There are different types.
The gloss on photos is partly the surface finish of the paper and partly the emulsion that makes the photo.
Lithographed cards can have a layer of Gloss which is a clear substance like ink that's applied with the press as a separate color pass.
I've had a few postcards where the gloss was peeling like a thin layer of plastic. I believe that's actually a layer of celluloid or similar plastic that's been added to make the card shiny.
Steve B
|