Negative Emulsions Positive Emulsions Gelatin
In general Photographic materials are of two types;
Negative materials and Photographic papers are made by coating an emulsion upon the base. Negative emulsions must have a relatively wide flexibility in terms of sensitivity since they are used under conditions mostly beyond the control of the unprofessional photographer.
Typically Photographic film contains tiny crystals of very slightly soluble silver bromide "grains" suspended in a gelatin matrix and the resulting gelatin dispersion is melted and applied as a thin coating on a polymer base. However the exact preparation methods are closely guarded secrets, only the basic procedures are well known.
"Negative emulsions are suspensions in gelatin of sliver bromide containing a little iodine, from which most of the soluble salts resulting from the precipitation have been removed by washing." 1
Three separate operations make up the preparation of negative emulsions;
Solutions containing 10% or more of the soluble halide plus Silver Nitrate make up photographic emulsions. The finished emulsion contains a lot more gelatin than that used in emulsifiaction. Silver is added to the bromine solution under fixed temperature and rate of addition, to produce the despersion required. The silver bromide that precipitates is ripened by heat in the presence of a solvent until the process of growth and of coalscence and recrystallisation are completed. On finishing the previous processes gelatin is added, an amount sufficient so that on cooling the emulsion sets to a firm jelly. It is then squeezed through a grid under pressure to cut it into small pieces. Then the emulsion is washed in chilled water to remove soluble salts, made up of alkali nitrate produced from the reaction and excess of alkali halide, this is done until the diffusion is complete. Now the emulsion is remelted and a further quantity of gelatin is usually added. It is held at a moderate temperature until the required sensitivity and contrast is produced by after ripening.
The two stages of digestion are different in the effects they produce and their functions. Stage one establishes the grain distribution of the silver bromide. The process of after ripening affects their distribution only a very little, if at all. Before washing, the emulsion has low sensitivity and contrast. During the after ripening the grains undergo changes in their reation to light, owing to their aquisition from the gelatin of the sensitized materials.
"A finished liquid negative emulsion contains about 6% of gelatin (dry weight)and 4% of silver halide. It is coated on the support to a depth of about 0.3mm, from which it dries down to about 0.025mm in thickness. The coating on paper shrinks from about 0.1mm, when wet to 0.04mm when dry. Negative materials therefore contain about 1.5mg of silver halide per square centimeter, and papers, about 0.5mg."1
Paper emulsions may also consist of silver bromide free from soluble salts; but the emulsion of most photographic papers is composed of AgCl or a mixture of chloride and bromide which has not been freed from soluble salts by washing. The exact conditions of precipitation, the gelatin and the formula all effect the quality and sensitivity of the emulsions. The silver chloride grains produced can only be seen clearly under a microscope and are very very finely dispersed. Paper is an absorbant support which is used instead of an impermeable support because of the presence of soluble salts which may crystallise.
Gelatin is a protien originally extracted by the boiling of animal hides and bones, but now synthetically produced, and is a hydrophillic (water loving) polymer. Gelatin used for photographic processes is usually made by an alkaline extraction process using bovine bones. Photographic gelatin is much purer than that used in foods. The quality and origin of the raw materials used in the gelatin process, the conditions (ph, temperature, etc.) and the presence of contaminents are very important to the production of the gelatin.
Gelatin solutions act as liquids when warm and set relatively hard when cool. When coated and dried on a substrate it is flexible and resistant to a certain amount of damage, but absorbs water readily, as in the development process. Formaldehyde is one substance used as a hardener in the dispersion of gelatin to improve the physical characteristics of the coating material. The processes of development, fixation and washing means that the emulsion needs to be able to resist disitergration or dissolving at normal temperatures. Gelatin does this and swells rapidly, absorbing water and dissolving development chemicals.
Gelatin can significantly affect the properties of the final photograph. In preparing the emulsion, coating and adhesing it onto the film base, the wetting characteristics of the coating material in the development, the physical characteristics of the dried developed product, and for the long-term stability of the developed image, it is essential to control the characteristics of the gelatin.
Gelatin also acts as an anti-coagulant or stabilizing suspention of particles in the preparation of a photographic emulsion. Throughout the preparation, ripening, and coating process the silver halide formed in fluid gelatin remains evenly distributed and does not precipitate out of solution. By regulating the concentration of the gel, the temperature, and the rate of addition of the components, the grain size and distribution can be controlled to meet particular requirements.
Gelatin molecules, in the silver halide dispersion, adsorb at the surface of the silver halide grain,stabilizing the dispersion by surrounding the grain and creating a barrier. The radiation sensitivity of the grain is affected by the adsorbed layer and this also makes reduction by developers, more controllable.
Author: Lee Slimming (document modification date: 28th May 2004)