Traditional photographic paper is coated with a gelatin emulsion containing light sensitive metallic sliver halides. These halides turn dark when exposed to light, but are otherwise invisible. The exposure work is done in a darkened room under a “safelight” which does not affect the light sensitive silver.
Silver bromides and silver chlorides are most often used, usually in combination with each other to deliver neutral toned papers. If a greater concentration of bromides are used, the photographs take on a “cool,” or bluish black tone. Conversely, if more chlorides are used, the final prints are more “warm” in color, tending more toward a brownish black.
Most photo papers strive to reach a neutral tone.
Traditional fine art papers are of a high quality fiber. Many less expensive papers are plastic coated. These are used in numerous places, including for most student work.
Fewer such papers are available today, as most of the world has switched to digital capture and printing, which uses inks and printers instead of silver particles and chemicals to develop the prints.