Causes and solutions for film yellowing
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During the plate making process, the yellowing of the copy, camera, and computer output film, or browning, is mainly caused by a water-insoluble sulfide, sulfur, in the emulsion film. There are several factors that cause the photographic film to yellow.
1. When preparing the developer, it is not prepared strictly according to the proportion of the formula. The alkalinity of the developing solution is too strong, the developing solution is easily oxidized and discolored, the developing solution is used for a long time, the developing effect is remarkably lowered, the developing solution is oxidized to reddish brown, and the developing performance is easily weakened or even disappeared, so that The film exhibited by these developing solutions is dyed yellow or brown due to discoloration of the surface film.
Under normal circumstances, the photographic, copying, and electric splitting sheets all use hard line pieces. The high contrast and high stability developing solution all use a bag of medicine, which can continuously wash 8m2 film. The developer should be used in time. Replace with new drugs.
2. The fixing solution is sodium thiosulfate + water + sodium sulfite. Formulation of fixing solution: sodium thiosulfate: 350 g, sodium sulfite: 8 g, water: 1000 mL.
The purpose of the fixing is to permanently fix the image of the density and contrast obtained after the development is completed. The function of the fixing solution is to neutralize the alkaline substance of the developing solution with its acidic substance to stop development, fix the image, and dissolve the silver halide which is not photosensitive. The sodium thiosulfate in the fixing solution is subjected to too strong acidic substances, or the temperature is too high, and free sulfur is precipitated. These free sulfur can chemically react with the silver particles on the film to form silver sulfide. The color of silver sulfide is brown, so the color of the film is dyed brown. Therefore, the acidity of the fixing solution should not be too strong, and the temperature of the fixing solution should not be too high.
3. The improper concentration of the fixer can also cause the film to yellow. Due to the speed of the fixing speed of the film, it is related to the amount of sodium thiosulfate added in the fixing solution. The concentration of the fixing solution is high, and the fixing speed is fast. If the concentration of the fixing solution exceeds 45%, the fixing liquid is too thick, and the film is too thick. The unsensitized silver halide has the least solubility, but reduces the solubility, so that it has a reverse effect. Therefore, when the fixing concentration is not properly controlled, the effect of the fixing solution can be weakened, and the fixing is not deep and yellow.
4. The fixing solution is old and insufficiently fixed. If it is not thorough, it can make the film yellow. After the fixing solution is used for a long time, the performance of fixing is degraded, and the fixing effect is lacking, so that all of the unsensitized silver halide is dissolved. These undissolved silver salts, which remain in the emulsion film, cause the film to appear yellow after rinsing.
5. After the photographic film is placed in the fixing basin during fixing, be sure to pay attention to the shaking operation method. It is required to shake the developing pot to shake the fixing liquid, or to flip the film up and down. The purpose is to completely dissolve the photographic film and thoroughly dissolve the silver halide in the fixing solution. If the photosensitive film is placed in the fixing basin, it will not be flipped or shaken. When it is fixed in a standing state, insufficient fixing occurs, and residual sulfide adheres to the film, which is yellow and brown.
6. Yellowing caused by insufficient washing after fixing. After the film is fully fixed, it is placed in the rinse tank.