Whenever possible, the glass sides of plate negatives should be wiped dry with a soft cloth ; in addition to the fact that drying is thereby slightly accelerated, it is easier at this stage to get rid of small particles of gelatine or other foreign matter which may soil the back of the plate.
When once negatives have been placed in the racks or hung up, they must be left to themselves until they are dried, no matter how great the impatience to examine them may be.
a very humid atmosphere a higher tempera ture leads to an increase of density and contrast. Whereas the evaporation of the water is rapid in a dry atmosphere, thus cooling the gelatine which is kept fairly firm even in comparatively warm air, evaporation is retarded in a moist atmosphere and cooling becomes negligible, so that the gelatine can become softened and allow of a re-arrangement of the grains of silver. Variations of 25 per cent have been noted, after drying in various conditions, in densities that were equal before drying ( J. Crabtree ; l). R. White, 1022) Drying should never be undertaken in winter in a place where there is a risk of the tempera ture falling so low as to freeze the water with which the negatives are impregnated ; " ice flowers," which would be formed under such conditions, would leave their distinct impression in the gelatine.
Certain insects, cockroaches, ants, etc., especi ally in warm climates, devour gelatine. In places infested by these creatures it is a good plan to protect negatives during drying by a mosquito net.
In warm, humid, and stormy weather, the gelatine of negatives is sometimes attacked during drying by colonies of microbes. During the course of very slow drying which such weather causes, these microbes may liquefy the gelatine in places, leaving the glass support bare. This trouble occurs especially with non-hardened negatives, when straw packing or other such material containing mildew is disturbed near the negatives during drying. In such atmo spheric conditions it is well to hasten drying by bathing the plates after washing in water to which alcohol has been added (§ 428), or at least to finish washing in an antiseptic bath such as 3 per cent solution of phenol.
426. Distortion of the Image during Drying. Very slight deformations, quite negligible in the common applications of photography, occur in the film of gelatine during drying, especially near the edges. They may need to be taken into account in high-precision measurements (carto graphy, astronomy, etc.).
These deformations are mainly due to local inequalities of drying. They occur chiefly on negatives developed with a tanning developer (pyrogallol), on account of the fact that the denser portions of the image, which are tanned, contain less water than the surrounding gelatine. The presence of drops of water adhering to the gelatine surface, or even to the glass, causes similar deformations due to local retardation of drying. Every negative has a margin of about half an inch in which these deformations are fairly considerable. Distortion, which is due to inequality of drying, may be considered as temporary if the gelatine has not been hardened with alum, and with this exception it is possible to remedy the defect by allowing the negative to swell in water and to dry again with all necessary precautions, particularly by hastening drying by treatment with alcohol (§ 428).
427. Acceleration of Drying by Heat. The drying of glass negatives or paper prints may be greatly hastened by a current of warm air, or by placing the material to be dried near a source of heat, provided that the gelatine has been hardened so as to raise its melting-point to a sufficient degree. Unless the heat is greatly moderated, this method, when applied to films, often causes a permanent deformation of the film base (notably curling at the edges), which, in a measure, is an obstacle to perfect sharpness of prints.
Negatives fixed in solutions containing alum, and especially in a fixing solution containing a large quantity of chrome alum (§ 409), will generally stand a temperature of i2oe F. to r4o° F. They may be dried in full sunlight, provided that no shadow falls on them, since shadows would cause local inequalities of drying. Drying in the sun is indeed often the only pos sible method in very warm and humid climates.