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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1312, October 26, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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674 THE PHOTOGRAPHIC NEWS. [OcTOBER 26, 1883. detail them here, as it would make this article too lengthy. We must, therefore, reserve them for another occasion, as well as a few experiments showing that the precipitate produced in the manner mentioned is nothing more nor less than metallic silver, though the latter is a question that the chemist is hardly likely to doubt. Granted that this precipitate is identical with green fog, it throws considerable light on various questions connected with that defect, of which we can only indicate a few, on account of the space at our disposal. It explains, for instance, why some emulsion makers do not get green fog with plates prepared by ammonia methods, whilst others, following out the same instructions, get a plentiful crop of it. In the latter case, the emulsion has not been sufficiently washed, and the films have contained ammoni- acal bromide of silver ready to produce green fog immedi ately on coming into contact with the developer. Again, with boiled emulsions, when one is found particularly prone to give green fog, it may possibly be connected with a very fine state of subdivision of the bromide. The reason why green fog is not found, or is less prominent on those parts of the plate which have been exposed to light, will readily appear from an article in the preceding number. In such cases, the particles of silver already beginning to form the image attract the particles thrown down by pyro from the minute quantity of bromide of silver which the ammonia has been able te dissolve. THE PREPARATION OF GLASS PLATES BEFORE COATING WITH EMULSION. The cleaning of the glass plates to be coated with collodion was one of the most troublesome parts of the wet process, any dirt which might be left, resulting in a deposit of silver on the part. When plates are to be coated with gelatine emulsion, there is not by any means so great care necessary as with collodion. In fact, we have often coated with emulsion, glass plates which were perceptibly dirty, without any evil resulting to the negative which was afterwards produced. This is not always the case, however. Very often, very slight dirtiness of a plate results in green fog on the negative, especially if these have been under-exposed. If glass is carelessly cleaned, it is by no means uncommon to see lines and smears after the plate is developed, these clearly following the direction of the cloth which was used for polishing. In spite of this, it may be said, in respect of markings produced by imperfect cleaning, that these are less likely to appear in the case of a gelatine plate than in that of a collodion plate. There is, however, another point to be considered, and this appears to be one in which gelatine emulsion is even more exacting than is collodion. All who have used both collodion and gelatine emulsion must have observed that, as a rule, the former flowed over a plate much more easily than the latter, and that whereas the former almost always flowed with the same degree of ease, there are great variations in the case of the latter. At one time it will flow almost as easily as collodion does ; at another it is almost impossible to make it cover the plate at all. The fact is that in the case of plates to be coated with emulsion we have to give quite as much consideration toso operating as to produce a surface on which emulsions will freely flow, as to the mere removal of dirt. It must be understood, in connection with this, that the surface produced must, in a certain sense, be looked upon as a substratum. Different methods of cleaning produce plates which may be coated with greater or less ease. It is perfectly certain that, if these methods all produced per fect and absolute cleanness or absence from foreign matter on the surface of the glass, then there would be no difference in the, ease with which emulsions would flow. The fact would appear to be that a thin film of the sub stance used for cleaning is left on the glass. We bad, some time ago, an experience which will illus trate this matter well. We were about to coat some dozen 12 by 10 plates with gelatine emulsion. We had about a couple of dozen use less negatives, and we removed the films from these, after wards polishing the glass in the manner which we shall presently describe. We also ordered from a glass mer chant a couple of dozen of plates of the same size ready polished. These latter were most beautifully cleaned, and, on comparison with those polished by ourselves, the usual test of breathing on the surface being used, appeared to us by far the cleanest. We commenced coating, the plates being placed all in a pile, those of our own cleaning uppermost. We coated in the ordinary way, pouring the emulsion on to the centre of the plate, and guiding it by “tipping,” as with collodion. So long as we were coating the top two dozen of plates, all went well, the emulsion flowed almost like oil, and scarcely a drop was spilled into the flat dish which was placed below to prevent waste. When we took the first of the second batch of plates, all was changed; the emulsion, on tipping the plate, instead of gently spreading itself in the desired direction, suddenly darted off the plate, forming a streak from the central pool to the edge. After several trials we had to stop coating till we could re-polish the plates. An enquiry as to the method used to clean the plates elicited the fact that dry sawdust had been employed. Doubtless, in spite of the apparently perfect cleanness of the glass, there was a very thin film of some resinous sub stance on the surface of it, and this repelled the emulsion. We have tried all the various substances which have from time to time been recommended for giving such a polish to glass that emulsion will readily flow on it, and have found the best to be the homely whitening, the same that enters into the composition of whitewash. We have tried substrata, French chalk, acids, and methylated spirit, and none of these has given us so good a surface as whitening. Before applying the polishing agent, it is necessary, of course, if old negatives be used, to strip the films from the glass. If these be collodion, or varnished gelatine films, then soda bath has to be used. The plates are placed in a hot saturated solution of washing soda, after which the films may readily be removed. If the negatives be on dry plates, and be not varnished, an acid bath will remove the films. This may consist of an ounce of hydrochloric or sulphuric acid to a pint of water. If glass, as it comes from the glass merchant unpolished, be used, it is necessary to remove the evident dirt—putty and such like—which is on it. This can generally be done by mere friction with a cloth, especially if a little soda solution be used ; or the sawdust, as mentioned above, may be made use of. When the glass is fairly clean, whitening is applied in the form of a mixture with water, having about the con sistency of cream. A cloth may be used to smear this on the glass. An old shaving-brush is an excellent imple ment. Only a thin coating should be given, otherwise un necessary trouble is involved in removing it. The whitening may be at once rubbed off with one dry cloth, a second cloth being used to thoroughly polish the plate. The best method, however, if many plates are to be cleaned, is the following. The plates are smeared with whitening one by one, as described, and are reared against the wall, or in racks, to dry. The first will possibly bo dry by the time the last is coated with whitening. A slightly damp cloth is now taken, and the whitening is by its means removed from the glass. Immediately after-
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