Unexposed raw film stock — used for camera tests, sound playback, or sync work. Saves expensive print stock.
You need material for testing, for sound work, or to simulate editing offsets – but you don't want to use your expensive negative. This is where blank stock comes in: virgin raw film material that you use up in the camera or at the editing bay without wasting a second of the actual shot. The economic advantage is obvious – especially in the analog world, every meter was cost-critical.
In practice, you use blank stock wherever image content is irrelevant. During the sync or ADR process, you need material that has the correct length and film speed so that the magnetic sound reels and drive rollers run in sync – the visual content is completely unimportant. You simply run blank stock through while the actors re-record their dialogue. Similarly in editing: when you do timing tests, try out transitions, or simply need placeholders to spatially organize the edit sequence, don't waste your precious positive or negative material. A few meters of blank stock are perfectly sufficient.
In the digital age, blank stock has largely disappeared from the workflow – here you work with proxy files and unlimited digital blanks. But in traditional analog workflows, especially in archival work or with 16mm productions, it remains standard. Some editors still stock various formats – 16mm, 35mm, different perforations – because the costs for incorrect tests are simply too high.
An important point: blank stock must have the same film emulsion and perforation as your working film, otherwise the running length will not match your actual recordings. Anyone who is careless here will produce unpleasant surprises later in the edit – suddenly the length doesn't fit, the sound tracks shift. You should store blank stock cool and dry like any other raw film material; light is your greatest enemy, even if there's nothing usable on the emulsion anyway.