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Photographic paper
From Wikipedia, the free encyclopedia
Until the advent of digital photographic processes, the sole meaning of photographic paper was paper coated with light-sensitive chemicals.
So-called photo papers of today are often specially coated papers for use in inkjet or laser printers to make digital prints. This article focuses on traditional photographic papers.
Photographic paper may be exposed to light in a controlled manner either by placing a negative in contact with the paper directly (contact printing) or by using an enlarger (enlarging) in order to create a latent image. Photographic papers are subsequently developed using wet chemicals to create a visible image.
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Photographic papers have been used since the beginning of all negative-positive photographic processes as invented by Nicéphore Niépce (France/1824-Heliography) and popularised by William Fox Talbot (Great Britain/1841-calotype).
Traditional photographic papers are still sold commercially today.
I. Papers used for negative-positive photographic processes. (e.g. the gelatin-silver process). This is the most common form of analogue photography. Even though slide film produces a positive image of a positive scene, the common process for the development of photographic transparencies (E-6) is actually a positive-negative process applied twice with an intervening bleach step, which removes the negative image.
II. Papers used for positive-positive photographic processes in which the "film" is the same as the final image (e.g. the Polaroid process).
III. Papers used for positive-positive film-to-paper photographic processes in which a positive image is enlarged and copied onto a photographic paper (e.g. the Cibachrome process).
Basic photographic paper (1) consists of at least two layers:
A) Photographic emulsion
Even though different photographic processes use different chemical mixtures to record the exposure of the paper(s) to light, all photographic papers use a sensitised metal as a reactant to the exposure of light, suspended in gelatin which is called the photographic emulsion. The metal is usually silver, but alternative processes exist that use iron (cyanotype), platinum or palladium (platinotype), uranium (uranotype), or other metals. Photographic emulsion can be sensitised for any wavelengths of light. In traditional black-and-white photographic papers, the photographic emulsion is sensitised for green/blue light which allows processing under red/orange safelighting.
B) Paper base
Photographic papers are made up mainly of a paper base which can have a number of different surface characteristics. Classical surfaces are glossy, stipple/lustre, and matte. Specialised papers with unusual surface characteristics such as watercolour paper are also produced commercially (e.g. Kentmere's Art Document). If the printer produces his/her own photographic paper, any paper surface that permits the application of photographic emulsion is available. The thickness of the paper base is specified by its "weight" (single, double or triple weights are available).
Fibre-based (FB) photographic papers (2) consist of a paper base covered with a baryta layer that whitens the paper and holds the photographic emulsion.
Supercoated fibre-based photographic papers (3) include a clear gelatin layer made up of hardened gelatin which protects the photographic emulsion underneath against scratches etc. especially during processing. Photographic emulsion is particularly vulnerable when it expands after contact with water.
Fibre-based papers are mainly used for exclusively high-quality prints and to maximise archival stability.
The paper base of resin-coated photographic papers (4) is sealed against the chemicals used for processing the paper by two polyethylene layers. Since no chemicals and no water is soaked into the paper base the time needed for processing, washing and drying of the paper is significantly shorter than the time needed for fibre-based papers. In a traditional black-and-white darkroom, an RC print can be finished and dried within 10-15 min, whereas the washing time of an FB print alone may take up to 60 min.
Most resin-coated photographic papers are also supercoated.
All colour photographic materials available today are coated on either RC (resin coated) paper or on solid polyester. The photographic emulsion used for colour photographic materials consists of three colour emulsion layers (cyan, yellow and magenta) along with other supporting layers. The colour layers are sensitised to their corresponding colours and shielded against the intrusion of light of a different wavelength than the actual layer by colour filters which dissolve during processing.
The emulsion layers include the colour dyes (Ilfochrome) or "colour couplers" which react with colour developers to produce colour dyes (Type C prints).
The emulsion itself is made up from light sensitive silver halide crystals suspended in gelatin.
Photographic papers are distinguished by the characteristics inherent in their different photographic emulsions. Black-and-white photographic paper is available in different grades which are usually numbered 0 to 5, with 0 being the softest, or least contrastful paper grade and 5 being the hardest, or most contrastful paper grade.
Photographic emulsions are also produced in a variable contrast type which permits the selection of any grade between 00 and 5. Variable contrast photographic paper is actually coated with a mixture of two types of emulsion, one of which is very low in contrast (0 or 00) and one of which is very high in contrast (5). The low contrast layer is activated by green light, the high contrast layer by blue light. The use of filters activates each layer in different proportions, thereby creating all contrast grades from 0 (or 00) to 5.
The trade names of variable contrast photographic papers typically include a reference to contrast variability, such as Variocontrast, Multigrade, Varycon or similar. Some companies simply designate variable contrast type papers with the words variable or polycontrast.
The contrast of photographic papers can also be controlled during processing or by the use of bleaches or toners.
The actual life span of any given photographic paper will vary with the environment the paper is stored within and how well the paper was processed.
Fibre-based black-and-white photographic paper that has been processed archivally (i.e. has been fixed and watered properly) is considered archivally "stable" and should last at least 70 years (the life-span of a typical resin-coated print is usually around 30-40 years.) Some special processes include photographic emulsions that are, if processed correctly, inherently more stable than the paper base they are printed upon, such as platinum prints.