When completing a collection survey, you may come across negatives and be uncertain of whether or not they should be frozen. There are many ways to determine what the base of the negatives could be, including various chemical tests and burning tests. Purchasing chemicals can be expensive, and dangerous to house and dispose of. While burning tests work, they require sacrificing a piece of the negative and lighting a fire.
A safe, reusable, environmentally friendly, and inexpensive method of analyzing negatives is to look at one between two linear polarizers. Linear polarizers are a type of grey, transparent film used in photography as a camera filter. When purchasing linear polarizer film, it is important not to order circular polarizer film as it will not perform the same way.
How to use it:
- Cut two pieces of film from a sheet.
- Create a frame for both pieces using acid-free mat board. This will help protect the film from abrasions and fingerprints.
- Sandwich the negative between the two pieces of matted film and hold up to natural light.
- Rotate one piece of matted film.
What to look for: When the one of the matted film pieces is rotated you will see one of two things: The negative and film will both appear to darken, or colours will be seen, called birefringence.
Birefringence will only be encountered when looking at a polyester negative. If you see the film darken, the negative is either nitrate or acetate. Polyester negatives are relatively stable and do not need to be frozen. Cellulose nitrate and cellulose acetate negatives should be frozen as they off-gas nitrous oxide and nitrogen dioxide or acetic acid, and they rapidly degrade. Cellulose nitrate is also a fire hazard because it can auto-ignite, and the fire cannot be put out.
To further identify between cellulose nitrate and cellulose acetate, you can consult the Northeast Document Conservation Center’s guide to visually identifying negatives: https://www.nedcc.org/assets/media/documents/05PH_01FilmBaseGuide.pdf