PRESERVATIVE PROCESSES. The object of these processes is to preserve the sensitiveness of an excited collodion plate for a longer or shorter time, as may be required, and thereby to do away with the inconvenience of preparing the plate and developing the picture on the spot whence the view is taken.
The best dry preservative processes at present known are undoubt edly those of Dr. Hill Norris, and Mr. Fothergill, described under the head of " Dry Collodion Processes;" q. v. With respect to the moist preservative processes, various plans have been suggested, (some right and others manifestly wrong in principle,) which consist in removing more or less of the free nitrate of silver from the sensitive plate by washing it and then pouring over it such a substance as glycerine, golden treacle, oxymel, or honey, which retains its mois ture for a great length of time. After the exposure, and immediately before the development, the plate is well washed in order to remove the preservative substance, and the picture is then developed, either with gallo-nitrate, or pyrogallo-nitrate of silver, and fixed in the ordinary way.
Of the substances named as preservatives, golden syrup, or glycer ine, or a mixture of golden syrup, and metagelatine, are probably the beat, and oxymel or honey the word. The objection to honey eonsists in its containing a large quantitiy of grape sugar, which is a powerful reducing agent and tends to fog the plate without increasing its eensi tiveness ; while golden syrup is entirely uncrystallizable and a very feeble reducing agent, therefore a much more suitable substance than honey to employ. The reducing action of the grape sugar contained in honey is kept at bay somewhat by adding vinegar to it, and forming oxymel ; but when an acid is present in the preservative, not only is the time of exposure greatly increased, but the latent image is grad ually destroyed by it between the time of exposing the plate and de veloping the picture. Some time ago an amusing spectacle was exhibited by two photographers, each contending for the honour of the discovery of the honey process. We believe more honour would
have been gained at the time by any one who had then pointe,d out the dieadvantages which attend, and the error of principle which is involved in the use of it. Glycerine appears to be inert and well adapted for a preservative agent, particularly when the nitrate of silver is not thoroughly washed off.
In any preservative process it is always well to add a chloride, say chloride of magnesium, to the ioclizing solution, and the collo dion should not be too hard and contractile.
The deliquescent salts, nitrate of magnesia, and nitrate of zinc, have been tried as preservatives, but they do not appear to answer.
The theory of preservative processes appears to be this :—The sensitiveness of the excited collodion plate is impaired, but not alto gether destro)ed, by washing off the free nitrate of silver. An image perfect in all its details may be impressed upon a washed col lodion plate, by sufficiently increasing the time of exposure ; but in the development nitrate of silver must be added to replace that which has been removed. The impressed image cannot be removed by simple washing in water, although it may be dissolved out by im mersion in an acid nitrate bath, or by acid oxymel, or acid gelatine or any acid developer, or other chernical agents. The effect of a moist preservative solution poured over the plate is therefore (sup posing it to be quite inert), merely to retain the moisture of the film during the time that it is in contact with it.
A good moist preservative process is chiefly valuable to the pro ssional photographer (who must of necessity work collodion in a m or tent near the spot where the picture is taken), for taking in riors, or subjects which require a very long exposure. To the nateur it is useful for enabling him to excite and develop the plate the inn where he is staying. If an excited plate be required to be " kept " more than a few hours, Dr. Norris's dry process is certainly far better than any moist preservative process. See " Collodion."