What is microfilm and what is it made from?

Microfilm can be made from three different substances:

• Cellulose nitrate: These films are highly flammable and over time release hazardous gases which are subject to natural decomposition. By the 1950s production ceased, but obviously it can still be available within archives.
• Cellulose acetate: Less flammable but degrades over time, producing a vinegar smell.
• Polyester: More stable so better for preservation, with a life expectancy of over 500 years.

And the images can be created through:

• Silver gelatine: Usually used when creating the master film/fiche, a negative image is created, which can produce positive images when duplicated. It can be used on all three substances above.
• Diazo: Usually used when directly copying the master film through contact printing. The diazonium salts combine with dye couplers to produce strong, dense colours. It can be used on polyester or acetate. But fading can be relatively quick when exposed to light, including a film reader.
• Vesicular: An inexpensive way to create copies from Diazo, utilising the nitrogen produced from diazonium salts as they decompose. To duplicate, the film is placed in direct copy with the master, exposed, and developed by heating the film. As a result of the heat, the image will always have slightly raised areas made from salt bubbles.
• Polyester can not be used in this process, as it cannot tolerate the heat.
• Colour: As enticing as coloured microfilm is, the life expectancy of most 35mm colour film falls short of the general need.

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