Photographic Reproduction Processes Part 3

"As to the proofs in gallate (or tannate) of iron, they can be transformed into Prussian blue in a solution of pota.s.sium ferrocyanide (yellow prussiate of potash) 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 alb.u.men 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. (46 deg. C.) 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 alb.u.men 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 cliche or drawing.

THE CUPROTYPE. (BURNETT'S PROCESS.)

_This process gives positive impressions from negative cliches._

Uranic nitrate 10 parts Cupric nitrate 2 parts Water 100 parts

Float for a minute strong, well-sized paper on this solution and let it dry spontaneously in the dark. Expose until the image is visible, then develop by floating on a solution of pota.s.sium ferricyanide at 5 per 100 of water-the image appears at once with a rich brown color. When developed, wash it in several changes of water until the unaltered salts are eliminated. The proof is then fixed, and, if too intense, can be reduced in water slightly acidified with hydrochloric acid. A fine black image is obtained by toning in a solution of platinic chloride at 1 per 100 of water.

The chemical actions giving rise to the formation of the metallic ferrocyanide, of which the image consists, are quite complicated. Under the luminous agency the uranic nitrate is first reduced, then the uranous oxide acts on the cupric nitrate, forming cupric oxide, which is finally reduced to the metallic state. This metal now converts the ferricyanate in the ferro compound, which, by another action, forms both cupric and uranic ferrocyanate.

The following uranium process gives black impressions:

In a saturated solution of tartaric acid dissolve freshly precipitated ferric oxide, and keep the solution-ferric tartrate-in the dark. To prepare the sensitizing solution, dissolve 20 parts of uranic nitrate and from 1 to 3 parts of tartaric acid in 100 parts of water, and add a small quant.i.ty of ferric tartrate, the proportion varying with the tint desired: an excess gives a blue black. With this solution brush the paper over, and, when dry, expose under the negative cliche, then develop with a solution of pota.s.sium ferricyanate at 4 per 100 of water. To fix, it suffices to wash in water, renewed three or four times.

As pointed out by Mr. B. J. Burnett (see Introduction), many photographic processes can be devised by basing them upon the various chemical changes, of which uranous oxide, reduced by light from the uranic nitrate or sulphate, is susceptible by means of metallic or organic reagents.

In the Appendix some of the most important processes, with or without silver salts as reagents, will be described.

THE ANILINE PROCESS.

The aniline process was published in 1865, by Mr. Willis, the inventor of the platinotype.(11) It is based on the oxidation of aniline by chromic acid, thus: A sheet of paper brushed with a solution of pota.s.sium bichromate and sulphuric acid, dried, and after insolation under a cliche exposed to the fumes of aniline which, in reacting with the chromic compound not reduced by light, forms a blue-black image. _The process gives, consequently, a positive impression from a positive cliche._

There are various methods of operating; we will briefly describe them.

SENSITISING SOLUTION.

1. Pota.s.sium 6 parts bichromate Sulphuric acid 6 parts Magnesium chloride 10 parts Water 150 parts

Willis recommended 10 parts of solid phosphoric acid instead of sulphuric acid; the latter forms a preparation about twice more rapidly reduced.

2. Pota.s.sium 10 parts bichromate Manganous sulphate 4 parts Pota.s.sium 20 parts bisulphate Water 300 parts 3. Ammonium 5 parts bichromate Ammonium chloride 5 parts Cupric sulphate 1 part Sulphuric acid 8 parts Water 150 parts

Good well-sized paper should be employed. Rives is too tender and absorbs too much. Steinbach is better. For small sizes, whatever be the paper selected, it is well to size it with starch and, if possible, to calender it on a hot steel plate, or, in lieu, to iron it. This is not, however, a sine qua non. The paper is sensitized by brushing or by floating. To sensitize by floating, it should be left but for a few seconds on the solution and removed by dragging it on a gla.s.s rod in order to remove the superfluous liquid. Only the surface of the paper should be impregnated, otherwise the whites would be more or less tinted and the image imbedded not as sharp.

Sensitized, the paper must be dried as rapidly as possible. It does not keep, and should be employed the day it is prepared or the day after, keeping it well wrapped in paper.

As said above, it is exposed under a positive cliche, plans, designs, etc., drawn on tracing paper or linen. The more transparent the material, the more rapid the chemical changes. During the insolation-and it is very short-the chromic compound is reduced, the parts corresponding to the ground, that is, the transparent parts of the cliche, are discolored, while those under the design remain unaltered; the image being, therefore, faintly visible, and being formed of the chromic mixture, it is developed by the fumes of aniline in a blue black tone. Therefore, if the paper be not sufficiently exposed, the ground is colored like the image, although not as deeply, since the dye formed is proportionate to the more or less quant.i.ty of unreduced compound, and if exposed too long the image is imperfectly developed or not at all by excess.

The discoloration of the ground, which turns to a greenish hue, easily indicates when the exposure is sufficient. But, to ascertain it, the beginner should use _tests_ as in the cyanofer process. Mr. Endemann regulates the time of exposure by partly covering a strip of the sensitive paper with a piece of the tracing material upon which the design is made, and exposing the whole until the covered part of the paper a.s.sumes the same shade as the part directly exposed to light.

To develop the print is placed in the bottom of a tray, which is then covered with a lid upon which is pinned blotting paper well imbued with an aniline and benzine mixture, or the reverse; that is, exposing the print fastened to the lid and placing the aniline on the bottom of the tray.

The tray should be hermetically closed; that is a condition to obtain a fine and equal coloration. For this purpose the lid should be well lined with sheets of blotting paper and a weight placed over it during the operation. Large prints are necessarily developed in a fumigating box made ad hoc. The aniline solution consists of

Aniline (commercial for 8 parts red) Benzine, rectified 100 parts

In place of benzine, ether U.S.P., sp. grav. 0.837, may be used.

When the proof is not over-exposed the development commences in a few minutes. The image first takes a dirty black olive color which turns blue in water, then the tone darkens to a dark-brownish tint. The time of exposure to the aniline fumes depends on the time of insolation; if short, the ground is soon tinted, and consequently the development should then be stopped; if over-exposed, the development proceeds slowly. The darkest tone is obtained by a rather full exposure which admits a long fumigation.

Sometimes the image takes a green color; it suffices then to wash the proof in water rendered alkaline by a few drops of aqueous ammonia to obtain the normal color.

To somewhat improve the tone of the image and, if objectionable, to remove the chromic oxide which tinges the ground greenish, the proof should be immersed in a dilute solution of sulphuric acid 1:100, then washed twice, and finally pa.s.sed in ammoniacal water 1:100.

Mr. Hermann Endemann has published, in 1866, the following process in the _Journal of the American Chemical Society_, pp. 189 et seq.:

The paper, which must be well sized with glue, 1:50, is sensitized with the following solution and exposed when dry, but still slightly damp:

A. Pota.s.sium 1 ounce or 480 bicarbonate parts Salt 1 ounce or 480 parts Sodium vanadate 2/3 grain or 0.66 part Water 20 ounces or 9,600 parts B. Sulphuric acid 2 ounces or 960 parts Water 10 ounces or 4,800 parts

When cold mix to A.

"From the composition of the solution," says Mr. Endemann, "it is evident that it must be strongly acid; but when this solution is exposed to light, in the presence of the organic substances of the paper, the acidity of the solution disappears, we obtain pota.s.sium and sodium sulphates, basic chromium sulphate, salt and vanadic acid. While, therefore, the unchanged parts of the paper remain acid, the changed parts acquire a neutral reaction, and while the first will readily a.s.similate bases, the second will not. Exposed in an atmosphere laden with water and aniline, the aniline will be absorbed in those parts where the solution remains acid and in proportion to the remaining acidity."

To develop the image the paper is spread over the opening of a frame tightly placed on a pan, in the bottom of which is heated a solution of aniline in water, 1:50, until the image appears brown, and for further development in a box laden with steam water, which, according to Mr.

Endemann, requires two hours to obtain a deep black coloration. To remove the chromium compound the picture is immersed in a solution of aqueous ammonia, 1:6, then washed and dried.

A few years ago the aniline process was improved by developing the image with the aniline-benzine mixture vaporized by steam in a box made specially for that purpose, whereby a reproduction can be obtained in less than ten minutes.

In the photographic department of Messrs Poulson & Eger's Hecia Architectural and Ornamental Iron Works, which is directed by Charles Bilordeaux, this process is worked in the following manner:

The developing is made of sheet iron with a door sliding up and down, it being balanced by a counterpoise, and provided with a chimney. In the box is a gutter, extending the whole length of the bottom, covered with muslin and connected to a steam pipe; there is also a coil similarly connected.

After the insolation, which requires about one minute in sunshine, the print is suspended in the box, the muslin brushed over with the solution of aniline, and live steam allowed to pa.s.s through the gutter for only two minutes, whereby the aniline being vaporized acts on the chromic salt and develops the image; then the steam is allowed in the coil, and, in from three to four minutes, the paper is dry and the picture finished. The image stands on a slightly greenish ground, which is not objectionable for the purpose the reproductions are made.

The sensitizing solution is similar to that published by Mr. Endemann, viz.:

Pota.s.sium bichromate 460 grams Sodium chloride 460 grams Ammonium vanadate 0.75 gram Sulphuric acid 1 liter Water 13 liters