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The photographic news
- Bandzählung
- 27.1883
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- 1883
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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. 1272, January 19, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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decomposition sets in, and it is not until later, during the deve lopment, that any imperfection is discovered. To obtain uniform work, the strength of the bichromate bath must be regulated according to the temperature. The warmer the weather, the weaker the bath must be. In winter, to one part of bichromate of potassium thirty parts of water are added ; in summer, fifty ; and in very hot weather, a hundred parts of water will be re quired. After repeated use, the bath becomes dark coloured, and must then be thrown out. In summer, the bath must be kept as cool as possible ; if necessary, the vessel containing it should be placed in ice. The stronger the bath is, the softer the prints will be ; the weaker the bath, the harder will be the prints. Therefore, to print from a hard negative, a stronger bath should be used ; to print from a very weak negative, a very weak bath is necessary. It must be remembered that tissue sensitized in a weak bath is less sensitive, and, therefore, requires longer ex posure. Too weak a bath yields no half-tones ; too strong a one produces reticulation (a network structure in the print). .When the tissue has been cut to the required size, it is carefully wiped with a soft, smooth duster to remove any dust that may be sticking to it. It is then immersed in the bath. Air-bubbles which attach themselves to both sides of the tissue are removed by means of a soft, fine sponge, and then it is allowed to remain, face downwards, in the bath until the gelatine film feels soft, or until the edges of the tissue begin to curl upwards. If it is taken out sooner, the film will not have been equally impregnated, and spots will be produced. For the same reason a liberal quantity of the solution should be used. The time the tissue should be allowed to soak depends both on the solubility of the gelatine and the temperature of the bath. It ranges from one to four minutes. When the tissue has been removed from the bath, it is either hung up at once by wood clips, or over a roller on which are several thicknesses of blotting-paper; or, what is much better, it is placed, face downwards, on a clean sheet of zinc or glass, and the superfluous moisture removed by passing a squeegee gently over it length-ways and cross-ways. A squeegee is merely a strip of india-rubber fixed to a wooden handle. The tissue is then hung up to dry, which will take several hours. If it dries too slowly, exces sive sensitiveness is developed, and it becomes tough and insoluble, causing uncertainty in exposing, and great difficulty in develop ment. On the contrary, if it is dried too quickly, it requires a very lengthened exposure, and even then there is a deficiency in the half-tones. The dried sensitized tissue, if well kept from light and dampness, should keep good from ten to fourteen days. As there is no apparent change in the tissue after it has been printed, it is necessary to have some means of regulating the exposure. For this purpose, we have a small instrument called an actinometer or photometer. There are a great many different forms of this instrument; but, as usual, the simplest is by far the best; in fact, the only practicable one I have tried. Suffice it to say that almost every actinometer that has been proposed for testing the rapidity of gelatine plates has been tried for regulat ing the exposure of gelatine tissue, with this difference, that a piece of silver paper prepared in a very acid bath to make it slow is put in the actinometer instead of a gelatine plate. This actinometer, introduced by Mr. Johnson, and called the cube photometer, consists, as you see, of a little disc-shaped tin box, with a double lid; in the upper lid there is a round opening of 3 of an inch in diameter, covered with glass, on which, with oil paint, the chocolate-brown colour-tone is painted, which silvered albumen paper takes in about one and a-half minute’s exposure to sunlight. In the middle of the glass there is a narrow slit half-an-inch long and an eighth of an inch in width, which is left uncoloured, as you see ; inside the box is a roll of durable sensitized silver paper half-an-inch in width, and which can be drawn past and in contact with the inner side of the transparent space in the glass, without its being necessary to open the box. The silver paper used is albumenized paper sensitized with nitrate of silver solution to which a little citric acid has been added; paper prepared in this way will keep good for a year. The number of tints (that is, the number of times the paper will have to be shifted after assuming the tone that has been painted on the glass) which any negative will require will, of course, depend entirely on its density. A negative of medium density will require about four tints ; but a little practice is all that is required to be able to tell at once how many will be needed. If the printing is done in direct sunlight, the expo sure must be comparatively shorter than in diffused light, silver paper and carbon tissue not possessing an equal ratio of sensi tiveness in weaker light. If a negative requires five tints in diffused light, only four will be required in the sun. Any ordinary negative will do to print from, the only difference being that it must have a safe edge—that is, an edging of some perfectly opaque material. Any ordinary printing-frame may be used, provided the packing be kept dry. It is a good plan to put a piece of oil paper or American cloth, a little larger than the tissue, between it and the packing. Before being put in the frame, the tissue should be examined, and any dust (which sticks to it very readily) mnst be brushed off with a soft cloth. When the tissue has been printed, a great time should not elapse before development, as the change produced by the light’s action is continued in the dark. When prints have to be left overnight, about one tint is allowed for the extra printing that goes on. The tissue is now ready for development. Originally this was done by merely washing away the unacted-upon gelatine by soaking in warm water; but, do what they would, nothing could be got but a black and white picture ; half-tone, the chief beauty of the silver print, could not be obtained. The explanation of that is as follows:— The photogenic film, consisting of gelatine and pigment, pos sesses a certain thickness, and lies wholly on the surface of the tissue. Where the light finds free entrance through the negative, it acts quite through the film, and fixes it in these places to the paper, so that when the film is washed, the black gelatine remains and forms the shadow of the picture. In the lights, the sensi tive film is protected by the negative from the action of light, and these, being still soluble, are dissolved out in the washing, and form the lights or whites of the picture. In the half-tones, the light cannot act quickly, and reaches, as the action commences from above, perhaps to but half the entire thickness of the film. In washing, therefore, only half of the film would dissolve away, and through the remaining half the white paper would be partly visible, and thus form a kind of grey, representing the half-tones. Thus, in theory, the carbon process must yield half-tones. But one little matter has been overlooked. As the half-tone forms the outer part of the film, it does not come into direct contact with the paper; and as in the washing the soluble part under it is dissolved away, the half-tone is also carried away with it. The next experiment tried was printing the picture on the back of the gelatine, through the paper, which was waxed, to render it transparent; but that did not succeed, owing to grain of the paper, which could not be entirely got rid of; and they were also wanting in sharpness, owing to the thickness of the paper being between the negative and the gelatine. Mr. Swan at last entirely removed that difficulty by transferring the picture to a second paper, and then washing away the super fluous gelatine from the back. There are two modifications of this process, called the single and double transfer processes. When these processes were first brought out, the double transfer," in the form of chromotypes or Lambertypes, was the great favourite ; but now, comparatively little is done that way, and it is almost solely the single transfer process that is worked com mercially. I will therefore describe that first. The transfer paper is prepared by coating a hard-sized paper with a solution prepared by dissolving one ounce of gelatine in 1 lb. water. When the gelatine is thoroughly dissolved, 6 } drams of an aqueous solution of chrome alum, prepared by dissolving one ounce of chrome alum in twenty ounces water, is added, drop by drop, whilst it is being continuously stirred round. It is then filtered through fine muslin, and must be used while warm, as it will not re-dissolve if it is once allowed to set. It is then brushed over the paper with a broad camel-hair brush. Another very good method of preparing single transfer paper is, by coat ing the paper with an aqueous solution of shellac, obtained by boiling three parts of white shellac and one part of borax in thirty parts of water ; or, if an unbleached shellac of red brown colour is used, it imparts a very pleasant rose tint to the paper. This shellac transfer paper possesses one great advantage over gelatine transfer, inasmuch as it keeps good for any length of time ; while the gelatine transfer paper, contrary to what is gene rally supposed, only keeps at its best about a fortnight. The double transfer paper is prepared in exactly the same manner as the first single transfer paper described, only there is less chrome alum added. The proportions are— Gelatine 1 ounce Water 6 ounces Alum solution 24 drams A piece of single transfer paper is now cut a little larger than the tissue to be developed, and both it and the tissue are placed in cold water till the tissue has become soft—generally about half a minute ; they are then taken out, and placed face to face on a flat piece of glass, or zinc, or slate, and a squeegee is passed
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