كيمياء الضوء .. كتاب فن التصوير الفوتوغرافي

The chemistry of light

We learned how to display a film in a basin, and it is a satisfactory method for obtaining normal results, but the photographer who is not satisfied with ordinary results can reach better results if he understands what happens in each endorsement step, so he can choose the appropriate film for the type of image, and he can endorse it in the right way. To reach certain results that he wants, and he will not only get a better image, but also increase his enjoyment in practicing this art.

As we explained previously, the film consists of a transparent plastic film, one of which is coated with a thin layer of a light-sensitive substance, and in addition to that layer, there is usually a preservative layer of transparent gelatin over it. And there are in the ordinary wrapped films, in addition, a lining of colored gelatin. This lining prevents the film from wrinkling, and absorbs light that penetrates the photosensitive layer. During the endorsement process, the colored lining is completely bleached.
The 35 mm films do not contain this lining, but form part of the plastic membrane.

When light falls on the silver compound in the sensitive layer, it disintegrates and releases silver in the form of additional metal.
This reaction is very slow and absorbs a large amount of light to liberate an acceptable amount of silver, but if you dip a sensitive layer exposed to a little light in a chemical solution called appearance, this solution completes the reaction initiated by the light during the exposure process. The apparent solution, then, helps to cause a precipitation quickly, and with a sufficient density to cause a photo.

- Appearance The Developer

Appearance consists of a mixture of several chemical solutions in certain proportions, each of which has a specific and important function. The task of the manifestation is to break down the silver bromide, and to free the silver metal from it, but its delicate balance composition makes it need help to start its work. This assistance is provided by the microscopic silver granules that are liberated by the light falling on the sensitive layer during the exposure process. These liberated granules play the role of an agitator, so the film starts its work and releases a larger amount of silver that collects around the first granules. The amount of silver deposited by the effect of the film is directly proportional to the degree of exposure received by the film.

All types of display solutions, except for a few, contain the following elements:

1- Endorsement factor.
2- Accelerator.
3- Preservative.
4- Retarder.
E - water.

Since the proportions of these ingredients, in appearance, vary greatly according to the intended results, knowing the role of each of them allows you to visualize the expected results when you move from using one formulation to another if you are mixing the appearance yourself.

1 - The Developing agent

The endorsement agent is the basic material in the endorsement solution. It is the chemical that completes the light-initiated decomposition of silver bromide upon exposure. Exposure factors differ from each other in several areas, such as their vitality and ability to distinguish between exposed and unexposed silver bromide (and this determines the way the silver granules produced by the contrast are placed), the size and intensity of their interaction with air oxygen. Each of these properties has specific cases of application, and its quantity can be chosen in the solution to reach the best results according to different conditions.

2 - Accelerator

The accelerator is the alkaline substance (the opposite of the acid) added to the endorsement to speed up the endorsement process.

There are many types of alkali. If what is required is a high-speed appearance that gives a contrasting endorsement, we use a strong alkaline substance such as sodium hydroxide or a highly concentrated solution of sodium carbonate. But if a medium-fast appearance is required, we use a medium-strength alkaline substance such as sodium carbonate or sodium metaborate, and we use Borax to obtain a gentle appearance with a slow effect. You can dispense with all the previous alkaline substances and rely on the alkaline effect of Sodium Sulfite, which is used as a preservative in the appearance used to obtain fine granules.

3- The preservative

There is a tendency for all bleaching solutions to interact with the oxygen present in the air, which makes them useless after a period of time even if they are not used. Therefore, sodium sulfite is added to all bleaching solutions, which delays this reaction to an infinite degree, and plays the role of preserving the appearance.
But the disadvantage of this substance is that it dissolves silver bromide to a good degree, and this bromide appears under the influence of the bleaching agent in the solution, and gives an additional amount of silver that may precipitate at the previous amount that was deposited and thus increase the density of the crystal or scatter as the crystal is disturbed. In order to prevent the occurrence of these unwanted amounts of silver, a Restrainer is added to the appearance to prevent this distortion.

4- Retarder

The inhibitor works in two ways, either by delaying the dissolution of silver bromide or by forming a complex with the dissolved bromide that is difficult to develop. We use potassium bromide in most cases for this purpose, and it works in the first way, that is, it delays the dissolution of silver bromide, and it limits the process of endorsement itself.

5 - water

Water is in many respects the most important ingredient in endorsement solutions. This water must be pure, free of dissolved and suspended matter to an acceptable degree. Fortunately, drinking water can be considered sufficiently pure and suitable for photographic purposes. As for water that contains a high percentage of dissolved lime salts, it tends to form stratified sediments with most imaging solutions, and we usually add a small amount of a substance that removes lime from the solution and keeps it pure.