Benzene Varnish for Films. A 2 per cent solution of gum dammar in crystallizable benzene is used. The film should be allowed to dry for several days, otherwise it might adhere to anything with which it is pressed in contact.
479. Application of the Varnish.
Plates to be varnished should be thoroughly dried by keeping them for some time near a source of heat.
For cold varnishing, the plates are first dried in this way and should then be left on a marble slab until quite cold, and carefully dusted just before the application of the varnish. The knack of coating the varnish can be acquired after several trials, but its description is somewhat lengthy ; Fig. 178 shows the various stages of th€ operation. The plate is balanced on the fingers of the left hand and kept level under th€ thumb at the left-hand corner. The varnish iE poured on to the centre of the plate until ii covers about one-third of the surface. It is ther allowed to spread until the liquid reaches the two sides of the corner opposite the thumb. B3 continuous and imperceptible movements, thiE corner is inclined, and the other corners it succession, care being taken that the varnisi does not reach the thumb and run up the arm When the whole of the plate is covered, it iE raised first at an angle of about and ther vertically, allowing the excess of varnish t( drain into the bottle. While draining, the plat( should be rocked from side to side to prevent streakiness. The negatives should then be put to dry, away from dust.
For hot varnishing, both the plates and th( varnish are first warmed, and the coating carriee out preferably over a hot slab or a stove observing the same precautions as given for colt varnishing.
Films can be varnished by immersion in dish which is filled with varnish to such a dept.' that the film is completely covered. After a fev minutes, having ascertained that there are no air bubbles under the film, the latter is slowly lifted out, allowed to drain for a minute or two, and finally hung up by a corner until completely dry.
When using a water varnish, the films can, if required, be varnished as they come from the last wash water.
480. De-varnishing of Negatives. Glass nega tives, which have been varnished, and which have been subsequently found to require after treatment, can be de-varnished by allowing them to soak for some time in denatured (industrial) alcohol in which 2 per cent of caustic soda or caustic potash has been dissolved. The varnish
very soon becomes milky and can then be easily removed by gentle rubbing with a wad of cotton wool soaked in the alcoholic solution of soda. The negative is then washed in several changes of water and put to dry.
A water-varnished film negative may be de-varnished by immersing in a very weak aqueous solution of caustic soda (about i per cent) to which about 20 per cent of industrial alcohol may be added to avoid excessive swelling of the gelatine. The operation is completed by gentle rubbing with a tuft of cotton-wool, and washing in several changes of water.
Gum-dammar varnish can be removed from a film negative by prolonged immersion in rectified benzine (crystallizable benzene would evaporate too quickly). The cleaning is completed by rubbing with a wad of cotton-wool impregnated with crystallizable benzene, and the negative put to dry 481. Oiling of Paper Negatives. Prints may be made from paper negatives without treating the latter specially, but exposure is considerably prolonged, since the paper absorbs an appreci able proportion of the incident light.
To shorten the exposure it is usual to make the paper translucent by impregnating it with a varnish or a fatty substance. It is essential in this case not merely that the paper be dry, but it should be dried by heat immediately before applying the treatment ; otherwise the water imprisoned in the fibres prevents the penetration of the substance into the pores in places, and considerably accentuates the grain of the paper, which is then more marked than if the paper had not been oiled. The paper should be dried before a fire or in a hot-air oven.
The most practical method of oiling consists in painting the back of the negative (placed on the bottom of an upturned dish to avoid any projections) with a brush dipped in vaseline oil. Two applications should be made with an interval of one hour between, and the oiled paper left for about two hours, afterwards removing any surplus oil by pressing between blotters.