Read Ebook: Photographic Reproduction Processes by Duchochois Peter C

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Another and very sensitive preparation is the following:

A. Iron perchloride, 40 parts cryst Oxalic acid 10 parts Water 100 parts B. Potassium 20 parts ferricyanate Water 100 parts Mix

The printing frame is that used by photographers. The lid is divided, according to the side, in two, three and even four sections, held by hinges and fastened for printing by as many cross-bars, in order that by opening one section, from time to time, the operator can follow the progressive changes resulting from the action of light on the iron salts. To print, the frame should be placed in the light in such a manner as the luminous rays fall perpendicularly upon the drawing or clich?. The reason of this is obvious, since the sensitive paper is not in direct contact with the design, but separated by the material upon which it is drawn.

During the insolation--whose time depends necessarily from the more or less transparency of the clich?, and, also, from the intensity of the light--the paper assumes first a violet tint, which gradually intensifies to a dark shade; then this tint fades, becomes brownish, then pale lilac, while the parts under the lines--that is, the design--upon which the light has, therefore, no action, are visible by keeping the original yellow-green tint of the prepared paper. It is when the lilac color is produced that the exposure is sufficient.

The image is developed and fixed by washing in water two or three times renewed. The water must be free from calcareous salts; these salts converting the iron into carbonates which impart an ochrey tinge to the proof. Rain water--any water in which no precipitate is thrown down by the addition of a few drops of a weak solution of silver nitrate--may be used with safety.

During the development the ground takes a blue color which rapidly intensifies, while the iron compound, not acted on and imparting a yellow green tint to the design, is washed out from the white paper. If the print has not been sufficiently exposed the ground remains pale blue, more or less; the reason has been explained. In this case the development should be done quickly, as the blue is always discharged by washing. On the other hand, whenever the whites are tinted by excess of exposure, they can be cleared partly or entirely by a prolonged immersion in water, but the ground is also to some extent lightened.

When the proof is well developed and fixed, that is, when the soluble iron salts are eliminated, the blue color can be brightened by adding to the last but one washing water a small quantity of citric acid, or of potassium bisulphate, or a little of a solution of hypochlorite of lime .

The action of light in this, as well as in the other photographic processes with metallic salts described in this work, is one of deoxidation, as shown by Herschel. The chemical changes which produce the blue precipitate is quite complicated. It is evident that both the ferric citrate and the ferric cyanate are partly reduced to ferrous salts under the luminous influence, and react in presence of water with the unreduced part of each of these compounds, the ferric citrate with the ferrous cyanate forming Prussian blue , and the ferric cyanate with the ferrous citrate giving rise to Turnbull's blue . The blue of the print is consequently a mixture in a certain proportion of the two compounds; and as the color of Prussian blue is quite different from that of Turnbull's, it follows that by varying in a certain measure the percentage of the two ferric salts forming the sensitizing solution, the color of the blue may be varied thereby. Hence the difference in the formulas given by different authors.

The blue color of the image can be changed into black or dark green. But to that purpose the paper should be, although not exactly necessary, well sized as before directed, and sensitized with extra care to prevent the imbibition of the iron solution into the paper. After exposure the proof should necessarily be thoroughly washed to eliminate the soluble iron salts, then immersed for a moment in water acidified with nitric acid, 1:100, and this done and without washing treated by a solution of aqueous ammonia at 2 per 100 of water. In this the blue color disappears, being changed into a red brownish tint, which indicates that the Turnbull's and Prussian blues are transformed, the former into ferroso-ferric hydrate, with formation of ferrocyanate, and the latter into ferric hydrate. It is by the action of tannin on the ferric oxides thus formed that the black is produced, and by that of catechu-tannic acid contained in the extract of catechu that one obtains a dark green, almost black color.

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 potassium 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:

A. Oxalic acid 10 parts Water 100 parts B. Caustic potassa 12 1/2 parts Water 100 parts

The blue prints are permanent. When drying they darken a little from oxidation; exposed to sunshine for some hours, they bleach considerably; but in the shade the faded pictures progressively absorb oxygen from the air and assume 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.

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 constitutes 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. Baum?

Mix gradually B to C, then C, by small quantities, 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.

For brushing, 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 passing 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.

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 sunshine 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.

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 potassium ferrocyanate ; 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 ; in the latter the ground is more or less stained.

The blue stains, the lines for corrections, etc., are erased with the the potassic oxalate whose formula has been given.

A. Ferric chloride, 4 parts dry Water 350 parts B. Potassium 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 acts as does potassium bichromate 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.

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

Captain Pizzighelli'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

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.

"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 clich? 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 quantity of a solution of potassium 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 potassium ferricyanide , 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."

"As to the proofs in gallate of iron, they can be transformed into Prussian blue in a solution of potassium ferrocyanide slightly acidified by sulphuric acid."

The paper most suitable for this process is that which has been previously well sized with starch, as explained in a special paragraph of this pamphlet. Paper prepared with a film of coagulated albumen gives also good results. It may be prepared by brushing as well as by floating, but in either case the paper should be wetted on the surface only and dried rapidly at a temperature of about 115 deg. Fahr. and kept in a dry place. It does not keep for more than from ten to fifteen days, owing to the hygroscopicity of the iron compound. Mr. Colas, who prepares the paper for the Parisian market, I think, states that he avoids its deterioration by keeping it wrapped in blotting paper, between two sheets of India rubber, to exclude air and dampness. Silvered albumen and plain paper, well desiccated, could be kept in that way for a certain period, especially if the blotting paper is impregnated with sodium bicarbonate and well dried.

Mr. A. Fisch advises to discard the preliminary washing and to develop just on the removal of the proofs from the printing frame. In operating in this manner the development is best made by floating, taking care that the solution does not run off the back of the proof.

The developer may consist of a dilute solution of nutgalls or of

Tannin or gallic acid 4 parts Oxalic acid 0.15 parts Water 1,000 parts

After developing the proof should be washed rapidly--under a jet of water, if possible--for were the iron salt and the reagent not soon removed, or any remain in the paper, the ground would be tinted violet. And whatever be the care taken, it very seldom occurs that the whites are pure when the proof is dry. This for half-tone pictures has not a great importance, but for the reproductions of plans it is sometimes objectionable. In fact it must be acknowledged that none of the processes now at our disposal--if we except the so-called Artigues process described further on--gives an entirely satisfactory result. A simple and expeditious process, yielding intense black impressions on a white ground, is yet to be found for the reproduction of plans, maps, etc., without resorting to a negative clich? or drawing.

THE CUPROTYPE.

Uranic nitrate 10 parts Cupric nitrate 2 parts Water 100 parts

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