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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. 1277, February 23, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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114 THE PHOTOGRAPHIC NEWS. [FEBRUARY 23, 1883. We shall give a couple of extreme examples of plates of opposite characteristics. We had some plates which gave a clear image with a developer containing pyrogallic two grains, ammonia two minims, bromide nil. There was only the smallest possible tendency to reversal with this developer, and the sensito meter figure given was 14. An addition of one-eighth grain of bromide left matters the same, but with no ten dency to reversal. A quarter grain showed one figure less. With one grain the time of development was immensely protracted, and the figure 10 only appeared. It will be seen that this plate was amenable to compensation with a very small quantity of bromide. Another was taken of exactly opposite characteristics. With an unrestrained developer 20 figures flashed out instantly, and then supervened dense fog. With one grain of bromide there was slight tendency to reversal, and the figure 22 was visible. With two grains of bromide the same figure was visible, and the only apparent difference was that all tendency to reversal had ceased, and that the image was more powerful. These may be taken as two typical cases. The one in which a quarter grain of bromide acted as a restrainer—in the sense of keeping back detail entirely— in the presence of two minims of ammonia, the other in which two minims of bromide to the same quantity of ammonia did not act as a restrainer. Nevertheless, over- exposure in the case of the last plate might have been as completely compensated for as in the case of the first, the bromide being increased to three or four grains. We should say that, in all our experiments, the plate was left in the developer till all action appeared to have ceased. The time necessary was trom ten seconds to ten minutes. We have supposed the ammonia constant, and the bromide variable; but almost the same result might be brought about by keeping the bromide constant, and varying the ammonia. The only difference is, that there appears to be for all plates a certain minimum of ammonia necessary to render the pyrogallol active enough to give density. The lesson to be learned from all this is, that it is possible with bromide or other restrainer to compensate for over-exposure in any plate to about an equal extent, but that the quantity of restrainer necessary varies enormously —in our experiments, by as much as ten to one— and that with plates showing a tendency to reversal, a large quantity of bromide may be used without a longer expo sure being necessitated; whilst, with plates showing little tendency to reversal, even a comparatively small portion of bromide necessitates a protraction of exposure. THOROUGH WASHING AND EFFECTUAL FIXATION OF GELATINE NEGATIVES. Fuw have a just idea of the difficulties which accompany complete removal of all traces of soluble matter from a gela tine film; especially when the considerable hardening influences which ordinarily accompany exposure and deve lopment have rendered the gelatine more or less impervious to aqueous fluids. The first point to be considered is the removal of the haloid salts of silver from the developed negative, the solvent being in all ordinary cases a strong solution of sodium hyposulphite. As a chemical exercise it may be interesting and instructive to calculate how much silver bromide, chloride, or iodide, a given weight of sodium hyposulphite will render soluble ; but from a practical point of view such calculations have but little value, and they are likely to divert the attention of photographers from the urgent necessity of making use of such a considerable excess of hyposulphite as shall amount to several hundred times the quantity which is theoretically necessary for dissolving the silver salts. Those who have been accustomed to wet plate work, and have experienced the ease with which a collodion film yields up its silver bromide and iodide to the fixing solution, are especially liable to under-estimate the considerable time re quired to fix a gelatine negative. The wet collodion film is of the nature of an open network, comparable to a sponge ; this structure being apparent at once when a microscopical examination is made. The gelatine film is, on the other hand, quite free from anything approaching a cellular structure, and when a section is magnified to the utmost a clean and continuous surface is presented. Considering the importance of thoroughly removing every trace of silver which does not form an integral part of the photographic image, and the low price at which sodium hyposulphite is sold, it is advisable to carry out the late Mr. Thomas’s suggestion, of always using a second bath of fresh hyposulphite. Four ounces of hyposulphite to one pint of water is probably the best strength to make the bath, and not less than sufficient to cover the plate to a depth of half-an-inch should be used in the first instance. It is well to allow the plate to remain in this bath just double the time which is required for all traces of bromide or iodide to disappear from the sight. The old hypo sulphite solution, having now been drained away, is replaced by a fresh portion ; but in this case about half the quantity, or as much as will cover the plate to a depth of a quarter fo an inch, will suffice; and the negative should be allowed to remain in this bath for about the same time as it remained in the first. Immediately the plate is taken from the hyposulphite solution, it should be placed in water, or the picture may be weakened or lost by vit tue of the increased solvent action of the hyposulphite solution as it becomes concentrated by the partial drying of the film ; and the diffusion of silver salts through the film which accompanies this reduction of the image may introduce those very evils which ordinarily accompany imperfect fixation. Simple solution, when unaccompanied by chemical reaction, is always accompanied by the absorption of heat or the production of cold ; and as a consequence, one finds that a freshly prepared solution of sodium hyposulphite is considerably colder than the atmosphere. This cold solu tion acts far less energetically on silver salts than when it is at a normal temperature, and not only for this reason, but for others also, it should never be used for fixing gelatino- bromide plates. If, on the other hand, the solution be warmed by artificial means, there is considerable risk of so far increasing its solvent powers as to endanger the photo graphic image itself. The best plan is to prepare a moderate stock of the solution, and to keep it in some locality where the temperature is tolerably constant, say between 8° and 16° centigrade (46° and 61° Fahr). We pass on now to the question of washing, and unless this is done effectually, the whole work of the photographer is rendered of no avail, the negative gradually becoming weaker by the influence of light and time ; while varnish refuses to hold properly to the film, and silver from the printing paper stains it. All these circumstances are easy to understand, as any trace of hyposulphite remaining, becomes concentrated and dissolves more or less of the metallic image, while the silver thus dissolved diffuses all over the film, and leads to tbs ultimate darkening of the transparent portions. Varnish will never satisfactorily adhere to a film containing hypo., as the salt retains moisture, and prevents a fair union between the resinous coating and the gelatine; white the same circumstance fully explains the readiness with which an imperfectly washed negative becomes stained during the operation of printing. No rule whatever can be given as to the amount of washing required; but we have generally found that soak ing for ten minutes, in each one of six changes of water, is sufficient. In order that some idea may be gained as to the extent to which washing may be necessary for any particular batch of plates, it is convenient to test for hypo sulphite of soda occasionally. The most convenient test is a very dilute solution of the blue iodide of starch, hyposulphites possessing the property of decolourising this fluid.
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