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Printing Papers and Printing Methods 491

silver, emulsions, light, image, substance and sensitive

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PRINTING PAPERS AND PRINTING METHODS 491. The Principal Printing Processes. A great number of photo-chemical reactions are known on which processes for producing photographic positives can be based. There arc also various reactions by means of which the substance of the image produced by the action of light can be modified, thereby altering its colour (toning). For this reason the study of these processes must be necessarily limited to those in current use, and to any others of particular or special interest.

Any classification of working methods is to a great extent arbitrary ; in the following pages a classification based on the nature of the photo-chemical reaction has been adopted.

492. Methods of printing with silver salts may be conveniently divided into two principal groups_ according as the image is entirely formed by the action of light (these are known as print-out processes, also called P.O.P.), or whether it is obtained by the development of a latent image by a process very similar to that used for the production of negatives.

In the first case a halogen salt of silver (most usually the chloride) is used in conjunction with either a soluble salt of silver or some other substance capable of absorbing the halogen liberated by the action of light (a developer, for example, or some other reducing agent). The sensitive compounds may be formed either in the support itself, or in a " sizing " with which it has been previously coated (albumenized or salted papers), or an emulsion of the sensitive material may be prepared with gelatine or collodion and afterwards coated on the required support (gelatine P.O.P. ; collodion P.O.P.). Certain papers belonging to this latter category contain the necessary toning materials (gold salts, or, more usually, selenium or tellurium compounds), incorporated in the actual emul sion, and are then known as self-toning papers.

A considerable quantity of light is required to form an image on these various sensitized papers (exposures of the order of a quarter of an hour in good diffused daylight are required when printing from a negative of average density) and the various manipulations can be carried out in weak daylight. Thus it is not

practicable to use such papers for projection printing (enlarging or reducing). The final tones of prints obtained in this way usually vary from reddish-brown to purplish-brown.

In the second case (development papers), apart from having a very much slower speed, the emulsions do not differ essentially from negative emulsions. For a long time manufac turers have made such sensitive papers in the form of gelatine emulsions, although several attempts have been made to use collodion for the purpose_ The sensitive substance may be either silver bromide, silver chloride, or a mixture of the two. In all cases, such emulsions are too fast to allow of their being used for daylight printing. Gelatino-chloride emulsions are considerably slower than gelatino-bromide ones, and are insensitive enough to allow of their being handled in weak artificial light without the use of a safe light. This fact has led to their being incorrectly called gaslight papers (to be handled in gaslight). Silver bromide or silver chloride papers normally give black tones, whilst chloro-bromide emulsions, which are of inter mediate sensitivity, are used as a rule for the direct production of warm tones. These various emulsions, and particularly the more rapid ones, constitute the best material for enlarging purposes.

All the above-mentioned varieties of silver papers are often made in several grades for the printing of vigorous negatives (soft papers), normal negatives, and weak negatives (contrasty papers) respectively.

Toning of the silver image can be done either by replacing it by other metals (gold, platinum, etc.) or by converting the silver into a coloured compound (brown silver sulphide), or into a colourless substance by which a certain quantity of a coloured compound can be absorbed. The last-named process allows of a very wide range of colours being obtained.

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