Photographic Reproduction Processes Part 2

To obtain the black tone it suffices to immerse the proof on its removal from the ammoniacal in a solution of tannin at 5 per 100 of water, and when toned, to wash it in a few changes of water.

The process to turn the blue color into a green was devised by Mr. Paul Roy. It is as follows: Dissolve 7 parts of borax in 100 parts of water, and acidify the solution with sulphuric acid added drop by drop until the litmus paper becomes red; then, in the same manner, neutralize with aqueous ammonia not in excess, but just enough to show an alkaline reaction; this done dissolve 1 part of powdered catechu and filter. In this the proof is immersed after development until the desired effect is attained. Wash, etc.

To clear the lines, or to make additions, or to write on the blue margin of the proof a solution of pota.s.sium oxalate is employed. It dissolves the blue without leaving scarcely any trace of it. The solution can be prepared by mixing the two solutions whose formula is given below:(9)

A. Oxalic acid 10 parts Water 100 parts B. Caustic pota.s.sa 12 parts Water 100 parts

The blue prints are permanent. When drying they darken a little from oxidation; exposed to suns.h.i.+ne for some hours, they bleach considerably; but in the shade the faded pictures progressively absorb oxygen from the air and a.s.sume their original intensity and color in a period so much the longer as the insulation has been more prolonged; it may take weeks if the picture were much bleached.

THE CYANOFER. (PELLET'S PROCESS.)

_This process gives blue impressions on a white ground from positive cliches, and white impressions on a blue ground from negative cliches._ It is termed "positive ferrotype process."

The cyanofer is an application of one of the numerous and useful inventions for which photography is indebted to A. Poitevin. In 1863 he discovered that certain organic substances were rendered insoluble by ferric chloride, and that they again became soluble; when under the influence of light the ferric chloride has been reduced to a ferrous salt.

This curious phenomenon is the base of the process now to be described.

As usual the process has been modified by compounding the sensitive solution in various ways and by minor details in the manner operating.

But although these modifications have rendered the process easier to work with, there is not a great difference in the results obtained. We give two formulas. Aside from the addition of gum arabic, which was suggested by Mr. Pellet, and which const.i.tutes the capital improvement of the process, the formula is substantially that devised by Mr. Poitevin.

Prepare three solutions as follows:

A. Gum arabic, best 50 parts quality Water 170 parts B. Tartaric acid 12 parts Water 80 parts C. Ferric chloride 35 parts in volume solution at 45 deg. Baume

Mix gradually B to C, then C, by small quant.i.ties, in agitating briskly.

It is important to prepare the solution as directed, for by adding the ferric chloride before tartaric acid, the gum arabic would be at once coagulated. When the ferric chloride is mixed, the solution at first thickens, but becomes sufficiently fluid for use in a certain period. It does not keep, and should be employed the day it is made if possible.

The paper, which should be well sized and calendered, and which, when not giving good results by too much absorbing the sensitive solution, must be starched as before directed, is coated either by brus.h.i.+ng or by floating.

By the first method a roll of paper five yards long can be prepared without great trouble, and give, perhaps, better results than if prepared by floating; but the latter method is by far the the most convenient: one does not generally prepare by brus.h.i.+ng sheets of paper larger than about 3040 inches.

For brus.h.i.+ng, the paper is pinned on a board, then, with a large badger brush dipped in the sensitive solution, the latter is applied as evenly as possible; after which, by lightly pa.s.sing the brush over, the striae are removed, the coating well equalized, and the paper hung up to dry. The coating should not be very thin, and, above all, not too thick, for then it would require an unusually long exposure to allow the light acting through the whole thickness of the film, which is a sine qua non to obtain a clear ground, i.e., not stained blue.

To prepare by floating, pour the solution in a shallow tray, which needs not to be more than 2034 inches, 30 inches being the width of the drawing paper usually employed; then roll the paper and place it on the solution.

Now, taking hold of it by two corners, draw it out slowly: the paper will unroll by itself. This operation can be done by diffused daylight, but, of course, the paper should be dried in a dark room. It dries rapidly.

Endless rolls are prepared by machinery. To expose, the drawing is placed in the printing frame, face downwards, and the sensitive paper laid over it. The whole is then pressed into contact by interposing a cus.h.i.+on between the lid of the frame and the paper, and exposed so that the rays of light fall _perpendicularly_ upon it.

The cyanofer preparation is quite sensitive. From half a minute to two minutes exposure, according to the intensity of the light and the thickness of the coating, is sufficient in suns.h.i.+ne to reproduce a drawing made on the ordinary tracing paper. In the shade, by a clear sky, the exposure is about five times longer, and varies from half an hour to an hour and more in cloudy weather, but then the design is seldom perfectly sharp.

The progresses of the impression is followed by opening one side of the printing frame and examining the proof. The exposure is sufficient when the paper is tinged brown on the parts corresponding to the ground of the design. The image appears then negative, that is, yellowish on a tinged ground.

Another and more safe method of ascertaining the correct time of exposure, which can be employed concurrently with the other, is to place a few strips of the same sheet of sensitive paper between the margin of the design, upon which a few lines have been traced, and the paper, and, without opening the frame, to draw one of them, from time to time, and dip it in the developing solution. If the whole strip be tinted blue, the proof is not sufficiently exposed; but if the lines soon appear with an intense coloration on the yellowish ground of the paper, and the latter do not turn blue in a minute, at the most, the exposure is right. By excess, the lines are with difficulty developed or broken.

For developing, we provide with three wooden trays lined with lead or gutta-percha, or, more economically, coated with yellow wax. The wax is melted, then applied very hot, and, when it is solidified and quite cold, the coating is equalized with a hot iron, whereby the cracks produced by the contraction of the wax when cooling are filled up.

One of these trays should contain a layer, about three-quarters of an inch thick, of an almost saturated solution of pota.s.sium ferrocyanate (the developer); the next be filled with water, and the third with water acidified by sulphuric acid in the proportion of three per cent. in volumes.

All this being ready, the margin of the proof is turned upwards-so as to form a disk of which the outside is the impressed surface-in order that the ferrocyanate solution does not find its way on the back of the proof, which would produce stains. Now the proof is laid, the lower edge first, on the developer, and gradually lowered upon it, when, taking immediately hold of it by the two corners nearest to the body, it is lifted out and held upright to allow one following the development of the image; and, presently, if any air-bubbles are seen on the proof, they should at once be touched up with a brush wetted with the ferrocyanate solution; the reason explains itself.

The image appears at once. As soon as the fine lines are well defined, the blue intense, and, especially, when the ground has a tendency to be tinged blue, the proof is placed in the tray filled with water and in this turned over two or three times, when it is immersed in the diluted sulphuric acid. In this bath the print acquires a deep blue coloration, consisting of Prussian blue, and the ground becomes tinted with a blue precipitate without adherence, which is easily washed off by throwing the liquid on the proof with a wooden spatula, or, better, by rubbing with a rag tied to a stick. When the ground is cleared, and after three or four minutes immersion to dissolve the iron salts acted on, the proof is rinsed in water several times renewed to free it from acid, and hung to dry.

There are two causes of failures in this process, viz., over and under-exposure. In the former case the fine lines are broken or washed out in clearing the proof (which may also arise from the drawing made with an ink not opaque enough); in the latter the ground is more or less stained.

The blue stains, the lines for corrections, etc., are erased with the the pota.s.sic oxalate (_blue salving,_ as it is termed) whose formula has been given.

The additions, corrections and writing are made with a _Prussian blue ink_ prepared by mixing the two following solutions:

A. Ferric chloride, 4 parts dry Water 350 parts B. Pota.s.sium 15 parts ferrocyanate Water 250 parts

The precipitate being collected on a filter and washed until the water commences to be tinged blue, is dissolved to the proper consistency in about 400 parts of water. This ink does not corrode steel pens.

It has been stated that the cyanofer process keeps for years if preserved from the combined action of dampness and the air. The writer found in his practice that the ferric salts in presence of the organic matters (the sizes) acts as does pota.s.sium b.i.+.c.hromate and renders, in a certain period, the cyanofer film insoluble even after a prolonged insulation. Paper freshly prepared is always more sensitive and gives better whites and generally finer results.(10)

The prints can be toned black in operating as in the cyonotype, but the results are seldom good.

Captain Pizzigh.e.l.li's formula is as follows: Prepare

A. Gum arabic 15 parts Water 100 parts B. Ammonia ferric 45 parts citrate Water 100 parts C. Ferric chloride 45 parts Water 100 parts

For sensitizing mix _in order_:

Solution A 100 parts Solution B 40 parts Solution C 20 parts

The mixture very much thickens at first, but becomes sufficiently fluid for use in a few hours. It keeps well for two or three days. Leaving out B and replacing it by rain water, this makes also a good solution for the cyanotype.

THE BLACK OR INK PROCESS. (FERRO-TANNATE PROCESS.)

_This process gives black positive impressions on white ground from positive cliches, and negative impressions from negative cliches._ It has been attributed to Mr. Colas, but in reality it was invented by Mr.

Poitevin, who describes it as follows in his communication of May, 1860, to the Societe Francaise de Photographie:

"I make a solution containing-"

Iron perchloride, cryst 10 parts Tartaric acid 3 parts Water 100 parts

"I apply the paper on this mixture and let it dry spontaneously in the dark, and at the moment of using it I completely desiccate it at a gentle heat. Thus prepared the paper is of a deep yellow color. Light decolors it rapidly, and ten or twelve minutes' exposure through a positive cliche suffices to well impress it, that is, to reduce in the whites the iron perchloride to the state of protochloride."

"To print, one is guided by the decoloration of the paper, and even for more facility I add to the solution of iron perchloride and tartaric acid a small quant.i.ty of a solution of pota.s.sium sulphocyanide for the purpose of obtaining a red tint, which is more visible and disappears also under the influence of light in proportion to the decomposition of the perchloride. One obtains then after exposure a red design on the white ground of the paper. This red color is not permanent. It even disappears by keeping the proof in the dark."

"To develop and then to fix the design thus obtained I wash rapidly the paper in ordinary water, or better, in water holding chalk in suspension.

The red coloration disappears, a part of the iron perchloride is washed out, and in the parts which have not been acted on by light the perchloride is transformed into sesquioxide. I replace then the water by solution of gallic acid or of tannin and the image progressively appears in ink-black. When I judge the image to be sufficiently intense I wash the proof in rain water, in preference to ordinary water, which might cause the gallic acid and tannin to turn brown. I sponge between sheets of blotting paper and let the proof dry spontaneously."

"If in place of gallic acid I use a diluted solution of pota.s.sium ferricyanide (red prussiate of potash), Prussian blue is formed in the parts acted on by light. The preparation is even sensitive enough to permit one to obtain an impression in the camera obscura in developing by the ferricyanide."