International projectionist (Jan-Dec 1950)

VOLUME XXV JANUARY 1950 NUMBER 1 The 35-mm Projection Positive Film By ROBERT A. MITCHELL ///. The Preparation of Prints THE chief light-sensitive compound in photographic emulsion is, as we have seen, silver bromide, an insoluble pale yellow substance. Emulsion is prepared and kept in the dark until such time as it is exposed to the action of light in the camera or printer. The action of light on the emulsion is often not understood clearly. If an exposed film is taken out of a camera or printer and examined in the light, no change of any kind can be discerned. The emulsion looks exactly like unexposed emulsion. Yet a change of some sort undeniably takes place, for if an exposed film is immersed in a solution of a weak reducing agent, those portions of the emulsion which have been exposed to light turn black; those exposed to dim light turn gray, and those parts not exposed to light remain cream-colored. The invisible picture-image produced in the emulsion by exposure in a camera or printer is called the latent image. The developer reduces the silver bromide to finely-divided metallic silver wherever the emulsion has been acted upon by light; and the stronger the exposure, the more complete is the reduction effected by the developer. Developing Solutions An ordinary reducing agent will not do for use as a photographic developer, however, because it would reduce all of the silver bromide, whether it had been exposed to light or not, completely fogging the film. A developer is a special kind of very weak reducing agent. It is compounded to exert a selective action on the latent image, reducing only those particles of silver bromide which have been exposed to light. The reducing agents most commonly used in developing solutions are metol, hydroquinone, pyrogallol, glycin, and amidol. The action of each of these developers is increased and modified by the presence of other chemicals in the developing bath — an alkali, a restrainer, and a preservative. Occasionally a fifth chemical which serves as a "stain preventive" is encountered in developing formulae. Most developers (and here we are speaking of that component of a "complete developer" which does the actual developing) are neutral or slightly acid in water solution. In this condition they do not act readily as reducing agents. Comparison of printing processes. gjl | => Brll . 3 Hi 4i|ip Step-printed Continuous' printed The sodium salts of the common developers, however, actively reduce silver bromide. For this reason sodium corbonate or some other alkali is added to the developing solution. In addition to* converting the developer into an active sodium salt, the alkali softens the gelatine and accordingly permits the developer to penetrate it more readily and act upon all of the silver bromide particles affected by light. Restrainers, Preservatives, Anti-Stain But the alkali renders the developer so active that a chemical fog may result from reduction of silver bromide crystals which have not been exposed to light. To control the action of the developer, thus making it as selective as possible, a restrainer of potassium bromide is added. All reducing agents have a strong affinity for oxygen. In the case of photographic developing solutions, the air may oxidize them so quickly that they become discolored and unfit for use a short time after being prepared. A substance which combines with the oxygen of the air more readily than does the developer is accordingly added to the mixture. Sodium sulfite is such a preservative. In a few developing solutions, especially those intended for motion picture film, an anti-stain such as citric acid is utilized. Citric acid prevents stains and discolorations by combining with oxidation products and metallic impurities to form inert complex salts. A complete photographic developing solution therefore contains (1) developer, (2) alkali, (3) restrainer, (4) preservative, and (5) anti-stain. Many special INTERNATIONAL PROJECTIONIST January 1950 1^