A staining developer is a type of photographic developer in which the chemical oxidation products not only reduce silver halides to metallic silver but also stain the gelatin emulsion, adding extra image density. An example of such a developer is pyro, which produces a brownish stain that enhances the perceived contrast and depth of the image.
Staining developers are particularly valued in processes where subtle tonal gradation and increased sharpness are desired, as the stain contributes to the image density beyond the metallic silver itself. This makes them useful in traditional black-and-white photography, reproduction work, and certain fine art applications, where the added staining effect can improve both the visual impact and the archival stability of the photograph.