THIS is the next division of the subject requiring our attention. I may remark, that it is one of the most important divisions of the whole art.
The effect of light is to produce a slight decomposition of the iodide of silver in the collodion film, and we have to make choice of the best means of making apparent, developing, or bringing out, an exceedingly faint and delicate impression of outward objects, in all their delicacy and detail.
The development of the image is nothing more than a continuation of the change commenced by light, which, as has been before remarked, is the commencement of the precipitation of metallic silver. It is a branch of the subject not yet thoroughly understood; but, undoubtedly, it is worthy of much more consideration than has hitherto been given to it. Substances capable of developing a latent image have also the property of precipitating silver from solutions of its soluble salts; without this latter property they would be useless.
We take advantage of this property, and use it as long as it is convenient to do so; that is, until the picture is developed, or it begins to exert its power of precipitation upon the free silver solution on the plate. If, before this has commenced, the latent image be brought out, well and good, the developing solution has done its work, and may be thrown away; if, however, the precipitation commences before the picture is properly developed, the results will be unfavourable, if not altogether a failure.
The developing agents principally employed are Gallic acid, Pyro-gallic acid, Proto-sulphate of iron, Proto-nitrate of iron, and Fluoride of iron.
Fluoride of iron is one of the most stable of the iron salts when in solution. The above developing agents can be used separate, or mixed with each other in various proportions, as will be presently given.
The addition of acetic acid to the solutions serves two purposes; it retards the too rapid action of the solution when poured on the plate, and serves as a medium to make the liquid flow at once, evenly and quickly over the whole surface. The acetic acid is, however, not necessary for the latter purpose when baths of developing liquids are used; but in the former case it is a matter of much importance to have sufficient acetic acid with the developing liquid, to cause it to flow evenly, and without a break, over the whole surface. If the developing liquid does not do this, an unequal and patchy picture will be obtained, from those portions of the plate first covered beginning to develope, before the other parts have been touched by the liquid.
This is a most important point to be careful of, for there is no economy in employing a weak developing solution, and using four times as much of it in bulk as would be necessary if the solution had been used stronger, and with that proportion of acetic acid mixed with it which is found necessary to cause the developing liquid to mix freely with the liquid on the plate.
The strength of the solution may vary according to the intensity of the light, the time of year, and the temperature. No certain rule can be laid down on any of these points experience alone must be the guide.
Gallic acid can be used as a bath for developing collodion pictures; it is as certain as the pyro-gallic solution, but much slower.
I have left a plate in a gallic acid solution all night, to develope, without injury. As it is a much cheaper salt than pyro-gallic, it will, at times, be found a very serviceable reducing agent.
Gallic acid, saturated solution.
Acetic acid, 1 dr. to 8 oz. of the above.
Immerse the plate, after exposure in the camera, until the desired intensity is obtained. If the bath is large, many plates can be developed at the same time.
No. 2. Another kind of developing-bath can be prepared thus:
Gallic acid saturated solution
Tartaric acid .
Also the following
Sulphate of iron
The following are to pour over the plate in requisite quantities, according to their size:
For a plate 9" X 7", 3 drachms of solution will be sufficient to cover it well. There is no economy in diluting this solution with water previous to use.
Proto-nitrate of iron
Even less than half a drachm of acetic acid to each ounce will do when the proto-nitrate is to be mixed with pyro-gallic acid solution for developing.
For positives, a mixture of equal quantities of pyro-gallic and proto-nitrate solution will be found to give a more pleasing tone than either separately.
I prefer the proto-nitrate of iron solution to be made of the following strength, and to add 1 dr. of acetic acid to 4 oz. of the solution.
The solution of nitrate of baryta in water is first made by the assistance of heat; when the baryta salt is dissolved, add the sulphate of iron, and stir with glass rod until the solution is complete.
Allow the white precipitate of sulphate of baryta to settle, and drain the clear liquid into a clean bottle for use.
The liquid will keep good for about a fortnight, but it is always-best to prepare no more than can be used within three or four days.
The two solutions should be mixed in a cup just before development. The mixed liquids, when fresh, will assume a slight inky colour, which increases with the age of proto-nitrate solution; but it does not follow that, even when the mixed liquids are of a very dark colour, that they are incapable of producing a clear and distinct picture.
The two solutions will answer also for negatives, and I have found them preferable to the pyro-gallic solution alone.
Besides these solutions, there are several other mixtures of proto-nitrate or sulphate of iron with gallic and pyro-gallic acid.
Tartaric or citric acid
This solution will keep good for a week.
Instead of using proto-sulph. iron and water, one ounce of solution of proto-nitrate of iron, made according to formula given above, will answer as well; or gallic acid can be employed instead of pyro-gallic acid.
This last formula is good for negative pictures, but useless for positives, from the dark colour the deposit assumes when the picture is finished.
Formic acid (strong)
A developing solution containing a portion of wood naptha has been recommended. It will certainly give very black pictures, but the smell of the naptha will always be against its use.
The small portion of naptha it contains has the effect of preventing the decomposition of the salt of iron in the mixture.
The addition of a portion of fluoride of iron to the pyro-gallic acid solution will be found to accelerate and assist in giving a black deposit as a negative, and also a pleasing tone to a positive picture on glass.
It has one great advantage over the other salts of iron, from its being less liable to change when in solution; in fact, a very strong solution can be kept for a year without any change, by taking care that there is a very slight excess of acid in it.
One drop of hydrofluoric acid to each ounce of a saturated solution will be sufficient to preserve it.
There is one peculiarity attending the development in a sulphate of iron bath which requires notice; it is, that the development is not a continuous one; the image appears almost in full force at once; a certain density in the image is obtained, which is not improved by a continuous immersion.
I have found that the depth of this deposit has some reference to the strength of the exciting bath of silver, and that the stronger the nitrate of silver solution, the more dense will be the deposit.
It may also be remarked, that if, after the plate has been immersed in the sulphate of iron bath, it is taken out, and a small quantity of the solution of silver from the exciting bath is poured over it, a still further blackness is obtained; which may again be added to by another immersion in the sulphate of iron bath.
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