van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.
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IN this chapter it is intended to give a brief summary of the subjects to be subsequently described in detail under their several heads.
If a little pyroxyline or gun-cotton be immersed in a mixture of about one part in volume of alcohol, and two parts of ether, it will almost entirely dissolve. In order to obtain a sufficiently transparent solution, it must stand for twenty-four hours in a bottle well corked, and then be poured out into another bottle, taking care not to disturb the. thick part which remains at the bottom. This clear liquid is collodion.
If a small quantity of collodion be poured on a glass well cleaned, the ether and alcohol will evaporate, and leave on the glass a transparent film. This film is very firm, and adheres very strongly to the glass; it is that Which serves as a vehicle for the photographic materials; or, in clearer terms, it is that which is destined to form the surface which will retain the photographic image. It may easily be conceived that for the purpose of rendering this film sensitive to light, iodide of silver must be formed in its texture; this is done by dissolving iodide of potassium, or some other iodide, in the collodion.
It is important to choose a proper iodide, but for the present purpose the use of a collodion containing iodide of potassium. will suffice to trace what results and changes take place. On a well-cleaned;lass plate pour a certain quantity of collodion in such a manner as to cause it to flow over its surface; then incline the plate, that the excess of liquid may flow off: After the ether and alcohol. are evaporated,--or, in other terms, after the plate has become dry,--a coating of pyroxyline will be obtained; but this time it will have; an iodide intimately mingled with it.
In proportion as more or less gun-cotton is dissolved in the mixture of ether and alcohol, a liquid. of greater or less density is obtained, and consequently the thickness of coating of pyroxyline will also vary on the glass. The proportion of iodide added to the collodion also regulates the quantity which remains on the glass; from which it follows that it is not a matter of indifference what formula is employed in its preparation: on the contrary, it is necessary to study with the greatest care the relative quantities of the chemical substances which constitute photographic collodion. These quantities will depend upon the temperature, and yet more upon the results desired to be obtained.
The glass plate having the collodion spread over it, is now sensitised by being dipped into a solution of nitrate of silver, which converts the iodide of potassium into iodide of silver. sensitive to light.
It need hardly be mentioned that the iodide of silver being affected by light, the preceding operation of sensitising ought to be done in the dark. This word ought not always to be taken literally; in photography, we understand by darkness a light too feeble to affect the sensitive coating. A wax candle is generally used, or ordinary daylight neutralised by a yellow glass; for a glass of this colour prevents any action on photographic substances.
The sensitised plate now requires to be placed in a camera, an apparatus composed of a box of wood and an arrangement of lenses, which possess the property of forming with perfect accuracy an image of any required object on the sensitised plate. As a general rule, the collodion plate is left in this apparatus from ten to twenty seconds, according to the brightness of the object; it is then removed, and taken back into the dark room.
If at this moment the collodion film be examined with attention, no trace of an image will be perceived, but it can be made to appear in the same manner as has before been mentioned, by developing with gallic acid. There are also many other developers for bringing out the latent image, such as pyro-gallic acid, proto-sulphate of iron, proto-sulphate of uranium, &c. Whatever the developer may be that is used, it is dissolved in water, and poured over the coated surface of the glass. In a few seconds the image appears as a negative, and the reduction is allowed to proceed; or, in other terms, the proof left to darken, until it is judged to be sufficiently distinct. The glass is then plunged into water, which removes all soluble substances, then into a fixing solution, such as cyanide of potassium, or hyposulphite of soda, which dissolves the semi-opaque coating of iodide of silver; finally, the glass is washed in a current of cold water, and dried in the air.
If the proof be examined by transmitted light, it will be found to be a true negative; that is to say, supposing a view has been taken, the sky, the white houses, and in general all objects strongly illuminated, are shown of a black colour; while dark objects, on the contrary, appear transparent (Figs. 3 and 4).
The use of such a negative as before mentioned, is to give a number of other proofs either upon glass or upon paper; and if the tints be in good harmony with the original model, a satisfactory picture will be obtained.
To understand the above requires a proper comprehension of the principle, that in spite of the opposition of lights and shades shown in a negative, with respect to a given model, there must nevertheless be preserved a perfect harmony between the tints. This phrase may appear obscure, but a well selected example will make it clear. Suppose a series of ten bands be fixed on a wall, of which the first is absolutely black, the last perfectly white, and the others of intermediate tints. The first will then be black, the second of a greyish black, the third a little less dark, the fourth still less, and thus lighter and lighter, to the perfectly white.
If a corresponding figure be reproduced on collodion, it will be found, if the negative be a good one, that these ten tints are completely inverted. In the place of the first black tint of the model, will be one perfectly transparent on the negative, whilst the last will be of an opaque black, the intermediate ones having a regular gradation; for if it were otherwise, a proof taken from such a negative would not represent the true shades of the original model.
Figs. 5 and 6.
The Figs. 5 and 6 are intended to show this effect on three bands; but the result would be analogous for any larger series.
The conditions necessary to obtain this exactitude are, that the coating of collodion be of proper thickness, and sufficiently furnished with iodide of silver to yield a decomposition of such intensity as to produce a thick coating of reduced material; for if the light has not been able to decompose enough iodide in the coating of collodion, a perfect black can never be produced. It follows, then, to obtain, an intense negative, there must be employed a thick collodion strongly iodised, and a concentrated bath, of nitrate of silver.
The foregoing are the general details for producing negatives upon collodion. This process, however, like that of the Daguerreotype, can be made to give direct or positive proofs at one operation; but in that case the picture requires to be viewed by reflected, instead of by transmitted, light.
All the operations necessary to obtain a negative upon collodion require to be performed in rapid succession. But if it be wished to delay the exposure and development of the picture for some time after sensitising the plate, it is re-coated with gelatine, or other suitable substance. The advantage of this method of operating, besides allowing some time to elapse between the exposure and development, is that it does not require the whole of the materials to be taken to the place where it is wished to operate; and consequently, although much less sensitive than wet collodion, is very applicable for views, &c. This method is known by the name of the Dry Collodion Process.
Some remarks on the stereoscope, an instrument particularly adapted for viewing photographic pictures, and full details for printing positive proofs from collodion negatives, and a few notes on optical photography and other photographic subjects, will complete the work.
The following is a list of the subjects treated on in the respective chapters:
|II.||The Preparation of the Substances required in the manufacture of Photographic Collodion.|
|III.||The Preparation of Photographic Collodion.|
|IV.||Cleaning the Glasses.|
|V.||The Dark Room, or Photographic Laboratory.|
|VI.||Coating with Collodion and Sensitising the Plate.|
|VII.||Photographic Cameras, Lenses, &e.|
|VIII.||The Glass or Operating Room, and the Rules for taking Views and Portraits.|
|IX.||Development of the Image obtained in the Camera.|
|X.||Fixing and Strengthening the Negative Image.|
|XI.||Varnishing the Collodion Coating.|
|XII.||Dry Collodion, including the Collodio-Albumen and Tannin Processes.|
|XIII.||Positive Collodion Process.|
|XV.||Printing Positive Proofs.|
|Notes on Optical Photography and other Photographic Subjects.|