Base scratches

Base scratches on a piece of film prevent the light from passing through that area of the film because of light diffraction.

It will appear white on a film copied from it. So if we have a base scratch on a piece of negative we will see a white scratch on the print made from it.

If we have a base scratch on a positive and screen the film it will appear black because the light does not pass through the area of the scratch, if we make a negative from the scratched positive the scratch will be white in the negative and black in the subsequent print.