van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.
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As soon as the proper time has elapsed for exposure, the sensitised plate must be withdrawn from the light by closing the shutter of the camera-back. which is then taken into the dark room. After carefully closing the shutter in front of the yellow-glass window, the plate is removed from the frame. If it be now examined, there will very rarely be any traces of a picture; it can, however, be made apparent by covering the surface which has been exposed to the light with a solution of some substance capable of reducing the salts of silver to the metallic state.
Among the reducing agents which can be employed in photography, may be mentioned protonitrate of iron, sulphate of iron, pyrogallic acid, protosulphate of uranium, protosalts of osmium and titanium, &c.; but of all these, pyrogallic acid will be found the best for developing the negatives, and protosulphate of iron for obtaining direct positives on glass.
Pyrogallic acid is a white crystalline solid, without smell.7 It is soluble in water, alcohol, and ether, and its solution in either of these menstrua becomes rapidly decomposed on exposure to the air and light, especially if the solution be alkaline. This substance alters very soon even when it is dry. If a small amount of moisture be present, however, the decomposition proceeds with increased rapidity, being expedited by the foreign matter which it contains. Thus it is better-first, to keep the pyrogallic acid bought in shops in small stoppered bottles; secondly, to keep it in the dark; thirdly, never to prepare with it more developing solution than is likely to suffice for a day's consumption.
It is usual to add to the pyrogallic some other acid, as much to preserve the solution as to develop the picture uniformly; distilled water also must be employed, or at least rain-water, well filtered: that which is obtained during a storm contains ammonia or nitric acid, and is therefore unfit for use.
Solution of pyrogallic acid for developing, is thus prepared: A flask of about 1 pint capacity is obtained, and made perfectly clean; to this is adapted a funnel, furnished with a filter, as shown in Fig. 11. Upon this filter throw 15 grs. of pyrogallic acid; then 15 oz. of distilled water being measured off; and 1 oz. of crystallisable acetic acid10 being added thereto, the whole is well stirred with a glass rod, and poured on to the filter containing the pyrogallic acid, which latter dissolves as the liquid is passing through. The filtration being complete the funnel is removed, and the flask stopped lightly with a cork, and set aside for use in the dark.
MM. Davanne and Girard have proposed the substitution of citric for the acetic acid, in which case the following formula may be employed
|Distilled Water||12 fluid ounces.|
|Pyrogallic Acid||15 grains.|
|Citric Acid||15 grains.|
Whichever formula is adopted, the method to be followed to develop the picture is precisely the same; and it is important to note that the development should take place as soon as possible after the plate is sensitised, and on no account should a longer time than five minutes be allowed to elapse between these operations.
The plate, on removal from the frame, is held by one corner (the same by which it was held when the collodion was poured over it); then a sufficient quantity of the developing solution to cover the plate being placed in a glass (Fig. 58), is poured by a continuous operation over the collodion surface, and the plate inclined alternately in every direction, in order that the whole film may be covered without delay. Then suddenly turn the plate upright, holding it by the opposite corner to that from whence the liquid flows into the glass.
Fig. 59. Developing a Negative Proof.
Fig. 60. wash-bottle.
The same solution is again poured on the plate, inclining it constantly from right to left, in order that the liquid may be kept in continual motion. The image gradually appears; and when it is considered sufficiently developed, the plate is washed by dipping it, at first very carefully, into a shallow porcelain tray, filled with water; it is then placed under a small stream of water, after which it is fixed. It is desirable, for washing the plate, to use a little apparatus represented in Fig. 60. It is simply a flask, filled with distilled water. The cork is perforated, so as to admit of the passage of two tubes. The highest-that which is to be blown through-passes through the cork, and terminates directly under it; the other, on the contrary, rests in the water at the bottom of the bottle. The draughtsman has reversed this arrangement in the figure by mistake.
Fig. 61. Washing the Film.
In using this wash-bottle, it is held by the neck while the operator blows through the higher tube, by which means the water is projected through the exit tube, and may be directed over the surface of the film, beginning at the centre, and passing gradually to the edges, thus avoiding disruption. Fig. 61 shows the operation better than any verbal description.
Fig. 62. Pneumatic Plate-holder.
In order to avoid taking the plate in the hand, an India-rubber plate-holder is sometimes employed with advantage. Fig. 62 shows a glass plate attached to such a plate-holder. To do this the plate is placed on a level table, and the holder grasped by the globe part in such a way as to force out the air; the edges are then moistened, in order to make them adhere more perfectly, and the pressure is withdrawn at the moment of bringing the plate-holder in contact with the glass by pressing the edges of it. Although the air has been forced out of the bottle, its elastic sides re-assume their shape, and the vacuum left inside causes the plate to adhere very firmly to the plate-holder. To detach it, it is only necessary to compress the bottle as before. The picture having been developed and washed, is now fixed, an operation to be described in the next chapter.
The process of developing the picture, although apparently simple, is, nevertheless, a very delicate one, and one on which depends, in great measure, the success of the positive proof; it will not, therefore, be out of place to enter a little more into detail. The latent image produced by the action of light upon the iodide of silver, is brought out by the action of pyrogallic acid, aided by the nitrate of silver, with which the collodion film is impregnated. If too much pyrogallic developing solution is poured from the developing glass on to the plate, the details of the picture appear but slowly, and if too little there will not be enough to cover the plate completely; thus the medium lies between the two extremes.
Sufficient developing solution should be poured into the glass to cover the plate freely and completely, and no more; and this should be poured on and off, and allowed to flow in various directions, in order to facilitate the mixture of the developer with the adherent nitrate of silver. By this method of manipulating it will be found that the development proceeds uniformly, and may be observed from time to time by examining the plate by transmitted light, as indicated in Fig. 59.
The development is continued according to the preceding directions until the required amount of intensity is obtained, and the plate is then rinsed with water to remove the developing fluid, so as to prevent its continued action.
At an ordinary temperature the picture is generally sufficiently developed in two minutes; but if it is necessary to force the development, it will seldom prove a successful picture. At a higher temperature it sometimes becomes necessary to stop the development in half a minute, to prevent the high lights from becoming too dense. Experience alone can give the knowledge necessary to determine exactly the when and how in this delicate operation.
As soon as the solution of pyrogallic acid covers the plate, the sky and the high lights of the picture begin to appear on the primrose-tinted film of iodide of silver; a few seconds after, the minor details make their appearance, becoming more and more vigorous, and, as often occurs in a landscape, the sky darkens so much, that it is even difficult to see the sun through it. This will give an idea of the amount of intensity which the image acquires; and, without experience, it is very difficult to indicate the exact moment when the pyrogallic acid should be removed by treatment with cold water.
Fig. 63. Positive Proof. From a Stereoscopic View by a M [illegible]er
The quantity of acetic acid added to the pyrogallic also exercises an important influence on the results, and in proportion as it is increased the more slowly the image appears, and the more vigorous it is in character. When, for example, a negative has to be developed taken from a group of persons among whom there are some having on white dresses, a large proportion of acetic acid is necessary; while, on the contrary, a dull, grey, sombre monument develops best with a small proportion of acetic acid. The time and character of the development of a picture also enables a conclusion to be arrived at, as to whether the exposure in the camera has been too long or too short.
Fig. 64. Negative Proof, examined on tile Side of the Glass not covered with Collodion.
In this latter case it is with difficulty that the sky and the other high lights of the picture are developed, and detail in the shadows is never obtained; even after long waiting the image will not appear, in which case it is necessary to take a fresh negative.
If, on the contrary, the exposure has been too long, the negative is red and uniform; after fixing, there is an absence of vigour in the blacks and there is a general fogging spread over every part of the image; the development also has taken place very rapidly. In each case another negative must be taken.
The most frequent error is over-exposure, arising from an inability to realise the possibility of taking a picture in so short a time.
For the purpose of illustration, suppose the monument represented at Fig. 63, representing a portion of the Acropolis at Athens, is to be reproduced.
The sun illuminates these statues with all its meridian splendour; strong shadows are therefore a necessary con. sequence, which, in order to be correct, ought to be in the negative.
Beginning with a negative which not have been exposed in the camera more than five seconds, and successively increasing the time of exposure, five seconds in a series of five negatives, the following will be the result:
The first negative will be very slow in developing; after several minutes the sky will with difficulty be traced, and some faint indication may be obtained of the parts most strongly lighted.
In the second these portions will have acquired much more vigour, and the shadows even will begin to yield some indication of detail. The third negative will be a good one; but the fourth will be, in every respect, all that could be desired, possessing a due amount of vigour, and presenting the aspect of Fig. 64.
The fifth negative, although exposed a longer time, will have acquired a greyish colour. A good negative is blue; but too long an exposure gives it a dull grey appearance. The shadows, instead of being transparent, like glass itself, have upon them a sensible deposit, owing to the too rapid development.
In proportion as the exposure is increased in any subsequent negative, the grey colour tends to become red; there is a difficulty almost in distinguishing the shadows and the sky; and, in one word, the negative loses all its vigour.
The above are the principles upon which the photographer should proceed in the correction of his negatives.
It would be useless to pretend that an accurate judgment can be formed except by long experience. It is not until after several months of assiduous research, and after continual disappointment, that the beginner will acquire that accuracy of judgment at a glance which will guide him in the fugitive indications of the amount of exposure necessary for a. given subject; but, when once this knowledge is acquired he will be amply rewarded by the small number of defective negatives, as well as by the amount of photographic knowledge acquired during the apprenticeship.
It frequently happens that negatives are spotted, or covered with curious markings, either in parts or entirely.
The four accompanying figures are some examples of these spots, taken from M. de la Blanchere's work; the description of them, together with practical directions for their avoidance, will be found in Note 9 at the end of the volume.