When only a very few prints have to be dried in the shortest time possible, they may be bathed in alcohol, pressed almost dry between blotting paper, and then singed one by one. For this purpose, the print is held by a lower corner in an almost vertical position, the upper corner being held to a blue gas or spirit flame ; the alcohol with which the print is still impreg nated burns, the flame gradually creeping down the print. When it arrives about half-way the print is quickly turned round, taking hold of the corner at which the alcohol was originally lighted with the free hand. The flame then spreads upwards until it reaches the corner which was originally held in the hand. When the operation is properly carried out there is no risk of burning either the print or the fingers.
613. Deformation of the Image during Wash ing and Drying. All papers tend to stretch when wetted and to contract during drying, the final dimensions being usually less than the initial dimensions. In the case of machine-made papers—and photographic papers usually come within this category—the variations in length are not proportional to the variations in breadth. In such papers the fibres actually lie in a direction parallel to the length of the band of paper, so that although the fibres themselves swell when wetted their length remains prac tically constant. I The expansion which occurs when the paper is wetted, and which varies from one paper to another, and even for the same paper dried to the same extent, may amount to 2-5 per cent of the width of the paper and o-5 per cent of its length. When dried with free access of air, the contraction, relatively to the initial dimensions, may attain I per cent of the width and o-5 per cent of the length. When the same paper is alternately wetted and dried several times, similar variations occur each time, their magni tude often being slightly greater but soon reach ing a limit.
In ordinary photographic practice, these variations in the dimensions of photographs were particularly important at the time when photographs were mounted by the wet process ; the prints were fixed in position by the mountant when expanded to their fullest extent, appre ciable deformation thus resulting. In that case, one endeavoured to place the subject in relation to the paper in such a way as to minimize the effect of the deformation, for example, by length ening a very stout sitter, or broadening a person who was rather thin. The use of dry-mounting processes, now becoming more and more general, has allowed the deformation of the image to be sufficiently reduced so as to be no longer a source of difficulty in purely artistic work.
In all cases where measurements have to be made on photographic images paper prints are usually avoided. If, however, great accuracy is not essential, two metrical scales may be printed on the margins of the sheet of paper, parallel to two adjacent sides respectively, and measure ments relative to these scales may be taken.
For work requiring precision, paper coated on the back with plain gelatine may be used. Before printing, the paper is expanded by soaking in plain water for some time, and is then laid face upwards on a sheet of plate glass; any air bubbles and excess of water are expelled, and the whole is then left to dry. Owing to the adherence of the gelatine, the paper remains fixed at its maximum distension. When devel oped and dried, the image will have exactly the same dimensions as the latent image (H.
Roussilhe, 1922). The use of sensitive emulsions coated on thin sheets of matt aluminium has also been suggested.
614. Glazing and Enamelling of Prints. The processes of glazing and enamelling, when applied to prints on glossy paper, bring about a considerable increase in contrast and clearness of the details of the image.
The enamelling process, which was formerly applied to prints on albumen or collodion paper is no longer used. A sheet of flawless plate-glass was thoroughly cleaned, dried, and coated with a layer of an approximately i per cent solution of collodion. When the collodion had set, a solution of gelatine was run on top, and the print applied face downwards on the gelatine solution, the excess being removed with a flat squeegee. A sheet of stout paper was applied to the back of the print by the same method, and turned over behind the glass to keep the whole in place. When thoroughly dry, a cut was made with a penknife, and the print could be removed coated with a layer of collodion as highly polished as the plate-glass on which it had been moulded.
Similar results, although not as lasting, are obtained more simply by glazing prints on gelatine-coated papers. The print is applied while wet to a sheet of plate-glass, polished celluloid, or a piece of japanned iron plate (ferrotype plates). If the operations have been correctly carried out, the prints may be detached quite easily when completely dry. The gelatine surface is then as glossy as the support with which it had been in contact, but if damped afterwards at any time it loses its glaze.