The Cyanotype Or Blue Process

This process gives white impressions on a blue ground with diapositives

or drawings on transparent or semi-transparent materials, and blue

impressions on a white ground from negatives. It is commonly known under

the names of "blue print process," "negative ferrotype process" and

"ferro-prussiate process."

The process is indeed exceedingly simple. A sheet of paper, impregnated

or sensitized, as it is ter
ed, with a solution of ferric citrate and

ferricyanate is impressed under a cliche,(5) then immersed in pure water,

whereby the image is developed and at the same time fixed. It is on

account of the great advantages offered by its simplicity that this

process is generally preferred by civil engineers and architects for the

reproduction of their plans.

The sensitizing solution is prepared in mixing by equal volumes the two

solutions following:

A. Iron, ammonio 20 parts


Water 100 parts

B. Potassium 15 parts

ferricyanate (red


Water 100 parts

Although the mixture keeps pretty well for a certain period in the dark,

it is best to prepare only the quantity wanted for actual use.(6)

The paper is preferably sensitized in operating as follows:

Take hold of the paper by the two opposite corners and fold it into a

loop, lay it on the iron solution, the center of the sheet first placed in

contact with the liquid, and then gradually spread it by lowering the

corners with a little pressure. No solution should run over on the back

of the paper; it would be a cause of stain. This done, and without

allowing the liquid to penetrate in the paper, immediately take hold of

the two corners near the body and withdraw the paper by dragging it over

on a glass rod for this purpose fixed on the edge of the tray. Now pin up

the paper to dry, which should be done rapidly, and sensitize a second

time in proceeding in the same manner. If this second sensitizing be

found objectionable, let float the paper for no more than ten seconds; of

course this method of sensitizing is not applicable to prepare larger

sheets of paper. In this case the paper is pinned by the four corners on

a drawing board or any other support, lined with blotting paper and

quickly brushed over with a sponge sparingly imbued with the sensitizing

mixture, so as to wet the paper with a very small excess of liquid.

The rationale of this manner of sensitizing is to impregnate only the very

surface of the paper with the ferric salts, and thereby to obtain an

intense blue with very good whites, which latter it would be impossible of

obtaining should the sensitizing solution be allowed to reach in the

fibers of the paper, for, in this condition, it is impossible, owing to

the exigencies of the process, to wash out thoroughly the iron salts to

prevent the chemical changes which cause the whites to be tinted blue. It

is for this reason that better results are also obtained with well sized


The sensitizing should be done by a very diffused daylight, and the

drying, of course, in a dark room. When sensitized the paper is yellowish

green. It should be well dried for keeping, and rolled or wrapped in

orange or brown paper and preserved from the action of dampness and of the

air. It does not keep well, however, no more than two or three months,

perhaps, in good condition; but the sooner it is employed the finer the

proofs, the better the whites and more rapidly is the paper impressed.

There is in the market a paper which keeps for a long time. It is

prepared by adding a small quantity of gum arabic or of dextrine to the

sensitizing solution. Good for the reproduction of line work, it does not

give very satisfactory results for pictures in half tones.

The following compound gives a paper much more sensitive, but not keeping

so long, than that prepared according to the formula previously given:

Tartaric acid 25 parts

Ferric chloride, solution 80 parts (in volume)

at 45 deg. Baume

Water 100 parts

When the acid is dissolved, add gradually concentrated aqueous ammonia,

just enough to neutralize the solution--170 volumes, about. The chemical

change consists in the formation of ferric tartrate. Let cool the

solution, then, after adding the following, keep it in the dark:

Potassium ferricyanate 211/2 parts

Water 100 parts

Another and very sensitive preparation is the following:

A. Iron perchloride, 40 parts


Oxalic acid 10 parts

Water 100 parts

B. Potassium 20 parts


Water 100 parts


Printing.--The process we describe yields negative impressions, that is a

positive image from a negative cliche, and a negative image from a

positive cliche, exactly as the silver printing-out process ordinarily

employed in photography. Consequently, for the production of non-reversed

proofs from plans, etc., the original drawing should be placed face

downwards on the glass plate of the printing frame, and, upon the back,

the sensitive paper is laid and pressed into perfect contact by means of a

pad, felt or thick cloth.

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 cliche. 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 cliche, and, also, from the intensity of the

light(7)--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.

To ascertain when the exposure is correct, a few black lines can be traced

on one of the edges of the margin of the design, and strips of the

sensitive paper placed upon them to serve as tests in operating, as it

will be explained in the description of the Cyanofer process. When one of

them is taken out and show, by being washed in water, a clear white line

on a deep blue ground, the exposure is at an end. One understands that

the blue color of the ground is more or less intense according to time of

insolation, for the chemical actions between the reduced and the

non-reduced iron salts is so much more complete as the salts acted on are

more or less deoxidized, that is, reduced to ferrous salts; and that to

obtain the maximum of effect, which, therefore, depends on the allowable

time of exposure, the drawing ink should be opaque and non-actinic as far

as possible, because when, on testing, the lines are tinted the exposure

should be discontinued. However, a slight coloration of the lines is not

very objectionable, for it disappears by a longer washing after the


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

(bleaching powder).

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 (ferric-ferrocyanate), and the ferric

cyanate with the ferrous citrate giving rise to Turnbull's blue (ferrous

ferricyanate). 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.(8)

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 (gallotannic acid) 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:(9)

A. Oxalic acid 10 parts

Water 100 parts

B. Caustic potassa 121/2 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.