Having exposed the plate the necessary time, the next step is the development. The solution I usually employ is prepared with protosulphate of iron. I do not find distilled water absolutely necessary (during the summer months I fancied the tones were improved by using ordinary water, perhaps from containing a little lime), and the acetate acid is not glacial, but a description termed Beaufoy's, much less expensive. The proportions are—
| Water | 2 ounces. |
| Acetic acid | 1 drachm. |
| Protosulphate of iron | 8 grains. |
| Nitric acid | 2 drops. |
Mix the water and acetic acid first; then dissolve the iron; and, lastly, add the nitric acid, which, by varying the quantity, produces different effects. On pouring the solution over the plate, there is sometimes a difficulty experienced in causing it to flow evenly. Sometimes a little more acetic acid in the developing solution, or, if the plate has been out of the bath some time, redipping it, will prevent this; but if this does not remove it, and the resulting picture is hard and unpleasant in tone, a new bath is necessary. For positives, the resulting picture is more pleasing and delicate by using the developing agent rather weak. After it has remained on sufficiently long to bring out the image, the undecomposed iodide is to be removed by hyposulphite of soda. I always use the same solution, pouring it on and off until exhausted. Having sufficiently washed, the picture may perhaps appear with many black spots, this may in future be obviated by adding a little alcohol to the collodion:—or it may be covered with white spots; in that case the collodion requires settling, or rapidly filtrating through an old piece of loose silk. Sometimes it will look all black and white (a common fault with collodion positives), without middle tints: by adding a little more acetic acid, or an extra drop of nitric acid, to the developing solution, or the addition of a few drops of ordinary pyrogallic solution, this disagreeable effect may be overcome. In taking portraits, it is often caused by having the sitter placed with too much front light. Then, again, the should-be whites of the picture may be dull and greenish by reflected, and red by transmitted, light. This effect I generally find remedied by putting less nitric acid in the developing solution. During the development, by watching the colour (by holding a piece of white paper underneath), this red tendency may be observed; in that case the drawing may be preserved by leaving the plate for about a minute after pouring the developing agent off, and before removing the iodide. Some change appears to take place by its contact with the air; it gradually gets more opaque, and when finished, though not so white as many, yet presenting an extremely rich brownish-yellow tone.
During the late dull weather, many of my plates have shown a tendency to an uniform leaden-looking deposit, destroying the blacks of the picture. A little more nitric acid in the bath will sometimes overcome this, but I have not yet found a sufficient remedy. During the summer months I was in the habit of using double the quantity of
iron I have stated, diluting the solution more; then was compelled to diminish the quantity to twelve grains, and now I use eight. I have tried the proportions recommended by French photographers, but they seem to contain too large a proportion of iron. I prefer the use of the protosulphate to the protonitrate of iron from its cheapness, and the ease with which it is made up. It will also keep for any length of time, rather improving than otherwise.
I back with liquid jet from Suggitt, opposite the House of Correction, Mount Pleasant, Clerkenwell. It dries rapidly, and brightens the appearance.
G. H. P.
Mr. Weld Taylor's Iodizing Process.—The process I sent to your columns last month, for iodizing paper, is applicable only to the paper of Canson Frères; and I may further explain, that if the solution does not answer well, it may be washed over again with a solution of iodide of potassa only of the usual strength, and then set on a dish of slightly-acidulated water, to assist the separation and set free the potash. To make the matter clear to Mr. Shadbolt, I may observe, to one who is in the habit of iodizing paper, a considerable amount of the passage relating to cyanide of potassa could not be misunderstood; the nitrate of silver being added to the iodide of potassa, forms at once a precipitate which it is required to take up. The old double iodide says, add iodide of potassa till it does so, and it will do so; but the cyanide of potassa does it much better, and the cyanogen is lost as the paper dries, otherwise it would take no image at all. In the process I gave it merely requires an equivalent, "and cyanide of potassa is always of use in many of these processes." That equivalent is of course best arrived at by a solution, as, if the cyanide of potassa were added in the lump, it would be lost or be in excess.
Further, I may enlighten Mr. Shadbolt by assuring him that the iodizing paper with the ammonio-nitrate of silver, which I never saw published yet, is the best way; and I may confidently assert that the better ways of iodizing papers are not published at all. It is a tedious process to do, but it is as certain as taking a positive from a negative. At present I have not space to give my way of doing it. I may also add, that it will not answer with all papers. In fact, all samples of paper require some modification of the process, as the chemicals are different in the various modes of bleaching paper by different manufacturers. The ammonio-nitrate is perfect with Whatman's paper; indeed it is a subject of much regret, that this maker has not turned out a paper as thin and hard as the Canson Frères. The latter gentlemen have added some chemical, probably iodine, to their paper, which renders it almost impossible to iodize it at all. I believe it to be iodine, because the paper becomes perfectly black over free iodine, which no English paper will do. At all events, this paper is very uncertain, although it has a quality in appearance that is unsurpassed by any other.
Weld Taylor.