صناعة الطبقات الحساسة .. كتاب التحميض والطبع والتكبير

Sensitive Layers Industry

The sensitive gelatin layer is prepared by dissolving two quantities of potassium bromide and potassium iodide in a warm solution of gelatin, then silver nitrate is gradually added to them.

In complete darkness, they react according to the following equation: Potassium bromide + silver nitrate = silver bromide + potassium nitrate, potassium iodide + silver nitrate = Abu Deir silver + potassium nitrate. Silver bromide is included in the composition of fast sensitive films, and adding a little silver iodide increases the speed of its sensitivity. Silver iodide is not used alone in the composition of the sensitive layer except in the mechanical imaging processes using the wet collodion method. And if the amount of silver iodide exceeds the necessary limit, it results in damage called chemical fog.

Silver bromide is used in the manufacture of positive sensitive papers, which become susceptible to light (these papers are called Bromide Papers) and are suitable for enlarging images. As for the papers used in contact printing, it is better that they be slow in sensitivity, and silver chloride is included in their manufacture (see Chloride Papers). There are papers whose sensitivity ranges between two degrees, in which both silver chloride and silver bromide are used together. They are called Chlorobromide Papers and are used either for printing or for enlarging.

The silver salts resulting from the previous two equations do not dissolve in the gelatin solution, but remain deposited in it in the form of small grains with a diameter of stars and an inch = 0.075 millimeters, and the sensitivity speed increases with the increase in size.
Then the mixture is stirred well to ensure that the salts contained in the gelatin are spread and evenly distributed in all parts of the mixture.

Speed ​​of Sensitivity: Speed ​​of Sensitivity The speed of sensitivity of the sensitive dough depends on several factors, which we show in the following: (1) The temperature of the mixture.

(b) The ratio of the amount of silver salts to the amount of gelatin.

(H) The type of silver salts and the type of gelatin.

(d) The degree of maturation of the mixture. What is meant by the word ripening is the period that the mixture remains at a certain temperature until it reaches the required sensitivity speed, as with increasing ripening the sensitivity rate increases,

(e) Adding some substances that help ripening, such as adding ammonia (1)

or sulfur compounds to silver nitrate. After the settlement processes (i.e. ripening) are completed, the cooling stage (Fig. 1) follows, by leaving the mixture at a low temperature, and the mixture is then carrying many products of chemical reactions between silver salts and other added salts, which requires washing the mixture with water, but due to the difficulty The penetration of water into the frozen gelatin mixture, so it is necessary first to break the frozen mixture into infinitesimal parts so that it is easy for the water to penetrate into it and then the reaction products (such as potassium nitrate and others) to be disposed of, while the silver salts (chloride - iodide - silver bromide) remain precipitated In gelatin due to its insolubility in water, the fragmentation process was carried out by placing the frozen gelatin in a large cylindrical container coated with silver and with holes 1121

Then it is cut into very small pieces in the form of “noodles” and washed well with water throughout. This stage (Fig. 1).

Then comes a new stage, which is the stage of re-dissolving the gelatin that carries silver salts, raising its temperature to a certain amount, and it remains for a specific period on which the speed of sensitivity of the sensitive layer depends, as well as some other characteristics that distinguish the sensitive layers, such as the contrast feature.

Other materials may be added at this stage that affect the spectral sensitivity, which are often dyes with specific properties. Other materials may also be added to resist damage to the sensitive layer as a result of long storage, or to help the gelatin spread evenly on the support.

Pour the sensitive paste onto the support:

And after the stages of preparing the sensitive dough are over, the last stage begins, which is pouring the sensitive dough onto the support, which may be made of glass, celluloid, or paper. The support requires a special preliminary treatment to ensure the adhesion of the sensitive dough to the support when it is poured on it, and this is done by means of an adhesive layer (its shape) that is usually a mixture of gelatin and barium sulphate and is known as Baryta. The function of the last mixture is not limited to just ensuring the adhesion of the sensitive paste to the support, but also to limiting the spread of rays, ensuring smoothing the surface of the gelatinous layer, and to it is attributed the glossy appearance of the casting.

Paper or film before acquiring knowledge and shiny papers.

We see in Figure (1) an explanatory statement about all the previous stages, which finally end with cutting sensitive papers and films according to the required standards, then packing them in boxes or cans after wrapping them in a special way that prevents them from being affected by light or weather factors.