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The photographic news
- Bandzählung
- 24.1880
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- 1880
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- Bandzählung
- No. 1141, July 16, 1880
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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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346 THE PHOTOGRAPHIC NEWS. [July 16, 1880. development commences during the exposure : whereas such an attempt made with ordinary alkaline or ferrous oxalate developer would prove a failure. The great objection to the use of this compound is its price; it is expensive, but doubtless any demand for it would quickly reduce it to nearly the same price as pyrogallic acid. It is at present more a chemical curiosity than of any commercial use, but if it should prove to be a good developer, it would speedily find its way into the price list of manufacturing chemists at a more moderate rate than that at which it is now shown. THE INFLUENCE OF TEMPERATURE ON THE SENSITIVENESS OF GELATINE EMULSION. BY DR. J. M. EDER. Many formula for the preparation of gelatine emulsion are given, but few of them agree as to the temperature at which the emulsification is to be carried on. One recom mends emulsification at ordinary temperatures, another at from 30° to 40° C., a third at 60° C., and some go even so far as 100° C. Now it is well known that temperature has a great influence, not only on the formation of the more sensitive modification of silver bromide, but also on the rapidity with which this modification is formed. I have, therefore, in conjunction with Capt. Toth, made an attempt to arrive at some determinate conclusion on the conditions of this important question. In all our experiments, silver nitrate and potassium bro mide were used in the proportion of 5 :4 ; there was, conse quently, always an abundant excess of bromide present. 1 .—At ordinary temperatures—that is, from 15° to 20° C.—there is formed only the less sensitive modification of silver bromide, which transmits the red rays. If an emulsion of gum-arabic be prepared in the cold (according to Mawdsley’s formula) we shall get a comparatively non sensitive emulsion. Equally non-sensitive is an emulsion which has not been digested, but is at once flowed over the plates after the silver nitrate and the alkaline bromide have been mixed. A gum-arabic emulsion, prepared cold, does not improve by standing, even for three weeks ; at the end of this time it still contains the silver bromide in a non sensitive condition. Negatives taken with it are, it is true, clear and dense, but the plates possess so little sensitive ness that they cannot be compared with those coated with an emulsion prepared with the aid of heat. Even the addition of from 1'5 to 2 per cent, of ammonia produces scarcely any effect; the sensitiveness certainly is heightened after standing for from twelve to twenty-four hours, but still not sufficiently so, and even at the end of a week the bromide has not been converted into the sensitive modifi cation. Increasing the proportion of ammonia to 6 per cent, is not an effective substitute for wanning the emulsion. Hence we conclude that emulsifying in the cold will not produce very good results. By the cold method it will never be possible, even after a great loss of time, to arrive at that high degree of sensitiveness which, by the hot process, is attained in a few hours. 2 .—At a temperature of from 32° to 48° U. the silver bromide passes gradually over into the sensitive condition. At the end of twenty-four hours the conversion is not complete, but after from three to six days the emulsion contains bromide in its most sensitive form. This process, which was first mentioned by Mr. Bennett, gives excellent results, but if the emulsification be carried beyond six days, fogging will ensue. Ammonia (1-5 to 2 per cent) promotes, at from 30° to 40° C., the formation of this modification to an extraordinary extent, as was first pointed out by Van Monckhoven. But even at this temperature the change does not take place immediately, but requires a period of from one to two hours. Occasionally, when there is a great excess of solu ble bromide, the conversion will not have taken place in from six to eight hours. If the emulsion be heated with ammonia for too long a time—say from sixto ten hours— it will show signs of fog. It follows from these remarks that the conversion of the bromide into its sensitive modification is greatly promoted by ammonia, but that, in this case also, much advantage is obtained by heating it. When ammonia in a nascent state, is made to act on silver bromide, the conversion is effected more quickly, and at a lower temperature. For instance, if a solution of potassium bromide be mixed with one of the amtnonio- nitrate of silver, or an ammoniacal solution of potassium bromide with an aqueous solution of silver nitrate, the complete conversion of the silver bromide will take place in from fifteen to twenty minutes at a temperature of from 28° to 359 C. This is the quickest method of producing the highly sensitive form of silver bromide, and I hope soon to be able to describe a process founded on it. 3 .—At 60° C. the silver bromide is modified much more rapidly than at from 30 c to 40° C. ; this was first observed by Stuart Wortley. In fact, the emulsion is in great part already modified at the end of a quarter of an hour. After the lapse of four hours the change is as complete as— perhaps even more complete than—it would be in four days at 40° C. After cooking for twelve hours the emulsion gives plates of great sensitiveness and clearness, but I do not think that the digesting can be carried on beyond that time without harm. Digested with ammonia at this temperature an emulsion will assume its highly sensitive condition in a few minutes, but it is apt to work foggy; in fact, a gelatine emulsion will not stand stewing with ammonia at a high temperature. When ammonia is present the temperature should never be raised beyond 40° C., otherwise fogging will result, and the emulsion will mostly be altogether spoiled. A very advantagebus method is to introduce the silver nitrate with a hot (60° to 70 C.) solution of potassium bromide and gelatine; when both the solutions are hot at the time of mixing, the conversion of the silver bromide is effected much more quickly than when the solutions are mixed cold, and then digested over heat. A high temperature, therefore, produces the modification particu larly quickly if the silver bromide be exposed at once in its nascent state to heat.* 4 .—At 100° C. the conversion takes place most rapidly; after a quarter of an hour’s boiling it is nearly, and in half an hour quite, complete, this sensitive ness being then about equal to that obtained by a five days’ simmering at 35® to 40% C. Should the boiling, however, belong continued, the emulsion will fog, and an hour-and-a-half's boiling will completely spoil it. The process of emulsification at boiling heat was, in my opinion, first made public by Mansfield in a communica tion to the Photographic Society of Ireland on the 13th of August, 1879, and, as will be familiar to the readers of the Photographic News, it has been strongly recom mended by Captain Abney. I have also observed that an emulsion which has been boiled for half an hour increases materially in sensitive ness by digesting it still further with ammonia, care being taken that during this latter operation the temperature is not allowed to rise above 40° C. From this observation I have worked out a second method for producing very sensitive emulsion, and hope to publish it very shortly. The general result of my investigation is, therefore, that heat is an excellent means of promoting the sensitiveness of emulsions, and that it should consequently never be dispensed with. But it must be recollected that the higher the temperature the more liable is the emulsion to fog quickly; hence, the higher the temperature the greater must be the care used and the precautions adopted in emulsifying. • This fact was discovered both by Van Monckhoven and by ,mxsel. simultaneously, but quite independently of each other; compare the aa of our respective publications.
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