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The photographic news
- Bandzählung
- 24.1880
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- 1880
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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. 1130, April 30, 1880
- Digitalisat
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- Paginierfehler: S. XI als S. X gezählt.
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Zeitschrift
The photographic news
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- Register Index 631
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Band 24.1880
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2yotograpkir Jts. Vol. XXIV. No. 1130.—APRIL 30, 1880. CONTENTS. pag Photography In and Out of the Studio 205 Canary Medium r. Ruby Glass. By Thos. Charles Bridges ... 206 At Home 206 Stray Thoughts. By J. Crosthwaite 207 French Correspondence. By L on Vidal 208 Patents. Compiled by Mr. Des Vceux 200 On the Proportion of Gelatine to Sensitive Salts in a Gelatine Emulsion 210 Mr. Edwards’ Glycerine Developer 210 Notes 211 Topics of the Dav 212 Signals by Sunshine 213 Principles of Optics Involved in Lantern Construction ; and on a New Enlarging Lens, Especially Designed for Use with the Magic Lantern. By J. H. Dallmeyer, F.R.A.S., &c. 214 Silver Printing. By John Matthews 214 Correspondence 215 Proceedings of Societies 215 Talk in the Studio 216 To Correspondents 216 ON THE PROPORTION OF GELATINE TO SEN SITIVE SALTS IN A GELATINE EMULSION. Several correspondents have asked us the best proportion of gelatine to use in making gelatine emulsion, and we have thought that our own experience may be of use in helping them to arrive at the principles which should govern it. To begin with, as we have recently pointed out, there are various qualities of gelatine, commencing with the very fine and soft specimens, such as a Nelson’s No. 1 photo graphic, and ending with carpenter’s glue. Taking these two extremes, the quantity required for holding the sensi tive salts in situ is very different in the one case to the other. Of the very soft kind probably not less than thirty- five grains to each ounce of washed emulsion would be required, whilst of the glue one-third of that quantity •would suffice. It is this fact that makes the formulae given by various writers so misleading when the quality of the gelatine is not specified. For instance, we know some cases in which as much as fifty grains of gelatine were recommended to each fluid ounce of emulsion, and if any but the very softest sort were used, the resulting films would be almost impossible to develop. It may be taken as an axiom that the less permeable to water—that is to say, the harder—the gelatine, the more difficult is development, and therefore a less quantity will restrain the action of the developer ; for it must be recollected that the great and useful sensi tiveness of the gelatine emulsion is owing to the restraining power of gelatine on the activity of the developer. Taking a good sample of medium hard gelatine, we find that about twenty grains to the fluid ounce of emulsion is sufficient; thus, with Nelson’s flake gelatine, this is amply sufficient to hold in situ the silver bromide which is formed by adding twenty-five grains of nitrate of silver to an excess of some soluble bromide. But this is not a hard- and-fast rule, since the proportions may be somewhat altered if chrome alum be added to the emulsion. Chrome alum has the property, as no doubt our readers are aware, of hardening and toughening the film of gelatine, and the more added the harder and tougher does it become. The above proportions suffice when one-eighth of a grain of chrome alum is added to each ounce of the emulsion previous to coating the plate. If it be omitted altogether, three equal parts of nitrate of silver and gelatine may be advantageously used, whilst if the chrome alum be increased to a quarter of a grain per ounce of emulsion, the gelatine must be diminished to about seventeen grains to each twenty-five of nitrate of silver. When a very hard gelatine is used, there is the game apparent loss of sensitiveness as when much chrome alum is used with a softer sample, and we believe that the ] loss is more apparent than real, and is probably due to the physical qualities, and not to any chemical action of the gelatine or chrome alum, since by using means to soften the film the sensitiveness is at once increased in a very marked manner. From the foregoing remarks it will be seen that, in our opinion, the softer the nature of the gelatine used, the greater will be the sensitiveness that can be obtained in a plate. Unfortunately, the great bugbear of the gelatine plate—frilling—comes to the front when the extremely soft specimens are used in practice ; hence it is necessary to use either a mixture of a hard with a soft gelatine, or else one of medium tenacity. We believe that there is no theoretical limit to the sensitiveness of the emulsion, but the physical defects inherent to the gelatine on approach ing it are so serious and so difficult to overcome that we believe practically we have come to a point beyond which we cannot travel much further. Chrome alum must always be used with caution, but it is almost a necessity to use it, in order to get plates which will stand the fixing solu tion, and we should say that with a medium strong gela tine, l-20th of a grain per fluid ounce of emulsion is the maximum that ought to be required, and that never should more than half of a grain be used with any gelatine. If more be required to prevent frilling, a different sample of gelatine should be tried, and our advice is, that having found one which works well; a change from it should only be made if it can be demonstrated that some other speci men is very superior to it. We have said nothing regard ing the unsuitableness of some gelatines owing to impuri ties found in them, as that is beside the question now treated of. MR. EDWARDS’ GLYCERINE DEVELOPER. There is no question about it that Mr. B. J. Edwards has signally improved alkaline development by the addition of glycerine to the pyrogallic acid and ammonia solutions; the development is effected far more rapidly, and, what is as much to the purpose, you can watch well the progress of the image towards completion. We have had an oppor tunity of comparing it with the doings of two oxalate developers: the one made up and manipulated by Mr. Alexander Cowan, in that gentleman’s laboratory ; the other prepared according to Eder’s formula, and worked by ourselves. In both cases the development was effected more quickly by the Edwards’ formula than by the oxalate. The plates were similarly exposed, and put into develop ing dishes for a period of sixty seconds. In half that time the glycerine developer had acted with sufficient vigour ; on the other hand, the minute scarcely sufficed to fully develop the plate in the oxalate dish. A further experi ment with the Eder developer seemed to prove pretty conclusively that with plates similarly exposed, forty-five seconds of the Edwards’ developer gave as vigorous an image as two minutes’ development in the oxalate. The glycerine seems to allow the developer to come at once into intimate contact with the film, and to hasten the deve lopment in this way. But while thus recognizing the very great service which Mr. Edwards has undoubtedly rendered to photography, we must still uphold the value of the oxalate developer. The glycerine modification will be invaluable for use under many circumstances, but the fact that the oxalate yields a negative so like a model wet plate, with the clearest of shadows, and lights of that delicate matt appearance pho tographers delight in, is obviously an advantage not easily over-rated, 'there is an entire freedom from brownness, which is so characteristic of a gelatine film developed with ammonia and pyrogallic acid. If a plate will develop within two minutes, and the coming image is capable of being kept during that time under observation and con trol, there is little else to be wished for.
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