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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 termed, 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.

Next: The Cyanofer Pellet's Process

Previous: Choice Of Paper Sizing

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