Components of a photographMost photographs are made up of two components: base and emulsion.
A base is the material on which a photographic image appears. The most common materials are metal, glass, film, and paper. An emulsion is laid over the base and contains a light-sensitive material. Most emulsions are albumen, collodion, or gelatin, and the most common light-sensitive materials are silver salts. The exceptions to this are daguerreotypes and uncoated paper processes which do not include an emulsion. Instead, daguerreotypes are made with copper plates covered in a film of light-sensitive silver iodide while uncoated paper prints have the light-sensitive material soaked into the paper instead of applied as an emulsion.
The following video by the Getty Museum provides an excellent demonstration of the steps required to produce a daguerreotype. |
Processing a photographIn order to create a photographic image, a light-sensitive substance must be exposed to light under controlled circumstances. The exposure can occur with reflected light in a camera or transmitted light which is used to make a contact print or enlargement.
In order to create a photographic print, photographic papers must be used. Printing out papers (POP) are placed in direct contact with a negative and exposed to light through the negative until the image appears on the paper. Developing out papers (DOP) are either placed in direct contact with the negative or have an enlargement projected onto them. Their exposure to light is much shorter and no image is visible on the paper. In order to produce the image, DOPs must be chemically developed. In order to make the image permanent, it is necessary to desensitize the emulsion using a fixing bath. A chemical, commonly sodium thiosulfate, removes the unexposed silver so that the image will not continue to darken when it is exposed to light. The print must then undergo washing, often using chemicals, to remove the fixer. Until the invention of collodion wet plate negatives in the early 1850s, the majority of photographs were direct positives, meaning that they were unique images created as positives from which copies could not be made. While paper negatives did exist, they were not widely used due to the fuzziness and paper grain that they created in the prints. Wet collodion negatives were created on glass plates and could be used to print many copies of the same image, often as albumen prints. Please see the following video for a demonstration of collodion wet plate negatives and albumen prints. |