Mr. Archer's method is as follows ; and I believe no better course can be pursued.
Prepare a saturated solution of iodide of potassium in alcohol, say 1 oz., and add to it as much iodide of silver as it will take up. Or to 1 oz. of alcohol add an excess both of iodide of potassium and iodide of silver; after a day or two, and with repeated shaking at intervals to facilitate the operation, a saturated solution of the two salts will be obtained, and if this is filtered off into another bottle, it will always be found ready for use. The first bottle can be kept as a stock bottle, to obtain a still further supply by replenishing it with alcohol, and adding now and then small additional quantities of the two salts. The iodide of silver can be readily obtained by precipitation. For instance, take 1 oz. of solution of nitrate of silver used in the process, 30 grs. of nitrate of silver to 1 oz. of water, and add to it sufficient of a solution of iodide of potassium in water as will throw down the whole of the nitrate of silver as an iodide. When this precipitated iodide of silver has settled, which it very readily does, the liquid above must be poured off, and fresh water added, repeating this washing several times. The iodide of silver after this is dried, and then put into a bottle with a small quantity of alcohol, just sufficient to keep it moistened. The quantity of the solution of iodide of silver which can be added to 1 oz. of collodion, must depend upon the quantity of alcohol in the collodion. Iodized collodion is liable to decomposition, and iodine is liberated. This is prevented by putting a little piece of pure metallic silver in the collodion, which preserves it colourless and of a constant strength. The collodion process now resolves itself into:
One of the most successful general manipulators in the ordinary forms of the collodion process is Mr. Home, to whom I am indebted for much important information on this process. A variety of substances, such as tripoli, nitric acid, spirits of wine, etc., have been recommended for cleaning the glass: but all these Mr. Home thinks are quite superfluous; the only articles actually necessary being a clean cloth or two, and a wash leather that has been well and thoroughly rinsed through several changes of clean water, to deprive it as much as possible of the dressing which a new one contains, and a little liquid ammonia, not strong, but the ordinary liquor ammoniś of the shops. If this is not at hand, a little caustic potash or soda will answer as well, the purport of it being to remove any greasy matter attached to the surface, as glass is frequently marked with soap; and although it might appear at first sight that clean water must thoroughly remove this article, the operator will be certain of spoiling many of his pictures if he depend upon water alone.
The plan Mr. Home recommends is as follows:—Pour upon the plate a few drops of ammonia, rub it well over both surfaces, and thoroughly rinse through two waters, allowing the water to flow over the plate either by pouring from a vessel or holding under a tap; now, with a clean cloth wipe perfectly dry, and finally well rub with a leather. simple as this may appear, there is much more in it than will be at first imagined, for unless the glass is free from stains it is quite impossible to be successful. The plate may be washed perfectly clean, but the surface not thoroughly dried. Then, again, some hands are very warm, and if the plate is allowed to rest too much upon any one part, or held too long in the fingers at any one particular spot, that will become warmer than the surrounding part, from the glass being a bad conductor of heat. The cloth and leather should therefore be sufficiently large, that the plate may be as it were insulated as much as possible from the hands, that no unnecessary heat shall be applied. At the same time the employment of a warm cloth is very useful, for the heat is then equally diffused over the plate, and, what is very essential, the surface perfectly and quickly dried.
It has already been pointed out how necessary it is to handle the plate as little as possible in cleaning; we therefore suppose the operator to have the plate in a clean dry leather, from which it is taken to receive the collodio-iodide of silver. The plate must be held by the left hand perfectly horizontal, and then with the right a sufficient quantity of iodized collodio should be poured into the centre, so as to diffuse itself equally over the surface. This should be done coolly and steadily, allowing it to flow to each corner in succession, taking care that the edges are all well covered. Then gently tilt the plate, that the superfluous fluid may return to the bottle from the opposite corner to that by which the plate is held. At this moment the plate should be brought into a vertical position, when the diagonal lines caused by the fluid running to the corner will fall one into the other and give a clear flat surface. To do this neatly and effectually, some little practice is necessary, as in most tilings, 1 but the operator should by no means hurry the operation, but do it systematically and quietly, at the same time not being longer over it than is actually necessary, for collodion being an ethereal compound evaporates very rapidly. Many operators waste their collodion by imagining it is necessary to perform this operation in great haste; but such is not the case, for an even coating can seldom be obtained if the fluid is poured on and off again too rapidly ; it is better to do it steadily, and submit to a small loss from evaporation. If the collodion becomes too thick, thin it with the addition of a little fresh and good ether.
Previous to the last operation it is necessary to have the bath ready, which is made as follows:—
Nitrate of silver.....30 grains.