outdated stock

Film is perishable. When it starts getting stale the dyes will shift color and the grain will build up, giving you a generally fogged, muddy and desaturated effect. It is only after about 2 or 3 years that this will start to happen, provided the film is refrigerated. Faster films tend to become outdated slightly faster than slow films. Likewise, color film will become outdated a little sooner than black and white. The flip-side is that outdated stock can be gotten quite cheaply, and often for free.