van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.

Chapter XIII.
Positive Collodion Process

Tat, positive collodion process is very similar in its general details to the negative, and will generally be found much easier of manipulation, from its not requiring the like amount of accuracy.

Preparation of the Collodion.

The pyroxyline and the iodide of cadmium are first introduced into a dry flask or bottle, and the alcohol poured upon them, and the mixture shaken violently for about a minute; the ether is then added and the contents further agitated, and finally set aside all night. The clear supernatant portion may now be carefully poured off from the white deposit at the bottom of the bottle, or instead of decanting, it may be preferable to use a small syphon bottle as shown at Fig. 23, the action of which has already been explained.

The positive collodion thus prepared should like the negative, be preserved in the dark in well corked or stoppered bottles.

Should the collodion produce a fogged image, or in other words, if the plate becomes covered with a film of reduced silver, underneath which there is obviously a very good picture, this defect may be remedied by adding to it a few drops of the following solution:

A small quantity of this liquid is added to the collodion, so as to impart a pale amber colour thereto.

Patent glass plates are not required for direct collodion positives, and the reason is very simple. When negatives are taken they are afterwards applied to sheets of sensitised paper with some degree of pressure, in order to obtain the positive impression, on which account it is necessary that the plate should be perfectly flat and free from striae, otherwise the striae would be reproduced upon the paper, and the glass very liable to fracture from the pressure. Such is not the case, however, with direct positives; here the purity and flatness of the glass is by no means so important, and ordinary flatted crown may therefore be used. Some photographers employ glass of a deep red or purple colour, in which case it is unnecessary to varnish the plate.

Whatever kind of glass plate be adopted. it should be chosen as flat as possible, otherwise there will be difficulty in getting them into the camera back; but the colour in no way affects the beauty of the image, any more than bubbles or other mechanical surface detects.

It has been previously explained, when describing the negative collodion process, how the plate is cleaned, and in what manner the collodion is poured on the glass; no further remarks are therefore needed on the subject beyond reminding the reader that the collodion should be poured upon the plate in the dark room, in order to be ready as soon as the film is set to plunge it into the bath, which is composed as follows--

Filter. Prepare also the following solution

And, after filtration, pour it into the bottle which contains the above solution of nitrate of silver. A crystalline precipitate is produced which requires several days to deposit before the bath will be ready for use. It may, however, be used a few hours after mixing by resorting to filtration; but it is always better after a few days' keeping.

The bottle containing the bath should have a funnel and filter adapted to it, through which the solution ought to be always filtered after use, so as to be ready for each day's work, the same filter serving almost indefinitely.

The mixture of acetate of silver with the nitrate not only produces increased rapidity, but tends to maintain it. It has, moreover, the advantage of preventing fogging; but to this end it is necessary to be always kept in the dark.

The details of manipulation, as before mentioned, are the same as have been described for the negative process, except that the exposure is reduced about three-fourths and the development effected by protosulphate of iron instead of pyrogallic acid.

A saturated solution of protosulphate of iron is prepared by pouring half a gallon of boiling water on two pounds of green vitriol, or ordinary protosulphate of iron, stirring well together; allow it to cool and then filter.

This saturated solution is, of course, much too concentrated for use; the developing solution is therefore pre. pared from it in the following manner

This solution should be contained in a vertical bath with a dipper, and the plate immersed therein for fifteen seconds after exposure, which will be found amply sufficient to develop the picture in all cases.

On withdrawal from the bath the image ought to possess very little intensity, if otherwise it might be regarded as an indication that the proof would not be a satisfactory one. However this may be, the plate is now well washed with water, fixed with cyanide of potassium, and dried as has been described under the negative process.

It is only after fixing that a correct idea can be formed as to whether the exposure has been correct or not, and practice alone will enable the operator to determine the point. If any trace of fogging should become apparent, a few drops of the alcoholic solution of iodine may be added to the collodion, the formula for which has been already given.

It may not be out of place here to give a few explanatory remarks in order that the reader may fully comprehend the essential difference between a positive and negative collodion picture.

A negative proof being intended to possess varying degrees of opacity to transmitted light, it is necessary that the film should have in it a sufficient quantity of material to suffer various amounts of decomposition in the process of development; to this end, not only is a thick collodion employed, but also one fully charged with iodide.

With a positive the case is entirely different, the film is extremely thin, for unless it were so the whole would be patchy, owing to the light having penetrated the film and effaced the delicate details by reflection. It is, therefore, of the highest importance to use the thinnest possible collodion, which yields, on leaving the bath, a film only faintly opalescent.

For fixing, cyanide of potassium is a more appropriate agent than hyposulphite of soda, as it yields a more agreeable tone. It requires, however, to be used with great care on account of its highly poisonous character. The removal of the unaltered iodide by its means may be effected either by immersion or by pouring the solution over the plate. The fixing solution is prepared by mixing together--

Longer contact of the cyanide than is absolutely necessary should be avoided, as the delicate detail is apt t suffer, and when the iodide is removed, which may b ascertained by the transparency which the plate acquires it should be freely washed with water, dried, and varnished.

The varnish, if the picture be taken on a colourless glass, must be black on the plain side, and colourless or " crystal varnish " on the film side; when purple glass however, is used, the black varnish may be dispensed with.

A solution of gum arabic is sometimes poured over the film side of the plate, instead of varnish. Some operator; omit all kind of protection; this is, however, a mistake; as pictures which are not covered either with gum arabic or crystal varnish, very quickly assume a tarnished and disagreeable aspect.

To communicate an extra amount of whiteness to direct positives they are sometimes treated with bichloride of mercury after the final washing; the general tone, however, is usually sufficiently cold, and therefore its employment cannot be recommended in an artistic point of view.

The method of operating is as follows:--Cold distilled water is allowed to take up as much bichloride of mercury as it will, and, the solution being filtered, the positive is immersed therein until it assumes an appearance as white as snow, which will be in about one minute, after which it is washed and varnished in the usual way. These proofs should be preserved from the direct action of the sun's rays, otherwise they become very much weakened in a few months.

The different methods of mounting in passe-partouts, &c., involve operations of so strictly mechanical a nature that they need not be entered into in this place, as the means of carrying them out are soon acquired by practice.

Glass positives are very easy to take, although some practice is necessary to attain to any great degree of excellence. The pictures should have a mirror-like aspect, and the blacks extremely pure. Sometimes a certain amount of fogging takes place, marring the brilliancy of the picture; this may be removed by washing the picture while still wet, immediately after fixing, with an alcoholic solution of iodine of 15 grains to the ounce. The length of time must be determined by the extent of the fogging, and can only be ascertained by practice. When it is considered that the action of the iodine has been sufficiently prolonged, it is washed off, and the plate again immersed in the cyanide fixing bath, and finally washed and dried.