Film Restoration Methods at Test: Scratches Cleaning - Text 2
If the scratch is on the emulsion of a positive film, it will show as a clear line during projection; on the negative’s emulsion, as a dark line on the positive copy struck from its matrix. In prints with multilayered emulsions (such as those of Technicolor films), scratches can show up in different colours, depending on the depth of the abrasion. If the scratch is on the base, in forming slightly different angles, the light’s refraction against each groove on the base of the positive film generates a shadow, seen as a dark irregular line in the projected image. Any dirt or dust deposited into the groove makes this line appear more pronounced. It should be noted that scratches were sometimes applied intentionally to the camera negatives in order to achieve visual effects, such as lightning and rain in King Lear (William V. Ranous, 1909).
