The room in which the plates are prepared must be carefully guarded from scattered pencils of white light; the films are exposed to injury from this cause during the whole of the time occupied in applying the preservative syrup; and hence anything short of absolute chemical darkness will be likely to cause fogging; especially so when free Nitrate of Silver is left upon the film.
The water used for washing away the free Nitrate of Silver before applying the preserving liquid, need not be distilled. Common hard water containing Carbonates and Chlorides, and producing milkiness with Nitrate of Silver, will often suffice. The water of the New River and of the River Thames, with which many parts of London are supplied, may certainly be used; but in the case of a very hard water, containing much Sulphate of Lime, it might perhaps be advisable to substitute clean rain-water, free from brown organic discoloration.
The preservative Oxymel must be carefully filtered, and kept covered, in order to protect it from dust. It will also be necessary occasionally, before using it, to run it through a piece of white cambric, to stop back suspended particles, which, if allowed to remain, would be a source of spots. If it becomes mouldy, or discoloured by Silver, or ferments and evolves gas, throw it away.
After the syrup is applied and the plates are drained, stow them in a grooved box perfectly protected from light; or place them in slides, which must be kept scrupulously clean, since any trace of impurity would be likely to produce a stain when the plate was left a long time in the slide. If the preserved plates are kept in a cupboard or box, see that no volatile matter, such as Ammonia, coal-gas, etc., can find entrance.
In changing the plates after the exposure in the Camera, use a large bag made of several thicknesses of black calico, with a square of yellow calico let in at the top; an elastic band securing it round the waist.
THE COLLODIO-ALBUMEN PROCESS.
This process, the theory of which has been briefly explained at [page 181], is more sensitive than the one last described, and has the additional advantage of giving dry plates, which do not attract dust, and are less liable to injury. The details of manipulation are complex, but this inconvenience is not so much felt when preparing a large number of plates.
Cleaning the Glasses.—Success will greatly depend upon the mode in which this part of the process is performed. The layer of Albumen which is applied to the Collodion film tends to swell and to raise the latter in blisters; the most effectual mode of obviating which will be to clean the glass so that the film adheres with unusual tenacity.
The Liquor Potassæ of the Druggists, diluted with three or four parts of water, and rubbed on the glass by a roll of flannel ([page 214]), is very effectual. A mixture of Tripoli-water and Nitric Acid may however, if desired, be substituted:—
| Tripoli | 1 | drachm. |
| Nitric Acid | 30 | minims. |
| Water | 1 | ounce. |