Invented in 1873 by the Englishman Peter Mawdsley, silver gelatin prints, also known as developing out paper (DOP), was the first photographic process to use chemically treated paper rather than light to expose images (Historical New Orleans Collection). It became the most popular photographic printing process of the twentieth century
Process. Silver gelatin prints were initially comprised of two layers: 1) a substrate, or foundation, of paper; and 2) an emulsion of potassium bromide and silver nitrate in gelatin. This emulsion then used to coat a fiber paper base to create a light-sensitive top layer that, following exposure of the negative and development in a chemical bath (usually acetic acid), formed the image. The exposed paper was then placed in another bath to stabilize a
photographic image before final processing Historical New Orleans Collection. Later, a middle layer of barium sulfate, or baryta, was added between the paper and the emulsion, acting to limit the penetration of the emulsion into the fibers of the paper, resulting in a more even emulsion and more uniform blacks.
The light-sensitive materials used to create silver gelatin prints are stable under normal storage conditions, and can be exposed and processed many years after their manufacture. This is in contrast to the collodion wet-plate process, which had to be exposed and developed immediately after coating.
Identification. Developing out prints generally render cool tone black and white images. Yet, they may also have warmer tones or a metallic appearance depending on the type of paper or chemical process used for processing. For example, resin-coated paper, which is visually identifiable by the sheen on the back of the paper, was introduced by Kodak in the 1960s. And, any more variants of paper and chemicals have been introduced over time (Stulik and Kaplan).
Image left: Busy day at the gin - Pharr, Tex (n.d.). Rio Grande Valley 20th Century Studio Photographs Collection. ELIBR-0174, 032. University Library, Special Collections and University Archives, University of Texas Rio Grande Valley, Edinburg, TX.
Image right: Crowded street in Mexico City during the Mexican Revolution (1913). Cheryl Shepherd's Ten Tragic Days Collection, BD-0020. University of Texas Rio Grande Valley Special Collections and Archives Digital Collections.