The silver grains which form the developed image are held in a layer of gelatine. This gelatine is used in making the emulsion which is coated on the support to make the sensitive film.
Gelatine is a very interesting substance, and its characteristics are markedly different from those of most other chemical substances. Most chemical substances form crystals, and many of them are soluble in water. When they are dissolved in water, the solution is quite homogeneous, that is to say, alike in its properties in all its parts. Substances generally will dissolve in water to a fixed extent, dependent on the temperature. We say of one material, for instance, that it is soluble to the extent of 30%, meaning that a hundred parts of water will take up 30 parts of the material. If we heat the solution it will usually dissolve more, but then when it cools again the material will crystallize out so that whatever we do we can only obtain the fixed 30 parts per hundred remaining in solution.
Gelatine behaves quite differently to this. In cold water it does not dissolve but it swells, as if, instead of the gelatine dissolving in the water, the water dissolves in the gelatine. If the water is heated, the gelatine will dissolve in it, and it will dissolve to any extent. You cannot say that there is a definite solubility of gelatine in water. The more gelatine is added, the thicker the solution becomes, but there is no point at which the gelatine will refuse to dissolve.
Fig. 38. Swelling of Gelatine Cube.
Fig. 39. Reticulation.
If we heat a gelatine solution it will become thinner and less viscous when hot, and will not recover completely when cool; it will remain thinner than if it had not been heated, so that the heating of the gelatine solution produces a permanent change in its properties. If we cool a gelatine solution, the gelatine will not separate from the solution in a dry state, but the whole solution will set to a jelly, which we might consider a solution of water in the gelatine. If we heat the jelly it will melt again, and we can melt and reset a jelly many times, but in doing so we shall produce a progressive change in the jelly, and if we continue the process too long, sooner or later it will refuse to set and will remain as a thick, gummy liquid.
Gelatine belongs to the class of substances which are called colloids, the name being derived from a Greek word meaning gummy.
When a gelatine jelly is dried, it shrinks down and forms a horny or glassy layer of the gelatine itself, smooth and rather brittle, and this dry gelatine when placed in water will at once absorb the water and swell up again to form a jelly.
An interesting and important property of the drying and swelling of gelatine is that it swells almost entirely in one direction, namely, that in which it was dried. This is illustrated in Fig. 38. In this, A represents a small cube cut out of a sheet of gelatine which was originally dried in the horizontal plane when it was made. If this cube is placed in water, it will not swell in all directions, becoming a bigger cube, but it will swell almost entirely in the direction in which it dried down, and will take the form B and, finally, the form C.
The explanation of this directional swelling of the gelatine jelly, and also of the fact that gelatine solutions change permanently with heating, lies in the fact that gelatine is not a uniform substance but has an internal structure. Probably, gelatine has a structure somewhat like that of a sponge, but the structure is very small and has not the elasticity of the sponge.
When the gelatine is in the jelly state, it is as though the sponge were full of water, and then it is fairly rigid, because of the water contained in the pores. When the water is dried out, the sponge structure shrinks down, and if it is stretched out in one direction by being coated on film or paper, for instance, it will shrink down vertically just as a sponge without elasticity would fall into a flat mass if placed on the table.
When the gelatine solution is heated and the gelatine dissolves, it seems at first to retain a certain amount of its structure, as if the sponge had disintegrated and was distributed through the solution but the sponge structure had not entirely disappeared. Then, if the temperature is raised, it behaves as if the structure were slowly breaking up and dissolving, so that after a considerable heating at a high temperature the whole solution becomes homogeneous. When this solution is cooled and, finally, set to a jelly, it has to re-establish a new sponge structure, and this will be different to the original one and probably of less strength. This explanation of the behavior of gelatine, that it has an internal structure which can persist even in solution, seems to account for most of its properties and behavior.
Fig. 40. Spot on Gelatine, Caused by Moisture.
When a gelatine jelly contains only such an amount of water that it still contains a considerable proportion of gelatine, over 10% for instance, the jelly will be strong and tough, but if the jelly contains much less gelatine than this, it will be weak and likely to rupture on any kind of strain. This is a very important matter in dealing with photographic films. When the film is first placed in the developer the gelatine at once commences to swell. As long as it does not swell too much it is easily handled, but if it swells too far, then it becomes very tender and is likely to be damaged by touch, and in extreme cases will swell so much that it will loosen from its support or wrinkle up in what is called "reticulation".