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Plates Papers

emulsions, black, light, surface, friction, development and paper

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PAPERS, PLATES, AND FILMS FOR POSITIVE PRINTS BY DEVELOPMENT 547. Different Kinds of Emulsions and their Characteristic Qualities. The different emulsions to be considered are : Gelatino-bromide for black tones ; gelatino-chloride for black tones—these sometimes contain a small proportion of silver bromide ; and gelatino-chlorobromide of very fine grain for obtaining warm tones by simple development and fixing. It should, however, be recognized that this division is somewhat arbitrary. Warm tones can generally be ob tained with the emulsions described as being for black tones, especially when it is intended that they should be viewed as transparencies. And the emulsions described as being intended for warm tones can always be made to produce black images. This division is, however, justified by the general character of each class.

In each class, emulsions coated on paper' are usually prepared in several varieties, each corres ponding to a given range of extreme densities of the These different grades are not necessary in the case of emulsions coated on glass or other transparent supports for the pro duction of images to be treated singly, since the contrasts of the image can be so easily modified by exposure and control of development. This method of modification cannot be exercised when the same emulsions are coated on an opaque support and the image is examined by reflected light.' In an image examined by transmitted light, the contrast increases progressively in propor tion to the progress of development, up to a certain maximum, which may be very high (§ 202, Fig. 14o). But the contrast in an image seen by reflected light increases rapidly at first and then becomes stationary As a rough guide, an approximate idea may be given of the relative sensitiveness of the various types of emulsions-3 extremely rapid. Its normal time is generally about 30 seconds.

548. Abrasion. All these papers, especially gelatino-chloride and those with a glossy surface, are very susceptible to friction. Stress marks, the result of friction or abrasion, show in the same manner as on negative emulsions (§ 199), as black lines on a light ground, or as light lines on a dark ground. For some years many glossy

papers have been rendered immune from stress marks by a very thin overcoating of plain gelatine,' the thickness being about „ „th to dOh of an inch (0.002 mm. to 0-005 mm.). These stress marks are principally confined to the outside surface of the film, particularly in the case of the black markings. The markings which result from abrasion or friction can be minimized by " neutralizing " the outside sur- face during development by adding a very small quantity of a soluble iodide to the developer, which superficially desensitizes the emulsion. Fixing is then very slow and necessitates the employment of a more concentrated bath. The effects of friction can also be lessened by the addition of small quantities of hypo to the developer ; but this introduces the risk of yellow stains of the same nature as dichroic fog.

Although these remedies are suggested, it is very desirable to avoid all friction or rubbing of the surface of a paper, however slight. Special care should be taken to prevent the sheets of a packet from sliding over each other.

549. Manipulation of Printing Papers before Exposure to Light. There is no uniformity in the methods of packing adopted by different makers. In some cases the sheets are packed in pairs, the two sheets of each pair being face to face ; in others, all the sheets of the packet are turned with their surfaces in the same direction the last sheet of the packet being placed the opposite way so as to prevent the surface of the emulsion from coming in contact with the pack ing paper.

The emulsion surface is generally recognizable by its slight concavity. In case of any doubt arising with matt or rough papers, a corner may be taken between the teeth ; the gelatine film will adhere to the The surface of the paper should never be touched by fingers which are either moist, greasy, or contaminated with chemicals, espe cially hypo. Whenever possible, the paper should be held by the edges. At the most, when a print is to be trimmed after development, it may be held by a corner.

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