Home >> Photography Theory And Practice >> Silver Iron Printing Papers 627 to Various Processes 691 >> The Negative GeneralRemarks_P1

The Negative General Remarks on Photographic Negative Processes 190

silver, process, image, plate, solution, emulsion and development

Page: 1 2

THE NEGATIVE : GENERAL REMARKS ON PHOTOGRAPHIC NEGATIVE PROCESSES 190. The Wet Collodion Process. Of the many different photographic processes which were practised before the introduction of modern gelatino-bromide plates and films, the only one which is still commonly used commercially (photo-mechanical reproductions, copying orig inals for enlargement, etc.) is the wet collodion process. In this process each sensitive plate is prepared by the user immediately before it is required (the plate not being dried until the various stages of the process are finished) by coating a sheet of glass with a layer of collodion containing a suitable mixture of bromides and iodides. As soon as the collodion is set, the plate is immersed in a solution of silver nitrate, and by interaction between the silver nitrate and the ammonium and cadmium bromides and iodides there are formed in the collodion film silver bromide and silver iodide, whilst the ammonium and cadmium nitrates remain in solution in the excess of the " silver bath." After draining, the plate is exposed in the camera and taken to the dark-room. However long the exposure given to the plate, no visible image appears, a condition of things which we describe by saying that the image is latent. It can be rendered visible only by the process of development, i.e. by treatment of the plate with a solution of reducing substances (combining readily with oxygen), e.g. a solution of ferrous sulphate, with the addition of an acid to retard the process and render it more regular, which reduces to the metallic state the silver of the silver nitrate impregnated in the film. This silver is deposited in minute particles only at points on the plate which have been exposed to light, the amount of silver deposit increasing with the amount of exposure and thus giving an image. This method of development by precipitation of the silver contained in the developer is always spoken of as physical development, After rinsing, this image is freed from the silver salts which served as a temporary support for it by treatment with a solvent of silver iodide, e.g. a solution of sodium cyanide. This process is called fixing. The image, viewed by transmitted light, is opaque at the points where the silver is deposited ; viewed by reflec tion, especially in front of a dark background, it appears white, as do almost all images obtained by physical development.

For details of these processes the special manuals on the subject must be consulted.

z 9 z . The Gelatino-bromide Process. The manufacture of gelatino-bromide plates, films, and papers is exclusively an industrial process. The emulsion of which the sensitive coating is made is obtained by mixing under suitable conditions, in the presence of gelatine, solutions of silver nitrate with the iodide and bromide of potassium. In this way very minute crystalline grains of silver bromide, with a little iodide, are formed. After washing, in order to get rid of the residual soluble salts, the emulsion obtained, which is a milky suspension of these crystalline grains in the gelatine, is coated on the desired base and put on the market after sorting, cutting-up, and suitable packing.

When such an emulsion is exposed to light, no change is directly observable, except perhaps that after a fairly long time a slight darkening of the emulsion (which is yellowish-white in its normal condition) takes place. The action of the light has resulted in the formation of a latent image, just as was shown to be the case in § 190.

The negative image only appears when the emulsion is treated with a complex solution, the developer, of which the essential constituent, the developing agent itself, is a reducing sub stance chosen from amongst a fairly limited number of chemical products, most of which are closely related to the aniline dyes. This process of chemical development is nearly always used, the image being formed by reduction to the metallic state of the silver which, in the grains affected by light, was combined with the bromine and the iodine. The silver obtained by chemical development is usually in the form of very minute matt black particles, the agglomer ations of which retain to sonic extent the appear ance of the original crystals, but have a spongy structure comparable with that of coke. After rinsing, the excess of silver halides is removed from the image, otherwise they would reduce the contrasts and after a time would cause a change of appearance. This fixation is carried out in a solvent of silver bromide (the principal constituent of the emulsion), which is generally a solution of sodium thiosulphate. The negative thus obtained is then washed to remove the soluble salts, and finally dried.

Page: 1 2