Photographic Principles

Polarity:  This term refers to photographic images as either negative or positive.  Negative images have lights areas that appear dark and dark areas appear light.  Positive images have type that appears black and the paper appears white.  Both negative and positives can be on paper, glass, or film.

Base and Emulsion: All photographic images have a base and a light-sensitive emulsion.  The base supports the emulsion and can be made up of any materials, but is usually on metal, glass, film or paper.  Emulsions are made up of many different substances are albumen, collodion or gelatin

Collodion Emulsions: First used in 1848 by Frederick Scott Archer.  The collodion emulsion was a fine grain emulsion with clear creamy highlights and gray shadows.  A mixture of gun cotton and alcohol mixed with potassium iodide or bromide was soaked in silver nitrate solution, placed in the holder exposed in a camera and developed.

Albumen Emulsions: Used to produce negative, prints and crystoleums from 1848-1895 by Louis Blanquart Evrard in France.  This process was not practical accepts for landscapes and required long exposure times.

Gelatin Emulsions: Introduced in 1871 by Richard Leach Maddox in England.  Used to form negative and positive images on glass, film and papers to form the basis for many photo-mechanical printing processes.  This was a faster process that is more sensitive to light.

Processing: Photographs create an image formation through the process of exposing the light under controlled circumstances onto a light-sensitive emulsion.  This exposure can produce reflective light after the exposure through a camera device, through an enlargement or making a contact print. 

Contact prints are made with a negative in direct contact with sensitized paper and exposing it to light.

Enlargements:  Enlarging a print did not emerge until the end of the 19th century because of the practicality of new technologies and better photographic paper.  An enlargement can be made by using an enlarger (a lens system) to project the images through the negative onto photography paper.

Fixing:  This process desensitizes the emulsion in a fixing bath agent.  This removes the unexposed silver so that it will not continue to darken upon exposure to the light.

Toning: In the 19th century many prints were toned, which gave them a specific color, like Albumen prints were toned gold, Printing-out papers were red.

         

Ritzenthaler, M. L., G. J. Munoff, M. S. Long (1984) Archives and manuscripts: Administration of Photographic Collections.  Society of American Archivists.  Chicago, Illinois.