This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
188. Until one becomes thoroughly trained in the required amount of exposure to be given they will occasionally misjudge the necessary exposure, and in the majority of cases such plates or film are over-timed. The object of this instruction is to correct these exposures in the development, and thereby produce good printing negatives, which, if developed, in the ordinary way, would be lost.
Construction Of Sensitive Emulsion. Before taking up the manipulation of over-exposed plates generally, we will consider of what the sensitive emulsion on a dry plate is composed, and how constructed. The emulsion of a dry plate is a mechanical mixture of some viscous substance, and sensitive salts of silver in extremely minute divisions. These minute particles of silver are held in suspension by the viscous substance, which may be either gelatine, collodion or albumen. For the dry plate, however, it is generally gelatine.
190. To more clearly explain why the silver salts are distributed in minute particles, and held in suspension in the emulsion, suppose we pour into a graduate a certain amount of water. To this we add a small quantity of common table salt, and then add a little nitrate of silver solution. The entire solution will immediately assume a milky appearance, because we have formed, when mixing these ingredients, silver chloride. This silver chloride will, in a very few minutes, fall to the bottom of the graduate, and the clear solution of water may then be decanted. If, however, you substitute a warm solution of gelatine, or some gummy substance, in place of water, and then add the salt and nitrate of silver, you will obtain the same milky appearance, but the minute particles of sensitive silver salts will remain suspended. With these ingredients for the principal basis, a sensitive emulsion is formed which can very readily be flowed over a glass plate or film. When this emulsion is set and dried upon the glass it is then termed a Dry Plate, which is very sensitive to light.
191. When we place the dry plate in the plate holder, and attach it to the camera, it is ready for exposure. The slide being drawn, and the shutter opened, or the cap removed from the lens, thereby exposing the plate, the light admitted through the lens effects these minute particles of silver which are suspended in the gelatine. A certain amount of light is required to perform this work, and this amount of light you measure by the length of the exposure. If more than the necessary amount should be given, the plate will require treatment in the development of the image, to overcome the excess exposure given.
192. Action of Light on Dry Plate as Compared to
Printing-Out Paper. - This action of the light on the dry plate is in some respects similar to that of sensitized printing-out paper. If you place a piece of printing-out paper on a negative, and place it in the sun, the light affects the parts of silver in the emulsion on the paper in the same way as on the dry plate, the only difference being that the emulsion on the paper is of necessity made less sensitive than the dry plate, because the emulsion used for sensitizing paper contains materials that cause the image to become visible as it prints. Therefore, you can see the image appear during the exposure on the paper, while it is invisible and does not show on the dry plate. The sensitive emulsion of a dry plate is also many times more sensitive than that of a printing-out paper; therefore, while practically the same action is taking place when the light comes in contact with either of the two, yet its action is much slower on the paper than on the plate.
193. If you were to continue printing sensitized paper beyond a certain stage, you would have over-printed; and the print would be too dark, and entirely worthless. This is exactly what happens to a dry plate when it is over-exposed. The plate, like the over-printed print, has become too dark, as it were, but unlike the print it is not lost, providing the proper measures are taken to cut off or remove some of the minute particulars of sensitive silver salts, which have been acted upon by the light.