Gelatin silver developing-out paper (DOP) was invented in 1873 by the Englishman Peter Mawdsley. It was the first photographic process that submerged exposed paper into chemicals, rather than using light, as the chief agent in developing an image. Due to its stability and ease of use, developing-out paper became the photographic process of choice by 1895 and dominated the twentieth century in terms of amateur and professional black-and-white photographic prints.

DOP was typically manufactured and purchased, rather than made by hand, and came in a variety of choices based on tonal and surface characteristics. Because these papers were much more light sensitive than any other paper of the time, photographers had to to adjust their practices to the increased sensitivity.

Developing-out papers are extremely light sensitive and must be handled in either total darkness or under dim light of a specific wavelength. The paper is made of potassium bromide and silver nitrate, which is combined with a binder, gelatin, to form an emulsion. That emulsion is used to coat a fiber- or resin-coated paper base. (Resin-coated paper was introduced by Kodak in the 1960s.) A latent image is formed either by contact printing (pressing a negative or object directly to the paper’s surface and exposing it to light) or by projecting a negative with an enlarger onto the paper. Due to their extremely sensitive emulsions, DOP prints require just a few seconds for ample exposure.

To process the image, the exposed paper is first developed in a standard black-and-white developer. Development comes to a halt when the print is placed in an acid stop bath, usually acetic acid. The print is then placed in a fixer bath of sodium thiosulfate, or hypo. Finally, the print is washed in running water to remove all residual chemicals.