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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. 1131, May 7, 1880
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- Paginierfehler: S. XII als S. XX gezählt.
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Zeitschrift
The photographic news
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Band 24.1880
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- Register Index 631
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Band 24.1880
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MAY 7, 1880.1 THE PHOTOGRAPHIC NEWS. 221 ON DEFECTS IN THE GELATINO-BROMIDE PROCESS. BY DR. J. M. EDER.* g. Hard, glassy, over-intense negatives are produced when the development is slow and lengthy.— when the high lightshave made their appearance, and continually increase in density long before the half-tones come out. This is generally due to under-exposure, but it is also often caused by too large a proportion of potassium bromide, either in the pyrogallic, or eke in the iron developer. In the first case, the bromide should be reduced in quantity ; in the second, it may be omitted altogether, and the action can he moderated by diluting with water. The defect itself is seldom observed when the alkaline pyrogalhc developer is used, oftener in the case of under-exposed plates deve loped with ferrous oxalate. When the same solution has been employed for developing a great number of nega tives, it may be found that the negatives are harder than when it was quite fresh. It requires practice to know when to break off developing, and to make it agree with the exposure. Over-intense negatives, in which all the details have been brought out, but which are too der.se (owing, for instance, to prolonged development after the normal exposure), may be, perhaps, cleared by treating with dilute (1 :80) hydrochloric acid, or with a very weak solution of chloride of iron, or with one of potassium cyanide ; but this treatment runs the risk of destroying all the fine details. In judging the quality of negatives, we are liable to be mistaken ; negatives developed with pyro gallic acid often copy hard, though they do not look like it, because the yellow colour is very non-actinic. Zi. Bubbles and wrinkles in the film, as well as its ten dency to spring off from the glass, are generally the con sequence of an inferior gelatine (see 21 a); but the fault first makes its appearance during development. It is generally observed when the developer contains too much alkali—that is, of course, in the case of the pyrogallic developer—but is scarcely ever seen when ferrous oxalate is used : the latter has, in the strict sense of the word, a tanning and hardening action on the gelatine ; it there fore prevents the formation of bubbles, and renders any other hardening unnecessary (see 21 b). After tanning with alum, the plates will even bear treatment with acids and alkalies without the layer of gelatine suffering any inj ury. i. White prints or round spots with a sharp outline, which, after fixing, have a glassy polished appearance, are caused by air-bubbles adhering to the plate, and pre venting the developer from penetrating. These bubbles may be avoided by rinsing the plates with water before development, or by a gentle movement of the pan during the same. They can also be removed by a camel’s-hair brush at the same time. k. The ferrous oxalate developer soon grows turbid after beingmixed, and deposits a yellowpowder, which, in settling on the plate, causes fog. This is simply owing to the fact that in mixing the sulphate of iron with the oxalate of potassium, the latter was not used in sufficient excess, or, in other words, that too much sulphate of iron was taken. Turbidity may also be produced by the addition of too much acid to the solutions, but is seldom caused by the impurity of the ingredients. I. If the gelatine plates have been kept for a long time in a dry place, they will take the developer with difficulty and unequally, and this causes spots. The remedy is to soften the plates in water before developing. m. Irregular, denticulated lines and spots make their appearance when there is too little developing solution flowed over the plate, so that portions only are covered. In this case also, if the plate be previously moistened with water, a less quantity of developer will be required. C. Defects caused by Haloes.—These surround the high * Concluded from page 202. lights ; they are almost always due to the plate being too thinly coated with emulsion, and disappear altogether directly the films are perfectly opaque. Haloes are gene rally more liable to appear when the emulsions have a small quantity of bromide in proportion to the gelatine. 7. Defects arising during Fixing.—a. The negatives get fogged or spotted when light is admitted before the fixing is complete. This fault is, however, seldom observed when, previously to fixing, tbe developer is well washed away, even if weak daylight be admitted during the fixing. b. Bubbles and wrinkles in the film can be avoided by tanning with alum, as was described among the defects during development. This defect is most often seen when a very strong fixing soda is used. c. If the ferrous oxalate developer have not been suffi ciently rinsed out, the fixing soda turns yellow, and com municates the same colour to the negative. As a remedy maybe recommended thorough washing before fixing, or the yellow colour can be often got rid of by mean of fresh fixing soda. 8. Faults Arising during Cleaning or Weakening e Nega tive.—Hydrochloric acid has a tendency to wrinkle up the film, if the precaution has not been taken to dry the plate beforehand, or to tan the film with alum. Iron chloride has the disadvantage of colouring the whole film yellow. In attempting to weaken the negative with cya nide of potassium, the details will be eaten away, if the solution be too strong—stronger than 1 : 50. 9. Defect of Intensification.—a. Red fog often appears when the plate is intensified with silver, more especially when every trace of fixing soda has not been removed. It will be best cured by a short bath in a weak, sherry-yellow aqueous solution of iodine in potassium iodide. When this remedy is adopted there is not much to fear from intensify ing with silver. In general, the pyro silver intensifier for gelatine plates is less certain than the ferro-silver intensi fier, especially when the latter contains sufficient acetic acid and a small quantity of gelatine solution. b. Intensifying also with mercury, uranium, and other substances gives a brown or yellow veil, if the plates are not properly washed after fixing. An exception is offered in the case of Edwards' method, in which the plates must be well rinsed, but need not be streamed or washed to excess. c. Intensification that has been too long continued pro duces fog in nearly every case, and with any method ; this is especially liable to occur in intensifying with silver. When mercury is used, the process must be carefully watched, as, if the action is allowed to continue too long, the negative becomes much too dense. This is also the case in the method of Mr. Edwards. d. After the film has been intensified, it is liable to alter stillf urt her, eitherin the direction of darkening or of fading. Both of these defects are likely to occur when the mercu rial intensifier is used ; for example, after the film has been intensified with chloride of mercury, and then flowed over with ammonia, it will grow paler, but with the double iodide of potassium and mercury it becomes darker, and sometimes the whole surface turns of a brownish-yellow colour. Edwards’ method gives a tolerably permanent in tensity, though some operators will have observed a change of colour in negatives intensified in this way. The silver intensifier causes sometimes over the plates a red brown veil, which appears in copying, and arises from the solu tion of silver not being completely washed out of the gela tine. It is consequently advisable, after the intensifying with silver is finished, to fix the image once more, in order to get rid of the last trace of silver that has penetrated into the film. By this means all liability to subsequent alteration is avoided. Besides the defects above pointed out, there are many others, which, in a great degree, depend on the quality— or, rather, on the faulty composition—of the different inten sifying liquids. These could only be thoroughly described by means of an exhaustive account of every one of the different methods of intensification.
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