Why do my photographic slides have a lighter area near the borders?

Question

I've just been examining my collection of 35 mm colour positive slides and I have noticed something unusual. The edge of the colour positive slide, where the image meets the inner rim of the border it is housed in, is a lighter shade compared to the rest of the picture. In some slides it may be the bottom edge and the right side of the image. In other slides it may be all four edges.

Does anyone know why this is?

Additional information: The lighter borders are only visible when viewing the slides over a dark area. In normal light the slides look perfectly normal.

Answer 1

Your slides are most likely duplicates. The white borders are where some light leaked through the dark borders surrounding the image in the original film. Duplication is performed by bringing the unexposed slide film's emulsion directly into contact with the exposed slide film's emulsion and "flashing" the image onto the dupe.

To learn more about "vintage" slide duplication techniques you could check out manuals for those devices. Beseler dual mode slide duplicator - A camera based duplicator Polaroid Polaprinter - Duplicated slides onto Polaroid

It is also possible that this is a form of in-camera masking. White-light masking is very rare and generally only used in scientific devices.

An even less likely explanation is that your slides are negative duplicates of a negative (two negatives makes a positive.) If that were true, though, you would have likely figured it out since the borders would have the characteristic beige base+fog of a normal color negative. This has, to my knowledge, never been common practice but it is possible using the technique mentioned above.