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The photographic news
- Bandzählung
- 27.1883
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- Bandzählung
- No. 1295, June 29, 1883
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THE PEOTOGRAPTTC NEWS. Vol. XXVII. No. 1295.—June 29, 1883. CONTENTS. PAGE The Quantity of Gelatine in Emulsions 401 Production of the Photographic Image on the Wood Block of the Engraver 402 A New Departure in Photo-Mechanical Printing 402 A New Method of Silver Printing 403 Screens and Reflectors in the Studio 404 The Glass Roof 405 By-the-Bye.— On Re-Sitting 406 Photo-Lithography and Photo-Zincography. By Major J. Waterhouse, B.S.C 407 Notes 408 412 413 414 415 415 416 , 416 416 PAGE Patent Intelligence 410 Influence of Moisture, &c.,on Silver Prints. By W. M. Ashman 411 Fixing a Number of Negatives in Same Solution of Hypo. By M. A. Davanne Echoes from the Orient. By E. L. Wilson Correspondence Proceedings of Societies Talk in the Studio To Correspondents The Every-Day Formulary Photographs Registered THE QUANTITY OF GELATINE IN EMULSIONS. In the making of an emulsion, by whatever process we resort to, a question to be considered at every step is what proportion of gelatine we are to use. Indeed, this would seem to be one of the most important questions in connec tion with the matter. There are two distinct and separate matters to consider. The first, what quantity of gelatine shall be used during the progress of the process used for gaining sensitiveness ? The second, what proportion shall exist, in the final emul sion, between gelatine and silver bromide ? In considering the first question, it is well to understand, as far as we can, what is the true function of the gelatine- It appears to be a question, never yet settled, whether or not there is any true chemical combination between the gelatine and the silver salts ; nor does this appear to be a question easily settled. Of course, if there is such a com bination, one, and in fact the chief, function of the gela tine is to provide the material necessary to form it. There are, however, other functions of the gelatine which are more readily ascertainable, and a proper understanding of which will tend to success in the working of the emulsion process. The first of these is the formation of a fine grained bromide in emulsions; and a second is the keeping of this bromide in a fine state of division whilst some process is carried on which will increase its sensitiveness. It appears uncertain whether or not sensitiveness of the highest degree is compatible with the very fine state of division which is indicated by a red colour of transmitted light; but it is quite certain that the highest sensitiveness is compatible with a very fine grain of bromide ; in fact, that any size of particles beyond a certain point results cer tainly in a low degree of sensitiveness, for the reason that with a granular emulsion the developer reduces the silver bromide without the assistance of light. The stronger the gelatine solution—or rather, the greater the amount of gelatine in proportion to the silver bromide —the easier it is to obtain a finely-divided emulsion at the first start, and the longer may the sensitizing process be continued before granularity will result. On the other hand, the larger the proportion of gelatine beyond a cer tain point, the slower will be the change—whatever it be —which causes sensitiveness. We now see what we have to aim at in proportioning the gelatine to the silver bro mide which has to be rendered sensitive in its presence. We have on the one hand to avoid a proportion of gelatine so small that granularity will ensue before the change to sensitiveness is complete ; on the other hand, we have to avoid a quantity of gelatine so large that the time neces sary to bring about this change will be greatly prolonged. As to whether there is any ultimate limit to sensitive ¬ ness or not, whether anything other than the advent of fog need prevent the prolonging of any process causing sensi tiveness, is a question unanswerable in the present state of our knowledge. This, however, is certain: the advent of fog is practically our limit, and the point of sensitiveness at which this limit is reached may be varied by compara tively small changes in the process worked—amongst others, by varying the proportion of gelatine used. The fog limit may show itself in one or two forms. We may continue our process till granularity is produced, and this in most cases is the limit, but it is not always so. There are conditions under which an exquisitely sensi tive bromide may be obtained without any tendency to granularity, in which the process might be long continued without producing any size of particle necessarily giving fog. In such cases the fog limit shows itself in quite a different form. A point is reached beyond which the plates produced appear to be so sensitive to atmospheric influences that fog usually appears during drying. In such a case the process may be continued apparently almost indefinitely without a foggy emulsion, but beyond a certain point will be sure to give a foggy plate. This may appear to involve a distinction without a difference, but it is not so. Beyond a certain point a plate coated with the emulsion, caused to set quickly, and developed at once, will show no fog. To dry this plate without fog making its appearance will be a matter of great delicacy. Fog will in almost every case show itself on the surface of the plate. The weather has much to do with the intensity of this. In a windy, dry, and cool day, fog might be avoided entirely. In a damp, warm day, it will be so dense as to leave no trace of a visible image. It is evidently a thing desirable to know what are the conditions necessary to obtain an emulsion, the only limit to the sensitiveness of which is reached when plates are produced of a nature so delicate that there is difficulty in drying them. In the proportion of gelatine used during boiling, the chief factor of success would appear to lie. We have found a proportion of gelatine somewhat larger than is commonly advised useful at first, but the thing most ensuring success we have found to be the addition, as the process continues, of small quantities of gelatine. At the commencement we use a quantity of gelatine, equal in weight to a fifth of the silver nitrate. As the pro cess of boiling goes on, and the viscosity of the gelatine— which is probably what prevents granularity.—is destroyed, we add every half hour a quantity of gelatine equal to a quarter of that used at first. So far we have found no limit to the length of time during which boiling may con tinue without any appearance of granularity, and therefore without any fog but such as may result during the drying of plates. Of course, for practical purposes, it is necessary
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