Fixing

Fixing, in Photography, is the term applied to the operation of rendering permanent the image formed on a "negative" or positive print [Photography], by dissolving away the unacted upon and unchanged sensitive constituents. It is of course necessary that the solution used for this purpose should not act upon the deposit which forms the image itself. In those cases where salts of silver, as the chloride, bromide, or iodide, are the sensitive substances employed, as in ordinary silver prints, negatives, or bromide papers, sodium thiosulphate (Na₂S₂O₃), known commercially and generally as sodium hyposulphite, is almost invariably employed. Where iron salts are used, as in the platinotype and chrysotype processes, dilute hydrochloric acid is used, while in the carbon process the print is fixed during development, as the latter consists in washing away all the unaltered parts.