Photographic FilmsThe first flexible roll films, dating to 1889, were made of cellulose nitrate, which is chemically similar to guncotton. A nitrate-based film will deteriorate over time, releasing oxidants and acidic gasses. It is also highly flammable. Special storage for this film is required.
Nitrate film is historically important because it allowed for the development of roll films. The first flexible movie films measured 35-mm wide and came in long rolls on a spool. In the mid-1920s, using this technology, 35-mm roll film was developed for the camera. By the late 1920s, medium-format roll film was created. It measured six centimeters wide and had a paper backing making it easy to handle in daylight. This led to the development of the twin-lens-reflex camera in 1929. Nitrate film was produced in sheets (4 x 5-inches) ending the need for fragile glass plates.
Triacetate film came later and was more stable, flexible, and fireproof. Most films produced up to the 1970s were based on this technology. Since the 1960s, polyester polymers have been used for gelatin base films. The plastic film base is far more stable than cellulose and is not a fire hazard.
Today, technology has produced film with T-grain emulsions. These films use light-sensitive silver halides (grains) that are T-shaped, thus rendering a much finer grain pattern. Films like this offer greater detail and higher resolution, meaning sharper images.
- Film Speed (ISO) — An arbitrary number placed on film that tells how much light is needed to expose the film to the correct density. Generally, the lower the ISO number, the finer grained and slower a film. ISO means International Standards Organization. This term replaces the old ASA speed indicator. The slower the film, the more light is needed to expose it.
Photographic PrintsTraditionally, linen rag papers were used as the base for making photographic prints. Prints on this fiber-base paper coated with a gelatin emulsion are quite stable when properly processed. Their stability is enhanced if the print is toned with either sepia (brown tone) or selenium (light, silvery tone).
Paper will dry out and crack under poor archival conditions. Loss of the image can also be due to high humidity, but the real enemy of paper is chemical residue left by photographic fixer. In addition, contaminants in the water used for processing and washing can cause damage. If a print is not fully washed to remove all traces of fixer, the result will be discoloration and image loss.
- Fixer (Hypo)—A chemical, sodium thiosulfate, used to remove residual silver halides (grain) from films and prints when processing them. Fixer "fixes" the remaining silver halides in place on either film or prints. Fixer is also called hypo.
At first color prints were not stable because organic dyes were used to make the color image. The image would literally disappear from the film or paper base as the dyes deteriorate. Kodachrome, dating to the first third of the 20th century, was the first color film to produce prints that could last half a century. Now, new techniques are creating permanent color prints lasting 200 years or more. New printing methods using computer-generated digital images and highly stable pigments, offer permanency for color photographs.