In this way a developed print on gelatino bromide paper may be either converted into an image in greasy ink (oil or Bronzoil process), which can be further transferred to plain paper, or it can be used to insolubilize the gelatine of a carbon tissue previously soaked in reagents which react with silver (ozotype process).
Finally, there may be mentioned various print ing processes utilizing photochemical reactions caused only by the extreme ultra-violet and not by luminous rays, the images therefore not hav ing to be fixed. (It is evident that the negatives must have a base transparent to the rays used, cellophane for instance.) The paper can be sensitized, for instance, with a solution of amino phenol to which an alkaline nitrate has been added.
496. Supports for the Photographic Image. Photographs can be made on, or afterwards transferred to, a large variety of materials : papers of various textures, tints and substances, fabrics (silk, canvas), metals or metallized papers, glass, celluloid, etc. Of all these, the most commonly used support is paper, and the words " photograph," or " photographic print " are always understood to mean a paper print. Glass and celluloid (and its non-inflammable substitutes) are chiefly employed for diapositives or transparencies for ordinary and motion picture projection.
An emulsion containing salts of silver has been sprayed on to the inside walls of a room, first covered with plastic paint, for the purpose of their decoration by producing enlargements on the walls (E. Mollo and H. C. Merrett, 1934).
497. Photographic Papers. Positive photo graphic papers (other than albumenized or salted papers which are made solely from rags) are usually coated with a thin layer of a sus pension of barium sulphate, and sometimes kaolin, in gelatine, before coating with the emulsion, after which the paper is burnished with rotary brushes and calendered. This baryta coating has two purposes. It insulates the sensitive emulsion from impurities in the paper (chiefly metallic particles), and increases the brilliancy of the whites of the picture. The baryta coating is particularly thin on papers which are to exhibit a textured surface.' Photographic papers are made in various thicknesses : thin (chiefly used for small-sized prints), thick (used for large prints, and also for making prints with wide margins which are not to be mounted), thin card or cardette (which is used almost exclusively for post-card printing).
Photographic papers can be obtained with smooth surfaces, or with surfaces which show a more or less marked texture (grained, rough, imitation linen, etc.). The smooth finishes (glossy, semi-glossy, or matt)' are obtained as much by the composition and the method of application of the baryta coating as by the introduction into the emulsion of various sub stances (e.g. crude starch) which are used to increase the matt effect.
A picture is always sharper and more vigorous on glossy paper than on the other varieties, the blacks being denser for the same quality in the whites. For this reason, such papers are used for purely record purposes, and for making prints for photo-mechanical reproduction. 2 Glossy or semi-glossy papers are usually preferred for small or medium-sized prints, and, with the exception of the cases already men tioned, matt and grain-surfaced papers are generally chosen for large-sized prints and enlargements. In this way the excessive detail nearly always present in any photograph is minimized, whilst surface reflections, which are difficult to avoid when viewing a large picture, are reduced in some measure. At the same time, the spectator is obliged to some extent to step back, so that the picture is seen from the best viewpoint with regard to its perspective (§ 25).
Photographic papers are made with either a white or slightly-tinted base (pink, mauve, chamois, for print-out papers; cream or chamois for development papers), and occasionally strongly-coloured bases are obtainable (e.g. blue for pseudo-night effects). However, a print on white paper can be given any desired tint or depth of tint quite easily by immersing it in a bath of suitable dye.
To decrease the overcrowding of records, use is sometimes made of papers with emulsion on both surfaces, a temporary stain preventing the light used for printing one surface from acting on the other one.