عمليات الغسيل والإيقاف .. كتاب أسس التصوير الضوئي

(washing and stopping operations)

This is done in a stream of running water, the temperature of which is close to the temperature of the apparent try, so as not to cause any damage to the gelatin layer as a result of the temperature difference, and the washing period must be sufficient to remove the effect of the apparent solution attached to the gelatin. But due to what may happen during the washing period of the continuation of the effect of the solution appearing on the sensitive layer, it is better to stop the reduction process immediately by means of the stopping solution after dipping the film or the sensitive plate in water, and this is sufficient.
The washing takes between one and a half minutes (stop bath).
We have already mentioned about the compounds that make up the appearance solution

We always add an alkaline substance within the solution’s appearance compounds, which is often sodium carbonate, and the purpose of adding alkali is to neutralize the acidity of the preservative compound first, then increase the alkalinity of the solution to activate the reduction process, as the reduction process is activated in an alkaline medium and stops in an acidic medium
We return now to our topic, which is the stopping bath, so we say that it is a very dilute acid solution, and by placing sensitive film, board, or sensitive paper in it, the reduction process stops immediately. There are acids or materials that perform this purpose, including very diluted acetic acid. However, it was observed that if the alkali in the apparent solution was sodium carbonate, then carbon dioxide gas would escalate as a result of the interaction of sodium carbonate with the acid as a component. Thus, gas bubbles appear on the surface of the gelatin layer.

It was possible to dispense with acetic acid by using a dilute solution by shaking the solution in order to prevent the formation of a precipitate of chrome hydroxide, and sometimes other materials are used other than those we mentioned, whose solutions are acidic mediums that stop the reduction process. Potassium Chrome Alum Note with When the color of the stop acid changes to a prepared color, it must be changed as well, and it must be changed after it is finished, as it becomes ineffective when the colored show solution reacts with the stopping acid forming a neutral solution