With printing-out paper, light alone creates a complete and visible image, with no chemical development required. Washing, toning, and fixing are typically the only processing procedures. The manufacture of printing-out paper made it easier than ever before for people to pursue photography.
The chemicals used in printing-out paper processes (POP) are sensitive to ultraviolet light and must be applied either in the dark or under low light. Ammonium chloride, sodium potassium tartrate, and silver nitrate are combined with a binder, either gelatin or collodion, to form a light-sensitive emulsion that coats the paper support. Contact printing, or pressing a negative or object directly to the surface and exposing the paper to ultraviolet light (historically sunlight), produces an image without the need of chemical development agents.
POP prints can take minutes for ample exposure. To process the image for permanence, the paper is washed in a running water bath to remove silver nitrate residue. To increase image stability, toners, such as gold chloride, are used at this stage. The print is then fixed in a sodium thiosulfate bath, or hypo. The final step is to wash the print in water to rid the paper of extraneous chemicals.