There are many types of photographic negatives. Knowing how to differentiate between types of negatives will help you to accurately assess the conservation and preservation needs of each type. Glass negatives were the first kinds of negatives, and were replaced with sheet film made from cellulose nitrate, followed by cellulose acetate and polyester. Cellulose acetate and polyester films are also called “safety.”
While there are tests that can be performed to determine the negative type, they can be destructive to the negative. Before any testing, first check to see if the type is printed on the negative. Many manufacturers printed “Safety” or “Nitrate” on the edge of the negative. There might also be notches on the negative that indicate the type. Nitrate negatives have all “V” notches, whereas safety negatives have a “U” notch before the “V” notches.
Be sure the words “Safety” and “Nitrate” are imprinted on the negative itself. They will be slightly embossed. Sometimes negatives are copied, and the original negative’s type is transferred to the new negative. Therefore, a safety copy negative might appear to be nitrate because of the negative image of the word “Nitrate.”
Polyester film is a type of safety film, but is made of polyethylene terephthalate (PET) rather than cellulose acetate. When held to a light source, polyester roll film transmits more light than acetate roll film.
For other ways to differentiate between safety, nitrate, and polyester negatives, try the identification tests. Beware: some are destructive to the negative, and some tests are dangerous to perform.
Glass negatives are easily identified — they’re glass. However, the two types of glass negatives, collodion wet plate and gelatin dry plate, are difficult to differentiate without chemical analysis. If a wet plate negative is placed on a dark background, the image should appear positive. Gelatin dry plate images can also be identified because of manufacturing standardization, such as smoothness, sizes, thickness, and an even coating of gelatin.
Depending on how obvious negative (rather than positive) image is on a paper negative, it could be confused with a print. Paper negatives are usually thin, and sometimes the paper is waxed.
Knowing the type of film can also help you determine a date range. See types of negatives for more details about the dates of manufacture for each negative type.
archiving negatives 