Most black and white pictures are made on paper (barited paper or RC -“resin coated”-). Its emulsion is gelatin based and contains the silver salts that form the image. That’s why they can suffer dichroic (silver mirroring), and all typical damages seen in paper: wrinkles, tears…
Two examples of collage GATCPAC panels from Col·legi d’Arquitectes de Catalunya:
Top left: before treatment: dirt, wrinkles and ripples. Right: During the emulsion cleaning process.
Top left: before treatment: dirt, wrinkles and ripples. Center: After cleaning the emulsion. Right: After flattening the photographic paper.