POSITIVE COLLODION PROCESS. By this process, a direct positive is produced on a collodionized tablet, by developing the image that is obtained in the camera.
The collodion positive and negative processes differ in the following important particular, viz : In the negative process the object is to obtain a picture, in which the material of the image shall be more or less opaque when looked through ;—in the positive process, to obtain a picture in which the ma terial of the image shall be a dead white when looked at. In a negative, density of various gradations is what is required, without any reference to the appearance of the surface deposit. In a positive, whiteness of the surface deposit is what is required, without any reference to density. In the negative process, organic matter is introduced, first as acetic acid or acetate of soda in the nitrate bath, and secondly, as gallic or pyrogallic acid, acidified with acetic or citric acid for the developer ; the object being to obtain density in the image by combining organic matter with the reduced silver, so as to obtain sufficient opacity, and and not a grey, feeble, metallic image. In the positive process, on the contrary, the object is to avoid, as much as possible, the intro duction of organic matter in any part of the formula, first by acidi fying the nitrate bath with nitric, instead of acetic acid, and secondly, by developing with an inorganic developer, such as a mixture of the proto-salts of iron with nitric acid, and in this way endeavouring to produce not an opaque organic image, brown on the surface, but a thin, white, metallic deposit.
This principle being borne in mind, the rationale of the positive process will easily be understood, and as the manipulation very much resembles that for negatives it may be more briefly described, and the points of difference only between the formulte indie,ated. The plain collodion is the same both for positives and negatives, but the iodizing solution for positives should contain iodide of ammonium, instead of iodide of potassium, in about the same quantity, i. e. from
12 to 44 grains to the ounce of alcohol. The alcohol of the iodizing solution should be chemically pure, and free from the fusel and grain oil contained in spirits distilled from grain or roots. These organic matters introduce,d into collodion, although comparatively harmless in negathe collodion, affect injuriously the tone of a positive by producing an admixture of organic silver salt with the white metal.
The nitrate bath should be rather stronger in silver than that for negatives, and should be acidified with nitric acid. The formula is as follows :— Distilled water , • • . 1 ounce Nitrate of silver , . . . 40 grains Nitric Acid . „ . . 1 minim The time of exposure is about half that required for negatives. The Developer is made thus :— Dissolve one ounce of powdered nitrate of baryta in 16 ounces of clistilled water, and add 2 draoluns of nitric acid, S.G. 1.4. Next add 1-3/4- ounces of powdered proto.sulphate of iron. Shake well until the iron salt is dissolved, The mixture becomes white and turbid, in consequence of the formation of sulphate of baryta. Let it stand a few hours until this has settled to the bottom of the vessel ; then decant and filter the solution, which, if right, will be of an apple green colour. Add two ounces of alcohol to enable it to flow freely over the collodion film. It is now ready for use, but gradually deteriorates by keeping. It may be kept about a month in a 'cool place. The nitric acid slowly oxidizes the proto-salts of iron and the solution turns yellow. In this state it is much slower in its action, and a little fresh Foto-sulphate should be added to it. An ounce of proto-sul phate of iron decomposes about an ounce of nitrate of baryta, forming insoluble sulphate of baryta, and soluble proto-nitrate of iron. The remaining half ounce of undecomposed proto-sulphate of iron forms the energetic part of the developer.