The photographic processes with the salts of iron are all derived from the
researches of Sir John Herschel. The investigations of that great
philosopher are so valuable, so full of instructions that we are led to
reprint them, together with those of Mr. C. J. Burnett, on the salts of
uranium, etc., as an Introduction. It will be seen that the process by
which blue prints are to-day obtained is exactly that Sir John Herschel
devised in 1840.
“It is no longer an insulated and anomalous affection of certain salts of
silver or gold, but one which, doubtless, in a greater or less degree,
pervades all nature, and connects itself intimately with the mechanism by
which chemical combination and decomposition is operated. The general
instability of organic combinations might lead us to expect the occurrence
of numerous and remarkable cases of this affection among bodies of that
class, but among metallic and other elements inorganically arranged,
instances enough have already appeared, and more are daily presenting
themselves, to justify its extension to all cases in which chemical
elements may be supposed combined with a certain degree of laxity, and so
to speak in a _tottering equilibrium_. There can be no doubt that the
process, in a great majority, if not in all cases, which have been noticed
among inorganic substances, is a deoxidizing one, so far as the more
refrangible rays are concerned. It is obviously so in the cases of gold
and silver. In the case of the bichromate of potash it is most probable
that an atom of oxygen is parted with, and so of many others. A beautiful
example of such deoxidizing action on a non-argentine compound has lately
occurred to me in the examination of that interesting salt, the
ferrosesquicyanuret of potassium described by Mr. Smee in the
_Philosophical Magazine_, No. 109, September, 1840, and he has shown how
to manufacture in abundance and purity, by voltaic action on the common or
yellow ferrocyanuret. In this process nascent oxygen is absorbed,
hydrogen given off, and the characters of the resulting compound in
respect of the oxides of iron, forming as it does Prussian blue with proto
salts, indicate an excess of electro-negative energy, a disposition to
part with oxygen, or which is the same thing, to absorb hydrogen (in the
presence of moisture), and thereby to return to its pristine state, under
circumstances of moderate solicitation, such as the affinity of protoxide
of iron (for instance) for an additional dose of oxygen, etc.”
“Paper simply washed with a solution of this salt is highly sensitive to
the action of the light. Prussian blue is deposited (the base being
necessarily supplied by the destruction of one portion of the acid, and
the acid by the destruction of another). After half an hour or an hour’s
exposure to sunshine, a very beautiful negative photograph is the result,
to fix which, all that is necessary is to soak it in water in which a
little sulphate of soda is dissolved. While dry the impression is of a
dove color or lavender blue, which has a curious and striking effect on
the greenish yellow ground of the paper produced by the saline solution.
After washing the ground color disappears and the photograph becomes
bright blue on a white ground. If too long exposed, it gets
‘over-sunned,’ and the tint has a brownish or yellowish tendency, which,
however, is removed in fixing; but no increase of intensity beyond a
certain point is obtained by the continuance of exposure.”
“If paper be washed with a solution of ammonio-citrate of iron and dried
and then a wash passed over it of the yellow ferro-cyanuret of potassium,
there is no immediate formation of true Prussian blue, but the paper
rapidly acquires a violet-purple color, which deepens after a few minutes,
as it dries, to almost absolute blackness. In this state it is a positive
photographic paper of high sensibility, and gives pictures of great depth
and sharpness, but with this peculiarity, that they darken again
spontaneously on exposure to the air in darkness, and are soon
obliterated. The paper, however, remains susceptible to light, and
capable of receiving other pictures, which in their turn fade, without any
possibility (so far as I can see) of arresting them, which is to be
regretted, as they are very beautiful, and the paper of such easy
preparation. If washed with ammonia or its carbonate, they are for a few
moments entirely obliterated, _but presently reappear with reversed lights
and shades_. In this state they are fixed, and the ammonia, with all that
it will dissolve, being removed by washing in water, their color becomes a
pure Prussian blue, which deepens much by keeping. If the solution be
mixed there results a very dark violet-colored ink, which may be kept
uninjured in an opaque bottle, and will readily furnish by a single wash
at a moment’s notice the positive paper in question, which is most
sensitive when wet.”
“It seems at first sight natural to refer these curious and complex
changes to the instability of the cyanic compounds; and that this opinion
is to a certain extent correct is proved by the photographic impressions
obtained on papers to which no iron has been added beyond what exists in
the ferrocyanic salts themselves. Nevertheless, the following experiments
abundantly prove that in several of the changes above described, the
_immediate action_ of the solar rays is not exerted on these salts, but on
the iron contained in the ferruginous solution added to them, which it
deoxidizes or otherwise alters, thereby presenting it to the ferrocyanic
salts in such a form as to precipitate the acids in combination with the
peroxide, or protoxide of iron, as the case may be. To make this evident,
all that is necessary is _simply to leave out the ferrocyanate_ in the
preparation of the paper, which thus becomes reduced to a simple washing
over with the ammonio-citric solution. Paper so washed is of a bright
yellow color, and is apparently little, but in reality highly sensitive to
photographic action. Exposed to strong sunshine, for some time indeed, its
bright yellow tint is dulled into an ochrey hue, or even to gray, but the
change altogether amounts to a moderate percentage of the total light
reflected, and in short exposures is such as would easily escape notice.
Nevertheless, if a slip of this paper be held for only four or five
seconds in the sun (the effect of which is quite imperceptible to the
eye), and when withdrawn into the shade be washed over with the
ferrosesquicyanate of potash, a considerable deposit of Prussian blue
takes place on the sunned part, and none whatever on the rest; so that on
washing the whole with water, a pretty strong blue impression is left,
demonstrating the reduction of iron in that portion of the paper to the
state of protoxide. The effect in question is not, it should be observed,
peculiar to ammonio-nitrate of iron.”
“The ammonio and potasso-tartrate fully possess and the perchloride
_exactly neutralized_ partakes of the same property; but the experiment is
far more neatly made and succeeds better with the other salts.”
“The varieties of cyanotype processes seem to be innumerable, but that
which I shall now describe deserves particular notice not only for its
pre-eminent beauty while in progress, but as illustrating the peculiar
power of the ammoniacal and other parsalts of iron above-mentioned to
receive a latent picture susceptible of development by a great variety of
stimuli. This process consists in simply passing over the
ammonio-citrated paper on which such a latent picture has been impressed,
_very sparingly and evenly_, a wash of the solution of the common yellow
ferrocyanate (prussiate) of potash. The latent picture, if not so faint
as to be quite invisible (and for this purpose it should not be so), is
negative. As soon as the liquid is applied, which cannot be in too thin a
film, the negative picture vanishes, and by very slow degrees is replaced
by a positive one of a violet blue color on a greenish yellow ground,
which at a certain moment possesses a high degree of sharpness and
singular beauty and delicacy of tint. If at this instant it be thrown
into water, it passes immediately to a Prussian blue, losing, at the same
time, however, much of its sharpness, and sometimes indeed becoming quite
blotty and confused.”
“To prevent this confusion gum arabic may be added to the prussiated
solution, by which it is hindered from spreading unmanageably within the
pores of the paper, and the precipitated Prussian blue allowed time to
agglomerate and fix itself on the fibers. By the use of this ingredient
also, a much thinner and more equal film may be spread over the surface,
and _when perfectly dry_, if not sufficiently developed, the application
may be repeated. By operating thus I have occasionally (though rarely)
succeeded in producing pictures of great beauty and richness of effect,
which they retain (if not thrown in water) between the leaves of a
portfolio, and have a certain degree of fixity—fading in strong light and
recovering their tone in the dark. * * *”
“If paper be washed with a mixture of the solutions of ammonio-citrate of
iron and ferrosesquicyanate (red prussiate) of potash, so as to contain
the two salts in about equal proportions, and being then impressed with a
picture, be thrown into water and dried, a negative blue picture will be
produced. This picture I have found to be susceptible of a very curious
transformation. To effect this it must be washed with a solution of
protonitrate of mercury, which in a little time entirely discharges it.
The nitrate being thoroughly washed out and the picture dried, a smooth
iron is passed over it, somewhat hotter than is used for ironing linen,
but not sufficiently so to scorch or injure the paper. The obliterated
picture immediately reappears, not blue, but brown. If kept for some weeks
in this state between the leaves of a portfolio, in complete darkness, it
fades, and at length almost disappears. But what is very singular, a
fresh application of heat revives and restores it to its full intensity.”
“This curious transformation is instructive in another way. It is not
operated by light, at least not by light alone. _A certain temperature_
must be attained, and that temperature suffices in complete darkness.
Nevertheless, I find that on exposing to a very concentrated spectrum
(collected by a lens of short focus) a slip of paper prepared as above
(that is to say, by washing with the mixed solutions, exposure to
sunshine, washing and discharging the uniform blue color so induced, as in
the last article), its whiteness is changed to a brown over the whole
region of the red and orange rays, _but not beyond_ the luminous spectrum.
Three conclusions seem unavoidable: first—that it is the heat of these
rays, not their light, which operates the change; second—that this heat
possesses a peculiar chemical quality which is not possessed by the purely
calorific rays outside of the visible spectrum, though far more intense;
and third—that the heat radiated from obscurely hot iron abounds
especially in rays analogous to those of the region of the spectrum above
indicated.”
Sir John Herschel then proceeds to show that whatever be the state of the
iron in the double salts in question, its reduction by blue light to the
state of protoxide is indicated by many other agents. “Thus, for
example,” says Robert Hunt, “if a slip of paper prepared with the
ammonio-citrate of iron be exposed partially to sunshine, and then washed
with the bichromate of potash, the bichromate is deoxidized and
precipitated upon the sunned portion, just as it would be if directly
exposed to the sun’s rays.”
“I have proved this fact with a great number of preparations of cobalt,
nickel, bismuth, platinum and other salts which have been thought hitherto
to be insensitive to the solar agency; but if they are partially sunned
and then washed with nitrate of silver and put aside in the dark, the
metallic silver is slowly reduced upon the sunned portion. In many
instances days were required to produce the visible picture; and in one
case paper being washed in the dark with neutral chloride of platinum was
sunned and then washed in the dark with nitrate of silver; it was some
weeks before the image made its appearance, but it was eventually
perfectly developed, and, when quite so, remained permanently impressed
upon the paper.”
The following process, discovered at the same time as the cyanotype, and
termed chrysotype, is thus described by Sir John Herschel:
“In order to ascertain whether any portion of the iron in the double
ammoniacal salt employed has really undergone deoxidation, I had recourse
to a solution of gold, exactly neutralized by carbonate of soda. The
proto-salts of iron, as is well known to chemists, precipitate gold in the
metallic state. The effect proved exceedingly striking, and, as the
experiment will probably be repeated by others, I shall here describe it
ab initio. Paper is to be washed with a moderately concentrated solution
of ammonio-citrate of iron and dried. The strength of solution should be
such as to dry into a good yellow color, not at all brown. In this state
it is ready to receive a photographic image, which may be impressed on it
either from nature in the camera obscura, or from an engraving on a frame
in sunshine. The image so impressed is, however, very faint, and
sometimes hardly perceptible. The moment it is removed from the frame or
camera, it must be washed over with a neutral solution of chloride of gold
of such strength as to have about the color of a sherry wine. Instantly
the picture appears, not, indeed, at once of its full intensity, but
darkening with great rapidity up to a certain point, depending on the
strength of the solutions used, etc. At this point nothing can surpass
the sharpness and perfection of detail of the resulting photograph. To
arrest this process and to fix the picture (so far at least as the further
agency of light is concerned), it is to be thrown into water very slightly
acidulated with sulphuric acid, and well soaked, dried, washed with
hydrobromate of potash, rinsed and dried again. * * *”
“In point of _direct_ sensibility, the chrysotype paper is certainly
inferior to the calotype; but it is one of the most remarkable
peculiarities of gold as a photographic ingredient, that _extremely feeble
impressions once made by light go on afterwards, darkening spontaneously
and very slowly, apparently without limit so long as the least vestige of
unreduced chloride of gold remains in the paper_. To illustrate this
curious and (so far as applications go) highly important property, I shall
mention incidentally the results of some experiments made during the late
fine weather on the habitudes of gold in presence of oxalic acid. It is
well known to chemists that this acid, heated with solutions of gold,
precipitates the metal in its metallic state; it is upon this property
that Berzelius has founded his determination of the atomic weight of gold.
Light, as well as heat, also operates this precipitation; but to render it
effectual, several conditions are necessary:—First—the solution of gold
should be neutral, or at most _very_ slightly acid; secondly—the oxalic
acid must be added in the form of a neutral oxalate; and thirdly—it must
be present in a certain considerable quantity, which quantity must be
greater the greater the amount of free acid present in the chloride.
Under this condition, the gold is precipitated by light as a black powder
if the liquid be in any bulk; and if merely washed over paper, a stain is
produced, which, however feeble at first, under a certain dosage of the
chloride, oxalate and free acid, goes on increasing from day to day and
from week to week, when laid by in the dark and especially in a damp
atmosphere, till it acquires almost the black of ink; the unsunned portion
of the paper remaining unaffected, or so slightly as to render it almost
certain that what little action of the kind exists is due to the effect of
casual dispersed light incident in the preparation of the paper. I have
before me a specimen of paper so treated in which the effect of thirty
seconds’ exposure to sunshine was quite invisible at first, and which is
now of so intense a purple as may be well called black, while the unsunned
portion has acquired comparatively but a slight brown. And (what is not a
little remarkable, and indicates that in the time of exposure mentioned
the _maximum_ of effect was attained) other portions of the same paper
exposed in graduated progression for longer times, viz., one minute, two
minutes, and three minutes, are not in the least perceptible degree darker
than the portion on which the light has acted during thirty seconds only.”
“If paper prepared as above recommended for the chrysotype, either with
the ammonio-citrate or ammonio-tartrate of iron, and impressed, as in that
process, with a latent picture, be washed with nitrate of silver instead
of a solution of gold, a very sharp and beautiful picture is developed of
great intensity. Its disclosure is not instantaneous; a few moments
elapse without apparent effect; the dark shades are then first touched in,
and by degrees the details appear, but much more slowly than in the case
of gold. In two or three minutes, however, the maximum of distinctness
will not fail to be obtained. The picture may be fixed by the
hyposulphite of soda, which alone, I believe, can be fully depended on for
fixing argentic photographs.”
“The best process for fixing the photographs prepared with gold is as
follows: As soon as the picture is satisfactorily brought out by the
auriferous liquid, it is to be rinsed in spring water, which must be three
times renewed, letting it remain in the third water five or ten minutes.
It is then to be blotted off and dried, after which it is to be washed on
both sides with a somewhat weak solution of hydriodate of potash. If
there be any free chloride of gold present in the pores of the paper it
will be discolored, the lights passing to a ruddy brown; but they speedily
whiten again spontaneously, or at all events on throwing it (after lying a
minute or two) into fresh water, in which, being again rinsed and dried,
it is now perfectly fixed.”
As the chrysotype will be no more referred to, we shall state, first, that
the image can be developed with a plain solution of silver nitrate or one
acidified with citric or any other organic acid, which generally gives a
brown impression that can be toned with an acid or alkaline gold bath, the
color varying with the solution employed; and secondly, that the process
may be employed to obtain outlines of any picture on paper or canvas to be
colored in oil-paints. The impression developed with gold terchloride is
pale blue, _quite permanent_, and does not at all interfere with the work
of the artist. The canvas should first be washed with a mixture of
alcohol and aqueous ammonia, then dried and rubbed with pumice stone
powder to give a _tooth_. The modus operandi suggests itself.
The researches of Mr. C. J. Burnett on the application of uranium salts
and other compounds to photography are recorded in the _Photographic
Notes_ of Ths. Sutton for 1857. We give in the following lines the most
interesting parts of the two papers of Mr. Burnett:
* * * “The next class of processes are dependent on the sensitiveness to
light of the salts of uranic oxide or sesquioxide of uranium, U2O3.”
“In the first process, the paper being charged with the uranic salt and
exposed to the solar influence under the negative to be copied, is washed
with a solution of the ferridcyanide or red prussiate of potash. The
‘Harvest Scene’ in the exhibition, being from an albumen negative lent me
by Mr. Ross, the well-known Edinburgh photographer, is an example, the
salt of the sesquioxide of uranium being in this case the hydrofluate, and
the time of exposure from the strength of the albumen negative fully an
hour of good sunshine. I have used for the solution of the uranic oxide
for this process a variety of acids with very similar results; the
sensitiveness of the prepared paper to light varying much, however. For
instance, a collodion negative with the hydrofluate paper producing a very
good print in half an hour of unsteady sun, while with a paper prepared
with the tartaric acid solution of the oxide, it gave an equally good
impression in less than five minutes of the same intermitting sunshine,
indicating thus a difference of sensitiveness of six to one in favor of
the tartrate.”
“The rationale of this process is the reduction of the sesqui-oxide of
uranium, U2O3, on those parts of the paper exposed to the solar influence,
to a lower state of oxidation, the photo-oxide UO, the salts of which have
the property of forming with soluble alkaline ferridcyanides a rich
chocolate-brown precipitate, while the salts of the sesquioxide are
destitute of this reaction. Hence the brown deposit on the parts of the
picture on which the sun has been allowed to act when the developing
solution is applied, and the absence of any such appearance on those parts
which have been protected from its influence.”
“As to the manipulatory details of this process, the paper is floated on
the solution in a dark room and hung up to dry, and then preserved from
light in a portfolio. If carefully secluded from light it appears to keep
well. After exposure for the proper time under this negative, there is in
some cases scarcely any visible impression; while in other cases,
particularly when using the tartaric solution, I have found the impression
very distinguishable, of a brownish or blackish shade, although still
quite faint. The development is best conducted by floating it, anything
like rubbing the picture being very objectionable.”
“When the picture has fully come out, which is generally from three to ten
minutes at the very most, it is removed from the developing bath, placed
in cold water and washed very gently for a few minutes, the water being
frequently changed till it ceases to acquire a yellow tinge from the
dissolved red prussiate. The picture is then drained from the water,
pressed between folds of blotting paper, dried (I dry in the dark), and
the process is complete. * * * I may state, as one recommendation of this
process to ladies and other lovers of clean hands, that any brown stains
left by it on the fingers or elsewhere are at once removable by a little
weak ammonia or soap and water. * * * I would particularly suggest, as
deserving of notice, the development of the salts of sesquioxide of
uranium, and still more iron, by the metals and metallic-cyanic alkaline
salts, as also by the mellonides and nitro-prussides, and the latter also
by itself and as developed by many metallic salts.”
“I have since had the opportunity of trying the nitro-prusside of sodium,
which, by itself, gives a blue and white picture, in color like that
obtained from the red prussiate of potash.”
“When mixed with a solution of ammonio-nitrate of copper, previous to its
application to the paper, the color obtained is pale purplish pink or
peach-blossom color. By mixing it in the same way with ammonio-oxalate of
sesquioxide of iron, we get a dull green picture, changeable through
intermediate stages into brown by alkaline carbonates, and that into a
_dirty_ black by gallic acid. It may be well to know that the blue of the
picture given by the red prussiate in the process of Sir John Herschel may
be considerably modified or entirely changed to another color, in many
ways, without interfering with the purity of the white ground, by steeping
the picture, after the undecomposed red prussiate has been washed out, in
solution of salts of various metals, copper, uranium or cobalt, for
instance, and that the colors so produced may be modified as desired,
according to the stage at which the action is stopped.”
“There remains but one class of uranic photographs to be described,
namely, that obtained when we develop with a salt of silver or gold (or
platinum?). This class may be made to print much more rapidly than our
ordinary silver printing process, approaching sometimes more nearly to the
calotype development in this respect. We get the _minutest details_ with
great fidelity, and the picture is effectually fixed by a simple fresh
hyposulphite solution, with a good color in many cases, or by ammonia,
which will be considered an advantage by those who hold the hyposulphite
an enemy to durability. Different shades of color are produced according
to different solvent acids and different details. I have got a good black
perfectly like that of an engraving, by the nitrate of uranic oxide,
developed by ammonio-nitrate of silver (or plain nitrate) and fixed by
plain hyposulphite without any coloring bath. * * * I have tried the
hyposulphite of gold on some of the silver-developed prints prepared with
the hydrofluate of the uranic oxide and fixed with ammonia, which had an
exceedingly unpleasant raw-red color, a very agreeable gray was at once
obtained. I have succeeded in getting very beautiful impressions by
development of the uranic paper by chloride of gold alone.”
In another communication to the _Photographic Notes_, more interesting
perhaps than the foregoing, Mr. Burnett says:
“The clearest and brightest of my results have been obtained by the action
of gallic acid, tannin, or especially a _mixture of tannin and carbonate
of ammonia_, potash or soda, on the blue pictures obtained by the
solarization of paper prepared with ferridcyanide of potassium,
ferrocyanide or ferridcyanide of ammonium. * * * I have also experimented
with the bichromate and iron, with gallic, tannin and other developer; but
I must confess to not having been, in this particular way, so successful
as Mr. Sella appears to have been in the preservation of the whites, owing
possibly to my not having taken the trouble to wash out sufficiently the
iron before toning.”(1)
“I have experimented most extensively in many ways with the chromates and
bichromates, and have succeeded in various ways in getting _very good_
results. A very capital process for many purposes is to float or steep
your paper in a mixed solution of bichromate of potash and sulphate of
copper. As for E. Hunt’s chromotype process,” (2) I have mixed gelatine,
or occasionally grape sugar, or both, with the solution, but instead of
developing it by a silver solution, as in the chromotype, wash out the
salts unacted on by light, and develop by floating on a solution of
ferrocyanide of potassium. The color of the red copper salt which now
forms the picture may be modified or changed in many ways, viz., by
soaking the picture, after the ferrocyanide of potassium has been washed
out of the lights, in a solution of sulphate of iron (or the iron salt
may, but not so advantageously, have been applied to the picture before
the application of the ferrocyanide). Solutions of chloride of tin,
gallic and tannic acids, alone or with alkalies or alkaline carbonates,
may also be employed to modify or change the color. Instead of developing
by ferrocyanide you may develop by the cobalt or chromo-cyanogen salts, or
by an alkaline _mellonide_ arsenite, etc. Sulphureted hydrogen, or a
sulphide, will give a _brown_, or _black_ tone, which may be protected
against oxygen and dampness by a resinous varnish.
“Of all the simple pictures obtainable with bichromated papers, without
complications or other tonings, those obtainable by the combination of a
salt (say the sulphate) of _manganese_, with the bichromate in the paper
preparation, are about the best; these pictures being, however, capable of
being toned and modified in many different ways if desired. This may be
accomplished by the use of toning baths of ferridcyanide or ferrocyanide,
or other metal cyanogen salts, etc., or by either mixing the salts of
other metals, as copper or iron, with the cyanic toning baths, or using
them in the original solution, or by soaking the paper in them, as in
Sella’s process, previously to the application of the metal cyanic,
mellonic or other toning baths. Alkalies and alkaline carbonates may also
be used to remove the chromic acid, and leave a subsalt, or the very
stable oxide or carbonate of manganese, which may be peroxidized by the
use of chloride of lime, peroxide of hydrogen, or ozone.”
“In all the processes with metallic salts, alone with bichromates, the use
of sized or unsized paper along with gelatine, etc., has some advantages.
I have got good results by such processes on albumen paper, the albumen
tending to prevent mealiness in the print; also on paper soaked in
gelatine before the application of the bichromic solution. * * * There is
great interest connected with the action of all such papers, along with
the tannin and vegetable coloring matters. I have long been of opinion
that by the steeping of papers or textile fabrics, containing the salts
not only of iron, as recommended by Mr. Sella, but of tin, copper,
bismuth, lead, etc., in solutions of cochineal, red cabbage, beetroot,
grass or the most ordinary foliage, etc., that the most useful results
might be obtained; though for _certain_ permanence I am not sure but that
some of the other processes which I have briefly run over with the
cyanogen acid salts or metallic acid salts, as precipitators, may be more
to be depended upon. The processes with _precipitated oxides_, such as
the one with manganese and similar ones, with other metals which I have
described, I also consider as deserving of more attention than almost any
processes which have been stated, on the score of probable permanence; but
perhaps the best process for black, or generally useful neutral tint,
without silver, that has yet been offered to the public, I believe to be
the process alluded to with the bichromate of potash and sulphate of
copper, toned by an iron salt. * * * This process, the cuprotype (as
also the uranotype and manganotype) is applicable perfectly to films of
_albumen_ or gelatine on glass or porcelain, textile fabrics, parchment,
paper, tiles and many other substances besides paper.”