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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- F 135
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1317, November 30, 1883
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
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Band
Band 27.1883
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- Titel
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754 THE PHOTOGRAPHIC NEWS. [November 30, 1883. make a marked difference in the emulsion. If it is at all alkaline, as it frequently is, its solution should be rendered at least neutral before mixing with the silver and gelatine, and most workers even prefer to make the solution dis tinctly acid. Litmus papers are cheap, and may always be handy; the extra trouble is insignificant. Less frequently, potassic bromide may contain a trace of bromate, but this is hardly likely to work mischief. Bromide of ammonium is more variable than the potassium salt, as it is apt to become acid on keeping. With both salts a little acidity more or less in the emulsion alters the time required for boiling considerably, and the worker should depend upon the colour of the emulsion, rather than the time of boiling. Iodide of potash, though even more subject to variation in quality as bromide, is used in such small quantities that the only point that requires attending to is its colour, denoting the presence or absence of free iodine. The presence of free iodine is not a disadvantage, but it is an item which must be taken into account. The British Pharmacopeia allows a small per-centage of carbonate of potassium to be present to prevent change of colour, and a small quantity of acid is necessary to neutralize this, or it may have a deleterious effect on the emulsion. Bromides and iodides may also contain traces of chlorides and sulphates. The first of these is seldom present in sufficient quantity to seriously affect the emul sion, and the latter may be ignored. It is important, again, to see that all the chemicals one uses for making emulsions are kept in well-stoppered bottles, for the presence of a small amount of moisture diminishes the excess of soluble bromide that the worker is supposed to be using. The developer, again, demands some amount of careful attention. If we compare the components of the alkaline with that of the iron developer on the score of uniformity, the first-named will probably have the best of it. Happily, ft is rare now-a-days to hear complaints of the quality of pyro, though the time was when such complaints were long and loud. The ammonia solution, however, is apt to vary considerably in strength if its bottle is not kept well stoppered, but troubles on this score are generally the fault of the worker, unless he is so situated that he has to depend on the local chemist for his supplies, and then indeed this “ amm.fort. -880 ” may not be all that the label sets forth. The iron developer is subject to many variations, but they are not very difficult to avoid. The potash oxalate should be rendered slightly acid to litmus paper if it is not so already, and its freedom from chloride should be ascer tained. This is readily done by dissolving a crystal in a little water in a test-tube, adding a few drops of nitric acid and a drop of nitrate of silver solution. If free from chloride, no permanent precipitate should be produced. Ferrous oxalate is met with in the market in two very dis tinct forms. In one form it is a bright yellow powder, apt to cling together and form lumps, soft to the touch, and readily soluble. The other form is duller in colour, has a sandy sort Of appearance, and seems more difficult to dis solve. Both forms make up a good developer, but we prefer to use the former of them. When the readier and more popular form of the iron developer is used, the satu rated solutions of oxalate of potash and sulphate of iron, a frequent source of variability in the strength of the deve loper is to be found in the differences in solubility of the articles at varying temperatures. This has but little effect on the final result so far as rapidity and quality of the plate are concerned; it is when, as in warm weather, there is danger ot a precipitation of ferrous oxalate, that trouble is caused. This subject is, we need scarcely say, no new one, and in some form or another has been treated of before in these columns. It is one of those points that the worker should keep continually before him if he would attain successful results. The prices charged for photographic chemicals are so low that absolute purity can scarcely be expected, and yet the slight variations in purity have an important effect on photographic work. With the exercise of in telligence and care, however, the effect of slight variations may be minimized. PHOTOGRAPHIC PATENTS. Expiration of Edwards’s “ Heliotype ” Patent. A most notable point in connection with photegraphi: patents is the circumstance that only a very small propor tion are considered sufficiently valuable to be kept in force for the full term of fourteen years, and of those taken out in 1869 we find that all but two became void before the commencement of the present year. Of these, one is a " Carbon ” patent of Johnson, which expired on the third of February last (see page 74), and the second is the “ Heliotype ” patent of Edwards. This latter becomes public property on the eighth of the present month. The following extracts from the specification embody all the important points of Edwards’ patent:— In carrying these my improvements into effect, I proceed as follows :—Upon a surface of metal, wood, stone, glass, porcelain, slate, or enamel, clean and free from grease, I place a layer of gelatine, gum, albumen, fibrine, or compounds of these substances, one or more of them, hardened and rendered insoluble in water by means of ammonia, alum, and other varieties of alum, chrome alum, tannin, chlorine, or any of the other substances that are known to have the property of tending to produce insolubility of gelatine, gum, albumen, fibrine. This layer is rendered sensitive to light either during its preparation, or subsequently by treating with a printing from a film of bichromated gelatine which has been acted on by light and subsequently moistened, has not been found, on account of the nature of such a film, to give satisfactory results, either as regards numbers, or vigour, or firmness of line, besides showing a certain grain ; but gelatine may be rendered hard and insoluble, and converted into a very tough and durable substance by treatment with alum, and by other means; and I have discovered that after such treatment it still retains its pro perty by being acted on by light, &c., and has, moreover, been converted into a substance which has all the qualities of stability, depth, and firmness necessary for a good printing plate, while the surface has been rendered completely free from grain, chromate, or bichromate. The sensitive layer thus obtained is to be sub mitted to the action of light and washed m water; a second layer of the insoluble gelatine is applied, and the operation repeated till the plate is again sensitive to light, after exposure, under a nega tive, till all the details appear. The plate is freed from bichro mate by washing in water, and is ready for printing, or it may be dried and placed on one side for future use. Where the plate is of glass, light may be allowed to act on the reverse side of it till the image has almost disappeared. Where a few copies only are required to be printed, the first coating may be omitted, or the sensitizing and exposure to light of the first coating may be omitted. It is not possible to fix the quantity of alum or other sub stance to be added to the gelatine, as this varies with the sample and description'of gelatine used ; but it is to be understood that it is Ito be such a quantity as will render the gelatine film, after being dried, insoluble in boiling water. When it is desired to print from the plate, it is thoroughly wetted and placed in a press in which the pressure is vertical. I prefer to use an ordinary print ing press ; the plate having been freed from superfluous moisture, the image appears on the surface of the gelatine in relief and de pression, the depressed parts being the shadows, and the raised parts the lights. It is necessary to obtain a sufficient amount of pressure in the shadows without much more pressure on the lights, or without so much as to risk the breaking of the plates; to effect this a mould is made by warming a sheet of gutta-percha, placing it on the plate, and pulling the press, the mould being arranged that it falls each time exactly in its place. The plate may, as an additional precaution against breakage, be bedded in gutta-percha; other materials besides gutta-percha may be used, such as leather, india-rubber, paper, paper pulp. The mould having been obtained and the pressure adjusted, the plate has to be inked. In ordinary lithography there is a great adhesion to the plate, both from the ink and the roller, which, applied to this process, results in the tearing up of the gelatine film; toobyiate this, and also to obtain a finer surface, I use rollers or dabber made of india-rubber, and I use lithographic ink thinned down with tallow and olive or other oil, avoiding as far as possible t e
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