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127 9.]

TALBOTYPE, OR CALOTYPE PROCESS.

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the two were pressed firmly together between two plates of glass, and exposed to the direct rays of the sun, or even to diffused daylight, for a period of half an hour or an hour. The impression thus obtained was a negative one, that is to say, the shadows were represented by lights, and the lights by shadows; those portions of the surface which had been exposed to the strongest light becoming dark; in the half tints, where a feebler light had been transmitted, the blackening became less evident; and the parts corresponding to the deep shadows in the engraving remained white. The pictures were fixed by immersing them in a strong solution of common salt. Considerable improvements in detail have been introduced into this process since it was first published, but, in principle, this operation, which has been termed photographic printing, remains unchanged.

A very good paper for this kind of printing may be obtained as follows:Prepare a solution of sodic or ammonic chloride, containing 23 grammes to the litre, or 10 grains of the salt to each ounce of water. If French paper (which is sized with starch) is to be used, it will be improved by dissolving 2 grammes of gelatin in each litre of the solution of salt. Pour this liquid into a flat shallow dish, and having cut the paper into pieces of a convenient size, take a sheet of it by the two opposite corners, and bring it down upon the surface of the solution, so that the middle of the sheet shall be first moistened; then lower it gradually towards each corner, so as to exclude air-bubbles. After the lapse of a minute, it may be removed from the solution, and hung up to dry. In order to render the paper sensitive, prepare a solution of argentic nitrate, containing about 114 grammes to the litre, or 50 grains of nitrate to the ounce, and lay the sheet upon the surface of the solution in the same manner as before; in about three minutes' time the sheet may be removed: it must be raised by one corner with a pair of forceps tipped with sealing-wax, allowed to drain, and hung up to dry. The process of immersion in the silver bath and the drying must be performed in a darkened room.

Another sensitive paper, which is often used, may be prepared by forming a solution which contains 182 grammes of argentic nitrate to the litre, or 80 grains in each ounce of distilled water, and adding caustic ammonia until the precipitated argentic oxide is almost redissolved; the solution should be preserved in a dark place. The paper, having been previously salted, is excited by brushing it over with this solution by means of a pledget of cotton wool. The paper is allowed to dry in the dark, and should be used immediately.

In order to protect the picture obtained upon either of these papers from the further action of light, it is now usual to adopt the method introduced by Sir J. Herschel, which consists in soaking the picture in a concentrated solution of sodic thiosulphate (or hyposulphite of soda, as it is called in commerce) containing about part of the salt to 2 parts of water: this salt combines with the undecomposed salt of silver, and renders it soluble. By washing the picture for 5 or 6 hours in water, which should be frequently changed in order to ensure the thorough removal of the salts of silver and of the thiosulphate, the surface is secured from further change when exposed to light.

(127 g) Talbotype, or Calotype Process.-In 1841, Fox Talbot took out a patent for the very beautiful process to which his name has since been attached.

In this remarkable operation the surface of the sheet of paper is coated with argentic iodide, which is not sensitive per se to the action of light if the process of immersion in potassic iodide is the last operation previous to washing. In order to render it sensitive, it is washed over with a mixture of argentic nitrate, with gallic and acetic acids, and then exposed in the camera to the object which is to be copied. After the lapse of a few minutes (the time required varying with the intensity of the light), the paper is withdrawn from the camera. Unless the light has been very strong, no image is visible, or a mere outline only, but the compound has undergone a change of a very singular nature, for if the blank sheet be protected from the light, and washed over with the mixture of argentic nitrate with gallic and acetic acids, on gently warming it, a negative image appears with wonderful distinctness and fidelity,-the portions which have been exposed to the strongest lights assuming the darkest tints. The development of the image appears in this process to be due to the reducing agency of the gallic acid, which acts more rapidly upon the argentic nitrate in contact with those portions of the iodide which have been most freely exposed to the action of light. This dormant picture may be developed many hours or even days after it was produced, if the paper be preserved from the light. It seems as though the light, without actually producing a decomposition of the particles of the silver compound upon which it falls, gives to them a particular condition which predisposes them to produce decomposition in a reducible mixture consisting of argentic nitrate and gallic acid. The process may be conducted in the following manner :—

1. Preparation of the Iodized Paper.—A sheet of smooth writing paper, such as that manufactured by Turner, of Chafford Mills, of uniform texture, and free from stains and spots, is pinned upon a board by two of its corners, and brushed over uniformly with a solution of argentic nitrate containing 75 grammes to the litre, or 33 grains of the salt in an ounce of distilled water. The solution is best applied by means of a brush consisting of a flock of cotton wool partly drawn through a glass tube, which furnishes a convenient handle: whilst the paper is still moist, it is immersed in a bath of potassic iodide, containing 46 grammes of the iodide to a litre of distilled water, or 20 grains to the ounce, taking care to avoid the occurrence of air-bubbles. In about two minutes, or as soon as the paper has acquired a uniform yellow colour throughout, it is transferred to a vessel of water, where it is allowed to soak for two or three hours, changing the water three or four times, so as to remove all the soluble salts. Each sheet of paper is then to be hung up separately, and allowed to dry. These operations may be conducted by candle-light, or in diffused daylight. A stock of this paper may be prepared and kept for use.

2. Exciting the Paper for the Camera.—When required for the camera, prepare 1. a solution of argentic aceto-nitrate (consisting of 3 grammes of argentic nitrate, 30 cb. centim. of water, and 6 cb. centim. of glacial acetic acid, or 50 grains of argentic nitrate, I ounce of water, and I drachm of glacial acetic acid), and 2. an aqueous solution of gallic acid saturated in the cold. Add 3 or 4 drops of each of these solutions to 4 cb. centim. (or I drachm) of distilled water, and then in a darkened room apply the mixture freely with a pledget of clean cotton wool to the silvered surface of the iodized paper-when well soaked remove the superfluous portion with a sheet of clean blotting-paper the same sheet of blottingpaper must not be used twice for this purpose. Whilst still damp it is to be placed between the glasses of the camera slide. It will retain its whiteness for twelve hours or more.

3. Exposure in the Camera.-In order to take the photograph of a landscape, a sheet of the prepared paper is exposed in the focus of the camera, and

127 h.]

PHOTOGRAPHY ON COLLODION.

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after the lapse of from five to fifteen or twenty minutes, according to the amount of light, the picture may be withdrawn.

4. Development.-The image is developed by brushing the paper over, by means of clean cotton wool, with a mixture of equal parts of the solution of argentic aceto-nitrate and gallic acid. The two solutions must be mixed immediately before they are used, as they speedily undergo mutual decomposition. In a few minutes the picture gradually begins to appear. Any part of the picture which seems wanting in distinctness may be washed over with fresh solution of argentic aceto-nitrate. The development should be effected by candlelight, or in yellow light

5. Fixing the Impression.-As soon as the picture ceases to increase in distinctness, it is to be well washed with water, and immersed in a saturated solution of sodic thiosulphate till the yellow tint of the argentic iodide has disappeared. It is then to be washed thoroughly for several hours in clean water, frequently renewing the water. Unless all traces of the argentic thiosulphate be removed, the picture will gradually lose its intensity. Fox Talbot originally employed a solution of potassic bromide for fixing these pictures, but sodic thiosulphate is to be preferred. When dry, the photograph should be waxed by placing it between two sheets of blotting-paper saturated with white wax, and then passing a moderately heated smoothing iron over the whole. The negative pictures thus obtained may be employed to furnish positive prints, or prints with the lights and shadows as they occur in Nature, by Talbot's original photogenic' process, or by printing upon a second sheet of the prepared Talbotype paper.*

(127h) Photography on Collodion.-An important modification of Talbot's process was introduced by Mr. Archer, who substituted for the iodized paper a transparent film of iodized collodion spread upon glass, as the recipient of the negative picture. The process is thus rendered more certain and very much more rapid, at the same time the manipulation is simplified, whilst the positive pictures obtained by transference of the negative impression are much sharper in their outline. The operation requires to be conducted in a manner different from that which is practised when paper is employed. The following is the method to be pursued:

:

1. To Prepare the Bath of Argentic Nitrate.-Take of the nitrate 20 grammes, dissolve the salt in 60 cb. centim. of distilled water, and add I decigramme of potassic iodide dissolved in 2 cb. centim. of water: then add drop by drop a solution of potassic carbonate till a slight permanent turbidity is produced; afterwards add distilled water until the mixture measures 300 cb. centim.; filter, and add 3 drops of glacial acetic acid †

2. Preparation of Solution of Collodion.-A solution of iodized collodion,

For further details upon the subject of photographic printing, &c., the reader is referred to Hardwich's Manual of Photographic Chemistry.

+ Ordinary argentic nitrate is apt to contain a trace of nitric acid, which it is desirable to neutralize, because an acid solution is much less sensitive to the action of light than a neutral one. It is still more important, however, not to have any alkaline reaction, and as argentic carbonate is slightly soluble in the nitrate, the addition of acetic acid is subsequently made to guard against this: the potassic iodide is added in order to saturate the bath with argentic iodide; if this precaution were not taken, the film of iodized collodion would be liable to lose a portion of argentic iodide, since this salt is also somewhat soluble in a solution of argentic nitrate.

which is suitable for the formation of negative pictures, may be prepared as follows (Hardwich)*:-Take of

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First dissolve the iodides in the alcohol, then add the pyroxylin, and lastly the ether. Agitate the materials well, set them aside for twenty-four hours, and then decant the clear liquid, which will retain sufficient sensitiveness to admit of being used at the end of a month or more after its preparation.

3. Preparation of the Collodion Film.-In order to make use of this solu tion, a plate of glass cut to the size required for the camera (after being washed with a solution of potash to free it from grease, rinsed in water, dried, and wiped, with a clean silk handkerchief), is to be held horizontally in the left hand, and a portion of the collodion is to be poured steadily on the middle of the glass, and by slightly inclining the plate in different directions, made to flow completely over the upper surface: the excess of the solution is immediately to be poured back into the bottle.

4. Exciting the Plate for the Camera.-The nitrate bath having been introduced into a trough of glass or of gutta percha sufficiently wide to allow the introduction of the glass plate upon which the collodion is spread-the prepared plate, within half a minute after the film has been poured off its surface, is introduced into the solution of argentic nitrate; in from two to three minutes' time it is thoroughly impregnated with argentic iodide, and when withdrawn from the bath it will exhibit a cream-coloured opalescence. These operations must be effected in a room illuminated by light admitted through a yellow blind, or by the light of a candle screened by yellow glass (127 v).

5. Exposure in the Camera. The prepared plate is to be immediately introduced into the slide of the camera, in which it is to be exposed to the object for a few seconds (from 3 or 4 to 30 or 40) according to the nature of the object and the intensity of the light. The slide is then withdrawn from the camera, and the plate, when examined in the darkened chamber, will not be found to exhibit any image.

* A suitable pyroxylin for this purpose may be obtained in the following manner :-Take of

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Mix the acid and water, and add gradually the nitre, finely powdered and dried, stirring between each addition, until the whole of the salt is dissolved. Suffer the mixture to cool to 145° or 150° (62° or 65° C.), then add the cotton wool in small tufts at a time, taking care to plunge the cotton completely beneath the surface; cover it, and allow it to stand for ten minutes. Then press out the acid with a glass rod as completely as possible, and throw the pyroxylin into a large volume of cold water, and wash for half an hour; afterwards soak it well in water for 24 hours; lastly, wring it out in a cloth, and dry at a heat not exceeding 100° (38° C.). The substance thus obtained is completely soluble in a mixture of ether and alcohol. It is essential to attend to the strength of the

acid and to the temperature employed.

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PHOTOGRAPHIC ENGRAVING.

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6. Developing the Image.-A latent image, however, exists, and it may be developed by the use of a liquid prepared by dissolving—

Pyrogallic acid

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Half an ounce or more of this liquid is to be poured over the plate immediately after its removal from the camera. The negative image which is thus gradually developed, will be more intense if immediately before using the pyrogallic solution an addition be made to it of the same solution of argentic nitrate as is employed in the bath, in the proportion of 2 drops to each drachm of the developing liquid.

The exact reaction which occurs in this remarkable process is not known. The pyrogallic acid, however, is a substance which has a strong tendency to combine with oxygen; and under the conjoined action of argentic iodide and argentic nitrate (the presence of the latter salt in excess being necessary to the reaction) a portion of silver is reduced probably to the metallic state, and is deposited upon those parts of the film which have been exposed to the action of light.

Other solutions may be employed for developing the latent image. One which answers very well for this purpose consists of crystallized ferrous sulphate from 12 to 20 grains, glacial acetic acid 20 minims, alcohol 10 minims, and water I ounce. It is not, however, so well adapted for the production of intense negatives as the pyrogallic acid. When the picture is sufficiently distinct, it must be washed with clean water, and fixed by immersing it in a solution of sodic thiosulphate (I part of the salt to 2 of water) till the cream-coloured argentic iodide is entirely removed. A solution of potassic cyanide, containing from 4 to 12 grains of the salt in an ounce of water, may be substituted for the sodic thiosulphate for the purpose of fixing the image. The picture is again to be thoroughly washed in clean water; it is allowed to dry, then heated before a fire until it feels slightly warm, and the film is protected from mechanical injury by covering it with a coat of transparent spirit-varnish by a manipulation similar to that employed in coating the plate with collodion. This varnished photograph may then be employed for procuring positive pictures by means of the sensitive paper prepared with argentic chloride upon Fox Talbot's plan (p. 259). By employing a neutral nitrate bath free from all organic matter, and a collodion which when iodized with potassic iodide remains very nearly colourless, the sensitiveness of the film to the action of light may be so highly exalted, that moving objects, such as the waves of the sea, or a crowd of people, may be successfully depicted by the instantaneous action of light upon the plate.

(127 î) Albuminized Plates.-Niepce de St. Victor introduced the employment of glasses coated with albumin, prepared by beating up whites of eggs with 1 per cent. of potassic iodide: the liquid is to be placed for 12 or 24 hours in deep vessels, to become clear, after which the supernatant liquid is to be poured upon glass, so as to produce a uniform layer; it is then allowed to dry for 12 hours, and is fit for the bath of argentic nitrate. Albuminized glasses may be preserved for some weeks without injury; they may be excited by means of Talbot's mixture of argentic aceto-nitrate with gallic acid (p. 260). The image is developed by means of a solution of gallic acid after the plate has been exposed in the camera.

(127) Photographic Engraving and Lithography.-In the year 1827, Niepce published a process for obtaining pictures by the aid of light, the basis of which was the fact that the bitumen of Judæa, when exposed to the sun's rays, becomes insoluble in oil of lavender, whilst those parts which have remained

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