Volltext Seite (XML)
THE PHOTOGRAPHIC NEWS. Vol. VI. No. 199.—June 27, 1862. BROMIDES IN COLLODION. As discussion progresses, it becomes more and more appa rent that the influence of bromides in collodion remains but imperfectly understood. Our own conviction, held for many years, as wc have repeatedly remarked, is, that in proper proportions, they are in all ordinary cases an improvement, and in most cases accelerators, and no argument and experiment we have hitherto met with has disturbed that conviction. Mr. Dawson, of King’s College, has recently addressed a communication to our Liverpool contemporary on the subject, in which, admitting their value under some circumstances, he nevertheless urges that under some cir cumstances they are injurious. Without entering at large into the general subject, we will just make one or two remarks, before reproducing Mr. Dawson’s communication on the subject. We would, in the first place, call attention to Mr. Heisch's interesting paper in our last, on the im portance of the proper condition for rendering experiments in this direction fairly comparable. We next have to sug gest, briefly, for the consideration of experimentalists, a possible reason for the alleged retardation of bromides in the presence of a pyrogallic acid developer. The allegation is, that the addition of a bromide to collodion, in any pro portion, small or great, actually retards the production of an image, and renders necessary a longer exposure than when an iodide only is used; and it is often stated that exposure, which would suffice to give with the latter a passable pic ture, would, if a bromide were present, yield results thin, feeble, and entirely worthless. We believe this is true, but that it is, nevertheless, rather a question of intensity than of sensitiveness. An under exposed, simply iodized plate has generally excess of density, but lack of detail ; the same ex posure, if bromide were present, would probably yield more half-tone, but less density. The under exposed iodized plate might be made to print and produce one of the hard black and white pictures which have done so much to ruin the artistic reputation of photography, and impress art critics with the impression that it could not render soft re- flected lights and tender half-shadows. The bromo-iodized plate, under the same circumstances, would probably have Yielded nothing at all, and would at least have prevented the operator from producing an imperfect picture. Again, We have no hesitation in admitting that, as a general rule, bromides make development more tardy. The image of an iodized plate, whether with iron or pyrogallic acid develop- ment, generally springs much more rapidly into existence, and much more readily fogs under prolonged development, than that of a bromo-iodized plate. Those who have been chiefly accustomed to the use of an iodized collodion are, therefore, very liable to be deceived as to the amount of de velopment necessary and permissible with the bromo-iodized collodion, and failing to give the proper amount of develop ment, they are apt to imagine a plate under-exposed, and charge the collodion with slowness, simply because it was really under developed. Without entering further into the subject here, wc may just enumerate those advantages arising from the use of bromides, which have received uni versal acknowledgement, not only from those who advocate their use, but from those who regard them as in some respects injurious and retarding. In the Daguerreotype process, bromine was an un questioned accelerator, besides giving roundness, delicacy, an harmony to the pictures. It was the addition of bro- mne, or, as it was technically termed, “ a quick,” or “ quick stuff,” which rendered portraiture possible by the Daguer reotype process. In the waxed paper process it is an unquestioned accele rator. In the calotype, and all paper development processes, it is an unquestioned accelerator. By its aid only could the mag netic and meteoric registrations be preserved at the Royal Observatory at Greenwich and elsewhere, where the slightest perturbation is rendered by means of the momentary photo genic action of a gas jet, upon bromo-iodized paper. In solar camera printing, its accelerating principle is described by those who use it as enormous. In the dry collodion processes its accelerating influence is, we believe, unquestioned by any one. In the wet collodion process its accelerating and beneficial influence in the presence of an iron developer is generally admitted. We submit, then, that with so much admitted on all hands, it becomes necessary to pause before allowing that under its own best conditions and. fair experiment it is ever absolutely a retarder. We shall now quote Mr. Dawson:— “The question of bromides in collodion has again, I see, become the subject of strong contention in some of your con temporary journals. About five years ago, when the pyrogallic acid developer for negatives was almost exclusively employed I, in conjunction with a friend well versed in photographic chemistry, went through a series of experiments for the pur pose of settling the point at issue. We worked independently of each other with different chemicals, and only compared notes at the end of the series. Our conclusions were pretty nearly alike in all points; but in this opinion we were per fectly agreed, viz., that the use of a bromide, combined with an iodide, had a retarding instead of an accelerating effect, and tended strongly to the production of feeble negatives. It should be borne in mind that we worked only with pyrogallic acid and the potassium and ammonium salts. This conclusion, forced on us by the facts, seemed the more surprising, inasmuch as a previous series of experiments in connexion with the waxed- paper process showed that the addition of bromide, in the pro portion of one to two of iodide, reduced the time of exposure by nearly one half; while farther addition began again to exercise a retarding and fogging effect. We made some attempts to reach the cause of this curious anomaly, but failed in our en deavour. “ Having had occasion during the early part of the present spring to repeat my experiments, considerably modified, and on a more extensive scale, both with iron and pyrogallic deve lopers, and with the iodides and bromides of different metals, I still remain of opinion that with a pyrogallic developer the pre sence of a bromide in the collodion is a positive injury, and re markably so when conjoined with the iodide of potassium or ammonium. With the iodido of cadmium the effect is slightly different. For instance, supposing we have dissolved a suffi cient quantity of pyroxyline, free from all trace of acid, in pure fresh-washed ether, and the proper proportion of alcohol, spe cific gravity ’805, and iodized with cadmium, we shall find great difficulty, even in a slightly acid bath, of obtaining any thing better than a faint, blurred, foggy negative, until the collodion has been kept for several weeks. The addition of ono grain of any bromide to each ounce of the collodion at once corrects the fault; and, although it does not seem to me to in crease the sensibility, it undoubtedly gives greater clearness. But pyrogallic acid as a preliminary developer is fast giving place to iron, and here it is that the advantage of bromides is very conspicuous. My experience will not warrant me in say ing that their presence increases the sensibility of collodion or the opposite ; but they do give a clearness and incipient vigour to tho image, which is easily raised to a fine gradation of
