Another method of coating is the following, usually adopted by the writer. The glass plate is placed upon a leveling stand, and arranged perfectly level. The exact quantity of the emulsion is then poured on to the centre of it, and with a spreader made of a piece of bent glass rod or tubing, the emulsion is spread evenly over the plate.
Plates may also be coated by brushing a small quantity of the emulsion over with a wide badger hairbrush, and afterwards pouring on the necessary quantity For commercial purposes the process of preparing dry plates by hand would be too expen sive. Machines have, therefore, been constructed, by means of which several thousands can be coated per diem. The principles of the Edwards' machine are these: In a trough containing the emulsion a roller of silver or glass rapidly re volves. Upon one side of the roller is a metal plate, which rests upon it and scrapes off the thin film of emulsion that the roller takes up in revolving, the thickness of the film being regu lated by the speed of the roller. This thin layer of emulsion scraped off runs down on to the glass plates, passing one after another beneath, giving a perfectly even coating. The coated plates then run through a part of the machine, which is kept very cool, and they are very soon set. An operator at the other end then removes them one by one as they come out, and places them in a rack, which, when full, is sent up to the drying room. Another machine for coating plates is that of Mons. Hutinet. It has the appear ance of a long, skeleton table, about sixty or seventy feet long. Along this table endless bands of a stout material are constantly traveling, kept in motion by a small gas or hot-air engine. At one end of the table is the arrangement for coating. A small hot-water cistern, containing the jar with the prepared emulsion, is placed on a raised platform. A small pipe fitted with a glass stop-cock conveys the emulsion from the cistern in an even stream to an open glass cylinder. This cylinder is perforated on its lower part with a number of holes, through which the emulsion oozes, and spreads itself over an india-rubber roller below. Underneath this roller the glass plates are made to pass along by the endless band, and as it presses lightly upon them it applies the emulsion in a thin even layer. The plates, after
they have received the necessary coating, pass along. the lengthy machine, kept quite level, so that they are quite set before they reach the other end, from whence they are conveyed to the drying room.
The drying of gelatine plates is by no means an easy matter. The principal desideratum is a current of dry air, which carries with it the moisture from the gelatine, and escapes with it out of the drying chamber or cupboard. In Fig. i68 we have a good form of drying box. The direction of the arrows shows the passage of current of air. In the pipe which conveys the vitiated air out of the room is a gas flame, which can be regulated through the door. The heated air from this flame will rapidly ascend, drawing with it the air from the drying box in which the plates are placed. Some small holes are made to allow of the entrance of the fresh air.
Great care should be taken to prevent dust from reaching the plates during the drying op eration, also that the temperature of the room or cupboard be kept even. About 8o degs. Fahr. is about correct. Sudden changes of temperature will produce markings upon the surface of the plates. The door of the drying cupboard should never be opened after the plates are put in until they have been allowed ample time to dry.
The drying operation should take from six to eight hours. It should not be hastened, as quick drying is a great mistake, and often results in bad plates and frilling.
Plates may be dried in an hour, if urgently required, by placing each plate when thor oughly set into a dish of methylated spirit, quite free from resinous matter, for about ten minutes. When taken out it will dry in less than an hour.
With all these operations the most careful precautions must be taken to exclude all light, especially if the emulsion be very highly sensitive. In coating the plates the light used should be only sufficient to enable one to work carefully. All parts of the room should be carefully looked to. Fig. 174 shows a plan of a light-tight door, upon which principle it would be well to construct the door of the room or of the drying box.