The Optical Magic Lantern Journal (January 1890)

62 Curious Deterioration of Lantern Slides. T#aT lantern slides themselves occasionally fade is a fact that many are well aware of, this depending upon the treatment they receive by way of toning and fixing, but at a recent meeting of the London and Provincial Photographic Society, an illustration was given of an entirely new cause of degradation. Writing on this subject in the British Journal of Photography, the editor says :—* A few weeks ago we received a parce! of French lantern slides from a correspondent, who informed us that they formed portion of some which he had taken with him to India, where they had been for two years. Observing that they had gradually been becoming somewhat obscured by a species of mistiness, he undid the binding of one of them with a view of wiping off the scum which seemed to have settled upon the covering glass, when he found that all the area not protected by the mat had become corroded. Only those slides of French manufacture were thus effected ; those of English make suffered no change. Curious to know the cause of this erosion he sent them to us. : “Upon removing the binding and opening up some of these, the covering glass, where unprotected by the mat, presented a finely dimmed appearance, not SO coarse aseven the finest ground glass, but as if breathed upon, or as if it had been exposed to the fumes of hydrofluoric acid. The extreme fineness of this elicited from friends to whom it was shown warm expressions of desire that such could be obtained in sheets for focussing screens for cameras. Indeed so fine was it, that some at first considered it was only a scum of the nature of a varnish, which could be moved by scraping with a knife, especially as such scraping did remove a dusty deposit from the surface, only, however, to reveal the fact, under the microscope, that the dust was disintegrated glass, and that the Surface below was eroded. We had previously applied to the glass quite a variety of solvents of gums, including alcohol, ether, chloroform, and benzole, without any action being apparent. The image onthe slide having been formed on albumen, we were quite at a loss to account for the liberation of any corrosive, such as hydrofluoric acid—the only one we could then think ofas capable of having an action of this nature on glass. “As it was somewhat of a puzzle, we showed it to Opticians, experts in glass burning, and others likely to throw light upon the subject, but without avail. Knowing that the singular beauty of many of the French slides depends upon their being toned with mercury, we casually mentioned this when bringing the subject before the meeting of the London and Provincial Association on Thursday last week, and a The Optioal Magio Lantern Journal and Photographic Enlarger. clue to the difficulty was suggested by Mr. Everett, one of the members. Mercury, he said, was. well known to attack glass, presumably from its affinity for lead. “Following up this idea, we think a valid cause of the disintegration of the glass is to be found in the. fact that mercurial vapour, liberated from that. imbedded in the interstices of the image by the heat of India, operated upon the glass and decomposed it. We have obtained numerous formule employed by French glass makers in the manufacture of glass of a nature similar to that made use of in covering lantern slides, and we find that not only lead but arsenic and other metallic substances form component parts of the same. We here give as a specimen formula one ot these employed in actual practice :—Sand, trolbs. ; oxide of lead, 49$lbs.; potash, 393lbs.; arsenious acid, 1lb. Why slides of English manufacture should have enjoyed immunity from the erosion spoken of it is difficult to say, unless there is some difference in the composition of the glass, or perhaps there is no mercury employed in the toning. We have known innumerable instances in which the image on the slide has itself faded as a result of this toning, even when it was protected by a varnish possessing a good body, but in this case it was probably washed in an imperfect manner. Heat, at any rate, was not a factor in the destruction, as several of the slides in question. had never even been subjected to even the heat of the lantern; but never before have we known of a corroding action on the cover glass such as that described. The action of moist air we all know exercises an action upon optical flint glass, especially that of French manufacture, as most persons who have taken opera or field glasses to India are but too well aware, the action showing itself in the form of a brown stain on the flat side of the object glass. This is sometimes, too, the case with photographic lenses, although, happily, it is of less frequent occurrence, Owing perhaps to the fact that photographers in hot, humid climates probably keep the lenses of their photographic objectives cleaner than they do. those of their binoculars. In every case we have seen, it is the flint glass of the achromatic lense that has. suffered from atmospheric causes.” 102 Editorial Table. A CONVENIENT form of frame for holding transparencies has recently been introduced by Messrs. Mawson and Swan. These frames are made of brass, and are silver-plated. The picture is secured in place by a neat arrangement at the corners. A chain is supplied for hanging the frame, and it presents an attractive appearance.