All photographic papers consist of a light-sensitive
emulsion, consisting of
silver halide salts
suspended in a
colloidal material – usually
gelatin-coated onto a paper, resin coated paper or polyester support. In black-and-white papers, the emulsion is normally sensitised to
blue and
green light, but is insensitive to wavelengths longer than 600 nm in order to facilitate handling under red or orange
safelighting. In
chromogenic colour papers, the emulsion layers are sensitive to
red,
green and
blue light, respectively producing
cyan,
magenta and
yellow dye during processing.
Base materials Black-and-white papers Modern
black-and-white papers are coated on a small range of bases;
baryta-coated paper, resin-coated paper or polyester. In the past, linen has been used as a base material. Tints are sometimes added to the baryta to add subtle colour to the final print; however most modern papers use
optical brighteners to extend the paper's tonal range. Most fiber-based papers include a clear hardened gelatin layer above the emulsion which protects it from physical damage, especially during processing. This is called a supercoating. Papers without a super coating are suitable for use with the
bromoil process. The baryta layer has two functions 1) to brighten the image and 2) to prevent chemicals adsorbed on the fibers from infiltrating the gelatin layer. The brightening occurs because barium sulfate is in the form of a fine precipitate that scatters light back through the silver image layer. In the early days of photography, before baryta layers were used, impurities from the paper fibers could gradually diffuse into the silver layer and cause an uneven loss of sensitivity (before development) or mottle (unevenly discolour) the silver image (after development). The emulsion layers can include the colour dyes, as in Ilfochrome; or they can include colour couplers, which react with colour developers to produce colour dyes, as in
type C prints or chromogenic negative–positive prints. Type R prints, which are no longer made, were positive–positive
chromogenic prints. ==Black and white emulsion types== The emulsion contains light sensitive
silver halide crystals suspended in gelatin. Black-and-white papers typically use relatively insensitive emulsions composed of agb
silver bromide,
silver chloride or a combination of both. The silver halide used affects the paper's sensitivity and the image tone of the resulting print. These insensitive papers are suitable for contact printing, and yield warm toned images by development. Chloride emulsions are also used for printing-out papers, or POP, which require no further development after exposure.
Chlorobromide papers Containing a blend of silver chloride and silver bromide salts, these emulsions produce papers sensitive enough to be used for
enlarging. They produce warm-black to neutral image tones by development, which can be varied by using different developers. Filters in the enlarger's light path are a common method of achieving this control.
Magenta filters absorb green and transmit blue and red, while
yellow filters absorb blue and transmit green and red. The contrast of photographic papers can also be controlled during processing or by the use of bleaches or
toners.
Panchromatic papers Panchromatic black-and-white photographic printing papers are sensitive to all wavelengths of visible light. They were designed for the printing of full-tone black-and-white images from colour negatives; this is not possible with conventional orthochromatic papers. Panchromatic papers can also be used to produce paper negatives in large-format cameras. These materials must be handled and developed in near-complete darkness. Kodak Panalure Select RC is an example of a panchromatic black-and-white paper; it was discontinued in 2005.
Non Silver papers Numerous photo sensitive papers that do not use silver chemistry exist. Most are hand made by enthusiasts but
cyanotype prints are made on what was commonly sold as
blueprint paper. Certain precious metal including
platinum and other chemistries have also been in common use at certain periods. ==Archival stability==