The following are descriptions of some of the photographic formats of the original images from the Print and Picture Collection that have been digitized for this project.
Invented in 1850 by Louis-Desiré Blanquart-Evrard, the albumen process used paper coated with a mixture of egg whites and ammonium chloride. This was the first type of paper that could be prepared and stored well before use, making its manufacture possible. Photographers could purchase it in reams and sensitize it in a bath of silver nitrate when it was needed. To make the print, a collodion negative was placed in direct contact with the sensitized paper and exposed to direct sunlight for anywhere from ten to thirty minutes. The tone and hue could be changed by stopping the process at different times. Albumen prints were almost always toned with gold chloride. Prints can range from reddish to purplish brown. Albumen prints almost always show some deterioration through the appearance of yellow highlights. The albumen print was the most common type of photograph until the 1890s.
The cyanotype process was invented by John Herschel in 1842 and is based on the principal that iron salts are sensitive to light. The paper was brushed with ferric ammonium citrate and potassium ferricyanide and dried in a dark place. The object to be reproduced was then placed upon the sheet in direct sunlight. Sometimes the object was a negative, but often a plant specimen was used. After about fifteen minutes, a white impression formed where the light had not penetrated, leaving a blue background. The paper was washed in water, and the oxidation formed a bright blue. A variation of this process was used to duplicate architects' drawings.
The platinum print process is based on chemical reactions caused by the sensitivity of iron salts to light, which produced an image consisting of pure platinum. It is one of the most stable and permanent types of photographs with a richness and tone superior to other processes. It was invented in 1873 by William Willis, who refined it over the course of five years, eventually making the platinum papers commercially available through his company. Platinum prints were popular until the 1920s, when platinum became prohibitively expensive.
Gelatin silver prints are photographs printed on paper coated with gelatin containing silver salts. They generally displaced albumen prints in popularity by 1895 because they were more stable, did not yellow, and were simpler and quicker to produce. A modified version of the process is still in use today. Photographers of the 1880s and afterward did not have to coat their own papers because a variety of these papers were available from commercial sources. Gelatin silver prints generally have a high surface gloss. The tone depends on the type of silver salts used and the process.
A stereograph is a pair of photographic images of the same object, presented on a single sheet, each at a slightly different angle. They were made by using a dual lens camera, or two cameras side by side. Stereographs were mainly albumen prints, but were produced by every photographic process available during the 19th century. The images were viewed through a stereoscope, a binocular-like device that produced a three dimensional effect. Stereographs were introduced at the Crystal Palace Exhibition in London in 1851. These photographs were produced purely for amusement, and a stereoscope was a common fixture in parlors well into the twentieth century. In 1854 one of the first commercial stereographic businesses was started in Philadelphia by the Langenheim brothers.
Sources and Further Reading
Baldwin, Gordon. Looking at Photographs: a Guide to Technical Terms. Malibu, CA: The J. Paul Getty Museum, 1991.
Mace, O. Henry. Collector's Guide to Early Photographs.