The photographic news

• Bandzählung: 29.1885
• Erscheinungsdatum: 1885
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
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AUqU8E 28, 1885.] THE PHOTOGRAPHIC NEWS. 549 —will give a better print than a negative showing a pure black image; but more of this anon. Imy say that I have tried the substances which have been recently suggested as substitutes for pyrogallol—hydrokinone, and hydrochlorate of hydroxylamine—but have found them to give results decidedly inferior to our old friend pyro. Granted that pyro forms the most suitable developer for our modern plates, the next question which arises is, what alkali are we to use with it ? On this side of the Atlantic we have dung with remarkable tenacity to caustic ammonia. Our de velopers have for the last few years consisted of mixtures in various proportions of the pyrogallol, caustic ammonium, and a soluble bromide—generally bromide of ammonia and water. There has been no change in the substances used, but in the proportion in which they have been used there has been great variety, as also in the subsidiary substance used to prevent the .pyro from turning very brown in an aqueous solution. Even greater has perhaps been the variety of manners in which diffe- tent plate makers have continued to make confusion worse con founded by the extraordinary manner in which they have stated the formulas that they recommended. Each maker appears to be vicing with his neighbour in the task of making it as difficult as possible for the user of the plates to know in what propor tion the ingredients of the developer are present in the solutions actually poured over the plate. Pyro, with caustic ammonia, being the developer in this country, I, some few months ago, carried out a series of experi ments with the view of discovering what relative proportions of the three materials mentioned would give the best results in actual practice. The experiments were described in the British Journal of Photography. The following is an abstract of the descriptions there given : - • “ Forty-eight plates were exposed under a standard sensito meter screen. The exposures were made at three feet from a duplex paraffin lamp, each lasting for twenty seconds. As the experiments were intended to be comparative merely, it was not necessary to use the phosphorescent tablet. As all the exposures were made within an hour, it may be assumed that no appreciable variation in the intensity of the light took place, and, as a matter of fact, a plate exposed before those to be used for experiments, and another after, showed no appreciable differ ence when developed side by side. “ My object was to develop these plates, each with a different developer, so that there should be one developed by each of all the variously proportioned developers commonly in use. “ The annexed table will give an idea of the variations which existed between the various ingredients Table No. 1. Half minim ammonia One minim ammonia Two minims ammonia F our minims ammonia Half grain pyro, d-grain J-grain 1 grain bromide bromide bromide of am- of am- of am ¬ monia. monia. monia. 1 ... 2 ... 3 .. 5 ... 6 ... 7 .. 9 ... 10 ... 11 .. 13 ... 14 ... 15 .. 2 grains bromide of am monia. 4 . 8 . 12 . 16 Half minim ammonia One minim ammonia Two minims ammonia F our minims ammonia Table No. 2. One grain pyro. +-grain -grain 1 grain bromide bromide bromide of am- of am- of am monia. monia. monia. 17 ... 18 21 ... 22 25 ... 26 29 ... 30 2 grains bromide of am monia. 19 ... 20 23 ... 24 27 ... 28 31 ... 32 Half minim ammonia One minim ammonia Two minim ammonia Four minims ammonia Table No. 8. Two grains pyro. -grain -grain 1 grain bromide bromide bromide of am- of am- of am monia. monia. monia. 33 ... 34 ... 35 . 37 ... 38 ... 39 . 41 ... 42 ... 43 . 45 ... 46 ... 47 . 2 grains bromide of am monia. .. 36 .. 40 .. 44 .. 48 “ The quantities are in the somewhat unscientific form of grains to the ounce. It will be seen that pyro was used in quantities from half a grain to two grains, ammonia from half a minim to four minims, and bromide from a quarter of a grain to two grains. 1 1 do not profess to have observed any very refined degree of accuracy in the experiments. I have, however, kept well within the degree of accuracy commonly observed by photographers ; that is to say, I have employed with care, the usual druggist’s scales and weights, and the ordinary glass measures.. The pyro was not made up with any preservative, lest such, acting as a re strainer, should disturb the results. A four-grain solution in distilled water was made immediately before commencing the experiments. The ammonia solution was used in the form of one part of commercial ’88 ammonia to nine parts of water, the ammonia bromide in the form of a ten per cent, solution. When very small quantities were to be measured out, each of those solutions were first diluted in considerable quantity to one-tenth of its strength. “ In mixing each developer, solutions containing twice the quantities marked in this table were poured into a glass measure, and the whole was made up to two ounces. I imagine that no one will have any difficulty in understanding the tables ; but, lest he have, I give an example: “ If we take No. 24, we find at the head of the table No. 2, in which it is, ‘ 1 grain pyro.’ At the beginning of the line in which it is, we find ‘ 1 minim ammonia,' and at the head of the column in which it is, we find ‘ 2 grains bromide of ammonium.’ This means that plate No. 24 was developed with a solution containing 1 grain pyro, I minim ammonia, and 2 grains bro mide of ammonium in each ounce. “ In making a series of experiments with various developers, it is, of course, necessary to determine what shall be the limit of development. A time limit would be by far the easiest to work with, but I cannot see that any useful results would come from experiments performed with such a limit. For example, sup pose a limit of four minutes was taken—that is to say, that each plate was to be developed for four minutes. At the end of that time a developer which, at the end of ten minutes, would have produced a better result than on another in four minutes, might at the end of four minutes have shown scarcely any re sult at all ; whilst another, which would show an excellent result at the end of two minutes, might have entirely destroyed the plate at the end of four. “ On thinking this matter over, it appeared to me that, were it practicable, the best limit would be one of density; that is to say, that each developer should be allowed to act till it showed the first square of the sensitometer tablet of a certain fixed density, which should be about the greatest wished for in ordi nary negatives. According, then, to the highest number visible, and the clearness or the reverse of the unexposed parts of the plates, would the developer be judged. “ Although this criterion of density appeared to me by far the best one, I gave it up as impracticable. The judgment of density is at all times a very difficult matter, and, moreover, inaccuracies are liable to be introduced by variations in the thickness of the film. “ I decided, therefore, on a less scientific but more practicable limit. I developed in each case, either until the developer ap peared to have long ceased to produce any further change on the plate, or till fog became apparent. As life is short, I had to set, besides this, some time limit to the experiments. I therefore fixed the maximum time of development, in any circumstance, at half-an-hour. This I considered to be as long as it was likely that any development should be continued in practice. “ The plates used were of commercial make. The brand is fairly rapid, and in ordinary working gives very brilliant negatives, with exceedingly clear shadows. “ The difference in result produced by developers containing extremely different quantities of the ingredients is less than I should have expected. Thus, a developer containing in each ounce half a grain pyro, half a minim ammonia, and two grains bromide, which I should have expected to show scarcely any trace of image, shows one with the first few figures of great density, and shows only eight figures of the sensitometer less than one containing two grains pyro, four minims ammonia, and a quarter grain bromide.” Increase of pyro has, as might be expected, wrought increase of density. Increase of ammonia, when accompanied by increase of bromide, has had the same result, but without increase of bromide has resulted, as a rule, in decrease of density. Increase of ammonia has also resulted in increase of detail up to a certain point, after which, except when increase of bromide has accompanied it, actual falling off in detail has accompanied the increase of alkali. The following is the result of taking “ averages ” from the reading of the different plates :—