Oxalic acid is a white, water-soluble crystalline substance commonly used in photographic toning processes. It acts as a bleaching or reducing agent in certain toners, helping to adjust image contrast, colour, or density.
Photographers and darkroom technicians use oxalic acid carefully due to its chemical strength and potential hazards. When applied correctly, it allows precise control over toning effects, contributing to the final aesthetic quality and tonal range of black-and-white prints.