Photographic paper is a paper coated with a light-sensitive chemical, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then developed to form a visible image; with most papers the image density from exposure can be sufficient to not require further development, aside from fixing and clearing, though latent exposure is also usually present. The light-sensitive layer of the paper is called the emulsion, and functions similarly to photographic film. The most common chemistry used is gelatin silver, but other alternatives have also been used. The print image is traditionally produced by interposing a photographic negative between the light source and the paper, either by direct contact with a large negative (forming a contact print) or by projecting the shadow of the negative onto the paper (producing an enlargement). The initial light exposure is carefully controlled to produce a grayscale image on the paper with appropriate contrast and gradation. Photographic paper may also be exposed to light using digital printers such as the LightJet, with a camera (to produce a photographic negative), by scanning a modulated light source over the paper, or by placing objects upon it (to produce a photogram). Despite the introduction of digital photography, photographic papers are still sold commercially. Photographic papers are manufactured in numerous standard sizes, paper weights and surface finishes. A range of emulsions are also available that differ in their light sensitivity, colour response and the warmth of the final image. Color papers are also available for making colour images.
History The effect of light in darkening a prepared paper was discovered by Thomas Wedgwood in 1802. Photographic papers have been used since the beginning of all negative–positive photographic processes as developed and popularized by William Fox Talbot's 1841 calotype. After the early days of photography, papers have been manufactured on a large scale with improved consistency and greater light sensitivity.
Types of photographic papers
Photographic papers fall into one of three sub-categories:
Papers used for negative-positive processes. This includes all current black-and-white papers and chromogenic colour papers. Papers used for positive-positive processes in which the "film" is the same as the final image (e.g., the Polaroid process, Imago direct positive paper). Papers used for positive-positive film-to-paper processes where a positive image on a film slide is enlarged and copied onto a photographic paper, for example the Ilfochrome process.
Structure 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.
Fiber-based papers (FB) Fiber-based (FB or Baryta) photographic papers consist of a paper base coated with baryta. 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. Fiber-based papers are generally chosen as a medium for high-quality prints for exhibition, display and archiving purposes. These papers require careful processing and handling, especially when wet. However, they are easier to tone, hand-colour and retouch than resin-coated equivalents.
Resin-coated papers (RC) The paper base of resin-coated papers is sealed by two polyethylene layers, making it impenetrable to liquids. Since no chemicals or water are absorbed into the paper base, the time needed for processing, washing and drying durations are significantly reduced in comparison to fiber-based papers. Resin paper prints can be finished and dried within twenty to thirty minutes. Resin-coated papers have improved dimensional stability, and do not curl upon drying.
The baryta layer The term baryta derives from the name of a common barium sulfate-containing mineral, barite. However, the substance used to coat photographic papers is usually not pure barium sulfate, but a mixture of barium and strontium sulfates. The ratio of strontium to barium differs among commercial photographic papers, so chemical analysis can be used to identify the maker of the paper used to make a print and sometimes when the paper was made. 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 unevenly discolour (mottle) the silver image (after development).
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