Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/866

 830 PHOTOGRAPHY and dried with a clean cloth. They are then ready for receiving the emulsion, which, after being warmed to about 120 Falir. , is poured on them in sufficient quantity to cover well the surface. This being done, the plates are placed on a level shelf and allowed to stay there till the gelatin is thoroughly set ; they are then put in a drying cupboard, through which, by a simple contrivance, a current of warm air is made to pass. It should be remarked that the warmth is only necessary to enable the air to take up the moisture from the plates. They ought to be dry in about twelve hours, and they are ready for immediate use. Expo- Exposure. With a good emulsion and on a bright day the ex- sure, posure of a plate to a landscape, with a lens whose aperture is one- sixteenth that of the focal distance, should not be more than one- half to one-fifth of a second. This time depends, of course, on the nature of the view ; if there be foliage in the immediate foreground it will be longer. In the portrait-studio, under the same circum stances, an exposure with a portrait-lens may be from half a second to four or five seconds. Develop- Development of the Plate. To develop the image either a ferrous ment of oxalate solution or alkaline pyrogallic acid may be used. The plate. former is conveniently prepared as described on p. 826. No chemical restrainer such as bromide of potassium is necessary, since the gelatin itself acts as a physical restrainer. If the alkaline developer be used, the following may be taken as a good standard : ( Pyrogallol 50 grs. 1. &amp;lt; Citric acid 10 ( Water 1 oz. f Potassium bromide 10 grs. &quot; | Water 1 oz. j Ammonia, 880 1 dr. One drachm of each of these is taken and the mixture made up to 2 oz. with water. The plate is placed in a dish and the above poured over it without stoppage, whereupon the image gradually appears and, if the exposure has been properly timed, gains suffi cient density for printing purposes. It is fixed in a solution of hyposulphite of soda, as in the other processes already described, and then thoroughly washed for two or three hours to eliminate all the soluble salt. This long washing is necessary on account of the nature of the gelatin. Intensi- Intensifying the Xegative. Sometimes it is necessary to intensify fying the negative, which can be done in a variety of ways with mercury negative, salts. An excellent plan, introduced by the Platinotype Company, is to use a saturated solution of mercuric chloride in water, and a subsequent addition of 2 grains to the ounce of platinic chloride. This is put in a dish and the metallic solution allowed to act till sufficient density is obtained. With most other methods with mercury the image is apt to become yellow and to fade ; with this apparently it is not. Varnish- Varnishing the Xegative. The negative is usually protected by ing nega- receiving first a film of plain collodion and then a coat of shellac or tive. other photographic varnish: This protects the gelatin from moisture and also from becoming stained with the silver nitrate owing to contact with the sensitive paper used in silver printing. Printing Processes. The first printing process may be said to be that of Fox Talbot (see above, p. 824), which has continued to be generally employed to the present day (with the addition of albumen to give a surface to the print, an addition first made, we believe, by Fox Talbot). Paper for printing is prepared by mixing 150 parts of ammonium chloride with 240 parts of spirits of wine and 2000 parts of water, though the proportions vary with different manufacturers. These ingredients are dissolved, and the whites of fifteen fairly-sized eggs are added and the whole beaten up to a froth. In hot weather it is advisable to add a drop of carbolic acid to prevent decomposition. The albumen is allowed two or three days to settle, when it is filtered through a sponge placed in a funnel, or through two or three thicknesses of fine muslin, and transferred to a flat dish. The paper is cut of convenient size and allowed to float on the solution for about a ininute, when it is taken off and dried in a warm room. For dead prints, on which colouring is to take place, plain salted paper is useful. It can be made of the following pro portions 80 parts of ammonium chloride, 100 parts of sodium citrate, 10 parts of gelatin, 5000 parts of distilled water. The gelatin is first dissolved in hot water and the remaining components are added. It is next filtered, and the paper allowed to float on it for three minutes, then withdrawn and dried. Sensitiz- Sensitizing Bath. To sensitize the paper it is made to float on a ing bath. 10 per cent, solution of silver nitrate for three minutes. It is then hung up and allowed to dry, after which it is ready for use. To print the image the paper is placed in a printing -frame over a negative and exposed to light. It is allowed to print till such time as the image appears rather darker than it should finally appear. Toning Toning and Fixing the Print. The next operation is to tone and and fix- fix the print. In the earlier days this was accomplished by means ing print, of a bath of sel d or, a mixture of hyposulphite of soda and auric chloride. This gilded the darkened parts of the print which light had reduced to the semi -metallic state; and on removal of the chloride by means of hyposulphite an image composed of metallic silver, an organic salt of silver, and gold was left behind. There was a suspicion, however, that part of the coloration was due to a combination of sulphur with the silver, not that pure sulphide of silver is in any degree fugitive, but the sulphuretted organic salt of silver seems to be liable to change. This gave place to a method of alkaline toning, or rather, we should say, of neutral toning, by employing auric chloride with a salt, such as the carbonate or acetate of soda, chloride of lime, borax, &c. By this means there was no danger of sulphurization during the toning, to which the method by sel d or was prone owing to the decomposition of the hyposulphite. The substances which can be employed in toning seem to be those in which an alkaline base is combined with a weak acid, the latter being readily displaced by a stronger acid, such as nitric acid, which must exist in the paper after printing. This branch of photography owes much to the Rev. T. F. Hardwich, he having carried on extensive researches in connexion with it during 1854 and subsequent years. MM. Davanne and Girard, a little later, also investigated the matter with fruitful results. The following may be taken as two typical toning-baths : Auric chloride .......................... 1 part. Water .................................. 4000 arts. In the latter (a) and Q3) are mixed in equal parts immediately before use. Each of these is better used only once. A third bath is : Auric chloride .............................. 2 parts. Chloride of limn ........................... -2 ,, Chalk ...................................... 40 Water ..................................... SOOO ,, These are mixed together, the water being wanned. When cool the solution is ready for use. In toning prints there is a distinct difference in the modus operandi according to the toning- bath employed. Thus in the first two baths the print must be thoroughly washed in water to enable all free silver nitrate to be carried away from the image, that salt forming no part in the chemical reactions. On the other hand, where free chlorine is used, the presence of free silver nitrate or some active chlorine absorbent is a necessity. In 1872 Abney showed that with such a toning- bath free silver nitrate might be eliminated, and if the print were immersed in a solution of a salt such as lead nitrate the toning action proceeded rapidly and without causing any fading of the image whilst toning, which was not the case when the free silver nitrate was totally removed and no other chlorine absorbent substituted. This was an important factor in the matter, and one which had been overlooked. In the third bath the free silver nitrate should only be partially removed by washing. The print, having been partially washed or thoroughly washed, as the case may be, is immersed in the toning- bath till the image attains a purple or bluish tone, after which it is ready for fixing. The solution used for this purpose is a 20 per cent, solution of hyposulphite of soda, to which it is best to add a few drops of ammonia in order to render it alkaline. About ten minutes suffice to effect the conversion of the chloride into hyposulphite of silver, which is soluble in hyposulphite of soda and can be removed by washing. The organic salts of silver seem, however, to form a different salt, which is partially insoluble, but which the ammonia just recommended helps to remove. If it is not removed, there is a sulphur compound left behind, according to Spiller, which by time and exposure becomes yellow. The use of potassium cyanide for fixing prints is to lie avoided, as this reagent attacks the organic coloured oxide which, if removed, would render the print a ghost. The washing of silver prints should be very complete, since it is said that the least trace of hyposulphite left behind renders the fading of the image a mere matter of time. Whether this be due to the hygroscopic nature of the hyposulphite and its reaction on the organic salt of silver, or to the destruction of the hyposulphite and sulphurizing of the black organic snlt, seems at present to be an undetermined question. The stability of a print has been supposed to be increased by immersing it, after washing, in a solution of alum. The alum, like any other acid body, decomposes the hyposulphite into sulphur and sulphurous acid. If this be the case, it seems probable that the destruction of the hyposulphite by time is not the occasion of fading, but that its hygroscopic character is. This, however, as has already been said, is a moot point. It is usual to wash the prints some hours in running water. We have found that half a dozen changes of water, and between successive changes the application of a sponge to the back of each print sepa rately, are equally or more efficacious. On drying, the print assumes a darker tone than what it has after leaving the fixing-bath. Different tones can thus be given to a print by different toning- baths ; and the gold itself may be deposited in a ruddy form or in a blue form. The former molecular condition gives the red and sepia tones, and the latter the blue and black tones. The degree
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