gives, consequently, a positive impression from a positive cliche.



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



SENSITISING SOLUTION.

1. Potassium 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. Potassium 10 parts

bichromate

Manganous sulphate 4 parts

Potassium 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 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

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 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

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 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,

viz.:



Potassium bichromate 460 grams

Sodium chloride 460 grams

Ammonium vanadate 0.75 gram

Sulphuric acid 1 liter

Water 13 liters