COLLODION WET PROCESS.—A process in which collodion forms the vehicle to hold the sensitive salts. It is commonly known as the wet-plate process, in contradistinction to the dry-plate process, the film being exposed in the one case whilst wet and in the other when dry. Before describing the process in detail it will, perhaps, be better to give an outline of it. A col lodion is first prepared, in which are dissolved bromides and iodides. A perfectly clean glass plate is then coated with a thin film of this collodion, and as soon as this is set it is immersed in a solution of silver nits ate (see Silver Bath), the silver forming with the salts in the collodion, silver-iodide, or silver-bromo-iodide. The plate is then exposed in the camera, and afterward treated with a developing solution, which brings out the image. This is then intensified or strengthened and then fixed. Lastly, the collodion is given a protective coating of varnish, and the negative is ready for the printing processes.
Cleaning the importance of having the glass clean cannot be over-estimated. It should be well rubbed with a mixture of alcohol and tripoli powder, sufficient to form a cream; about ten drops of liquid ammonia is then added to each ounce of the cream. Should the plate have been used before, it should be previously laid in a weak aqueous solution of nitric acid. A soap commonly known as " Monkey soap " is an admirable cleanser of glass, removing all traces of grease and organic matter. Common whitening will also remove grease when dry. It should be made into a thick cream with water and applied to the glass, and rubbed off when dry. With regard to the selection of the glass, a careful perusal of the few remarks under Glass might be advantageous to the reader.
Salting the particulars regarding the necessary qualities required in the plain collodion are given under that heading. It will not, therefore, be necessary to repeat them here, but pass on to the addition of the iodide or bromide to the collodion. The following is the standard formula: Cadmium iodide 4} grains Cadmium bromide a grains Plain collodion i ounce This should be mixed about six months previous to using. For landscape work the following is
to be recommended: Ammonium iodide 4 grains Cadmium bromide 13( grains Plain collodion... ounce This may be used two or three days after mixing. A large number of modifications have from time to time been recommended. The following rules may guide the operator: Should a de crease of contrast and plenty of detail be required, add more bromide; but if great contrasts are necessary, lower the quantity of bromide, and increase the iodides. The lowest quantity of bro mide necessary to secure clearness in the shadows is about one-quarter grain to the ounce of collodion. The iodides and bromides used are usually ammonium and cadmium. If ammonium be used the collodion is more fit for immediate use; cadmium causes collodion to become gluti nous when first iodizing, and it is therefore necessary to keep it for several months previous to using. If an iodide alone be used the resulting image will be very dense, but lacking in detail in the shadows; and if a bromide only be used a much flatter image, but full of detail, will be ob tained. It will therefore be seen that by a judicious mixture of the two a suitable film can be made, which, when sensitized, will give the detail and delicacy of the bromide combined with sufficient density of the iodide, or a mean between the two extremes.
The plain collodion should always be kept in a cool, dry place, otherwise the ether is liable to decompose, and in its turn decompose the pyroxyline. If the collodion be made with a pure spirit and a neutral cotton it should be quite colorless after iodizing, but should any of the solv ents be impure a slight discoloration will result, as the contact of iodine with ether compounds will sometimes form an organic compound. Again, if the pyroxyline be acid, the collodion will become sherry colored immediately, and will keep good for any length of time.