HAVING given, in the last division of this subject, the mode of iodizing collodion, we must proceed to describe the nature of the Exciting Bath, and the various ways of preparing the solution.
The salts of silver available for this purpose are those soluble in water; they are used alone, or by admixture with each other; they are, the nitrate, chlorate, and fluoride of silver.
There are, however, only two which can be employed with advantage these are, the nitrate and chlorate of silver. The former, from its greater cheapness, is the only salt at present in use, although the chlorate of silver, could it be obtained at a reasonable price, would be found superior to the nitrate. I have used it at various times, and found it produce a more sensitive surface, and in solution it is not so liable to change. Although not so soluble as the nitrate, it is still sufficiently so for the preparation of the exciting bath. Our attention, however, must be confined to the nitrate.
The nitrate of silver solution should be prepared with the crystallized salt and distilled water. When distilled water cannot be obtained, boiled rain-water may be taken as a substitute. River and spring-water generally contain free lime.
Its presence will be detected at once by a discoloration of the silver solution, from the precipitate of a portion of oxide of silver. Besides this evil, it will have the effect of decomposing the pyro-gallic solution when in contact with it, whilst developing the picture, giving rise to a brown discoloration throughout the whole surface.
This evil can be remedied by the addition of one or two drops of acetic acid, and subsequent filtration.
To attain the greatest sensibility, the exciting bath should be perfectly neutral.
If it is acid, sensibility will be lost in proportion to its acidity; if alkaline, a brown precipitate will form in the collodion more or less rapidly after the development of the image has commenced.
The addition of acetic acid tending to take away sensibility, great care is necessary not to add more than is absolutely required to overcome the evil.
The precaution, therefore, of testing the solution after the addition of each drop of acid, should be attended to.
The most simple way of testing the purity and proper working-condition of the silver bath, is this: by the faint light of a candle, or lamp, shaded by yellow glass, prepare a strip of glass with iodized collodion, and immerse it in the silver solution to be tested, for one minute. On removal from the bath, expose a portion of the excited collodion to the unshaded light of the candle, or lamp, at a distance of six inches from the flame, for ten seconds, then develope with pyro-gallic solution; if the shaded part of the film preserve its clear appearance, whilst the exposed portion is blacking from the influence of the light, it may be considered that the chemicals employed are pure; if, on the contrary, the unexposed portion likewise discolours, it may be considered that the bath requires a slight portion of acid.
Acetic acid is preferable to nitric acid, for although the latter will answer the purpose, there is more chance with it of giving the bath an overdose.
If the surface of the developed picture assumes a dull, brownish grey colour, (fogging, as it is so commonly called,) it arises from the precipitation of silver itself, which may be entirely in the body of the collodion, or partly as a deposit on the surface, veiling the developed picture.
One other cause, which, if not guarded against, is certain to produce indistinctness in the picture, is, the want of due caution in protecting the iodized surface, when sensitive, from the action of even the faintest daylight, or direct light from a candle or lamp. This is often sufficient to cause a haze over the whole picture. If, on the contrary, light is allowed to strike on the plate after the development has commenced, a different action is set up. The parts of the picture which should be transparent, and correspond with the shadows, are first attacked, and the haze is produced over those parts only, which equally destroys the beauty of the positive image, and interferes with the deep shadows of the printed picture.
I need not say that it would be superfluous to add acid to a bath if clear and distinct pictures can be obtained without it, and should only be had recourse to as a remedy.
In my first experiments with collodion I used an acid bath, in fact, a bath of the same composition as that usually given for calotype paper; my iodized collodion was, of course, very little sensitive to light, and the development of the image was slow in proportion; but there was no fogging. My impression at that time was, that a solution of silver could not safely be used without some portion of acetic acid, to keep in check the spontaneous change which always comes on in the mixture of nitrate of silver and developing solution, when the development is prolonged; and my surprise was great when I first found that a picture could be fairly produced by exciting the plate with a perfectly neutral solution of silver; this is the chief cause of the superior sensibility of the collodion process over paper photography, and is a distinguishing feature in the process.
For ordinary use the silver bath need not be of greater strength than
Nitrate of silver
A stronger solution of nitrate of silver does not so much add to the sensibility of the collodion itself, as it conduces to the more rapid development of the image, which would come out equally well, with the same exposure to light, giving the development a little more time.
The case is different, however, when a developing-bath is employed ; one of sulphate of iron, for instance, in which case a fifty-grain solution of nitrate of silver, will give a deeper negative picture than a thirty-grain solution, with the same exposure to light.
It must be borne in mind, that the stronger the silver bath is employed, the more rapidly will the fogging and haze, just alluded to, come on, and obliterate the labours of the photographer.
A solution of nitrate of silver dissolves a small portion of iodide of silver; the quantity dissolved depending upon the strength of the silver bath.
A fresh solution of nitrate of silver will acquire it from the iodized collodion, when immersed in it; therefore, to prevent the iodide in the film being dissolved out from the first plates used in it, the exciting bath should be saturated with iodide of silver previous to use.
To effect this purpose, I generally prepare a glass with a very thick film of iodized collodion, and plunge it, when set, into the silver bath, and allow it to remain in for several hours, or a day ; the bath will acquire, by this means, sufficient iodide of silver to prevent it acting upon the prepared plate. I cannot recommend the putting ether or alcohol to the silver solution.
The stronger the solution of nitrate of silver the more readily will it dissolve out the iodide from the film, and will often remove when aprepared plate has been allowed to drain long in the frame of the camera, whilst receiving the impression, a considerable portion of the iodide, particularly from the upper part; in this ease the nitrate of silver solution becomes stronger from evaporation, and consequently acquires greater dissolving power the latter being in proportion to its strength
If the silver bath has become slightly brown from the presence of any impurity, it should be well filtered; if, after filtering, it still retains any colour, it should be put by, and exposed to daylight; and, by standing for a few days, the whole of the discoloration will be got rid of a black precipitate being formed at the bottom of the bottle.
When decanted, or filtered from the precipitate, it may again be used. This discoloration may arise from the presence of organic matter, or free alkali,
Instead of having; one large bath, containing all the stock of exciting liquid, it will be better to have a portion of the liquid put by, to fall back upon in case of any impurity getting into the quantity in use, which is sometimes the case even when the greatest precautions are taken.
The usual method of exciting the prepared plate is given above; my original plan however, as I first published it, was to wash the excited plate previous to exposure in the camera, and develope with pyro-gallic acid solution, with the addition of a few drops of nitrate of silver solution.
The plate by being thus washed previous to exposure, will lose much of its sensibility, but if put by in a moist state, it will be found to retain that sensibility for a considerable time.
Besides these two methods, the excited plate, just previous to exposure in the camera, can be washed with the pyro-gallic solution and immediately exposed; this renders the plate highly sensitive to light, and the image developes as soon as the light begins to act, consequently the picture is partly visible before the plate is removed from the camera. This method will give a very dense deposit to the negative, and the positive image will have a very peculiar effect.
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