COCKLING OF PRINTS Photographs mounted in a wet state upon thin cardboard, or upon the leaves of an album, invariably cockle or curl when dry, whereas prints mounted surface-dry do not cockle so badly. The defect is clue to uneven expansion caused by the wet mountant, and can be made worse by unskilful manipulation. Careful selec tion of the mountant minimises the trouble, and the following formula is as good as any in this respect : White dextrine . . 36o grs. 82 g.
Powdered alum . . 16 „ „Sugar . . . . 6o „ 13'6 „ Hot water . . . I oz. ioo ccs.
This, when thoroughly mixed, should form a thick cream, which should be allowed to stand u day before use. Take the trimmed dry print and lay it face downwards on a sheet of glass, and with a fairly stiff brush apply the smallest possible quantity of mountant to the back of the print, distributing evenly and quickly ; before the mountant has had time to soak through, place the print upon the mount and squeegee or rub down. Place two or three thicknesses of fluffiess blotting-paper over the picture and mount, and put into a copying press and screw down hard, or put under heavy pres sure for several hours. When dry, there should be little or no cockling.
Another plan is to brush the back of the dry print over with a strong solution of gelatine or soft glue, and to damp slightly the mount before placing the print in position, drying under pressure.
The theory of the subject is to prevent expansion of the print before pressing it in contact with the mount ; or, if this expansion is unavoidable, to expand the mount, as in the preceding paragraph, and let mount and print contract together. Another point is to use a thoroughly even mountant, because should one part of the print get wetter than another, cockling is almost sure to occur.
Photographers may learn something from the draughtsman's method of stretching drawing paper, and even if it is dangerous to mount prints in this way, they can adopt it when pasting brown paper on the backs of photograph frames. The draughtsman slightly damps the back of the paper, thus evenly expanding it all over, touches the margin all round with paste, and "lays " the paper on the board, thoroughly pressing the margin into contact. The paper contracts in drying and becomes as tight as a drum-head.