PLATES, PAPERS, ETC., AND THEIR PREPARATION, it is quite possible to become a skilful photo grapher, and yet know little or nothing of the materials which are used in the production of the pictures, it cannot be doubted but that even a slight knowledge of the nature and preparation of the plates and papers and other materials in common use can be otherwise than helpful in times of difficulty, I therefore make no apology for adding this cha,pter to the pages of " Elementary Photography." I can, however, only treat the subject in the briefest possible manner, and I would refer the reader who desires to extend his knowledge to Abney's " Photography with Emulsions," a book which all photographers should possess.
Astonishing as the statement may seen, there are not a few amateurs who, though able to produce very passable photographs, are totally unacquainted with the nature and mode of preparation of the sensitive plates and other materials which they are accustomed to use in the produc tion of their pictures ; but as I do not wish tbis reproach to apply to those whom this book may initiate into the mysteries of the art, I trust ray readers will peruse and not pass by this chapter.
The modern dry plate consists of a sheet of glass evenly coated with a sensitive preparation technically termed an " emulsion." This emulsion consists of bromide of silver, mechanically suspended in a very fine state of division, in a solution of gelatine. The emulsion is prepared by dissolving either bromide of potassium or bromide of ammonium in water, with a small quantity of gelatine. A second solution is made by dissolving nitrate of silver in water. Then, in the dark-room, the silver solution is added to the bromide solution slowly, and with vigorous stirring, when the bromide combines with the silver, and forms bromide of silver, which is the sensitive silver salt. It is very essential that the combination should take place slowly, in order that the silver bromide may be produced in a fine, and not a coarse condition, the former being found to give better results than the latter. This is effected commercially by
special mixing apparatus, not unlike a huge churn in appearance. An emulsion so prepared would be too slow for general purposes, and the vessel containing it is placed in a hot-water apparatus, in which the emulsion is kept at a high temperature, or even boiled, for a certain time, until the required sensitiveness is attained. When this stage is reached, the emulsion is allowed to cool and the necessary quantity of gelatine added ; for were the full amount used in the first place and during the boiling process, it would lose its setting properties, and the plates, when coated, would not dry. Before the emulsion is in a fit state for coating the plates it has to be " washed," in order to remove the soluble nitrate of potassium, which is formed at the same time as the insoluble bromide of silver. The emulsion is forced through coarse canvas, a silver grating, or broken up by other mechanical means into small particles. These are enclosed in a suitable apparatus, and a continuous current of water passed through until all the soluble salts are dissolved. The emulsion is then drained and re-melted, when it is ready for coating.
Formerly plates were coated by hand, but now all com mercial plates are coated by machinery. An endless band carries the glass plates underneath a vessel containing the liquid emulsion, by which they receive a thin and even coating of emulsion ; the endless band, which is kept travelling at a uniform rate of speed, takes the coated plates onward through a short metal tunnel, the outside of which is packed with ice ; this causes the films to set, and upon their emerging from the far end, an assistant lifts them off, and stacks them in racks, which, when full, are removed to the drying chambers or darkened rooms, through which a continuous current of air is passed, generally by mechanical means. When dry, the plates are, in all good factories, properly tested, and, if approved, passed on to the packers, who put them up in the boxes which are so familiar to photographers.