Preservation microfilm consists of several layers; the key layers are the base and the emulsion. Photographic images are reproduced in a silver-gelatin emulsion (in negative polarity) which is fixed to the base for rigidity.
Film Bases
Cellulose nitrate base
Developed in 1852 (by Benjamin Dancer) and used commercially until the 1950s, this type of microfilm is inherently unstable due to its nitrate base and has a tendency to self-combust.
Cellulose acetate base (including cellulose diacetate and cellulose triacetate)
This was largely produced between 1930 and 1980. Acetate has the tendency to degrade, releasing acetic acid (the cause of the notorious vinegar smell) which then accelerates the process of deterioration. The base layer tends to shrink (whilst the emulsion retains its normal size but deforms and buckles), resulting in warping of the microfilm and the appearance of blisters.
Polyester base
Invented in 1941, polyester is relatively stable chemically and resistant to shrinking. Under recommended storage conditions, polyester has a life expectancy of 500 years. Polyester base is currently the accepted standard for preservation microfilming.
Film Coating
Silver-Gelatin (or Silver-Halide) Microfilms
Modern silver-gelatin films are the only microform medium appropriate for archival purposes, being durable under recommended storage conditions. Silver gelatin film should be distinguished from dry silver film, which is of nonarchival quality.
According to the US EPA, archival-quality film contains images produced by using light-sensitive silver halide crystals suspended in a gelatin emulsion.
Diazo Microfilms
The US EPA specifies that diazo film contains images produced by exposing an emulsion of diazonium salts to ultraviolet light and then to ammonia fumes during the process of copying from a master microform.
Diazo film is available in color and black, and may have an acetate or polyester base. Black diazo resembles silver gelatin film but is glossy on both sides. Diazo film gradually fades, even in the dark. Care must be taken to minimize prolonged exposure to light (as in a film reader).
Vesicular Microfilms
The US EPA describes this film as containing images that are developed through exposure to ultraviolet light and then to heat during the process of copying from a master microform. A polyester film base is used to tolerate the heat. During development, as heat softens the polyester base, nitrogen escapes to form tiny bubbles which remain trapped when the film is cooled, giving an uneven raised appearance to the surface. Vesicular film is prone to damage through mechanical pressure which breaks the bubbles. At high temperatures, the base material softens and gas trapped in the bubbles expands, causing the bubbles to rupture with subsequent damage to the overlying image. Great care is required when using vesicular microfilm in a film reader (Dalton, 2000).
Alternative Microform Types
Color Microfilms and Microfiche
Color microfilms do not meet preservation standards in terms of life expectancy. Ilfochrome has a potential life expectancy of 200 years if protected from light, but further research is required.
Continuous-Tone Microfilms
Black and white microfilm produces a high-contrast resolution but does not capture gray tones. Continuous-tone microfilming maximizes gray scale reproduction, but is not an accepted standard for archival preservation.
Source: Northeast Document Conservation Center. (2000). Microfilm and microfiche. Retrieved February 25, 2011, from http://www.nedcc.org/resources/leaflets/6Reformatting/01MicrofilmAndMicrofiche.php
Microform Types
Microfilm is available in 16mm or 35mm roll. Microfiche is produced by inserting strips of microfilm into transparent microfiche jackets.
First generation microfilm refers to the master negative.
Second Generation microfilm (also a negative) refers to the print-master negative (also known as duplicate negative) which is copied from the master negative.
Third Generation refers to the service copy, use copy or distribution copy which is copied from the duplicate negative.
Source: Library of Congress. (2005). Preservation microfilm. Retrieved February 25, 2011, from http://www.loc.gov/preserv/supply/catmfmf.html