The papers are salted with a mixed Chloride and Citrate as in the formula for the Ammonio-Nitrate Process.[49] They are then rendered sensitive upon a Bath of Nitrate of Silver containing either Citric or Acetic Acid, which are used in Negative processes to preserve the clearness of the white parts under the influence of the developer.
[49] The formula at [p. 246] may be modified with advantage: use double the quantity of Gelatine, and half the amount of Citrate and Chloride.
The Bath of Aceto-Nitrate is prepared as follows:—
| Nitrate of Silver | 30 | grains. |
| Glacial Acetic Acid | 30 | minims. |
| Water | 1 | fluid ounce. |
Float the papers (Papier Saxe or Papier Rive) upon the Bath for three minutes, and suspend them to dry in a room from which actinic rays are perfectly excluded.
The exposure to light,—which is conducted in the ordinary printing frame, the Negative and sensitive paper being laid in contact in the usual way,—will seldom be longer than three or four minutes, even upon a dull day. It may be regulated by the colour assumed by the projecting margin of the paper; but it is quite possible to tell by the appearance of the image when it has received a sufficient amount of exposure:—the whole of the picture should be seen, excepting the lightest shades, and it will be found that very few details can be brought out in the development which were altogether invisible before the Gallic Acid was applied.
The developing solution is prepared as follows:—
| Gallic Acid | 2 | grains. |
| Water | 1 | fluid ounce. |
In very cold weather it may be necessary to employ a saturated solution of Gallic Acid, containing about four grains to the ounce; whereas in warm weather the image will develope too quickly, and Acetic Acid must be added (see the remarks at the end of the process, [p. 266]).
To facilitate the solution of the Gallic Acid, stand the bottle in a warm place near the fire. A lump of Camphor floated in the liquid, or a drop of Oil of Cloves added, will to a great extent prevent it from becoming mouldy by keeping; but if once mould has formed, the bottle must be well cleansed with Nitric Acid, or the decomposition of the fresh Gallic Acid will be hastened.