Volltext Seite (XML)
THE PHOTOGRAPHIC NEWS, Vol. XXVII. No. 1275.—February 9, 1883. CONTENTS. PAGE Fine and Coarse Silver Bromide in Emulsions 81 The Collotype Process 82 By-the-Bye.—About other Studios 82 French Correspondence. By Leon Vidal 83 Notes on Photography. By E. Howard Farmer 84 The South London Photographic Society’s Popular Lantern Meeting, and its Lessons. By William Brooks 85 Remarks on Parlour Portraiture. By Dr. Tulloch. 86 Notes 87 PAGE Patent Intelligence 90 Photo-Lithography and Photo-Zincography. By Major J. Waterhouse, B.S.C 90 Experiments on the Direct Photography of Sound Vibrations 92 Review 92 Correspondence 93 Proceedings of Societies 94 Talk in the Studio 95 To Correspondents 96 FINE AND COARSE SILVER BROMIDE IN EMULSIONS. When Plener's process of preparing emulsion by separating the silver haloids from a boiled or ammonia treated emulsion, and re-emulsifying them with fresh gelatine, was first de- Ecribed in our columns, it was pointed out that one of the great advantages of this process was, that in the case of a chemically fogged emulsion it was possible to remove the coarser and fog-causing particles of silver bromide, and thus get rid of the fog. Some time ago, when describing experiments on another process which brings about a similar separation of the silver haloids and the modified gelatine, we gave it as our opinion that there probably is iu every emulsion a certain percentage of bromide of silver which, if not actually in this fogged condition, isso nearly so, that the emulsion wouldbe improved by its elimination, and that, in fact, the residue tub, and not the films on the plates, was the best place for it. We men tioned our intention of experimenting in the direction indicated. We are glad to see that Dr. Eder is either acting on our hint, or, having been struck with the same idea, has been working in the same direction, making use of Plener’s machine. We have finished the experiments which we men tioned, and find they entirely confirm Dr. Eder’s. We shall describe them briefly, and give their results. We used the precipitation process, not having access to a centrifugal machine of the Plener type. We prepared an emulsion by the boilir-g process, using 400 grains of silver nitrate, boiling for three hours, and stirring at brief intervals. We allowed the emulsion to cool to a temperature at which it could be easily handled, when we once more stirred it vigorously. We now drew off a portion, and set it on one side to precipitate. The bulk of the emulsion we allowed to stand for one hour, when we de cantedit, and seton one side to precipitate. Wo found that in the hour a considerable amount of settlement had taken place, and that there was a layer of silver bromide at the bottom of the jar. We mixed this with water, and divided it into two equal portions. Each of these we allowed to precipitate, but retained one for the purpose of weighing the silver bromide, whilst the other we kept to mix with gelatine to make an emulsion. After precipitation and washing, it will be seen that we had : first, a portion of the silver bromide of the emulsion exactly as it was after boiling ; second, a portion minus the coarser bromide ; and third, the coarser bromide in two portions, one for weigh ing, the other for mixing with gelatine. We mixed each of these samples of silver bromide with gelatine to make an emulsion ; the result, as regards appear ance, was as follows :— The first and second were not to be distinguished. Each was an opaque emulsion, of a good colour, and with no perceptible difference in size of grain. The third was a greyish, granular, and very transparent emulsion. Plates were tested of all three, and the results were as follows :— The first and second bromide gave excellent results. When developed with a normal developer, there was not much difference between them ; but the second showed one figure of the sensitometer higher than the other, namely, 21; it was somewhat clearer in the shadows, considerably denser, and gave apparently a better gradation of density. It was when forcing was applied, however, that the differ ence was most apparent. With a developer containing to each ounce a grain and a-half of pyrogallic and one grain of bromide of ammonium; the first showed symptoms of fog when ammonia reached five minims. The secund, on the other hand, remained quite bright till twelve minims of ammonia were applied. By that time the figure 23 was visible. And now about the third. The appearance of the film was almost enough to condemn it. On applying a normal developer to a plate of this batch exposed under the sensito meter, a uselessly thin and fogged image was the result, and the figure 14, or thereby. The bromide of silver from the half of the third portion, which had been put on one side for weighing, was dried, and found to equal a shade under 40 grains; that is to say, altogether 80 grains of bromide of silver had been removed from the emulsion, and this, be it noted, to its absolute benefit. Not even its covering power was reduced, its sensitiveness was increased somewhat, and its quality, including resistance of a strong developer, had been increased much. Here we have a result which appears to us to be of some importance. Eighty grains of bromide of silver represents roughly 18 per cent, of the bromide, resulting from decomposition of 400 grains of silver nitrate, and this amount is recovered, and may be placed amongst the residues to the advantage of the emulsion. The saving in actual cash would be considerable in the case of a manufacturer working on a large scale. Another result which we obtained is a confirmation of the observation first made, we believe, by Dr. Eder, that the re straining influence of finely-divided bromide of silver is such, that in its presence, coarse-grained bromide of silver, which, if alone, would give nothing but fog, may be deve loped clearly. In our first emulsion we had present suffi cient bromide of silver in a coarse state of division to produce fog had it not been in the presence of the finer bromide; yet with a normal or even somewhat strong developer we got quite clear plates. At the same time, we see that although the restraining power of the fine silver bromide goes, to a certain extent, and prevents the coarse bromide from being reduced without the action of light and with a normal developor, yet it is not sufficient to prevent the