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Silver Print-Out Papers 520

image, light, colour, exposure, paper, yellow, salts and chloride

SILVER PRINT-OUT PAPERS 520. General. We shall consider here chiefly those papers in which the sensitive material is silver chloride associated with an excess of soluble silver salts (nitrate, and organic salts, such as citrate, tartrate, etc.).

According as the printing is carried out in weak or strong light, the colour of the image approaches blue-violet (the colour of image given by pure silver chloride), or red (the colour of image given by citrate or tartrate of silver ; these salts are less sensitive, and so require a very strong light). The degree of humidity of the film considerably affects the colour of the image. A damp condition increases the sensi tivity of the organic salts without affecting that of the chloride, and so, for a given exposure, the image is redder than that given by a dry film.

With these print-out papers the image con sists of silver in a very fine state of subdivision (colloidal silver), adsorbed (solid solution) in the excess of silver chloride, and thus protected against certain reagents which attack metallic silver.' The image is generally more contrasty when it has been printed by weak light than when strong light has been used (see § 51o, effect of coloured light If the exposure to light is prolonged suffi ciently to saturate the remaining silver chloride, the silver which is liberated may appear on the upper surface of the film in a coherent state with a greenish metallic lustre ; the image is then said to be bronzed or metallized.

Fixation of prints obtained by direct darken ing occasions a general weakening of the image which is chiefly visible in the lightest tones. This effect varies with the type of paper and also, though to a smaller extent, according to the conditions of printing (colour and intensity of the light). This regression of the image makes it necessary in all cases to continue the exposure until a darker image than is required is obtained ; a few preliminary trials will show the depth of printing required for retention of the finest details in the high lights.

During fixation the image becomes yellow" in colour and after drying acquires a disagreeable yellowish-brown colour. This is remedied by toning the image (before, during, or after fixing), that is to say, by substituting another metal (gold, platinum, etc.) for a portion of the silver, or by converting the silver into a coloured compound (e.g. selenium toning).

A very faint image on a print-out paper can be brought to a normal depth either by deposit ing silver on it by physical development (which, in this case, can be regarded as an intensification process), or by exposing the paper to an orange or red light.

521. The sensitive film of print-out papers is not affected in the least by the light which passes through a red, orange, or deep yellow filter, even on long exposure to sunlight, but the colloidal silver formed during a short exposure to light under a negative plays the part of a panchromatic sensitizer. The parts where an image has already been formed can, therefore, darken during a second exposure more where the silver has already been formed than in other parts. The second exposure to light thus acts as an intensifier (continuing radiation.) This phenomenon, which was noticed in Daguerreotype images by E. Becquerel in 184o, and which was explained by Liippo-Cramer in 1909, is of particular importance if soft prints are required from very vigorous negatives ; in this case exposure under the negative should be carried just far enough to obtain full detail in the shadows.

The continuing action of yellow or red light can be used for increasing the contrast of an image which has been made from a negative with weak contrast, but in this case the second exposure should be made through the negative in order to graduate the intensification of the image (H. J. Channon, 1909).

522. Deterioration of Print-out Papers. The ageing of print-out papers containing soluble silver salts is shown by the appearance of a yellow colour, due to the spontaneous reduction of minute quantities of silver in a very fine state of subdivision. This yellow colour changes to brown and finally to black with a metallic lustre. This change does not occur in complete absence of moisture, and so these papers are generally packed in waterproof paper after having been dried and separated by sheets of straw paper which acts as a desiccating agent. The deterioration of print-out papers is very rapid when they are exposed to the action of certain gases and vapours (hydrogen sulphide, formaldehyde, hydrogen peroxide formed during the oxidation of resins, etc.).

These papers should be kept in a dry place, away from any chemical operations. These recommendations are particularly important in the case of opened packets.

If the colouration of the paper by age is not very pronounced, it disappears in the course of somewhat longer fixing, unless the fog has been " consolidated " by toning before fixing.