The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-189100009
• PURL: http://digital.slub-dresden.de/id1780948042-18910000
• OAI: oai:de:slub-dresden:db:id-1780948042-18910000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

ON THE INTENSIFICATION AND REDUCTION OF GELATINE NEGATIVES.* BY ROLAND WHITING. Having made these preliminary remarks, I will now pro ceed to describe the various methods employed for increasing or reducing the printing density of a plate, and, as there is less to say about the latter than the former, I will treat upon that subject first. Reduction of density can be affected in two principal ways: the film can be reduced in thickness by mechanical means or by solution, or the silver forming the image can be changed into some substance which can be dissolved by a solvent. For reducing by the first of these methods, Mr. Debenham recommended the use of ozone bleach. After the plate has been immersed in a solution of chrome alum about one ounce to the pint, it is placed in a solution of ozone bleach of the strength of about one to six of water. Holmes’ ozone bleach is a substance which is used by laundresses for bleaching purposes, and can be obtained either at the oil shops or at several of the photographic chemists, and is very cheap. In the place of ozone bleach, a saturated solution of chloride of lime can be used in the same manner, but a substance which I like better than either of these is the hydrochlorite of potash, otherwise known as “ eau de Javelle." To prepare this, dissolve one ounce of chloride of lime in fifteen ounces of water, and two ounces of carbonate of potash in five ounces of water. These are mixed, boiled, and filtered, and when cold are diluted, one of the solution to five of water being the strength I most generally use. All these methods act very much in the same manner, viz., by dissolving the upper part of the film away. If reduction is to be carried to a great extent, the plate is placed in hypo, which dissolves out the silver that has been converted to the chloride. To reduce locally, a stronger solution is poured on the parts to be thinned the most, and, if necessary, these parts can be rubbed with the finger, or better still, with apiece of cotton-wool, until the required reduction is produced. Great care must be exercised in using these solutions. Do not handle the plate more than can be helped, or frilling may result; keep the plate gently rocking, or the reduction will not be even; do not rock too violently, or the edges of the film will be eaten away; do not have the solution too strong, or blisters will occur; and finally, let the tap be gently running so that the solution can be thrown off, and the plate be immediately washed without draining, or streaks and honeycomb markings will show themselves all over the plate. Local reduction can be very well brought about by rubbing the parts to be reduced with a piece of wpol dipped in strong alcohol, and, if carefully done, the result is very good. I have also heard of an operator who used to reduce locally with a mixture of very fine emery powder and some greasy material such as lard applied to the parts with a leather stump, but only very small places can be properly reduced by this means for obvious reasons. The grease is removed by a piece of rag dipped in turpentine. The chemical method of reducing is done either with ferric chloride or potassium ferridcyanide. For the first of these make a solution of ferric chloride, one drachm to six ounces of water. The plate, on being immersed in this, has some of its silver converted to silver chloride, which is dissolved out with hypo. The method with * Continued from page 715; potassium ferridcyanide, which was, I believe, introduced by the Polytechnic School of Photography, and which I prefer to all others, is as follows. A solution of red prussiate of potash (potassium ferridcyanide) is prepared. Some hyposulphite fixing solution is taken, and the prussiate added to it until it assumes a bright yellow colour. The plate is simply taken from the fixing bath and transferred direct to this, where it is kept gently rocking until reduction is complete. The beauty of this reducer is, no washing is required after fixing, and, also, you can be certain to what extent reduction will go. If the plate is to be reduced locally, it should be dried first if the parts have a sharp outline. The solution can then be applied with a camel’s-hair brush, or, as before stated, the negative can be varnished all over except in those parts to be reduced, and the plate immersed in the solu tions until thinned down enough. The chemical change which takes place with this reducer is : the silver, directly it is touched by the potassium ferridcyanide, is dissolved out by the hypo. One word more. As the ferridcyanide solution will not keep when mixed with the hypo, it must be made up a little while before using; and further, if reduction is to be carried to a great extent, the solution should be rendered highly alkaline with ammonia to pre vent the negative turning yellow. So much, then, for reducing. I will now proceed to treat upon the process of intensifying. There are three methods, so to speak, of strengthening the negative image, viz.: (1) the physical method, depending upon the crystalline attraction of silver in solution to deposit upon the silver forming the image ; (2) the chemical method, depending upon the reducing power of silver to reduce a metallic salt which is soluble to a sub-salt, which sub-salt is insoluble, and is thus deposited upon the image, or else, by acting again upon another substance, produces an extra deposit; (3) by strengthening the image by adding a dark material to the back of the negative over those portions which require strengthening. Under the first method comes Captain Abney’s formula for silver intensification ; it is as follows : — Solution 1. Pyrogallic acid ... 2 grains Citric acid ... 2 to 4 ,, Water ... ... ... ... ... 1 ounce Solution 2. Iron sulphate 5 grains Citric acid... ... ... ... ... 10 ,, Water ... ... ... ... ... 1 ounce Either of these solutions is taken, and a few drops of a 10-grain (to the ounce of water) solution of nitrate of silver is added just before using. It is then flowed over the plate. When the plate is sufficiently intensified it is washed, and placed in a solution of common sodium chloride, and, after again fixing for a few minutes in hypo, it is thoroughly washed. Any stains which may happen to appear may be removed with a 5-grain solution of potassium cyanide. Although this is theoretically the best form of intensifier, it cannot be recommended, owing to its liability to produce stains, especially if there should be traces of hypo in the film. Another method of intensification, which is known as the Polytechnic method, and which, I believe, was intro duced by Mr. C. Howard Farmer, is one of the best of its class that was ever invented. The intensified negative cannot be recognised from one which had been properly