THE PHOTOGRAPHIC NEWS. Vol. I., No. 22, February 4, 1859.
[The following letter was addressed to M. Regnault, of the Acacédmie des Sciences.]
I SHALL be obliged if you will have, at the next meeting of the Academy, a sealed packet opened, respecting the toning of paper photographic proofs, which was deposited by me on the 13th of January, 1858.
I beg you will at the same time rectify an erroneous statement which I believe I committed in that communication, and to submit to the judgment of the Academy the following improvements which I have introduced into my process.
The correction to be made is, instead of water salted at 50 per cent.,
|Water strengthened with 50 per cent of water salted to saturation||1000 parts.|
|Chloride of gold||6 ,,|
The modifications I have made in my process consist in the substitution of chloride of lime of commerce (hypochlorite of lime) for the chloride of sodium, and in the more exact quantities pointed out by experience.
In considering this new mode of fixing, my object has been to produce the same effects that I pointed out some years since, in a memoir on fixing with the chloride of gold acidulated by hydrochloric acid, in evading the inconvenience of the reaction of the acid on the hyposulphite of soda.
I had succeeded in the substitution of the alkaline salt for the acid salt indicated in my preceding communication, but the object was not entirely attained; the chloride of sodium did not completely remove the yellow tint that positive paper often acquires, especially albumenised paper, which has been prepared for any length of time.
It behoved me, therefore, to push my researches further, and I believe that I have at last succeeded, by availing my self of the decoloring principle of the chloride of lime.
The formulae and manipulations consist--
1. In freeing the proof, by washing for a few minutes in two waters, from the free nitrate of silver contained in the fibres of the paper.
2. In submitting the proof to the action of an auriferous bath thus composed:--
|Distilled water||1000 parts|
|Chloride of lime of commerce in white powder (hypochlorite of lime)||3 "|
|Filter, and add:--|
|Chloride of gold (dissolved in 100 grammes of distilled water)||1 "|
The picture acquires in this bath a black tone which gradually tends towards a blue, at the same time that the yellow tint is restored to a brilliant white. It requires from ten minutes to a quarter of an hour to produce the maximum effect of this method of toning. Practice will be the best guide to the attainment of any particular tone. Nevertheless, as some guide in preliminary essays, I will observe that by leaving the proof for one minute in the bath, a violet-red tint will be obtained after fixing with hyposulphite, and a very clear blue-black tone after a sojourn there of an hour or two.
In this time the proof passes from violet, through all the intermediate tones, up to a deep black in the shadows, and afterwards from the black to blueish tones, becoming gradually weaker and weaker; of course, I mean after the final fixing in the hyposulphite of soda.
There are, therefore, two periods; the one ascending in the scale of intensity, the other descending.
3. To pass the proof anew in a bath of pure water, twice charged, to remove the chloride of lime. This washing may be performed very rapidly.
4. To afterwards fix the proof in a hypo. bath composed of one volume of hyposulphite of soda in crystals to six volumes of water.
This bath ought to be used for only a small number of proofs, the object of it being to remove the chloride of silver not acted upon by the light, which is contained in the fibres of the paper. The effect is produced in from ten to fifteen minutes, according to the temperature.
As soon as put into this bath the proof loses a little of the blue-black tone which it had acquired in the chloride of lime bath, and passes to more violet tints.
If the tones thus obtained are satisfactory, the ordinary washings in water may be at once proceeded with, and the proof dried; but, notwithstanding, I would advise, with a view to its perfect stability, to carry it through the whole series of operations.
5. To bring the proof to the final tone in a bath thus composed
|Distilled water||1200 parts.|
|Hyposulphite of soda||200 ,,|
|Chloride of gold||2 ,,|
The proof ought not to be left in this bath less than fifteen minutes, as that is the minimum time necessary to insure the permanency of the picture, but it may be allowed to remain in it for as much longer as is requisite for obtaining the desired tone.
6. To continue the washings in water in use in the old processes, especially recommending a washing in warm water to remove all trace of the salts.
I must observe that the proofs obtained by this process, beside the fine qualities of tone which they offer, have the advantage of not changing with time, a result I have verified on portraits which I fixed by this means more than eight months since. . . The advantages of this new method of fixing, consist principally in this, that it avoids the decomposition of the hypo. bath by the exclusion of every trace of free nitrate of silver in the proof, and that it preserves a very harmonious black tone in the proof, communicated to it by the gold bath; a tone which is not destroyed by a lengthened sojourn in the hyposulphite of soda, the destructive property of which is well known.
I would also remark, that, in the old method of fixing with the bath of hyposulphite of soda and chloride of gold, the cause of the destruction of the pictures was the presence and formation of an acid occasioned by the use of this bath, which in time led to its decomposition. I have succeeded in restoring its original qualities to this bath by mixing carbonate of baryta with it, and then filtering. It resumes then the qualities of a new bath and gives pictures of great permanency.