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The photographic news
- Bandzählung
- 29.1885
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- 1885
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1378, January 30, 1885
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The photographic news
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Band 29.1885
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- Register Index III
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JANUARY 30, 1885.] THE PHOTOGRAPHIC NEWS 69 ■when a negative possesses too much contrast, but requires a slight strengthening of the image. There is still another example where the mercury solution will, if employed, yield results well nigh perfect—I refer to the intensifica tion of very thin images, as are frequently produced with highly sensitive commercial plates. In this latter case the silver deposit should be subjected to the action of the mercury till the entire image is attacked—often a matter of seven to ten minutes—when, of course, the negative preserves its original delicate gradation after the process of intensification. By mercurial intensification I mean the process of bleaching the silver deposit with a mercuric salt, and then treating with some compound capable of blackening the image, such as ammonia, sulphuretted hydrogen, sulphite of soda, or ferrous-oxalate. The effect of mercuric chloride (the salt usually used) on the silver image is to convert the latter into silver chloride, and, at the same time, to deposit locally mercurous chloride ; thus the bleached image consists of chloride of silver and mer curous chloride. In the process of blackening it is gene rally only the latter that is affected—e.g., ammonia forms the black amido-chloride of mercury; while, again, sul phite of soda reduces the mercury salt to the metallic state. With sulphuretted hydrogen, or sulphide of ammonium, both the chlorides in the image are converted into the corresponding sulphides, and also ferrous-oxalate reduces the two chlorides to the metallic state. When the process of reduction by sulphite of soda or ferrous oxalate is employed, it is possible to obtain almost any amount of intensity, and thus perfect printing nega tives may be obtained from mere phantom images. To obtain such results it is only necessary to repeat the process of intensification a sufficient number of times till the desired intensity is produced. The negative, after being thoroughly washed from the sulphite of soda or ferrous oxalate reducing solution, is plunged directly into the mercuric solution, and the process proceeded with as usual. There is one great defect always met with in mercurial intensification, more especially when ferrous oxalate or alkaline sulphide is used in the after treatment; I refer to the staining of the shadows due to the mercuric salt com bining with the gelatine of the film to form an insoluble compound, which is afterwards rendered visible by conver sion into sulphide or metallic mercury. No doubt the fading of mercurial intensified negatives is due in a great measure to the spontaneous decomposition of this organic mercury compound ; also to the presence of the latter, together with the reduction of silver chloride, may be due the increase of intensity of negatives after exposure of some hours to light during printing. It is thus, I think, very evident that some method of preventing the formation i this organic compound should be employed ; and 1 am glad to say that I have discovered a suitable means of carrying out the desired end. It is a well-known fact that free acids act as powerful restrainers in preventing the combination of metallic salts with organic substances ; as, for example, the use of citric acid in hindering the forma tion of albuminate of silver in sensitized albumenized paper. In the same way, I find that hydrochloric acid prevents the formation of the gelatino-mercury compound. In proof of this assertion, I will cite here an illustrative experiment. An ordinary gelatine negative was cut in half, one piece was treated with the usual neutral mercuric solution, and the other was immersed in a similar solution, but containing a small proportion of hydrochloric acid; both films were then thoroughly washed in the same dish. The two plates were next cut up into three, and one piece from each was treated with solutions of ferrous oxalate, ammonium sulphide, and dilute ammonia respec tively. On examination it was found that while all the films that had been immersed in the acid mercury bath presented beautifully brilliant negatives and quite colour less in the shadows, those pieces from the neutral bath were more or less stained. I should mention that the alkaline sulphide solution developed the most stain, and was of a very non-actinic brown tint; ferrous oxalate yielded a less conspicuous grey deposit, while the ammonia gave the least objectionable result. It was found, how ever, that the ammonia-treated film turned quite brown on after treatment with the sulphide solution, proving that the ammonia did not remove part of the mercury, but the latter remained to a certain extent in embryo. For the intensification of gelatine films in which a preliminary treatment with mercuric chloride is required, I recommend the following solution :— Saturated solution of bichloride of mercury 20 ounces Hydrochloric acid (strong) ... } dram A larger proportion of the restrainer might be added when treating hard films, but for general purposes the above formula is preferable, as the acid is liable to produce frilling. Although not generally used, and probably unknown to many, the most perfect method for intensifying gelatine films is that in which a mercury compound is used simply as a carrier for silver, but does not actually exist in the final result. The process to which reference is made de pends on the reduction of silver cyanide by a mercurous salt; the film is bleached in a solution of mercuric bro mide, and, after slight washing, is immersed in a bath of cyanide of silver dissolved in cyanide of potassium. By the first treatment, the silver image is converted into mercurous and silver bromides, as is illustrated in the following equation :— Mercuric Bromide Silver Mercurous Bromide Silver Bromide HgBr, + Ag = HgBr - AgBr In the second reaction the mercurous bromide reduces the silver cyanide to the metallic state, thus :— Mercurous Silver Bromide. HgBr. AgBr Potassium Bromide. KBr Potassic Mercuric Silver Cyanide. Cyanide. AgK(CN)j = Hg(CN), Bromide and Metallic Silver. + Ag.AgBr From the above it will seen that the intensified image is composed of metallic and bromide of silver, so that should the deposit, after the treatment, be too intense, it can easily be reduced by a bath of dilute hypo, which dissolves the bromide. The formula which I recommend for intensifying by this process, although possessing little that is novel, may prove of value to many of the readers who have no experience with the method. The following solutions are required :— A.—Bichloride of mercury 1 dram Bromide of potassium ... ... 1 ,, Water 16 ounces B.—Nitrate of silver... ... ... 1 dram Cyanide of potassium 14 drams Water 8 ounces The commercial cyanide of potassium is quite pure enough for this purpose; but if the pure salt be used, only 1 dram should be taken. The solution of cyanide should be made at least twenty-four hours before required, and the liquid shaken briskly from time to time, to ensure the saturation by the silver. Even after standing for the above period, a large precipitate will remain undissolved. A negative to be treated by this method is first soaked in A till the image is more or less bleached, according to the amount of intensification required ; it is then washed in two or three changes of water, and placed in another dish containing B, and there allowed to remain until the white deposit is blackened throughout the whole film. The latter is finally very thoroughly washed, preferably in running water, for about half an hour, in order to remove every trace of the silver. Negatives treated by this means ought to be permanent, as the cyanide acts like hydro chloric acid in dissolving out every trace of mercury.
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