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The photographic news
- Bandzählung
- 12.1868
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- 1868
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- Bandzählung
- No. 499, March 27, 1868
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The photographic news
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Band 12.1868
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- Titelblatt Titelblatt I
- Kapitel Preface III
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March 27, 1868.] THE PHOTOGRAPHIC NEWS. 149 half-way across said passage as seen at E and F, and paint the inside of an orange or yellow colour. Should light enough to do injury get in after this addition, then blame the writer.” ON COLLODIO-BROMIDE. BY W. B. BOLTON. In opening this subject, I do so more in the hope of inducing Others to try collodio-bromide than with the intention of in troducing anything new ; for though much still remains to be done before the process will be perfect, so much has already been accomplished that in the present state of pho tographic knowledge there is, I fear, little chance of any considerable improvement being made either in this or other processes ; and until new materials are utilized, or new methods of applying the old ones discovered, we must be content to take things as they are. At the same time there is no reason why we should incur unnecessary trouble in any of our operations when the means are at hand of dispensing with that trouble ; and it is certainly bad policy to occupy the whole of a fine summer’s evening pent up in a stifling dark room, with the temperature almost at fever heat, pre paring half-a-dozen or a dozen dry plates for next day’s use, when the same may be easily and comfortably finished in half an hour, and equally good results expected. The one advantage which collodio-bromide has over all other dry-plate processes is the extreme simplicity of all its operations; and the saving of time and trouble is a matter of the greatest importance, especially to those who have not much leisure to devote to photography. Amateur's, as a rule, belong to this latter class, and to them, therefore, it offers peculiar advantages. If a person takes up photography as a recreation, his aim should be to make it as much like enjoy ment and as little like work as possible ; instead of which some seem to try the very opposite plan. What would be thought of a man who, of his own free will, spent his leisure time in stone-breaking, that is supposing him not to be a geologist? or of another, who had a passion for voluntary exercise on the treadmill ? There are many, I know, who do both under compulsion, but they arc not to be considered as amateurs. Most of my readers have, at any rate, a slight acquaint ance with collodio-bromide, and do not, therefore, require to be told that it consists of collodion holding bromide of silver in suspension. The idea of dispensing with the use of the bath by using a collodion containing the sensitive silver salts is, I believe, very old, collodion containing iodide of silver having been used several years ago, and almost simul taneously with the publication of the collodio-bromide pro cess in 1864; then Liesegang published his experiments with a similar process, based upon the formation of iodide of silver in the collodion, but found it unsuitable for negative work ; indeed, it was not until bromide was substituted for iodide that any success was obtained. For some time it seemed doubtful whether the process would ever become popular, and up to last year its use was confined almost entirely to a few Liverpool photographers; but now, thanks to the exer tions of Mr. Mawdsley, it has been so practically worked out as to bear comparison with any of the standard dry-plate processes of the day. And now to proceed to details. We will commence with the collodion, as being the most essential part of the process. Though scarcely so dependent upon the collodion as most other dry processes, there are still certain requirements to be fulfilled if any degree of success is to be attained. The most important thing to be looked to is the age of the collodion ; before it is of any use it should possess that quality, or combination of qualities, generally termed “ ripeness.” The length of time required to ripen it, of course, depends upon the bromizing salts. I have found very little difference in the results obtained by different formulae, provided always the collodion was of the requisite age; I do not care how old it is, as long as it is old ■enough. The formula I generally work with is 6 grains of bromide of ammonium and 4 to 5 grains of pyroxyline to the ounce of collodion, the ether and alcohol in equal parts. This requires no more than two orthree weeks toput it into working order. Mr. Mawdsley’s formula I do not know exactly, but bromide of cadmium is the prevailing feature. My reason for preferring the bromide of ammonium to that of cadmium is the peculiar effect the latter salt has in rendering the collodion glutinous, which effect is only partially removed by age. It is of the utmost importance that the collodion should be as structureless as possible, for the horrors of a mottled or crapy collodion are doubly magnified in the collodio-bromide process. It is a well-known fact that if the ether predominate over the alcohol, the collodion will give a strong film, but very much inclined to mottling and crapiness ; an excess of alcohol, on the other hand, gives an open structureless film, more liable to break away in the operations of washing and developing, and also more sensi tive. I consider it a mistake, therefore, to use the ether in excess, but would rather incline to the opposite course, the only objection being the difficulty of keeping the film on the plate, which is easily obviated by the use of a substratum of albumen or india-rubber. The former I prefer, as there is no bath to contaminate, and it does its work much more effectually than india-rubber, and the trouble is so very small that it need scarcely be taken into account. With regard to the salts used in the collodion I may, perhaps, say a word or two. Cadmium and ammonium are at present the only ones used, but I see no reason why the list should not be extended to two or three more, the bro mides of lithium and magnesium amongst the number. The latter, which is a very unstable salt, rendering the collodion very limpid and structureless, might with advan tage be used to counteract the opposite qualities of bromide of cadmium. The quantity of silver used in sensitizing the collodion will, of course, vary according to the formula, in consequence of the difference in the atomic weights of various bases. A collodion containing 8 grains of bromide of cadmium, which may be taken as the standard quantity, would require, according to theory, exactly 10 grains of nitrate of silver to combine with its bromine; but in practice it is found neces sary to use a little more. With bromide of ammonium, of which the atomic weight is loss, 6 grains would be about equivalent to 8 of cadmium, the exact theoretical quantity of silver required being 10-4 grains. The bromides of lithium and magnesium being still lower in the scale, would combine with the same quantity of silver in the proportion of 51 grains and 54 grains respectively. The silver having been added, the next question is how long the collodio-bromide should be kept before use, and how long it will keep in a fit state for use. There are so many opinions on this subject that it is a difficult question to answer, most agreeing, however, that the sooner it is used the better. I find it to be in its most sensitive state about six hours after it is mixed, and I also find it impossible to do anything much with it when more than a few days old. If the collodion be properly made, no substratum will be required ; the edges, of course, must be tipped with india- rubber or albumen, but as a security against accidents it is better to take a little- extra trouble and use the substratum. A number of plates may be coated and packed away in a dry place until required for use. Everything ready, my plates “ tipped ” or albuminized as the case may be, my mode of procedure is as follows :— Having first filtered the collodio-bromide lightly through cotton-wool, I coat the first plate. More care is required in coating a plate with collodio-bromide than with ordinary collodion; if the surplus be run back into the bottle too quickly, a streaky film is the result. The first plate is then laid down to set whilst the second is coated ; number one is then transferred to a dish of clean rain-water, and number three coated, and so on, until the required number are
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