It will be seen from this brief account that the prepara tion of an emulsion is by no means an easy matter, and if the reader only knew the infinite care and trouble which the leading plate manufacturers take in order that they may be in a position to offer their customers a reliable article they would pause before attributing their failures to their plates rather than to themselves. Some plates possess better keeping qualities than others, but all are liable to deteriorate if exposed to damp or the fumes of gas ; therefore these adverse conditions should always be avoided.
Perhaps gelatino-bromide paper ranks in importance next to the dry plate. It is, as its name indicates, paper coated with gelatine emulsion. The emulsion is prepared in the same manner as that used for plates, the only difference being that it is far less rapid, and less gelatine is employed. Indeed, in some papers, particularly those with a rough surface, the quantity is reduced to a minimum.
Ordinary albumenised silver printing paper undergoes two processes in the course of its preparation. Large sheets of specially-made paper, usually of German manufacture, are first albumenised by floating on large dishes containing albumen, to which a certain proportion of some soluble chloride has been added. After flotation the sheets are removed, and dried in a warm room. So prepared, the paper will keep indefinitely. This part of the process is very often clone in Germany, from which country the bulk of the albumenised paper used in England is imported. The paper is made sensitive to light by floating it on a silver bath containing about 60 grains of nitrate of silver to the ounce of water. After floating for three minutes, the sheets are removed, and bung up to dry in a warm room. Paper so prepared will not keep very long, and must be used fresh.
The method by which the manufacturers confer keeping qualities upon their sensitised papers is a trade secret ; but the result may be achieved by floating the paper on a bath of citric acid, or by keeping the paper between folds of blotting paper, which have been soaked in a saturated solution of carbonate of soda, and dried. The secret of preserving ready-sensitised paper for a long period is to keep the air from it, and the best way of doing this is to store it under pressure, either in a large printing-frame or in a press. If it be first wrapped up in some old or discoloured sensitised paper it will be found to keep almost indefinitely.
Glass has until recently been almost exclusively adopted as a support for the sensitive material, but recent improve ments in the manufacture of celluloid have led to the adoption of the latter material for the same purpose, and celluloid films, as they are called, are now largely used by amateurs. Their chief advantage lies in the great reduc tion of weight and bulk, and the absolute immunity from any danger of breakage. The chief and perhaps only drawback to their use is -the difficulty experienced in getting them to lie flat in the dark.slide during exposure. This is greatly obviated by the use of a carrier, several patterns of which are now procurable, one of the best and lightest being England's cardboard carrier. The manipula tion of the ordinary thick films does not in any way differ from that of ordinary plates, but with the very thin rollable films, a little care is necessary in order to keep them beneath the surface of the developer. If a dish with a plain bottom be used and wetted, and the film then placed in it, the capillary attraction will be sufficient to cause it to adhere, and will prevent it from rising to the surface of the developer.