عملية الاظهار .. كتاب أسس التصوير الضوئي

DEVELOPING process

It is called the acidification process in colloquial language - and in the sites it is a process of reducing silver salts that have been affected by light and converting them into black metallic silver deposited in gelatin, so the latent image is thus transformed into a negative image.
Showing solutions differ in their composition. And accordingly, in its effect on the sensitive layers. But they usually all share the following components:

Presentation solution installation

First - Developing Agent:

Brightening agents are actually reducing agents whose function is to reduce the silver salt (which is usually silver bromide in most modern films) and turn it into black metallic silver.

There are many chemical compounds (1) that reduce. However, it is required that it is suitable as an appearance for images to distinguish between areas that have been exposed to light and those that have not been exposed. The following are examples of some of the reducing agents used in photography:

1) Metol

2) Hydroquinone

3) Glycine

4) Pyrogallol


e) Chloroquinol

6) Amidol

7) Para-Aminophenol Hydrochloride

8) Paraphenylene Diamine.

We will talk about the first three, which are the most used:

_ Metol: It is a salt that is easily soluble in water - and it may sometimes cause skin infections for those with sensitive skin. This effect decreases as the degree of purity increases. This salt is considered a reducing agent of moderate activity, so the degree of its effect in the areas that were not exposed to sufficient light is close to the degree of its effect in the areas that received a large amount of light. So . from the light. Therefore, this salt is considered a reducing agent that does not lead to high contrast.

_ Hydroquinone: It dissolves easily in warm water - its solubility decreases in cold water, and the effect of hydroquinone as a reducing agent stops at temperatures below 10 °C and its activity decreases between 10-15 °C. Sufficient before its effect begins in areas that did not receive a large amount of light. As a result, the contrast ratio in the images increases with the increase in the proportion of hydroquinone in the acid.

And reduces its sharpness in the acid appearance, the presence of potassium bromide salt. And by using an acid that contains each of hydroquinone, mitol, and potassium bromide, it is possible to obtain negative images with a medium degree of contrast in which details appear in both the shadows and high-light areas.

_ Glycine: a salt that is difficult to dissolve in water and dissolves quickly in alkaline attempts. It is frequently used in show solutions prepared to obtain fine granules. Glycine is an inactive reducing agent, as it takes a long time to complete the reduction process, and it leads to low contrast results.

Second - Preservative

Reducing agents are oxidized as a result of their combination with air oxygen. Therefore, compounds whose function is to slow down the oxidation of the presenting agent are added to the group of salts that make up the acid. They are called preservatives - and in addition to their work - they interact with the unwanted reduction products and turn them into materials that are harmless to the image. Sodium Metabisulfite
On the other hand, there is harm resulting from an increase in the percentage of preservatives than is necessary, as silver salts dissolve in solutions of preservatives. An example of conservative compounds are the following materials:

1) Sodium Sulphite

2) Sodium bisulfite
Sodium Bisulphite

3) Sodium metabisulfite
Sodium Metabisulphite

4) Potassium Metabisulphite and the most used of these substances is sodium sulfite. It can be obtained in the form of crystals or powder, and both forms perform the same function, noting the following:

A) Crystalline sodium sulfite is more susceptible to damage than powder, as it turns into sodium sulfate when exposed to air. And the last salt does not act as a preservative.

b) The required amount of crystals is twice the required amount of powder. In the sense that if it is necessary to put an amount of 50 grams of crystalline sodium sulfite as a preservative in the apparent solution, it is possible to suffice with its weight.
25 grams of powdered salt to replace it.

Third - Activator or Alkali Accelerator or Alkali:

The activity of the brightening agent decreases in the acidic medium - and as a result of the addition of preservatives, the acidity of the brightening solution increases