality photographs, as it prevents future image degradation and ensures long-lasting results. Photographers working in darkroom processing must handle hypo carefully, following safety guidelines, while understanding its role in stabilising both black-and-white and colour images.