DRYING 422. The Purpose of Drying. Photographic plates and films which have been washed at temperatures between 6o' F. and F., and have been wiped free of water clinging to the surface, contain an amount of water which is about six or ten times the weight of the gelatine in the emulsion, so that it may be as much as one or two grains of water per square inch of emulsion surface. In the case of films coated on the back with gelatine, an approximately equal quantity of water will be contained in this backing layer. The amount of water absorbed is considerably greater at higher temperatures.
Before wiping, the quantity of water adhering superficially to the surfaces of a plate or film amounts, for the two faces, to about 1-5 to 1.8 gr. per square inch, of which about three-quarters runs off in drops when the negative is held. vertically. A portion of the remainder accumu lates on the lower parts of the negative, whilst a certain amount evaporates.
In the process of drying, this water has to be evaporated without injury to the image (partial or complete melting, markings, reticulation, etc.) ; at the same time, the adhesion of dust to the moist gelatine must, as far as possible, be prevented.
Obviously, drying will be more rapid if super ficial water is wiped off at the start. 1 423. The Physics of Drying. The air always contains a certain amount of water in the form of vapour, this quantity varying greatly accord ing to circumstances. Comparatively dry air, when kept in the presence of water or moist bodies, or into which water vapour is introduced by a boiler, will take up moisture to a certain maximum, which increases according to the temperature of the air. When this limit has been reached, the air is said to be saturated with moisture. The ratio of the quantity of water vapour present in a given volume of air to the quantity which would be present in the same volume of saturated air is called the hygrometric condition, the fraction of saturation, or the relative humidity. On cooling the air, its relative humidity is increased, and it may become saturated (condensation of mist or deposition of dew). Conversely, the relative humidity of
air is decreased when the temperature is raised.' Dry gelatine absorbs water in a moist atmo sphere, whilst moist gelatine loses moisture in a dry atmosphere. In saturated air, which is unable to take up any more moisture, all evapor ation is prevented, and as a consequence moist gelatine cannot be dried in it, even though the amount of moisture to be removed be very small. The rate of drying is very nearly inversely proportional to the relative humidity of the air; air which is warmed, and of which the relative humidity is thus reduced, has its capacity for drying increased to a greater extent when the rise of temperature is great. The practical limits of drying by this system are, however, quickly reached because of the risk of melting the gelatine. Since the air becomes loaded with moisture as evaporation proceeds, it soon becomes saturated, so preventing further drying, unless it is continually renewed at the surfaces of the drying material.
Gelatine should never be completely desic cated ; it is considered to be dry when it contains no more than about io per cent or 15 per cent of moisture. Further drying renders it very brittle, a fault which would be specially evident with film negatives, and might also be attended with injurious effects in the case of glass negatives.
Drying in an atmosphere of less than 6o per cent relative humidity tends to dry the surface layer before the underlying portions ; the evap oration of the imprisoned moisture is thus retarded. Injury may occur to negatives which are stored, or to films which are rolled, in this condition when the imprisoned water becomes re-distributed through the whole thickness of the coating.
The drying of films in cinematograph printing works is carried out as a_rule in a current of air at 70 per cent or 8o per cent relative humidity, at temperatures between 78° F. and 87° F.