POP is printing-out-paper. It originated shortly after albumen paper as a way for portraitists (mainly) to quickly produce prints, since no developer was/is needed to see the resulting image and all processing may be carried out in normal room lighting. Usually the emulsion is a silver chloride on gelatin. Personally I love Chicago Albumen Works Centennial, which is available from Bostick & Sullivan. The tonal range is UNBELIEVABLE, and if you are interested in alternative print processes (e.g. platinum/palladium), I highly recommend trying POP first. Some things to consider first: 1. your negatives must have an extended density range to exploit the paper's ability to reveal extremely high values. Negatives with a range of only two to three zones in the middle-grey area print like mud; 2. you'll need to pick up gold toner, which will allow you to vary print tone across a range from a warm orangey-brown to an ice-cold blue- black. I like the two-part gold thiocyanate toner available from Photographer's Formulary.

Is it better for contacts than enlargements? Isn't EVERY paper? My point here is that if you are trying to produce fine art and want the sharpest prints possible, why wouldn't you make a contact? And Centennial is double-weight. It has a lovely lustre about it - kind of a semi-matte/semi-gloss finish - slightly textured, and it dry mounts very well.