الكيماويات .. كتاب فن التحميض والطبع والتكبير

Chapter III

chemicals

. Developing

It is customary to name the chemical treatment to show the image and fix it with a name

In order to understand what takes place during the acidification, it is necessary to explain the theory of image formation on the sensitive layer, whether in film or sensitive paper, as well as the theory of display and fixation.

Brief theoretical background on acidification:

When the photosensitive layer is exposed to light, instability occurs in the silver halide particles present in the sensitive layer, but this does not appear until after the chemical treatment called showing the image. Firstly . Developing:

This is a chemical process for reducing silver salts (mostly silver chloride and bromide) to silver. This is done by transferring electrons from the show solution to the silver salts, and the silver is deposited in the form of black granules. These ordered electrons come from .

Some organic substances, especially those with an aromatic ring, such as hydroquinone, metol, etc

The reduction process is aided by the presence of an alkaline substance, which facilitates the decomposition of the acidic hydroxyl group. So we add sodium hydroxide or sodium carbonate to the solution. Sodium sulfite is added. Sulphite to prevent
Hydroquinone reacts in atmospheric oxygen and removes hydroquinone oxidation products (i.e. acts as a preservative). It is noted that the brightener works on silver salts that have been exposed to light and those that have not been exposed, but its effect on salts that have been exposed to light is faster and stronger. Therefore, if the film is left in the show solution for a longer period than required, it becomes completely black and the film density increases significantly. Therefore, some bromide salts are added to delay the transformation of salts Silver that has not been exposed to light into silver, such as potassium bromide. Pot. (bromide

Second: stopping by using a stop bath solution:

After removing the sensitive film or paper from the exposure solution, we find that the reaction is still going on. To stop this reaction immediately, an acid or an acidic substance such as acetic acid or citric acid is added.

This solution can be dispensed with by removing the film from the show solution shortly before the end of the show time and washing the film in water before transferring it to the fixing solution.

Third - Fixing:

Fixation means the process of removing silver halides (bromide and silver chloride) that were not exposed to light and were not reduced during the exposure process, thus fixing the film and not blackening over time due to the disposal of the silver salts in it.

This is done by dissolving silver halides in a solution of a substance in which these salts are soluble. There are many materials, the best of which is sodium thiosulfate. Sod Thiosulphate, or what photographers call Hypo

And silver halides dissolve in thiosulfate solution to form a series of salts

Silver thiosulfate that dissolves in water. In the case of not using a stopping solution, it is preferable to add an acid to the fixing solution to stop the reaction of the show solution, which is still continuing to work in the depth of the sensitive layer. One of the best materials used for this purpose is Sodium Metabisulfite or Sod Sod. Meta-Bisulphite, Sod. Bisulphite

It may be added to the fixing solution a substance that increases the hardness of the sensitive gelatinous layer to make it less susceptible to scratching, and this substance that increases the hardness is Hardner. It may be a solution of formaldehyde gas in water called formalin, potash alum, or chrome alum.

Fourth - Washing solution:

The film is washed with water to remove any silver thiosulfate salts present in the layer. The sensitive film or paper is not exposed to change with the passage of time. Therefore, the sensitivity is even

We find that the washing time is long.

The water must be soft and not hard, otherwise we will find calcareous deposits on the surface of the film or paper. Hard water can be treated with water softeners such as sodium hexametaphosphate. Hexameta Phosphate In order not to appear spots on the film as a result of the drying of the water droplets on it, drops of a liquid that reduces the surface tension of the water can be added to the last amount of washing water. This liquid is called the wetting agent.

And some photographers add a substance that helps to get rid of the effects of hypo and silver salts called Hypo-neutralizer
Fifth: Solutions or chemicals that you need in the colored acidification:

. As a matter of fact, the color development is not different from the development of black and white films except in some additions. The sensitive surface of colored sensitive film or paper also contains silver halides that are reduced to silver, as happens in black and white acidification, but there are color problems that give a specific color in each layer in the areas where silver has been reduced. Therefore, the silver image is discarded to leave only the colored image. This is done with a solution that is dissolved with a solution of silver particles called bleach. This solution is usually added to the fixing solution to be called the Bleach-Fix solution. Due to the multiplicity of layers of the colored sensitive layer, we need a longer time to appear, and this is overcome by raising the temperature of the solutions. Special substances can also be added that help the chemicals reach the deep areas quickly, and this solution is called the permeation solution.

Colored films often need to be treated with a regulator solution to improve the properties of gelatin. Especially after treating it in high temperature solutions. The film must not be washed after being treated in the buffer solution.