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The photographic news
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- 27.1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1272, January 19, 1883
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- Titelblatt Titelblatt I
- Register Index III
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43 determining cause, and, as before stated, the granular con dition is the most sensitive. The finer modifications, if present in any quantity, reduce the sensitiveness. The sensitiveness also apparently varies somewhat according to the method of preparing the granular bromide; thus the granular bromide prepared by the action of ammonia is never so sensitive as that prepared by boding (Abney). b. Gelatine, by its affinity for oxygen aid the halogens, acts as a sensitizer to the bromide; to exert its most favourable action in this respect it should be as pure as possible. c. The gelatine should be as soft as is consistent with safety. d. F inished gelatine emulsions, when kept for a few days, frequently increase considerably in sensitiveness. This, Captain Abney believes to be due to the fact that the particles of silver bromide, to be in the most sensitive state, must be placed entirely beyond any state of strain. During preparation, a strain is probably given to the particles of silver bromide, and by subsequent keeping the emulsion in the state of jelly, this strain wears off. e. The excess of soluble bromide employed should be very thoroughly washed out. The presence of one per cent, excess of ammonium bromide reduces the sensitive ness to one-half (Eder). The cause of this energetic retarding action of the solu ble bromides has been fully explained by Captain Abney (Cantor lectures) ; he showed that at the same time that light reduces the silver bromide to sub-bromide, it also decomposes any ammonium (or other) bromide present, with the result of re-converting the silver sub-bromide to bromide ; thus the sensitiveness obtained is only the diffe rence between these opposite actions. f. Alkaline carbonates and alkalies, especially ammonia, added in small quantities to emulsions increase their sensi tiveness (Eder). Fuming a dry plate with ammonia in creases its sensitiveness (Col. Wortley). Treating a plate with very dilute silver nitrate and thorough washing before development, increases its sensitiveness (Eder). g. Free halogens, oxidizing agents, acids, soluble bro mides, iodides, oxidized organic matter, &c., reduce the sensitiveness to a greater or less extent, and in some cases destroy it altogether. Silver Chloride.— 1 This substance is not so sensitive as silver bromide, and is therefore not employed alone for producing negatives. A small quantity in conjunction with the silver bromide in a t emulsion gives density on development, an I increases the sensitiveness somewhat (Bolton). Silver Iodide—This body is very insensitive, and very difficult to develop alone with gelatine. Captain Abney, however, in 1880, pointel out that a small quantity in con junction with the silver bromide in an emulsion confers very valuable properties upon it without reducing the sensi tiveness. In the first place, it takes away the sensitiveness which pure granular bromide has to red light, and so enables a comfortable light to be employed in develop ment ; in the second place, it acts as a restrainer of fog, giving brighter images with clearness in the shadows; thirdly, it allows greater latitude in exposure ; and finally, by producing a more opaque film, prevents halation. Dr. Eder’s experience it reduces the sensitiveness slightly, has a tendency to give thin images, and prolongs the time of development. METHODS OF PREPARING EMULSIONS. Extremely sensitive silver bromide in conjunction with gelatine can be prepared in many ways ; thus, it may be prepared by careful precipitation from aqueous solutions, washing by decantation, and subsequent emulsification in gelatine (Abney); or by producing the fine orange bromide by double decomposition in a solution of gelatine, digestion with an excess of the soluble bromide, and subsequently washing the set emulsion (Bennett) ; or by precipitating a solution of bromide of ammonium in gelatine with ammonia nitrate of silver, and subsequent washing (Obernetter), &c. They nearly all, however, arrange themselves under one or other of three principal types, viz.:— 1. Precipitation processes. 2. Boiling or acid processes. 3. Ammonia processes. And of which the above three methods are examples. In some cases these methods are combined together, as in Mr. Burton's recent process. They may again be divided into those which contain silver bromide only, silver bromide and iodide, and silver bromide, iodide, and chloride, respectively. In England the second of these types, or the boiling methods containing silver bromide and iodide, seem in most general favour, and will, therefore, be employed by us. — GREEN FOG. BY C. RAY WOODS.* The experiments I wish to bring before your notice this evening are somewhat imperfect in their scope, but may, in connection with the experience of others and the discussion I wish to open this evening, be possibly of a little value. I should like first to give a few of my experiences in the preparation of gelatine plates. In the first emulsion giving green fog, that I prepared, I omitted to put in the two drops of hydrochloric acid that the formula required. I stopped the boiling considerably before the proper time, and have no doubt I checked the evil to some extent. Since then I was not troubled with green fog for some time, until I began to use a fresh sample of bromide, when I had some difficulty to prepare plates in which this pest did not appear. Proceeding to examine the two samples of potassium bromide, I noticed this :— Old Sample. Neutral, containing a trace of bromate. On addition of a drop of hydrochloric acid a trace of bromine was set free, which was rendered more apparent by the addition of potassic iodide, due to the consequent liberation of iodine. The emulsions in which I used the above samples of bromide contained chloride, bromide, and iodide. Emulsions of exactly similar character, but preparel entirely with salts of ammonium, gave no green founder ordinary circumstances. The chloride and bromide were both perfectly neutral, but became acid on boiling, and therefore no free acid was added in the mixing. The iodide was old, of a dark yellow tint, containing a certain amount of free iodine. In addition to these facts, I have noticed, so far as my own experience goes, that green fog seems to occur more frequently when the gelatine and silver are mixed first, than when the gelatine and haloid salts are mixed and then the silver added. I very much prefer the first method of mixing, and have tried to persuade myself that it is not more liable to bring about green fog. In spite of my wishes, however, I have not been able to bring myself to believe this is the case ; while very far from asserting that the bromide into silver plus gelatine has more tendency to bring about green fog, my own experience tends towards this view. It may be asked, if nitrate of silver added to gelatine tends to green fog, how can green fog occur when the silver is added to gelatine plus bromide where the haloid salt is ready to receive it ? It must be recollected that the bromide is surrounded by the gelatine, and combination between gelatine and silver may in this case also take place, the boiling with excess of bromide being unable to undo the mischief. I have also noticed, in connection with some experiments of another character, that if a plate coated with emulsion be treated with a solution of nitrate of silver, washed well after a short time, placed in a dilute solution of bromide to destroy all trace of free silver nitrate, and then developed, a fog is found on the plate, somewhat resembling what is known as green fog. This fog is the more intense the longer the plate has been in contact with the nitrate of silver ; and is this a sort of slow cooking ? • Read before the Photographic Society of Great Britain. New Sample. Very faintly alkaline. No bromate present.
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