“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 [pg 9] 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 [pg 13] 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 [pg 13] 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 [pg 14] 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.”