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 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.
1. Potassium 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 10 parts
Manganous sulphate 4 parts
Potassium 20 parts
Water 300 parts
3. Ammonium 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. 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 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
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 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
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 1 ounce or 480
Salt 1 ounce or 480
Sodium vanadate 2/3 grain or 0.66
Water 20 ounces or 9,600
B. Sulphuric acid 2 ounces or 960
Water 10 ounces or 4,800
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 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,
Potassium bichromate 460 grams
Sodium chloride 460 grams
Ammonium vanadate 0.75 gram
Sulphuric acid 1 liter
Water 13 liters