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 potassium bichromate and sulphuric acid, dried, and after insolation under a cliché 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 cliché.
There are various methods of operating; we will briefly describe them.
| SENSITISING SOLUTION. | ||
|---|---|---|
| 1. | Potassium bichromate | 6 parts |
| 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. | Potassium bichromate | 10 parts |
| Manganous sulphate | 4 parts | |
| Potassium bisulphate | 20 parts | |
| Water | 300 parts | |
| 3. | Ammonium bichromate | 5 parts |
| 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. [pg 48] 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 glass 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 cliché, 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 cliché, 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 quantity 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 assumes 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 [pg 49] 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 red) | 8 parts |
| 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 passed 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. | Potassium bicarbonate | 1 ounce or 480 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 potassium 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 assimilate 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 pass 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 [pg 51] 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.:
| Potassium bichromate | 460 grams |
| Sodium chloride | 460 grams |
| Ammonium vanadate | 0.75 gram |
| Sulphuric acid | 1 liter |
| Water | 13 liters |