ON THE DEVELOPMENT OF AN INVISIBLE IMAGE BY MEANS OF A REDUCING AGENT.

It has been shown in the previous Chapter that the majority of the Salts of Silver, both organic and inorganic, are darkened in colour on exposure to light, and, by the loss of Oxygen, Chlorine, etc., become reduced to the condition of Subsalts.

Many of the same compounds are also susceptible of a change under the influence of light, which is even more remarkable. This change takes place after a comparatively short exposure, and as it does not affect the appearance of the sensitive layer, for some time it escaped notice: but it was afterwards discovered that an impression, before invisible, might be brought out by treating the plate with certain chemical agents which are without effect on the original unchanged salt, but quickly blacken it after exposure.

It is a remarkable fact that the Silver compounds most readily affected by light alone, are not the most sensitive to the reception of the invisible image. Thus, of Photographic papers prepared with Chloride, Bromide, or Iodide of Silver, the former assume the deepest shade of colour under the influence of the sun's rays, but if all be exposed momentarily, and then removed, the greatest amount of effect will be developed upon the Iodide paper. Iodide of Silver therefore is the salt commonly used when sensibility is an object, but it should be noted that images nearly or quite latent can be impressed upon many other of the compounds of Silver, including those belonging to the animal and vegetable kingdoms.

Experiments illustrating the Formation of an Invisible Image.—Take a sheet of sensitive paper, prepared with Iodide of Silver by the method given in the fourth Chapter of Part II., and having divided it into two parts, expose one of them to the luminous rays for a few seconds. No visible decomposition takes place, but on removing the pieces to a room dimly illuminated, and brushing with a solution of Gallic Acid, a manifest difference will be observed; the one being unaffected, whilst the other darkens gradually until it becomes black.

Experiment II.—A prepared sheet is shielded in certain parts by an opaque substance, and then after the requisite exposure, which is easily ascertained by a few trials, treated with the Gallic Acid as before; in this case the protected part remains white, whilst the other darkens to a greater or less extent.

In the same way, copies of leaves, engravings, etc. may be made, very correct in the shading and much resembling those produced by the prolonged action of light alone upon the Chloride of Silver.

The object of employing a substance like Gallic Acid to develope or bring out to view an invisible image, in preference to forming the picture by the direct action of light, unassisted by a developer, is the economy of time thereby effected. This is well shown in the results of some experiments conducted by M. Claudet in the Daguerreotype process: he found that with a sensitive layer of Bromo-Iodide of Silver, an intensity of light three thousand times greater was required if the use of a developer was omitted, and the exposure continued until the picture became visible upon the plate.

To increase the sensitiveness of Photographic preparations is a point of great consequence; and indeed, when the Camera is used, from the low intensity of the luminous image formed in that instrument, no other plan than the one above described would be practicable. Hence the advancement, and indeed the very origin, of the Photographic Art, may be dated from the first discovery of a process for bringing out to view an invisible image by means of a reducing agent.

The present Chapter is divided into three Sections:—first, the chemical properties of the substances usually employed as developers;—second, their mode of action in reducing the Salts of Silver;—third, hypotheses on the action of light in impressing a latent image.

SECTION I.

Chemistry of the various Substances employed as Developers.

Development is essentially a process of reduction, or, in other words, of deoxidation. If we take a certain metal, we can, by means of Nitric Acid, impart Oxygen to it, so that it becomes first an Oxide, and afterwards, by solution of the Oxide in the excess of acid, a salt. When this salt is formed, by a series of chemical operations the reverse of the former it may be deprived of all its Oxygen, and the metallic element again isolated.

The degree of facility with which oxidation as well as reduction is performed, depends upon the affinity for Oxygen which the particular metal under treatment possesses. In this respect there is considerable difference, as may be shown by a reference to the two well-known metals, Iron and Gold. How speedily does the first become tarnished and covered with rust, whilst the other remains bright even in the fire! It is indeed possible, by a careful process, to form Oxide of Gold; but it retains its Oxygen so loosely that the mere application of heat is sufficient to drive it off, and leave the metal in a pure state.

Silver, Gold, and Platinum all belong to the class of noble metals, having the least affinity for Oxygen: hence their Oxides are unstable, and any body tending strongly to absorb Oxygen will reduce them to the metallic state.

Observe, therefore, that the substances employed by the Photographer to assist the action of the light, and to develope the picture, act by removing Oxygen. The sensitive Salt of Silver is thus reduced, more or less completely, in the parts touched by light, and an opaque deposit results which forms the image.[7]

The most important of the developers are as follows:— Gallic Acid, Pyrogallic Acid, and the Protosalts of Iron.

CHEMISTRY OF GALLIC AND PYROGALLIC ACIDS.

a. Of Gallic Acid.—Gallic Acid is obtained from Gall Nuts, which are peculiar excrescences formed upon the branches and shoots of the Quercus infectoria by the puncture of a species of insect. The best kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall Nuts do not contain Gallic Acid ready formed, but an analogous chemical principle termed Tannic Acid, well known for its astringent properties and employment in the process of tanning raw hides.

Gallic Acid is produced by the decomposition and oxidation of Tannic Acid when powdered galls are exposed for a long time in a moist state to the action of the air. By boiling the mass with water and filtering whilst hot, the acid is extracted, and crystallizes on cooling, on account of its sparing solubility in cold water.

Gallic Acid occurs in the form of long silky needles, soluble in 100 parts of cold and 3 of boiling water; they are also readily soluble in Alcohol, but sparingly in Ether. The aqueous solution becomes mouldy on keeping, to obviate which, the addition of Acetic Acid or a drop or two of Oil of Cloves is recommended.

Gallic Acid is a feeble acid, scarcely reddening litmus; it forms salts with the alkaline and earthy bases, such as Potash, Lime, etc., but not with the oxides of the noble metals. When added to Oxide of Silver the metallic element is separated and the Oxygen absorbed.

b. Pyrogallic Acid.—The term pyro prefixed to Gallic Acid implies that the new substance is obtained by the action of heat upon that body. At a temperature of about 410° Fahr., Gallic Acid is decomposed, and a white sublimate forms, which condenses in lamellar crystals; this is Pyrogallic Acid.

Pyrogallic Acid is very soluble in cold water, and in Alcohol and Ether; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue colour with Protosulphate of Iron, which changes to dark green if any Persulphate be present.

Although termed an acid, this substance is strictly neutral; it does not redden litmus-paper, and forms no salts. The addition of Potash or Soda decomposes Pyrogallic Acid, at the same time increasing the attraction for Oxygen; hence this mixture may conveniently be employed for absorbing the Oxygen contained in atmospheric air. The compounds of Silver and Gold are reduced by Pyrogallic Acid even more rapidly than by Gallic Acid, the reducing agent absorbing the Oxygen, and becoming converted into Carbonic Acid and a brown matter insoluble in water.

Commercial Pyrogallic Acid is often contaminated with empyreumatic oil, and also with a black insoluble substance known as Metagallic Acid, which is formed when the heat is raised above the proper temperature in the process of manufacture.

CHEMISTRY OF THE PROTOSALTS OF IRON.

The combinations of Iron with Oxygen are somewhat numerous. There are two distinct Oxides which form Salts, viz. the Protoxide of Iron, containing an atom of Oxygen to one of metal; and the Peroxide, with an atom and a half of Oxygen to one of metal. As half atoms however are not allowed in chemical language, it is usual to say that the Peroxide of Iron contains three equivalents of Oxygen to two of metallic Iron.

Expressed in symbols, the composition is as follows:—

Protoxide of Iron, Fe O.
Peroxide of Iron, Fe₂O₃.

The Proto- and Persalts of Iron do not resemble each other in their physical and chemical properties. The former are usually of an apple-green colour, and the aqueous solutions almost colourless, if not highly concentrated. The latter, on the other hand, are dark, and give a yellow or even blood-red solution.

The Protosalts of Iron are alone useful in Photography; but the following experiment will serve to illustrate the properties of both classes of salts:—Take a crystal of Protosulphate of Iron, and, having reduced it to powder, pour a little Nitric Acid upon it in a test-tube. On the application of heat, abundance of fumes will be given off, and a red solution obtained. The Nitric Acid in this reaction imparts Oxygen, and converts the Protosulphate entirely into a Persulphate of Iron. It is this feature, viz. the tendency to absorb Oxygen, and to pass into the state of Persalts, which makes the Protosalts of Iron useful as developers.

There are two Protosalts of Iron commonly employed by Photographers: the Protosulphate and the Protonitrate of Iron.

a. Protosulphate of Iron.—This salt, often termed Copperas or Green Vitriol, is an abundant substance, and used for a variety of purposes in the arts. Commercial Sulphate of Iron however, being prepared on a large scale, requires re-crystallization to render it sufficiently pure for Photographic purposes.

Pure Sulphate of Iron occurs in the form of large transparent, prismatic crystals, of a delicate green colour: by exposure to the air they gradually absorb Oxygen and become rusty on the surface. Solution of Sulphate of Iron, colourless at first, afterwards changes to a red tint, and deposits a brown powder; this powder is a basic Persulphate of Iron, that is, a Persulphate containing an excess of the oxide or base. By the addition of Sulphuric or Acetic Acid to the solution, the formation of a deposit is prevented, the brown powder being soluble in acid liquids.

The Crystals of Sulphate of Iron include a large quantity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly anhydrous, in which state it forms a white powder.

b. Protonitrate of Iron.—This salt is prepared by double decomposition between Nitrate of Baryta or of Lead and Protosulphate of Iron. It is an unstable substance and crystallizes with great difficulty; its aqueous solution is pale green at first, but very prone to decomposition, even more so than the corresponding Sulphate of Iron.

SECTION II.

The Reduction of Salts of Silver by Developing Agents.

The general theory of the reduction of metallic oxides having been explained, it may be desirable to enter more minutely into the exact nature of the process as applied to the compounds of Silver.

First, the Reduction of the Oxide of Silver will be taken, as the most simple illustration; then that of Salts of Silver formed by Oxygen-acids; and lastly, of the Chloride, Iodide, and Bromide of Silver containing no Oxygen.

Reduction of Oxide of Silver.—To illustrate this conveniently, the Oxide of Silver should be in a state of solution; water dissolves Oxide of Silver very sparingly, but it is freely soluble in Ammonia, forming the liquid known as Ammonio-Nitrate of Silver. If, therefore, a little of the Ammonio-Nitrate of Silver be placed in a test-tube, and solution of Sulphate of Iron be added to it, immediately it becomes discoloured, and a deposit settles to the bottom.

This deposit is metallic Silver, produced by the reducing agent appropriating to itself the Oxygen previously combined with the metal. As metallic Silver does not dissolve in Ammonia, the liquid becomes turbid, and the metal subsides in the form of a bulky precipitate.

Reduction of the Oxyacid Salts of Silver.—The term Oxyacid includes those salts which contain the Oxide of Silver intimately combined with Oxygen-acids; as e. g. the Nitrate of Silver, the Sulphate, the Acetate of Silver, etc.

These salts, soluble in water, are reduced by developing agents in the same manner as Oxide of Silver, but more slowly. The presence of an acid united with the base is a hindrance to the process and tends to keep the oxide in solution, especially when that acid is powerful in its affinities. To illustrate the effect of the acid constituent of the salt in retarding reduction, take two test-tubes, the one containing Ammonio-Nitrate, and the other ordinary Nitrate of Silver—a single drop of solution of Sulphate of Iron added to each will indicate an evident difference in the rapidity of deposition.

The precipitate of metallic Silver obtained by the action of reducing agents upon the Nitrate, varies much in colour and in general appearance. If Gallic or Pyrogallic Acid be employed, it is a black powder;[8] whilst the salts of Iron, and especially the same with free Nitric Acid added, produce a sparkling precipitate, resembling what is termed frosted silver. Grape Sugar and many of the essential oils, such as the Oil of Cloves, etc., separate the metal from Ammonio-Nitrate of Silver in the form of a brilliant mirror film, and are often employed in silvering glass.

In remarking upon these peculiarities in the molecular condition of precipitated Silver, it should be observed that the appearance of a metal whilst in mass is no indication of its colour when in the state of fine powder. Platinum and Iron, both bright metals, and susceptible of a high polish, are dull and intensely black when in a fine state of division; Gold is of a purple or yellowish brown; Mercury a dirty grey.

Reduction of the Hydracid Salts of Silver.—By the term Hydracid is meant Salts of Silver which contain no Oxygen or Oxygen-acids, but simply elements like Chlorine or Iodine combined with Silver. These elements are characterized by forming acids with Hydrogen, which acids are hence called Hydracids. Hydrochloric Acid (HCl) is an example; so also is Hydriodic Acid (HI).

The reduction of the Hydracid Salts requires to be discussed separately, because it is evidently different from that already described; the reducing agent tending only to absorb Oxygen, which is not present in these salts. The explanation is as follows: When a Chloride of a noble metal is reduced by a developer, an atom of water, composed of Oxygen and Hydrogen, takes a part in the reaction. The Oxygen of the water passes to the developer, the Hydrogen to the Chlorine.

To illustrate this, take a solution of Chloride of Gold, and add to it a little Sulphate of Iron. A yellow deposit of metallic Gold soon forms, and the supernatant liquid is found, by testing, to be acid from free Hydrochloric Acid. The following simple diagram, in which however the number of the atoms concerned is omitted, may assist the comprehension of the change.

The symbol Au represents Gold, Cl Chlorine, H Hydrogen, and O Oxygen. Observe that the molecules H and O separate from each other and pass in opposite directions: the latter unites with the Sulphate of Iron; the former meets Cl, and produces Hydrochloric Acid (HCl), whilst the atom of Gold is left alone.

Hence there is no theoretical difficulty in supposing a reduction of Iodide of Silver by a developer, if we associate with the Iodide an atom of water to furnish the Oxygen. Unless the sensitive plate however has been exposed to the light, the reduction does not readily take place; nor can it be produced under any circumstances, with or without light, when the whole of the free Nitrate of Silver has been washed away from the plate. Pure Iodide of Silver is therefore unaffected by a developer, and the compound which blackens on the application of Sulphate of Iron or Pyrogallic acid is an Iodide with excess of Nitrate of Silver.

The mode in which a Salt of Silver, such as the Nitrate, soluble in water, may act in facilitating the reduction of Iodide of Silver, is shown in the preceding diagram, which corresponds closely with the last.

Notice that the compound atom of Nitrate of Silver contains a molecule of Oxygen for the developer, one of Silver (Ag) for the separated Iodine, and an atom of Nitric Acid (NO₅), which is liberated, and takes no further part in the change.

The chain of chemical affinities is more complete in this diagram than in the last, where an atom of water only was present, the affinity of Iodine for Silver being greater than that of Iodine for Hydrogen. Hence it is possible that an excess of Nitrate of Silver may, by furnishing an elementary basis for which Iodine has an attraction, assist in drawing off that element, so to speak, from the original particle of Iodide of Silver touched by light.[9]

SECTION III.

The formation and development of the Latent Image.

It was shown in the second Chapter that the continued action of white light upon certain of the Salts of Silver resulted in the separation of elements like Chlorine and Oxygen and the partial reduction of the compound. We have also seen that bodies possessing affinity for Oxygen, such as Sulphate of Iron and Pyrogallic Acid, tend to produce a similar effect; acting in some cases with great energy and precipitating metallic Silver in a pure state.

In forming an extemporaneous theory on the production of the latent image in the Camera, it would therefore be natural to suppose that the process consisted in setting up a reducing action upon the sensitive surface by means of light, afterwards to be continued by the application of the developing solution. This idea is to a certain extent correct, but it requires some explanation. The effects produced by the light and the developer are not so precisely similar that the one agency can always be substituted for the other: an insufficient exposure in the Camera cannot be remedied by prolonging the development of the image. In the Photographic processes on paper it is indeed found that a certain latitude may be allowed; but, as a rule, it should be stated that a definite time is occupied in the formation of the invisible image, which may not be shortened or extended beyond its proper limits with impunity. There is a maximum point beyond which no advance is made; hence if the plate be not then removed from the Camera, those portions of the image formed by the brightest lights are speedily overtaken by the "half tones," so that, on developing, an image appears without that contrast between lights and shadows which is essential to the artistic effect. On the other hand, in a case of insufficient exposure, the feeble rays of light not having been allowed time to impress the plate, the half shadows cannot be brought out on subsequent treatment with the developing agent.

A careful study of the phenomena involved in this part of the process cannot fail to show that the ray of Light determines a molecular change of some kind in the particles of Iodide of Silver forming the sensitive surface. This change is not of a nature to alter the composition or the chemical properties of the salt. The Iodine does not leave the surface, or there would be a difference in the appearance of the film, or in its solubility in Hyposulphite of Soda.

The following diagrams may perhaps be useful in mechanically illustrating what is meant by a molecular change.

Fig. 1 represents a compound molecule of Iodide of Silver, the component atoms of which are closely associated.

Fig. 2. The same after the action of a disturbing force. The simple molecules have not altogether separated, but they are prepared to do so, touching only at a single point.

Now the effect produced on this combination by a developer is understood, if we suppose that in the first case the affinity of the Iodine for Silver is too great to allow of its separation; but in the second, this affinity having been loosened, the structure gives way, and metallic Silver is the result.

This hypothesis has the merit of simplicity, and is not opposed to known facts; it may therefore for the present be received. The point however on which a doubt must rest is—whether the molecular disturbance produced by light upon Iodide of Silver leads to a reduction of that Salt by the developer. No image can be produced on the application of Pyrogallic Acid unless the particles of Iodide are in contact with Nitrate of Silver; and hence it may be the Nitrate and not the Iodide which is reduced—that is, the impressed molecule of Iodide may determine the decomposition of a contiguous particle of Nitrate, itself remaining unchanged. This view is supported to some extent by Moser's experiments, shortly to be quoted; and also by the fact that the delicate image first formed can be intensified by treating it with a mixture of the developing solution and Nitrate of Silver, even after the Iodide has been removed by a fixing agent. The following experiment will serve to illustrate this.—

Take a sensitive Collodion plate, and having impressed an invisible image upon it by a proper exposure in the Camera, remove it to the dark room, and pour over it the solution of Pyrogallic Acid. When the picture has fully appeared, stop the action by washing the plate with water, and remove the unaltered Iodide of Silver by Cyanide of Potassium. An examination of the image at this stage will show that it is perfect in the details, but pale and translucent. The plate is then to be taken back again to the dark room and treated with fresh Pyrogallic Acid, to which Nitrate of Silver has been added; immediately the picture becomes much blacker, and continues to darken, even to complete opacity, if the supply of Nitrate be kept up.

Now in this experiment it is evident that the additional deposit upon the image is produced from the Nitrate of Silver, the whole of the Iodide having been previously removed. Observe also, that it forms only upon the image, and not upon the transparent parts of the plate. Even if the Iodide, untouched by light, be allowed to remain, the same rule holds good;—the Pyrogallic Acid and Nitrate of Silver react upon each other and produce a metallic deposit; this deposit however has no affinity for the unaltered Iodide upon the part of the plate corresponding to the shadows of the picture, but attaches itself in preference to the Iodide already blackened by light.

This second stage of the development, by which a feeble image may be strengthened and rendered more opaque, is sometimes termed "development by precipitation," and should be correctly understood by the practical operator.

Researches of M. Moser.—The papers of M. Ludwig Moser 'On the Formation and Development of Invisible Images,' published in 1842, explain so clearly many remarkable phenomena of occasional occurrence in the Collodion and paper processes, that no apology need be offered for referring to them somewhat at length.

His first proposition may be stated thus:—"If a polished surface has been touched in particular parts by anybody, it acquires the property of precipitating certain vapours on these spots differently to what it does on the other untouched parts." To illustrate this, take a thin plate of metal, having characters excised; warm it gently, and lay it upon the surface of a clean mirror glass for a few minutes: then remove, allow to cool, and breathe upon the glass, when the outlines of the device will be distinctly seen. A plate of polished Silver may be substituted for the glass, and in place of developing the image by the breath, it may be brought out by Mercurial vapour.

The second proposition of M. Moser is as follows:—"Light acts on bodies, and its influence may be tested by vapours that adhere to the substance."—A plate of mirror glass is exposed in the Camera to a bright and intense light; it is then removed and breathed upon, when an image before invisible will be developed, the breath settling most strongly upon the parts where the light has acted. A plate of polished Silver may be used as before instead of glass, the vapour of Mercury or of water being employed to develope the image. An iodized Silver plate is still more sensitive to the influence of the light, and receives a very sharp and perfect impression under the action of the Mercury.

It seems therefore from these experiments and others not quoted, that the surfaces of various bodies are capable of being modified by contact with each other, or by contact with a ray of light, in such a way as to impart an affinity for a vapour; and further, that many of the Salts of Silver are in the list of substances admitting of such modification. But it is also evident that the same condition of surface which causes a vapour to settle in a peculiar manner also affects the behaviour of the Silver Salt when treated with a reducing agent. Thus, if a clean glass plate be touched in certain spots by the warm finger, the impression soon disappears, but is again seen on breathing upon the glass; and if this same plate be coated with a very delicate layer of Iodized Collodion and passed through the Nitrate bath, the solution of Pyrogallic acid will commonly produce a well-defined outline of the figure even before the plate has been exposed to the light. This experiment, although it does not invariably succeed, is nevertheless an instructive one, and shows the necessity of cleaning the plates used in Photography with care. If there be any irregularity in the manner in which the breath settles upon the glass when it is breathed on, a condition of surface exists at that point which will probably so modify the layer of Iodide of Silver, that the action of the developing fluid will be in some way interfered with.

One more remarkable fact observed by M. Moser may be quoted. He finds that the action of light upon the Daguerreotype plate is of an alternating kind: it first gives an affinity for Mercury, and then removes it. "If light acts on Iodide of Silver," he says, "it imparts to it the power of condensing mercurial vapours; but if it acts beyond a certain time, it then diminishes this power and at length takes it away altogether." This is precisely in accordance with phenomena observed also in the Collodion process, where the deposit of metallic Silver is sometime less marked than usual if the plate has been exposed in the Camera beyond the proper period of time.

A curious perversion of the developing process is occasionally met with, in which on the application of the Pyrogallic Acid, the deposit of Silver takes place upon the shadows of the picture, and not upon the lights; hence on viewing the image by transmitted light, the usual appearance is reversed. This may perhaps be explained by an alternating action of the light as above suggested.

A phenomenon at first sight even more remarkable has occurred, in which, on developing the plate, two images start out instead of one. The secondary image in such a case is probably the remains of a previous impression which, although apparently removed by washing, had nevertheless modified the surface of the glass so as to affect the layer of Iodide of Silver; and if the glass were breathed upon before again coating it with Collodion, there is every reason to suppose that the outlines of the accidental image would be seen.[10]

[10] Since writing the above, the Author has perused with pleasure a paper by Mr. Grove on the production of latent images by electricity, with a mode of fixing them. In the experiments described, a plate of glass, electrized in certain portions only, was breathed upon, or exposed to the fumes of Hydrofluoric Acid. In either case the vapour settled exclusively upon the non-electrical part of the glass, thus developing a latent image. When the plate was first submitted to electrization, and then coated with Iodide of Silver upon Collodion, and exposed to light,—solution of Pyrogallic Acid produced a reduction of Silver only upon the parts of the glass corresponding to those on which the breath settled in the previous experiment; thus indicating that the electricity neutralized the effect of light upon the sensitive Iodide of Silver.