NA_l'URE_vo_L_J13_24_JANU_AR_v_11185______ pRQDLJCT REVIEW~----------JJJ The importance of photographic gelatin from Yves Lestra The demands of modern photography have led to the evolution of extremely sensitive photographic emulsions. The vital role of gelatin in the emulsion is easily overlooked. IN the emulsion of a photographic film, After coating, the gel formed keeps the water, with only the first extraction pure silver chloride and bromide darken and de­ silver salt crystals on the support during enough for the exacting requirements of compose into their constituent parts when drying and development, and when the photography. exposed to actinic light. This darkening is emulsion is exposed to light, the gelatin a very slow process unless it is accelerated prevents the inverse reaction of the photo­ Modified gelatins chemically by use of a reducing solution or lysis by fixing the liberated halogen. Gelatin It is possible to chemically modify gelatins 'developer', which liberates silver from the also prevents fading of the latent image in order to provoke floculation simply by exposed grains without affecting unexposed resulting from the photolysis. lowering the pH. This type of gelatin is grains. This process furnishes most of the During development, the swelling of the increasingly used for floculation of energy necessary to obtain an image. It is gelatin, caused by the action of the added emulsions, replacing sodium or ammonium worth considering how an emulsion is reagents, allows the reagents to diffuse sulphate or washing by dialysis. produced, because the properties of the through the emulsion layer. The property emulsion, almost always containing gelatin of the gelatin to be tanned by various sub­ Auxiliary gelatins as the major colloid, are crucial in deter­ stances allows rapid development at high Photographic materials do not consist of mining the quality of the image obtained. temperatures. a single light sensitive layer. Other layers Since gelatin is extracted from the -auxiliaries-are coated onto the support Emulsion manufacture collagen of bone or skin, most preparations before, or at the same time as, the emulsion The initial step in the manufacturing inevitably contain impurities. Some are layer. The support itself is usually synthetic process involves the precipitation of silver detrimental to the photographic process and its surface has little affinity for the chloride or bromide crystals from solutions and must be removed (dust, heavy metals, hydrophilic emulsion. An intermediate of alkaline bromides and silver nitrate. The grease and 'fogging agents'). Albumens, layer or substratum is required, containing reaction takes place in a colloidal medium polypeptides containing cystine and nucleic the necessary ingredients to establish a in order to keep the insoluble crystals of acids act as 'restrainers', blocking the bond between the emulsion and support. the silver salts in suspension. formation of sensitivity sites or inhibiting Colour films or papers contain several The emulsion then undergoes a period of the development of silver halide grains. emulsion layers, each sensitive to a differ­ physical ripening or ageing during which Some impurities, however, have useful ent radiation band. The layers must be the crystals grow larger. Any soluble salts properties acting as 'sensitizers' and faci­ separated by intermediate layers to prevent remaining in solution are then removed by litating the production of silver or silver diffusion from one colour layer to another. washing. For many years this involved sulphide centres on the halide grains. The surface of the emulsion layer must gelling of the colloidal suspension and di­ Photographic gelatins can be classified be protected mechanically from external alysis of the! soluble salts by washing the into four groups: Inert gelatin, without contact. This is the role of the supercoat. gel in cold water. Today, most manufac­ sensitizers or restrainers; active gelatin, rich Like all auxiliary layers, it must have no turers rely on flocculation of the colloid by in sensitizer and low in restrainer; retarded photographic influence on the emulsion. salts or organic reagents, including the gelatin, low in sensitizer and rich in re­ They are commonly inert gelatines. modified gelatins, which flocculate in acid strainer; and retarded active gelatin, rich In order to compensate for the tension conditions. in both sensitizer and restrainer. created on one side of the support by all Next comes a second, or chemical The task of the photograhic emulsion the layers, a backing layer is coated on the ripening. At this stage certain critical manufacturer is to find the best gelatin for other side of the support. This is often additions are made to encourage the form­ a specific application. The gelatin manu­ coloured in order to avoid the halo created ation of sensitivity centres on the crystals facturer must produce the gelatin to an by the light ray as it reflects on the film. of the silver salts. exact specification and ensure that quality Some colour films can contain as many as FinaHy, various additives are used to stop is maintained from batch to batch. a dozen layers of chemicals. The sophisti­ the ripening process, to encourage the cated coating machines in use today allow coating of the emulsion and to harden the Gelatin production the simultaneous coating of most of the emulsion and make it insoluble to the The conditions under which gelatin layers at speeds of 100 to 150 metres per developer. The emulsion is then coated into extraction occurs determine the properties minute. Each layer is only some 10 µm its support and cooled. It is coated with a ·of the end product. In particular, the liming thick, so it is easy to appreciate the import­ protective layer of gelatin which is photo­ process has to be closely controlled. ance of the physical qualities required of graphically inert. Chemical changes occurring at this stage the gelatin. include the hydrolysis of amide groups to To supply the customer with a photo­ Role of gelatin carboxylic groups (destroying the triple graphic gelatin with precisely the properties Gelatin is generally the colloid of choice for helix of collagen), the elimination of many required, the manufacturer must be able to the manufacture of emulsions. During pre­ impurities (mucopolysaccharides, tyrosine, control the production process very accu­ cipitation and physical ripening it keeps the sulphur), the transformation of the sulphur rately, and quality control must be silver salts in suspension and is strongly in cystine to thiosulphate and the stringent. In addition, it is useful for the absorbed at the surface of the silver halide degradation of ribonucleic acid to customer to appreciate the problems of the crystals, retarding their growth. The ability nucleotides. manufacturer when specifying require­ of gelatin to form a gel allows the elimin­ After liming, the raw material is washed ments for an emulsion. D ation of unwanted soluble salts by dialysis, with water to eliminate as much lime as Yves Lestra is an engineer at the head office of and its high viscosity facilitates the coating possible, and then 'delimed' in a bath of Rousselot S.A., 8 Rue Christophe Colomb, of the emulsion onto the support. dilute acid. Gelatin is then extracted in hot 75008 Paris, France. © 1985 Nature Publishing Group.