Printing-out
paper, usually abbreviated to POP, was the commercial successor to albumen
paper, and shares many characteristics with it. The main difference is
in the binder, which instead of albumen used gelatin. This progression
made possible the use of photographic emulsion on paper, albeit a basic
one, in which silver nitrate and a halogen chloride are mixed in a gelatin
solution. The slow emulsion formed this way will 'print out' to give a
visible image by the action of strong light. The silver nitrate is always
in excess, and is not washed out of the emulsion as in modern photographic
papers, leaving a reserve of silver which strengthens the image. As in
albumen printing, the image is a rich violet colour immediately after
exposure.
This image is unstable, and will fog if kept in light. To stabilise the print it must first be briefly washed to remove the excess silver nitrate. Fixing can follow, but the image will lighten to an amber colour. To keep most of the purple / brown colour, and a full tonal range, following washing the print should be gold toned, followed by rinsing, fixing and a final wash. Platinum and palladium toning are other options, giving an attractive and subtle brown colour.