This non-destructive test can be used to determine if a negative is either cellulose acetate or cellulose nitrate rather than polyester. This test cannot differentiate between cellulose acetate and nitrate bases.
It is important to identify if a negative is polyester, since polyester is more stable than acetate.
Place the negative in question in a polarizing viewer or between two polarizing filters/films. Make sure that you observe the thinnest possible density of the negative through the polarizing viewer. The clear outer edge is a good choice. With the viewer polarized (the film looks black), hold the viewer up to a light source, while tilting to observe any colour change. If no rainbow colours are visible, the negative is on an acetate support. If a pattern of pink, blue, green rainbow effect becomes visible, then the negative support is polyester. This occurs because polyester film is highly birefringent and easily identified by interference patterns (i.e. rainbow colours) that are produced when the film is viewed through polarizing filters.