When it is required to wash a fairly large number of small-sized prints, either a washing machine or a shallow cylindrical tank can be used, the latter being fitted with inlet pipes placed tangentially, so that a whirling move ment of the water is produced. A washing machine consists essentially of a perforated. metal drum, the axle of which is mounted hori zontally on bearings fitted to an overflow tank, in the inside of which it is kept rotating either by an electric motor or by the fall of the supply water on to the paddles of a water-wheel con nected with the drum itself. A secondary flow of water is generally directed through the perforations to prevent the prints from sticking to the drum. With such machines there is often a risk of the prints becoming torn or creased if an attempt is made to wash large-sized prints, while prints on stiff supports are often scratched when washed by this method. 2 When washing large-sized prints, the con sumption of water is usually reduced by using a number of tanks arranged in tiers (cascade washer), the water flowing from one tank into the tank immediately below (arrangements should be made to prevent the prints being carried over by the current of water). On taking out of the fixing bath, the prints are placed in the lowest tank and are transferred progressively through the various tanks, so that systematic washing is ensured (the prints are moved in the opposite direction to the flow of Before proceeding to dry the prints it is a good plan to remove all foreign matter (pieces of paper, particles of gelatine, (lust, etc.) from the surface of the prints by wiping them with a large soft sponge (an ordinary or rubber sponge) which has been well soaked in water and squeezed almost dry. For this purpose, the prints are laid on the flat bottom of a metal dish tilted against the side of the sink ; at the same time, the greater part of the superficially adhering water is removed, and the time required for drying is thus 609. Drying of Paper Prints. The old method of drying paper prints was to hang them, by one or two corners, according to their size, with clips on to lines stretched either between the walls of a well-ventilated room, or fixed to a frame which is fitted to the wall or hung from the ceiling by cords passing over pulleys. When dried by this method, prints on emulsion papers tend to curl up or roll themselves into small tubes, and the process of flattening them out involves an additional manipulation, during which cracks in the coating are sometimes produced. This is usually a very serious matter, and can only be avoided with great difficulty, even when the lower edges of the prints are weighted. It is true that the gelatine forming the support of the image could be kept moist by dipping the prints, before drying, in water containing a little glycerine, but if this remedy is applied the risks of deterioration of the image under the influence of impurities in the air is considerably increased. The method of drying prints by suspending them freely is therefore usually confined to prints on non-emulsion papers (such as salted papers, ferro-prussiate papers, etc).
6zo. The usual method of drying papers coated with emulsion is to lay them face down wards on a support which allows of free access of air, such as stretched linen, butter muslin, or galvanized wire netting. If necessary, drying may be hastened by a current of warm air. When the amateur has only a very small number of prints which require to be dried, they should be first pressed between blotting-paper, and then placed face downwards on dry sheets of fiuffless blotting paper (blotting cards generally give much longer service and are therefore more economical in use), which, when not in use, are kept protected from dust.
When a large number of prints are to be dried, a drying cupboard is generally employed for the purpose. Such a cupboard is fitted
inside with sliding frames covered with cloth or wire netting'; if asufficiently large number of supplementary frames are available, they can be filled with prints outside the drier and placed inside as the frames containing the dry prints are removed. Such a drier is sometimes fitted with a glass door, while the sides of the cupboard may be furnished with ventilation holes at the top and bottom, covered with fine gauze to keep out dust. The temperature of the air inside the cupboard is raised by means of gas jets placed under sheet iron, or by electric resistance heaters ; a fan may be used to accelerate the circulation of air inside the cupboard, while the moist air is evacuated by a chimney. A drier which is less bulky when not in use may be made by stretching between two vertical walls (partition walls, furniture, etc.) pieces of cloth, each piece being mounted on an automatic spring blind roller, so that it rolls up when the free end is released.
For the rapid drying of large numbers of prints continuous drying machines are often used ; the prints are inserted one by one between two endless aprons or belts of porous which are carried round a cylinder heated from the inside ; at the end of the travel the prints emerge dry and fall into a box placed in position for the purpose C. Jellinek, 1908).
Post cards may be dried by fitting them into wooden grooves, the distance between the bottoms of the grooves being slightly less than the length of the card. The cards are then placed in the grooves so that the coated side is convex, thus counteracting the tendency of the card to curve in the opposite direction.
The gelatine of positive paper emulsions is usually sufficiently hardened to be able to resist relatively high temperatures without damage. However, it should be remembered that there are certain papers, particularly glossy papers, which are coated with a relatively soft gelatine, and these cannot be dried face downwards, especially with heat, without having been hardened with (§§ 540 and 581).
The operation of hastening the drying by heat often brings about a modification in the tones of prints on print-out paper and on prints which have been developed or toned to warm tones ; it nearly always causes a slight alteration in the appearance of the surface, so that when quality is more essential than speed, methods by which the prints are dried in less than half an-hour should be avoided.
Prints should be removed from the driers before the gelatine becomes over-dried, especially if the drying is hastened by a current of warm air, as the gelatine tends to become brittle and curling of the prints is only prevented with great difficulty. Immediately the prints are dry, but while they are still supple, they should be piled up on top of each other, and can be slightly weighted without any risk of their accidentally adhering to each other. Special care should be taken to place all the prints of the same pile the same way up, i.e. facing in the same direc tion, otherwise, if the drying has not been quite sufficient, two prints with the gelatine surfaces in contact would stick together.
612. Rapid Drying of Prints. In cases where prints have to be delivered in a great hurry, the rate of drying may be considerably increased if the print is immersed for a few minutes in a bath of methylated spirits (§ 428) after it leaves the last wash water. When the water has been displaced by the spirit throughout the thickness of the paper, a fact which can usually be verified by the translucent appearance presented by the paper when examined from the back, the prints are squeegeed to expel the alcohol (which is then recovered), and pressed between blotting paper ; they are then dried by heat, using the drying apparatus at its maximum temperature.