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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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- Bandzählung
- No. 1273, January 26, 1883
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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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THE PHOTOGRAPHIC NEWS. 53 the silver nitrate is added, is, that if silver nitrate be added the washing, four drachms are now added in the same way. The emulsion should now measure about 6} ounces, and is ready for filtering ; this is done through a piece of wet chamois leather or swansdown calico which has previously base silver to combine with the weaker halogen thus— KI- AgBr = KBr+AgI and in this case, the particles of silver iodide formed are of the same size as the previous particles of silver bromide; for the same reason, whatever excess there remains of the soluble haloid salts will contain no iodide, but will be all bromide. On calculating it out in this case, we find that there is an excess of 16 grains of potassium bromide when I the silver has all been added, and it is this excess of potas- ium bromide which performs the important operation of converting the orange bromide to the granular bromide ; boiling the emulsion has the effect of very much hastening this operation, but the same change takes place in a much longer time in the cold. The washing is for the obvious purpose of removing the soluble nitrate, and now worse than useless excess of bromide ; the addition of the chrome alum hardens the gelatine and prevents frilling ; the alcohol facilitates coating the plates uniformly. spirit—this is conveniently done with a piece of chamois leather tied over a bung-—wash again under the tap, rinse in distilled water, and stack up on clean blotting-paper to dry. In the operation of coating, a large slab of plate glass or flat slate is required, which should be levelled by means of three small wood wedges and a spirit-level on a bench or table close to where the plates are going to be coated. with the potassium bromide, the following chemical change takes place :— AgNOa + KBr = AgBr + KNO, The reason for adding the potassium iodide after some of The plates being ready, immerse the flask containing the filtered emulsion in water heated to 120° F., and taking a plate on a pneumatic holder, pour on a pool of the emul- | sion (considerably more than is required to coat the plate), tilt the plate quickly all round so that the emulsion covers the whole of it, and return the excess emulsion to the jar; finally, give the plate a final tilt in the opposite direction to that in which the emulsion was poured off, and place on the levelled slab to set. Another way, equally efficient, is to place the plate on a small levelled block of wood, pour on sufficient emulsion, and spread with a glass rod ; finally, tilt the plate to render the emulsion uniform, and transfer to slab. As soon as the plates are set (which PHOTO-LITHOGRAPHY AND PHOTO ZINCOGRAPHY. BY MAJOR J. WATERHOUSE, B.S.C., Assistant Surveyor-General of India. Chapter VI.—The Preparation of the Photo-Transfer Paper. As noticed in the last chapter, the photo-lithographic transfer paper consists of paper coated with gelatine, gum, albumen, or starch, mixed with bichromate of potash or ammonia. Various methods of preparing the paper have been pro posed, depending on the use of one or other of the above colloids, either singly or mixed, and it may be made sensi tive either by mixing the bichromate with the colloid, or by- treating coated paper with a bath of bichromate. Good results may be obtained in many different ways, and the choice must be guided by the nature of the work to be done, the circumstances in which the operator finds him self placed, and the conveniences at his disposal. As a rule, gelatine, either alone or in combination with albumen, is preferred. Paper so prepared can be used in either the inking-up or washing-off methods, whereas papers prepared with gum or soluble albumen are only suitable for the washing-off methods, the coating being readily soluble in cold water. On the other hand, papers prepared with starch or flour-paste are more suitable for the inking- up methods. One of the simplest and at the same time most efficient ways of preparing the transfer paper is that originally adopted at the Ordnance Survey Office, Southampton, and published in Sir Henry James’s work on “ Photozinco graphy.” Choice of Paper.—The most suitable paper for this and I other methods is the fine tough wove paper, made from linen, known as bank post. The writer has always used white or cream wove, but some operators prefer the blue. Laid paper should not be used, being uneven in texture. For small sizes the ordinary thin quality may be used ; but for large transfers a thicker description is better, being , stronger and less liable to tear. The paper should not be , too highly sized, but this may be remedied by a short ; I soaking in boiling water before use. Rive paper also been well boiled and washed, the leather or calico is placed the silver nitrate is added, is, that if silver nitrate be added loosely in a funnel, and the liquid emulsion poured on it, to potassium iodide, the silver iodide produced has a great and as it filters through is collected in a glass flask. I tendency to form in a coarse condition. When, however, To prepare the glass plates, immerse them in nitric acid the emulsion contains some silver bromide, and the and water (1 to 10), then wash and rub them over with a potassium iodide is then added (no free silver nitrate being 10 per cent, solution of soda containing a little methylated I present), the more powerful base potassium combines with —il •1 • - ■' ’ ■ • • - 1 the more powerful halogen bromine, and leaves the weaker they should do in a few minutes), place them in a drying I cupboard, and dry in a minimum time of about six hours ; I or place them in methylated alcohol free from resin for I ten minutes, and stack up on blotting-paper, when they should dry in about an hour. Precautions in Hot Weather.—After mixing the gelatine 1 (No. 5) with the boiled emulsion, the whole is poured into a jam pot, which is immediately placed in iced water for the emulsion to set. When set, it is washed in the same I way, but the water employed should have some lumps of ice floating in it. The draining should only take about I half-an-hour, and the emulsion should then be melted, a I few drops of carbolic acid (or other antiseptic) added, and I again placed in iced water ; when set it is covered with alcohol, and allowed to ripen for a day. When coating plates, the slab should be previously cooled by being covered with small lumps of ice for half-an-hour. If I the emulsion appears too thin before coating, 20 grains of I hard gelatine should be melted in 1} drachms of water and added to it immediately before filtering. An Alternative Method of Washing.—Cool the mixed gelatine and boiled emulsion to as low a temperature as con- I venient without its setting, and pour into a large beaker or jar ; take a glass rod, and, stirring the emulsion round with it, pour in methylated alcohol until the liquid is seen to become clear, when most of the emulsion will be found adhering as a clot to the rod ; pour off the clear liquid con taining the soluble salts, and add alittle fresh alcohol ; this removes the last traces of water, and hardens the clot, which is broken up into small pieces and allowed to swell in clean water ; this is changed a few times, and the emulsion finally melted as in the other method. Rationale of the Process.—The object of emulsifying in a | Small quantity of the gelatine is to save as much as possible from the decomposing effects of boiling ; it also shortens the time necessary to produce the granular bromide. The ad dition of the drop, of hydrochloric acid serves two purposes : it aids materially in the production of a fine precipitate, and, by rendering the emulsion acid, prevents or destroys fog. Heating the solutions, the use of the spray apparatus, and the agitation of the liquid, are all for the purpose of securing a free precipitate. On the silver nitrate coming in contact
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