Protosulphate of iron, which I first introduced as a photographic agent in 1840, may be employed instead of the pyrogallic acid with much advantage. The beautiful collodion portraits obtained by Mr. Tunny of Edinburgh, are all developed by the iron salt. The following are the best proportions :—
Protosulphate of iron .... 1 oz.
Acetic acid.......12 minims.
Distilled water......1 pint.
This is to be used in the same manner as the former solutions.
This is simply the removal of iodine from the surface of the plate, and is effected by pouring over it, after the water, a solution of hyposulphite of soda, made of the strength of 4 oz. to a pint of water. At this point daylight may be admitted into the room; and, indeed, we cannot judge well of its removal without it. We then see, by tilting the plate to and fro, the iodide gradually dissolve away, and the different parts left more or less transparent, according to the action of light upon them.
It then only remains to thoroughly wash away every trace of the hyposulphite of soda, for, should any of the salt be left, it gradually destroys the picture. The plate should, therefore, either be immersed with great care in a vessel of clean water, or, what is better, water poured gently and carefully over the surface.
After this it must be placed upright to dry, or held before afire. We have now carried the operator carefully through every stage of the process, from the cleaning of plate to the fixing of image; but our remarks have reference to colloclio-iodide alone; that is gun-cotton dissolved in ether, charged with an iodide of silver. We cannot, however, consider our task finished without mentioning the addition of gutta-percha to the collodion. This valuable discovery was made by Mr. P. W. Pry, to which gentleman belongs some of the most important steps made in the art.
The sensibility of the plates appears to be more materially increased by the addition of the gutta-percha ; indeed, pictures by superposition may be obtained with absolute instantaneity, and in the camera obscura in less than a second of time.
The plan of proceeding to obtain this extreme sensibility, as recommended by Mr. Fry, is to obtain a thick and strongly charged collodio-iodide, and to two parts of this add one of a saturated ethereal solution of gutta-percha, allowing it to stand a day or two to clear itself, previous to being used.
The plate is then coated in the usual manner. As the ether evaporates a peculiar white film comes over, at which time it is ready for immersion in the bath. This must be conducted as previously described, and, from its extreme sensibility, with, if possible, greater precaution than before.
For the development of negative pictures, Mr. Fry recommends the pyrogallic solution, rather stronger than that previously given, about one grain to the ounce, with the addition of an extra portion of acetic acid, and the plate re-dipped in the nitrate bath, in preference to adding silver solution to the pyrogallic acid.
In fixing the image after development, it is necessary to keep the hyposulphite on longer than with the ordinary collodion, as the iodide is held with greater tenacity. In other respects the method of proceeding is precisely the same.
Mr. Thomas has modified the collodion process as follows :—
Roughen the surface of the edge about one-sixteenth of an inch all round with coarse emery paper ; this prevents contraction of the film, and enables you to pump upon it, if necessary, without any fear of coming off.
Make a mixture of spirits of wine and liquor ammonia equal parts, render it as thick as cream with tripoli; with a piece of cotton wool, kept for this purpose, rub a small quantity over that side scratched as described, wash well under a tap of water, and wipe dry with a piece of old linen washed without soap, and kept scrupulously clean for this purpose.
N.B. To clean a glass after having used it, when not varnished, wash off the collodion film with water, and dry as above.
Always wipe the glass just before use, and breathe upon it ; if clean, the moisture evaporates evenly.
Pour into the centre as much collodion as the glass will hold, and pour- off at that corner diagonal to the one by which the glass is held : prevent the formation of lines, by altering quickly the position of the glass before the film dries; with observation and practice dexterity is easily acquired.
As soon as the collodion ceases to run, plunge the prepared glass, without stopping, into the following bath:—
Into a 20-oz. stoppered bottle put Nitrate of silver.....1 ounce.
Distilled water.....16 ounces.
Iodide of potassium .... 5 grains.
Distilled water.....1 drachm.
On mixing these two solutions a precipitate of iodide of silver is formed. Place the bottle containing this mixture in a saucepan of hot water, keep it on the hob for about twelve hours, shake it occasionally, now and then removing the stopper. The bath is now perfectly saturated with iodide of silver; when cold, filter through white filtering paper, and add:
Sulphuric æther ... 1 drachm.
A convenient way of saturating the nitrate of silver bath with iodide of silver, is to dissolve the 1 oz. of nitrate of silver in 2 oz. of the water, add the solution of iodide of potassium to this strong solution; the precipitate thus formed is by shaking entirely dissolved; now add the remaining 14 oz. of water, when the iodide of silver is again thrown down, but in such a finely divided state as to render the complete saturation of the bath more perfect, obviating the necessity of frequent shaking. After half an hour, add the alcohol and ether, and filter.
Allow the prepared glass to remain in this bath eight or ten minutes : just before taking it out move it up and down three or four times; drain it, but not too closely; when in the frame place upon the back a piece of common blotting-paper to absorb moisture; the sooner it is placed in the camera the better.