CALOTYPE PROCESSES. CLASS I.
There are Two methods which come under this head. In the first, argento-iodized paper is prepared by what is called the " double wash ;" in the second, by the " single wash." In all other respects the two methods are identical.
To argento-iodize the Paper by the Double Wash.—Float the face of the paper on a bath containing 20 grains of nitrate of silver to the ounce of distilled water. Let it remain a minute or two on the bath, then hang it up to dry.. Next immerse it in a solution con taining 25 grains of iodide of potassium to the ounce of distilled water. Let it remain a minute or two in this solution, the exact time depending on the kind of paper employed, and requiring to be ascertained by experiment. If too short a time is allowed, the whole of the nitrate of silver is not decomposed, and the paper darkens in the light ; if, on the contrary, too long a time is allowed, the iodide of potassium in the bath dissolves the iodide of silver in the paper. The time of sojourn in the iodide batli is therefore rather critical. On removing the paper from the bath, let it drain, and then immerse it in a pan of water, in which it must be allowed to soak, the water being changed several times, until the whole of the free iodide of potassium is removed. This soaking operation is rather troublesome, and the texture of the paper is decidedly injured. by it. Should any excess of iodide of pot,assium remain in any part of the paper, it would decompose the weak exciting solution and produce insensitive iodide of silver, and consequently a white patch in the negative. When the paper has been sufficiently washed, hang it up to dry. It is of a yellow primrose colour. When dry, it may be kept for use in a portfolio. It is not sensitive to light.
To argento-iodize the Paper by the Single Frash.—Lay the paper on a board with a piece of blotting paper under it, and brush over it a solution called " double-iodiole " i.e., a solution of iodide of silver in iodide of potassium ; (see "Double-Iodide.") The best kind • of brush is a large round one of camel's hair, bound with string or silver vvife. Apply the solution copiously both longitudinally and
transversdy, inclining the board, and keeping a flowing edge. Hang up the paper to dry in a room having a pure atmosphere, free from sulphurous and acid vapours, which bleach the reddish tint that the paper ought to assume. When very nearly or even quite dry, anamerse the paper in a pan of water as before, in order to remove completely the excess of iodide of potassium which it contains. Two or more papers should not be, soaked in the same pan, but each paper should have a separate pan. When sufficiently washed— lin operation which requires several hours—dry the paper and put it away for use.
Argento-iodized paper may be kept for some weeks without losing its good qualities. Some persons even affirm that it may be kept indefinitely in a dry place. It is said, on good authority, to be much improved by exposing it for an hour or two to strong sun shine. When this is done, it should not be e,xcited inunediately after insolation, but kept for some days previously in the dark, since it has been found that paper absorbs light in sufficient quantity to react on a sensitive surface placed in contact with it in the dark.
In comparing the two methods of iodizing the paper which have been described, it will be seen that the first is the most economical, but at the same time the least certain. A considerable quantity of iodide of potassium is wasted in the second mode of iodizing, and the washing operation is rather more tedious.
The paper having been iodized by either of these methods, the remaining operations are the same, and are as follow :— To excite the Paper.—Make two solutions, one a saturated solu tion of gallic acid in cold distilled water, which call solution A ; the other, by dissolving 50 grains of nitrate of silver in 1 ounce of distilled water, and adding 1 dram of glacial acetic acid, which call solution B. Immediately before use, mix, in a chemically clean measure, 1 ounce of distilled water, 35 drops of solution B, and about as many drops of solttion A, the number of the latter de pending on the temperature and kind of paper employed.