CALOTYPE PROCESSES. CLASS II.
In these processes, the operations consist in first impregnating the paper with an alkaline iodide ; next, rendering it sensitive by immersion in a strong bath of aceto-nitrate of silver, and developing with gallo-nitrate of silver rather weaker in silver than in the fore going process. It is not found necessary in this process to intro duce organic matter into the paper, as that which the paper itself contains, in the form of size, is in general sufficient ; but should that not be the case, the effects and their remedy are obvious enough. With respect to the introduction of organic matter, it appears that an extremely minute quantity, in combination with sensitive iodide of silver, is capable of producing 'a very marked effect, and that the effect of organic matter is not increased by over loading the paper with it. This remark is very important, as it shows how wrong in principle are those processes in which a large quantity of organic matter is introduced.
To iodize the Paper.—Immerse a number of sheets, one at a time, in a solution of iodide of potassium containing 20 grains of the salt to the ounce of distilled water. Let them remain together in the bath for at least a couple of hours, then hang up to dry. When dry, the papers assume a reddish or even deep purple tint, according to the quantity of starch which they contain. When dry, put them away in a portfolio until wanted. They may be kept for a few Weeks in a dry place without deterioration.
To excite the Paper.—Immerse it in a bath of acceto-nitrate of silver, containing 35 grains of nitrate of silver, and two scruples of glacial acetic acid to the ounce of water. First, float the face of the paper for a few seconds on the bath, then immerse it entirely. Leave it about three minutes, and then remove it into a dish of dis tilled water. Rinse it well in this, then press it between blotting paper and put it into the slide, with its face to the front shutter.
It may be well, when the nitrate bath is first made, to saturate it with iodide of silver, and filter.
Die Bxposure.—This is about the same as in the former process.
To develop th,e Picture.—The paper will scarcely exhibit any trace
of an image. Immerse it in a solution of gallic acid, to which a few drops of aceto-nitrate have been added. The picture should begin to appear of a red and not a gray tint, which should darken in the course of the development to an intense black in the darkest parts. The dish should be chemically clean, and the developer should remain colourless throughout ; the whites of the picture will not then be discoloured, and the development will proceed much more quickly.
If the picture comes out grey and metallic, exhibiting all the detail, and having but little intensity, it indicates a want of organic matter in the sensitive film. To remedy this, add a little citrate of soda to the iodizing solution, and substitute a little chloride of sodium for the iodide of potassium. This will entirely alter the character of the picture, increasing the density of the blacks, and also the necessary time of exposure. Sugar, honey, gelatine, and sub stances of this kind added in large quantity to the iodizing solution produce but little effect. Serum of milk contains some organic salts, which are de,composed and form organic salts of silver, and this substance acts much more powerfully than those before men tioned, which require to undergo decomposition before they act with effect.
As a rule, it may be considered that organic salts which at once decompose nitrate of silver and form an insoluble organic compound with oxide of silver, act most powerfully in producing the effects due to organic matter ; and that, next to these come such organic substances, (gallic acid, for instance,) as, when added to nitrate of silver, form a mixture which is immediately decomposed by light, and an insoluble organic compound of silver produced.
lb fax the Picture.—Proceed as described in the former process.
Having now described the simplest form of the second class of processes, it remains to indicate the various modifications which it has assumed.
The first and most important of these consists in waxing the paper before iodizing it. This modification has received the name of the