THE BRITISH JOURNAL OF PHOTOGRAPHY. September 9, 1864, p.312.
From Le Bulletin Belge de la Photographie
I AM frequently asked what are, the methods, I prefer. for printing enlarged photographs: the object of this article is to indicate them in detail.
I will consider the processes according as the negative is perfect or imperfect, and so necessitating; a re-touching, of the enlarged picture. In, the first case I use albumenised paper; but .in the second plain paper, as, albumen paper does not readily admit of re-touching.
Among the various papers of all kinds which I have tried; I find a great disparity with regard to rapidity. One sort requires double the time to print that another does. The paper called imperial is the most rapid of those I have used, but it is necessary to use the papier fort to prevent tearing Paper very highly albumenised generally gives better results than paper only feebly albumenised, The Saxe paper and certain English papers are, in general, excellent.
It is important to keep the albumenised paper in perfectly tight boxes, or in rooms very dry, because if it becomes damp the salt enters into the pulp, and the print becomes spotted. This effect can be seen by examining the paper after it is sensitised, by holding it up to the light. The "refusing to tone," so often complained of with. regard , to albumenised paper, is caused by this.
As enlarged prints require the use of large sheets of paper; it is important not to keep them in a flat state, but rolled up; and instead of rolling them with the albumen side inwards, as is usual, they should be rolled with the albumen side outwards, because then it is comparatively easy to place them on the silver bath. Other, wise there is very great difficulty,
There are two methods of sensitising the paper. The first has. the great advantage of permitting the sensitised paper to be kept for many days--an inestimable advantage in a climate so variable as ours. The second method affords a paper which prints more rapidly and gives better tones, but it turns yellow very quickly:
The, bath is composed as follows:
|Distilled water||35 ounces|
|Crystallised nitrate of soda||25 drachms|
|Crystallised nitrate of silver||25 "|
|Nitric acid||10 drops.|
Measure out the distilled water and then dissolve in it the nitrate of soda; next add the nitrate of silver, and then the nitric acid. If the liquid is cloudy, it is because the nitrate of soda contains a chloride or a carbonate. It is, therefore; necessary for this process to test first the nitrate of soda by dissolving fifteen grains of it in half-an-ounce of distilled water; then add to it a few drops of a pure solution of nitrate of silver. No precipitate ought to result. If there is a precipitate another sample of nitrate of soda ought to be procured, or the quantity of the nitrate of silver ought to be augmented. The filtered liquid ought always to show a slight acid reaction when tested by litmus paper. .
The paper may now be sensitised. Pour the bath into a porcelain or English china dish, but not into a gutta-percha one. Gutta-percha and marine glue are exceedingly injurious, causing a discolouration of the paper. It is easy now to obtain superb dishes of English earthenware, twenty inches wide by twenty-six inches long, and they cost less than gutta-percha, while it is possible to employ them, as we are about to detail, in sensitising sheets of double these dimensions. We especially recommend the entire abandonment of dishes made of glass glued together. They occasion continual failures, and the heat of summer often causes the wood which supports the side to warp, and so displace the glass, occasioning disastrous consequences. Such dishes of a small size may answer for certain purposes, but should never be used for a silver bath.
The sensitising bath being poured into a proper dish will fill it to the height of about two inches, and must be carefully skimmed with a strip of paper as wide as the dish, so as to remove bubbles and dust.
The paper is now carefully lowered on the bath, and immediately raised to see that no bubbles are under it. When it becomes necessary to sensitise a sheet double the size of the dish (a highly desirable accomplishment, as in this way dishes of a very large size can be avoided) we begin by turning down a corner at each of the four angles of the sheet; an assistant then takes hold of it by the two corners and we by the two opposite corners. While the assistant holds the sheet perpendicularly above the bath we place it upon the bath, commencing at the lower end, and the assistant permits it slowly to descend: just here the advantage of keeping our paper with the albumen side rolled outwards is very evident. When the paper has been lowered sufficiently to touch the opposite edge of the dish, we raise that part from the bath, while our assistant slowly and regularly lowers the remainder of the sheet, until the whole sheet has thus been brought in contact with the bath; he then rapidly raises his end of the paper, and we bring our portion down upon the bath again, causing the surface of. the sheet to pass rapidly over the bath in order to retain as much of the liquid as possible. This backward and forward motion of the paper over the bath continues during four minutes. At the end we raise the sheet very slowly and hang it up. When the paper is raised rapidly too much of the bath is lost in the draining. This whole operation is much easier in the practice than it would seem from the description. It is necessary, of course, to perform the operation without stopping, and to take care not to let drops of the bath soil the reverse side of the paper, or else in these places there will be spots. But, whatever method may be adopted, it is essential that the paper should remain on the bath for four minutes. The plan I propose has the great advantage of making it possible, with a single earthenware dish twenty inches wide by twenty four inches long, and three or four wooden trays thirty, inches wide by forty inches long, and lined inside with India-rubber, to avoid incurring a heavy outlay for dishes, and a multiplication of sensitising and toning baths, for double sheets can be toned in dishes half their size without difficulty.
When the sensitised paper is quite dry it is placed in a box with chloride of calcium, and may be preserved white for many weeks. I constantly employ a cylindrical zinc box furnished with a muslin cylinder in the centre, in which the chloride of calcium is placed. These boxes are very convenient and quite economical. I first roll some blotting-paper around the muslin cylinder; then the paper, being perfectly dry, is also rolled on the cylinder. It is sometimes a little difficult to roll the paper in this way, but it very greatly facilitates the labour of arranging it upon the printing-frame at the time of its exposure to light in the enlarging apparatus.
We will now notice two or three matters worthy of attention. 1. The weakening of the silver bath. 2. The habitual alkalinity of albumenised paper. 3. The discolouration of the bath after being used a few days, especially in summer.
The deterioration of the bath must be rectified by strengthening it by the addition of thirty grains of nitrate of silver for every sheet twenty by twenty-four inches. It is not necessary to add nitrate of soda..
The habitual alkalinity of albumenised paper renders it liable to turn yellow a few hours after it has been sensitised. It is necessary, therefore, after sensitising a few sheets to test the bath with blue litmus paper, which ought to redden slowly; if it does not, a few drops of nitric acid should be added to the bath.
The yellow discolouration of the bath takes place a few hours after it has been used, especially with certain albumenised paper having an alkaline reaction. The bath maybe clarified by adding to it twenty minims of hydrochloric acid for every thirty-six ounces of the bath, then stirring vigorously and filtering. In this operation the hydrochloric acid foams a chloride of silver, which combines with the coloring matter of the bath,
The alkaline possesses the following advantages in comparison with the acid bath, which has just been described:--
It gives greater sensitiveness to the paper.
It gives much finer tone.
The prints tone more easily.
It is possible to work the bath down to three or four per cent. of silver, and yet get good resets.
But this bath has the following disadvantages:--
The sensitised paper must be used in a few hours after it is prepared, for it turns yellow very quickly.
The bath must be kept from the light; for it decomposes readily.
It is difficult to prepare it, and all the chemicals must be very pure.
The bath is prepared as follows:--Procure. some very pure soda, either in the solid form, or in. solution, which is more convenient. If it is solid, dissolve it in ten times its weight of water, and keep it in a bottle stopped with a cork, acid not with a ground glass stopper, because the soda causes a glass stopper to adhere very tightly to the neck of the bottle. The purity of this solution may be tested by dissolving a small crystal of nitrate of silver, in two drachms of distilled water, and adding five or six drops of very pure nitric acid; shake well, and then pour into this mixture three or four drops of the soda. No precipitate should be formed.; the liquid ought to:remain limpid.
Procure now some crystallised nitrate of ammonia, dissolve it in its own weight of distilled water, and filter. Take a few drops of it and add thereto a drop or two of a solution of pure nitrate of silver. The liquid ought to remain perfectly limpid, otherwise the nitrate of ammonia is impure, and must not be used.
We now take three ounces of crystallised nitrate of silver and dissolve it in thirty-six ounces of distilled water; we then add to it, in doses of two drachms at a time, the solution of caustic soda, stirring each time with a glass rod, and we stop adding soda when we see that there is no longer any precipitate. To be certain about this we decant a little of the liquid and add soda to it. It is easy to see at once if any precipitation takes place. If there is none we cork the flask, and permit the brown precipitate of the oxide of silver which has been formed to settle during an hour. At the end of thus time we carefully decant the brown liquid which covers the oxide of silver, taking care, of course, not to lose any of the precipitate. A quart of filtered rain water is them added, and the whole well stirred with a glass rod, and then allowed to settle for half-an-hour, when the clear or slightly discoloured water is poured off. This washing with water is again repeated, and after standing for two hour; is very carefully decanted.
The solution of nitrate of ammonia is then permitted to fall drop by drop upon the oxide of silver, stirring all the tune with a glass rod. The oxide of silver soon disappears; and the liquid assumes a cloudy appearance, which cannot be removed by adding nitrate of ,ammonia. It is necessary to be guided by the disappearance of the precipitate. It is very important not to add an excess of the nitrate of ammonia; on the contrary, it is preferable, that a little of the oxide of silver should remain undissolved than to add any excess of the nitrate. We now pour the liquid into a graduated measure, and add water enough to make thirty-six ounces; it is then filtered, the filter paper being preserved among the silver residues. The bath is now a solution of the oxide of silver in the nitrate of ammonia, and has a strong alkaline reaction with litmus paper. It contains besides some soda, which the oxide of silver, prepared as we have just indicated, always retains. This soda would dissolve off the albumen from the paper, at least partially; we therefore add (to the filtered bath of course), eighteen minims of nitric acid, which neutralises the soda and transforms a very small quantity of the oxide of silver into the nitrate.
This bath is poured into a porcelain dish, and the paper sensitised upon it precisely as we have already indicated above. The dried paper ought to be used the same day on which it is prepared, especially in summer.D. VAN MONCKHOVEN.
(To be continued.)