  ON THE THEORY OF POSITIVE PRINTING. The subject of Collodion Negatives having been explained in the previous Chapter, we proceed to show how they may be made to yield an indefinite number of copies with the lights and shadows correct as in nature. Such copies are termed "Positives," or sometimes "Positive prints," to distinguish them from direct Positives upon Collodion. There are two distinct modes of obtaining photographic prints;—first by development, or, as it is termed, by the Negative process, in which a layer of Iodide or Chloride of Silver is employed, and the invisible image developed by Gallic Acid; and second, by the direct action of light upon a surface of Chloride of Silver, no developer being used. These processes, involving chemical changes of great delicacy, require a careful explanation. The action of light upon Chloride of Silver was described in Chapter II. It was shown that a gradual process of darkening took place, the compound being reduced to the condition of a coloured subsalt; also, that the rapidity and perfection of the change were increased by the presence of excess of Nitrate of Silver, and of organic matters, such as Gelatine, Albumen, etc. We have now to suppose that a sensitive paper has been prepared in this way, and that a Negative having Other chemical operations are therefore required to remove the objectionable red colour of the print, and hence the consideration of the subject is naturally divided into two parts; first, the means by which the paper is rendered sensitive, and the image impressed upon it;—and secondly, the subsequent fixing and toning, as it may be termed, of the proof. The present Chapter will also include, in two additional Sections, a condensed account of the most important facts relating to the properties and the mode of preservation of photographic prints. SECTION I. The Preparation of the Sensitive Paper. In this Section the general theory of the preparation of Positive paper, in so far as it affects the tone and intensity of the print, will be described; the reader being referred to the second division of the Work for the formulÆ required. The Preparation of the Sensitive Paper.—The conditions which are required for producing a sharp and well defined print are—that an even layer of Chloride of Silver should exist upon the very surface of the paper, and that the particles of this Chloride should be in contact with a sufficient excess of Nitrate of Silver. These points have been already referred to at an early part of the Work (p. 19). The material used for sizing the paper is of importance. English papers are usually sized with Gelatine, which is a photographic agent, and acts chemically in forming the image. Foreign papers on the other hand being sized with starch only, require an addition of Gelatine, Caseine, or Albumen, to retain the Salt at the surface of the paper, and to assist in producing the picture: if otherwise, the print will be flat and "mealy," as it is termed. Albumen especially produces a beautifully smooth surface, and is advantageously employed in printing small portraits and stereoscopic subjects. The uniform surface distribution of the Chloride of Silver is sometimes interfered with by a faulty structure of the paper, causing it to absorb liquids unevenly, and in consequence the pictures, when removed from the printing frame, appear spotted. Another cause producing the same effect, is the employment of too weak a solution of Nitrate of Silver, or the removal of the sheet from the Nitrate bath before the Chloride of Ammonium has been perfectly decomposed; it is thus rendered unequally sensitive at different portions of the surface, and the prints have the characteristic marbled appearance above referred to. A sufficient excess of Nitrate of Silver being essential, it is important to bear in mind, that the quantity of this salt eventually remaining in the paper, is much influenced by the manner in which the solution is applied. If it be laid on by floating, then the proportion of Nitrate to that of Chloride of Sodium should be about as 3 to 1 (the atomic weights are nearly as 5 to 2); but if the plan of brushing The Darkening of the Sensitive Paper by Light.—The operator should be familiar with the changes of colour which indicate the progress of the reduction of the sensitive layer. Much in this respect depends upon the kind of organic matter used, but there is always a regular sequence of tints; in the case of a paper prepared simply with Chloride of Ammonium and Nitrate of Silver, it is as follows: pale violet, violet-blue, slate-blue, bronze or copper-colour. When the bronzed stage is reached, there is no further change. On immersion in the fixing bath of Hyposulphite, the violet tones due to Subchloride of Silver are destroyed, and the print assumes a red or brown colour, which is deepest and most intense in the parts where the light has acted longest. Hence we see, that, to produce a good print, it is essential that the Negative should possess considerable intensity in the dark parts. Pale and feeble Negatives yield proofs which are wanting in vigour, and have a flat and indistinct appearance. The combination cannot be exposed to light for a sufficient length of time to bring about the requisite degree of reduction of the Chloride of Silver; and hence the deepest shadows of the resulting Positive are not sufficiently dark, and there is a want of contrast which is fatal to the effect. A good Negative should be so opaque as to preserve the lights of the printed image beneath clear, until the darkest shades are about to pass into the bronze or coppery condition. If the amount of intensity be less than this, the finest effect cannot be obtained. CONDITIONS AFFECTING THE SENSITIVENESS OF THE PAPER AND THE INTENSITY OF THE IMAGE. Some of the principal of these are as follows:— a. The Strength of the Salting Bath.—The sensibility Highly sensitized papers darken rapidly, and pass very completely into the bronze stage. Those containing less Chloride darken more slowly, and do not become bronzed with the same intensity of light. A Photographic print, formed upon paper highly salted and sensitized, is usually vigorous, with great contrast of light and shade; particularly so when the printing is conducted in a strong light. Hence it will be an advantage, with a feeble Negative, and in dull weather, to double the ordinary quantity of Salt, whereas in the case of an intense Negative, and with direct sunlight, the deep shadows will be too much bronzed unless the quantity of Chloride and Nitrate of Silver in the paper be kept low. In proportion as Photographic papers are highly salted and sensitized, they become more prone to change colour spontaneously in the dark. b. Proportion of Nitrate of Silver.—The compound on which a positive print is formed is a Chloride, or an organic Salt of Silver, with an excess of Nitrate of Silver. Nothing is gained by increasing the proportion of Chloride of Sodium, unless at the same time an addition be made to the quantity of free Nitrate in the sensitizing Bath. A surface of Chloride of Silver with a bare excess of Nitrate, darkens on exposure, but it does not reach the bronzed stage; the action appearing to stop at a certain point. On placing the print in Hyposulphite of Soda, it becomes very red and pale, and when tinted, looks cold and slaty, without depth or intensity. c. The sensitiveness and intensity affected by substituting the Oxide of Silver for the Nitrate.—Many operators employ a solution of Oxide of Silver in Ammonia Ammonio-Nitrate paper has the disadvantage of soon discolouring when kept; but it is very serviceable in printing during the winter months. The proportion of Chloride in the salting Bath may, if desired, be considerably reduced; the intensity of action being greatly exalted by the use of the Oxide of Silver. d. Employment of organic matters.—Those recommended in this work are—Albumen, Gelatine, and Iceland Moss. Albumen adds much to the sensibility of the paper, and gives very fine surface definition. A less amount of Chloride is required than in the case of plain paper simply salted, the glutinous character of Albuminous liquids causing more of the fluid to be retained upon the surface of the paper, and the animal matter assisting the reduction. By varying the proportion of salt, both feeble and intense Negatives may be printed successfully upon albuminized paper. No process gives better results, either as regards sensitiveness, or in faithfully rendering all the finer details of the Negative, than the process with Albumen. Iceland Moss, when boiled in water, yields a mucilaginous liquid which is conveniently employed as a vehicle for Chloride of Silver; it increases the sensitiveness of the Gelatine is used in positive printing; it is analogous to Albumen in composition, and, like it, forms a red compound with Suboxide of Silver. It is serviceable in keeping the print at the surface of the paper, but does not alter the sensibility or the general appearance of the finished picture so greatly as Albumen. e. Impurities in Nitrate of Silver.—Nitrate of Silver used for Photographic printing should be free from even a trace of Protonitrate of Mercury, since it is known that the precipitation of Chloride of Mercury prevents the darkening of Chloride of Silver by light. The peculiar condition of Nitrate of Silver spoken of at page 101, in which it is thought to contain Oxides of Nitrogen, is likely to interfere with Photographic printing. This is probably the explanation of a faulty state of the Nitrate solution, in which it yields red and feeble positives, and does not darken in colour in exciting albuminized paper. The remedy will be, to fuse the Nitrate of Silver at a moderate heat before dissolving it. THE COLOUR OF THE IMAGE INFLUENCED BY THE PREPARATION OF THE SENSITIVE PAPER. This subject should be studied by those who desire to print with taste. By introducing a few simple modifications into the mode of preparing the sensitive paper, almost any variety of tint may be obtained. The tendency of the "toning" process, to which the print is afterwards to be submitted, is to darken the colour, and, if gold be used, to give a shade of blue. Hence, if the Positive be printed of a red tone, it will change in the gold Bath to a purple; whereas if left, after exposure to light and fixing, of a dark brown or sepia tint, it passes by toning into a pure black. The Positive should look warm and bright on its removal from the printing frame; but the tint which remains after immersion in Hyposulphite of Soda is the proper colour of the simply fixed print. The following points may be mentioned as affecting the colour and general appearance of the picture. a. The proportions of Salt and Nitrate of Silver.—Highly salted and sensitized papers give a darker image than those which, containing a small proportion of Chloride of Silver, are less sensitive to light. Hence in printing upon paper weakly sensitized, in order to bring out the finer details of a highly intense negative, we find the image unusually red after fixing, and of a brown or mulberry colour when toned. The above remarks apply also in some degree to the strength of the Nitrate Bath, and especially so when no organic matter excepting Gelatine is employed,—in such a case the image will be darker after fixing, if the proportion of free Nitrate of Silver be large. b. Effect of Oxide of Silver on the colour.—Prints formed upon Ammonio-Nitrate papers highly salted are of a sepia colour after fixing, and usually of a pure black or a purple-black when toned. With the increased facility of reduction by light afforded by use of Oxide of Silver, there is also less redness in the print. But if the quantity of salt used in preparing the paper be reduced to a minimum (one grain to the ounce or less), for the sake of economy or to improve the half-tone, then the usual red colour returns, and the Positive is brown or purple after toning, in place of black. Thus by employing a solution of Oxide of Silver, the operator is enabled, without the addition of organic matter, to print Positives of a pleasing variety of tint, combined with a peculiar softness and delicacy, which cannot easily be obtained with the simple Nitrate of Silver. c. The colour affected by organic matter.—Albumen is coagulated by Nitrate of Silver, and forms a permanent Iceland Moss affects the colour of the proof to a certain extent, but less than Albumen; the finished prints are nearly black if the paper is highly salted. The Gelatinous sizing used for the English papers, and obtained by boiling hides in water, and hardening the product by an admixture of Alum, has a reddening influence upon reduced Silver salts, analogous to that of Albumen, or of Caseine, the characteristic animal principle of milk. Positives printed upon English paper, commonly assume some shade of brown more or less removed from black; the darker tones being more readily obtained upon the foreign papers. Citrates and Tartrates have a marked effect upon the colour of prints. Paper prepared with Citrate, in addition to Chloride of Silver, darkens to a fine purple colour which changes to brick-red in the fixing Bath. The Positives, when toned, are usually of a violet-purple or of a bistre tint, with a general aspect of warmth and transparency. SECTION II. The Processes for Fixing and Toning the Proof. This part of the operation is one to which great attention should be paid, in order to secure bright and lasting colours: it involves more of delicate chemical change than perhaps any other department of the Art. The first point requiring explanation is the process of fixing; to which (p. 41) brief reference has already been made. The methods adopted to improve the tint of the finished picture will then be described. CONDITIONS OF A PROPER FIXING OF THE PROOF. This subject is not always understood by operators, and consequently they have no certain guide as to how long the prints should remain in the fixing Bath. The time occupied in fixing will of course vary with the strength of the solution employed; but there are simple rules which may be usefully followed. In the act of dissolving the unaltered Chloride of Silver in the proof, the fixing solution of Hyposulphite of Soda converts it into Hyposulphite of Silver (p. 43), which is soluble in an excess of Hyposulphite of Soda. But if there be an insufficient excess,—that is, if the Bath be too weak, or the print removed from it too speedily,—then the Hyposulphite of Silver is not perfectly dissolved, and begins by degrees to decompose, producing a brown deposit in the tissue of the paper. This deposit, which has the appearance of yellow spots and patches, is not usually seen upon the surface of the print, but becomes very evident when it is held up to the light, or if it be split in half, which can be readily done by gluing it between two flat surfaces of deal, and then forcing them asunder. The reaction of Hyposulphite of Soda with Nitrate of Silver.—In order to understand more fully how decomposition of Hyposulphite of Silver may affect the process of fixing, the peculiar properties of this salt should be studied. With this view Nitrate of Silver and Hyposulphite of Soda may be mixed in equivalent proportions, viz. about twenty-one grains of the former salt to sixteen grains of the latter, first dissolving each in separate vessels in half an ounce of distilled water. These solutions are to be added to each other and well agitated; immediately a dense deposit forms, which is Hyposulphite of Silver. At this point a curious series of changes commences. The precipitate, at first white and curdy, soon alters in colour: it becomes canary-yellow, then of a rich orange-yellow, afterwards liver-colour, and finally black. The AgO S2O2. But AgO S2O2 plainly equals AgS, or Sulphuret of Silver, and SO3, or Sulphuric Acid. The acid reaction assumed by the supernatant liquid is due therefore to Sulphuric Acid, and the black substance formed is Sulphuret of Silver. The yellow and orange-yellow compounds are earlier stages of the decomposition, but their exact nature is uncertain. The instability of Hyposulphite of Silver is principally seen when it is in an isolated state: the presence of an excess of Hyposulphite of Soda renders it more permanent, by forming a double salt, as already described. In fixing Photographic prints, this brown deposit of Sulphuret of Silver is very liable to form in the Bath and upon the picture; particularly so when the temperature is high. To obviate it, observe the following directions:—It is especially in the reaction between Nitrate of Silver and Hyposulphite of Soda that the blackening is seen; the Chloride and other insoluble Salts of Silver being dissolved, even to saturation, without any decomposition of the Hyposulphite formed. Hence if the print be washed in water to remove the soluble Nitrate, a very much weaker fixing Bath than usual may be employed. But if the proofs are taken at once from the printing frame and immersed in a dilute Bath of Hyposulphite (one part of the salt to six or eight of water), a shade of brown may often be observed to pass over the surface of the print, and a large deposit of Sulphuret of Silver soon forms as the result of the decomposition. On the other hand, with a strong Hyposulphite Bath there is little or no discoloration, and the black deposit is absent. The print must also be left for a sufficient time in the fixing bath, or some appearance of brown patches, In fixing prints by Ammonia the Author has found that the same rule may be applied as in the case of Hyposulphite of Soda, viz. that if the process be not properly performed, the white parts of the print will appear spotted when held up to the light, from a portion of insoluble Silver Salt remaining in the paper. Prints imperfectly fixed by Ammonia are also usually brown and discoloured upon the surface of the paper. More exact directions as to the strength of the fixing bath and the time occupied in the process, will be given in the Second Part of the Work; at present it may be noticed only that Albuminized paper, from the horny nature of its surface-coating, requires a longer treatment with the Hyposulphite than the plain paper. THE SALTS OF GOLD AS TONING AGENTS FOR PHOTOGRAPHIC PRINTS. The Salts of Gold have been successfully applied to the improvement of the tones obtained by simply fixing the proof in Hyposulphite of Soda. The following are the principal modes followed:— M. Le Grey's Process.—The print, having been exposed to light until it becomes very much darker than it is The rationale of the process appears to be as follows:— the Chlorine, previously combined with Gold, passes to the reduced Silver Salt; it bleaches the lightest shades, by converting them again into white Protochloride of Silver, and gives to the others a violet tint more or less intense according to the reduction. At the same time metallic Gold is deposited, the effect of which is not visible at this stage, since the same violet tint is perceived when a solution of Chlorine is substituted for Chloride of Gold. The Hyposulphite of Soda subsequently employed, decomposes the violet Subchloride of Silver, and leaves the surface of a black tint, due to the Gold and the reduced Silver Salt. M. Le Grey's process is objectionable on account of the excessive over-printing required. This however is to a great extent obviated by a modification of the process in which an alkaline instead of an acid solution of the Chloride is employed; one grain of Chloride of Gold is dissolved in about six ounces of water, to which are added twenty to thirty grains of the common Carbonate of Soda. The alkali moderates the violence of the action, so that the print washed with water and immersed in the Gold Bath, is less reduced in intensity, and does not acquire the same inky blueness. On subsequent fixing in the Hyposulphite, the tint changes from violet to a dark chocolate-brown, which is permanent. The Tetrathionate and Hyposulphite of Gold employed in toning.—After the discovery of Le Grey's mode, it was proposed, as an improvement, to add Chloride of Gold to the fixing solution, so as to obviate the necessity of using two Baths. The print, in that case, although darkened considerably, is less reduced in intensity, and the same amount of over-printing is not required. The chemical changes which ensue are different from before: they may be described as follows:— Chloride of Gold, added to Hyposulphite of Soda, is converted into Hyposulphite of Gold, Tetrathionate of Gold, and (if the Chloride of Gold be free from excess of acid) a red compound, containing more of the metal than, either of the others, but the exact nature of which is uncertain. Each of these three Gold Salts possesses the property of darkening the print, but not to the same extent. The activity is less as the stability of the salt is greater, and hence the red compound, which is so highly unstable that it cannot be preserved many hours without decomposing and precipitating metallic Gold, is far more active than the Hyposulphite of Gold, which, when associated with an excess of Hyposulphite of Soda, is comparatively permanent. When rapidity of colouring is an object it will therefore be advisable to add Chloride of Gold to the fixing Bath of Hyposulphite rather than an equivalent quantity of Sel d'or; and by dropping a little Ammonia into the Chloride of Gold so as to precipitate "fulminating gold" The Author explains the action of these Salts of Gold upon the Positive print as follows:—they are unstable, and contain an excess of Sulphur loosely combined; hence, when placed in contact with the image, which has an affinity for Sulphur, the existing compound is broken up, Sel Or employed as a toning agent.—This process, which was communicated to the 'Photographic Journal' by Mr. Sutton of Jersey, has been found serviceable. The prints are first washed in water, to which is added a little Chloride of Sodium, to decompose the free Nitrate of Silver. They are then immersed in a dilute solution of "Sel d'or," or double Hyposulphite of Gold and Soda, which quickly changes the tint from red to purple without destroying any of the details or lighter shades. Lastly, the Hyposulphite of Soda is employed to fix the print in the usual way. This process differs theoretically from the last in some important particulars. The toning solution is applied to the print before fixing, which experience proves to have an important influence upon the result, it having been found that when the print is previously acted upon by Hyposulphite of Soda, the rapidity of deposition of the Gold is interfered with;—thus, a dilute solution of Sel d'or colours a print rapidly, but if to this same liquid a few crystals of Hyposulphite of Soda be added, the picture becomes red and may be kept in the Bath for comparatively a long time without acquiring the purple tones. As Hyposulphite of Soda in excess lessens the action of the Sel d'or, so on the other hand the addition of an acid increases it. The acid does not precipitate Sulphur, as might be expected from a knowledge of the reaction of Hyposulphite with acid bodies (p. 137), but it favours the reduction of metallic Gold. Hence it is usual to add a little Hydrochloric Acid to the toning solution of Sel d'or, to increase the rapidity and perfection of the colouring process. THE CONDITIONS WHICH AFFECT THE ACTION OF THE FIXING AND TONING BATH OF GOLD AND HYPOSULPHITE OF SODA. Although the process of toning Positives by Sel d'or is very certain in its results and gives good tints, yet, as involving a somewhat greater expenditure of time and trouble, it is not at present universally adopted. The ordinary plan of fixing and toning in one bath has been proved to yield permanent prints if the proper precautions are observed, but it is quite necessary, in order to ensure success, that the conditions by which its action is modified should be understood. The more important of these are as follows:— a. The AGE of the Bath.—When Chloride of Gold is added to Hyposulphite of Soda, several unstable salts are produced, which decompose by keeping. Hence the solution is very active during the first few days after mixing; but at the expiration of some weeks or months, if not used, it becomes almost inert, a reddish deposit of Gold first forming, and eventually a mixture of black Sulphuret of Silver and Sulphur, the former of which often adheres to the sides of the bottle in dense shining laminÆ. When the Bath is constantly kept in use there is a loss of Gold, which, although it is less perceived than it otherwise would be, from the fact that sulphuretting principles are formed (see next page) capable of replacing the Gold as toning agents—yet makes the Bath work more slowly, and hence over-printing is required. b. Presence of free Nitrate of Silver upon the surface of the proof.—This produces an accelerating effect, as may be shown by soaking the print in salt and water, to convert the Nitrate into Chloride of Silver; the action then takes place more slowly. The free Nitrate of Silver increases the instability of the Gold salts; but if present in too great an excess, it is apt to cause a decomposition of Hyposulphite of Silver, c. Temperature of the solution.—In cold weather, the thermometer standing at 32° to 40°, the Bath works more slowly than usual; whereas in the height of summer, and especially in hot climates, it occasionally becomes quite unmanageable. The best temperature for operating successfully appears to be about 60° to 65° Fahrenheit; if higher than this the solutions must be employed more dilute. d. Addition of Iodide of Silver.—Some operators associate Iodide with Chloride in the preparation of sensitive paper for printing. Another source of the same salts is the admixture of a portion of the fixing Bath used for Negatives with the Positive toning solution. The presence of Iodides in the fixing and toning Bath is injurious: when in large excess, they dissolve the image, or produce yellow patches of Iodide of Silver on the lights; in smaller quantity, the deposition of the Gold is hindered, and the action proceeds more slowly. Bromides and Chlorides have not the same effect. e. Mode of preparing the paper.—The rapidity of toning varies with causes independent of the Bath: thus, plain paper prints are toned more quickly than prints upon albuminized paper, and the use of English paper sized with Gelatine retards the action. Foreign papers rendered sensitive with Ammonio-Nitrate tone the most quickly. On certain states of the fixing and toning Bath which are injurious to the proofs.—The object of using the Hyposulphite Bath is to fix the proof and to tone it by means of Gold. But it is a fact familiar to the photographic chemist, that Positives can also be toned by a sulphuretting action, and that the colours so obtained are not very different from those which follow the employment of Gold. Some of the conditions which facilitate a sulphuretting action upon the proof are as follows:— a. The addition of an Acid to the Bath.—It was at one time common to add a few drops of Acetic Acid to the fixing Bath of Hyposulphite of Soda, immediately before immersing the proofs. The Bath then assumes an opalescent appearance in the course of a few minutes, and, when this milkiness is perceptible, the print begins to tone rapidly and becomes nearly black. The chemical changes produced in a Hyposulphite Bath by addition of acid, may be explained thus:—The acid first displaces the feeble Hyposulphurous acid from its combination with Soda.
Then the Hyposulphurous Acid, not being a stable substance when isolated, begins spontaneously to decompose, Observe therefore that free acids of all kinds must be excluded from the fixing Bath, or, if inadvertently added, the liquid must be set aside for some hours until the Hyposulphurous Acid has decomposed, and, the Sulphur having settled to the bottom, the Bath has regained its original neutral condition. b. Decomposition of the Bath by constant use.—It has long been known that a solution of Hyposulphite of Soda undergoes a peculiar change in properties when much used in fixing. When first prepared it leaves the image of a red tone, the characteristic colour of the reduced Silver Salt, but soon acquires the property of darkening this red colour by a subsequent communication of Sulphur. Hence a simple fixing Bath becomes at last an active toning bath, without any addition of Gold. This change of properties will be found more fully explained in the abstract of the Author's researches given in the next Section (p. 156). At present we remark only that it is due principally to a reaction between Nitrate of Silver and Hyposulphite of Soda, attended with decomposition of Hyposulphite of Silver (p. 130); and hence, if the prints are washed in water before immersion in the Bath, the solution will be less quickly liable to change. Many operators state that the toning Bath having at first been prepared with Chloride of Gold, no further addition of this substance will be required. This no doubt is correct, but in such case the proofs will at last be toned c. Tetrathionate in the Hyposulphite Bath.—The Author has shown that the Tetrathionates, which are analogous to the Hyposulphites, have an active sulphuretting action upon Positive prints (see the papers in the next Section). Very fine colours can be obtained in this way; but toning by Sulphur having been proved to be wrong in principle, the formulÆ given in the first two editions of this Work have been omitted. The bodies which produce Tetrathionate when added to a solution of Hyposulphite of Soda, and hence are inadmissible in the toning process, are as follows:—Free Iodine, Perchloride of Iron, Chloride of Copper, Acids of all kinds (in the latter case the acid first produces Sulphurous Acid, and the Sulphurous Acid, if present in any quantity, by reacting upon Hyposulphite of Soda, forms Tetrathionate and Trithionate of Soda). Chloride of Gold also produces a mixed Tetrathionate of Gold and Soda when added to the fixing Bath (p. 133); but as the quantity of Chloride used is small, the prints are far less sulphuretted than in the case of toning Baths prepared by Tetrathionate without Gold. SECTION III. The Author's Researches in Photographic Printing. Having been long engaged in conducting experiments upon the composition and properties of the reduced material forming the Photographic image, and especially with a view of determining the exact conditions under which the picture may be considered permanent, the Author has thought it advisable to give the results of these researches in the form of an abstract of the original papers read at the meetings of the Photographic Society. A previous perusal of these papers will put the reader in possession of the principal facts upon which are founded the precautions advised in the next Section for the preservation of Photographic prints. In order to keep the Work as nearly as possible within its original limits, and also for the purpose of distinguishing the present Section from the others, as one referring principally to scientific details, the type has been reduced to the size of that used in the Appendix. ON THE CHEMICAL COMPOSITION OF THE PHOTOGRAPHIC IMAGE. The determination of the chemical nature of the Photographic image in its various forms is a point of much importance, both as indicating the conditions required for the preservation of works of art of that class, and also as a guide to the experimenter in selecting bodies likely to have an effect as chemical agents in Photography. It has been stated by some who have given attention to the subject, that the image is formed in all cases of pure metallic Silver, and that any observable variations in its colour and properties, are due to a difference in the molecular arrangement of the particles. But this hypothesis, although involving much that is correct, yet does not contain the whole truth, for it is evident that the chemical properties of the Photographic image often In investigating the subject, the principal point appeared to be to examine the action of light upon Chloride of Silver, and afterwards to associate the Chloride with organic matter in order to imitate the conditions under which Photographs are obtained. The following is an epitome of the conclusions arrived at:— Action of Light upon Chloride of Silver.—The process is accompanied by a separation of Chlorine, but its product is not a mere mixture of Chloride of Silver and Metallic Silver; if it were so, we cannot suppose that the darkening would take place beneath the surface of Nitric Acid, which it is found to do. A definite Subchloride of Silver seems to be formed, the most important property of which is its decomposition by fixing agents, such as Ammonia, and Hyposulphite of Soda, both of which destroy the violet colour, dissolving out Protochloride of Silver, and leaving a small quantity of a grey residue of metallic Silver. Inasmuch therefore as all Photographic pictures require fixing, we may conclude that if they could be produced upon pure and isolated Chloride of Silver (which however is not the case), they would consist solely of metallic Silver. Decomposition of organic Salts of Silver by Light.—Compounds of Oxide of Silver with organic bodies, are as a rule darkened by exposure to light, but the process does not always consist in a simple reduction to the metallic state. This assertion is proved by the employment of the following tests. a. Mercury.—Little or no amalgamation takes place on triturating the darkened salt with this metal. b. Ammonia and fixing agents.—These usually produce only a limited amount of action. Thus, the Albuminate of Protoxide of Silver is perfectly soluble in Ammonia; but after having been reddened by exposure to light, it is little or not at all affected. c. Potash.—Animal matters coagulated by Nitrate of Silver, and reduced by the sun's rays, are dissolved by boiling Potash, the solution being clear and of a blood-red colour. Metallic Silver, it is presumed, if present, would remain insoluble. d. Boiling Water.—Gelatine treated with Nitrate of Silver The above facts justify us in supposing the existence of combinations of organic matter with a low Oxide of Silver; and analysis indicates further that the relative proportion of each constituent in these compounds may vary. For instance, when Citrate of Silver is reduced by light, and acted on with Ammonia, a black powder remains, which was found to contain as much as 95 per cent, real Silver; but Albuminate of Silver treated in the same way yields on analysis less of metallic Silver, and more volatile and carbonaceous matter. The use of Ammonio-Nitrate of Silver in preparing the salt tends also to increase the relative quantity of metal left in the compound after reduction and fixing. The length of time during which the light has acted, has also a modifying effect of the same kind,—the product of reduction by a powerful light being more nearly in the state of metal, and containing less both of Oxygen and organic matter. Action of Light upon Chloride of Silver associated with organic matter.—Photographs formed on Chloride of Silver alone, would, after fixing, consist of metallic Silver, but such a process could not be carried out in practice. The addition of organic matter is absolutely necessary in order to increase the sensitiveness, and to prevent the image from being dissolved in the Bath of Hyposulphite of Soda. The blue Subchloride of Silver is decomposed by fixing, a very scanty proportion of grey metallic Silver remaining insoluble; but the red compound of Suboxide of Silver with organic matter is almost unaffected by Hyposulphite of Soda, or Ammonia. The increase of sensitiveness and intensity produced by the use of organic matter is accompanied also by a change in the composition of the picture; the image losing the metallic character which it possesses when formed on pure Chloride of Silver, and resembling in every respect the product of the action of light upon organic Salts of Silver. There are certain characteristic tests which may usefully be employed in distinguishing the metallic image from what may be termed the organic or non-metallic image. One of these tests is Cyanide of Potassium. An image formed upon pure Chloride A second test is the Hydrosulphate of Ammonia. If no organic matter be employed, the image becomes darker and more intense by treatment with a soluble Sulphuret; whilst the non-metallic image, formed on an organic surface, is quickly bleached and faded. The action of Sulphur upon the image is indeed a mode of determining the real quantity of Silver present. When existing in a very finely divided layer, Sulphuret of Silver often appears yellow; but in a thicker layer it is black. Hence the colour of the Photograph, after treatment with Sulphuretted Hydrogen, is an indication of the proportion of metal present, and the reason of the organic image becoming so perfectly faded is because it contains a minimum of Silver in relation to the intensity. We see, therefore, that the addition of organic matter to Chloride of Silver does not so much increase the actual quantity of Silver reduced by light, as it adds to its opacity by associating other elements with the Silver, and altogether modifying the composition of the image. The employment of oxidizing agents shows also that in an ordinary Photographic process by the direct action of light, other elements besides Silver assist in forming the image: the pictures being found to be easily susceptible of oxidation, whereas the metallic image formed on pure Chloride of Silver resists oxidation. Composition of developed images.—By exposing sensitive layers of the Iodide, the Bromide, and the Chloride of Silver to the light for a short time only, and subsequently developing with Gallic Acid, Pyrogallic Acid, and the protosalts of Iron, a variety of images may be obtained, which differ from each other materially in every important particular, and a comparison of which assists the determination of the disputed point. The appearance and properties of the developed Photograph are found to vary with the existence of the following conditions. 1st. The surface used to sustain the sensitive layer.—There is a peculiarity in the image formed on Collodion. Collodion contains Pyroxyline, a substance which behaves towards the salts of Silver in a manner different from that of most organic bodies, 2nd. The nature of the sensitive salt.—When Iodide of Silver is used to receive the latent impression, the image after development, although lacking intensity of colour by reflected light, is more nearly in the condition of metallic Silver than if Bromide or Chloride of Silver be substituted; and of the three salts, the Chloride gives the most intensity, with the least quantity of metallic Silver. This rule applies especially when organic matters, Gelatine, Glycyrrhizine, etc., are present. 3rd. The developing agent employed.—An organic developing agent like Pyrogallic Acid may be expected to produce a Collodion image more intense, but less metallic, than an inorganic developer, such as the Protosulphate of Iron. 4th. The length of time during which the light has acted.—Over-action of the light favours the production of an image which is dark by reflection and brown or red by transmission, corresponding in these particulars to what may be termed the non-metallic image containing an oxide of Silver. 5th. The stage of the development.—The red image first formed on the application of the developer to a gelatinized or albuminized surface of Iodide of Silver is less metallic, and more easily injured by destructive tests, than the black image, which is the result of prolonging the action. Developed photographs which are of a bright red colour after fixing, correspond in properties to images obtained by the direct action of light on paper prepared with Chloride of Silver, more nearly than to Collodion, or even to fully developed Talbotype Negatives. To conclude the Paper, the following may be offered in the way of recapitulation:—An image consisting of metallic silver, as a rule, reflects white light, and shows as a positive when laid on black velvet; but a non-metallic organic image is dark, and ON THE VARIOUS AGENCIES DESTRUCTIVE TO PHOTOGRAPHIC PRINTS. Action of Sulphuretting Compounds upon Positive Prints.—It was first noticed by Mr. T. A. Malone, that the most intense Photograph might be destroyed by acting upon it with solution of Sulphuretted Hydrogen or a soluble Sulphuret, for a sufficient length of time. The changes produced by a sulphuretting compound acting upon the red image of a simply fixed print are these:—the colour is first darkened, and a degree of brilliancy imparted to it; this is the effect termed "toning." Then the warm tint by degrees alters to a colder shade, the intensity of the whole image is lessened, and the half-tones turn yellow. Lastly, the full shadows pass also from black to yellow, and the print fades. Now in this peculiar reaction we notice the following points of interest. If at that particular stage at which the print has reached its maximum of blackness, it be raised partially out of the liquid and allowed to project into the air, the part so treated becomes yellow before that which remains immersed. Again, if a print toned by Sulphur be placed in a pan of water to wash, after the lapse of several hours it is apt to assume a faded appearance in the half-tones. The full shadows, in which the reduced These facts prove that Oxygen has an influence in accelerating the destructive action of the Sulphur compounds upon Positive prints; and this idea is borne out by the results of further experiments, for it is found that moist Sulphuretted Hydrogen has little or no effect in darkening the colour when every trace of air is excluded. When prints are washed in water they are exposed to the influence of the dissolved air which water always contains, and hence the change from black to yellow is produced. There are some substances which facilitate the yellow degeneration of Positives toned by Sulphur, a knowledge of which will be useful: they are—1st, powerful oxidizers, such as Chlorine, Permanganate of Potash, and Chromic Acid; these, even when highly diluted, act with great rapidity: 2nd, bodies which dissolve Oxide of Silver, as soluble Cyanides, Hyposulphites, Ammonia; also acids of various kinds, and hence the frequency of yellow finger impressions upon old sulphuretted prints, which are probably caused by a trace of organic (Lactic?) Acid left by contact with the warm hand. It was at one time supposed that the Photograph in the stage at which it appears blackened by Sulphur, consisted of Sulphuret of Silver, and that this black Sulphuret became yellow by absorption of Oxygen and conversion into Sulphate. MM. Davanne and Girard, who examined the subject, thought that there might be two isomeric forms of Sulphuret of Silver, a black and a yellow form; the former of which passing gradually into the latter produced the fading of the impression. But neither of these views are correct; for it is proved by careful experiment, that the Sulphuret of Silver is a highly stable compound, not prone to oxidize, and, further, that the change of colour from black to yellow has no reference to a modification of this salt. The truth appears to be that the image whilst in the black stage contains other elements besides Sulphur and Silver, but when it has become yellow by the continued action of the sulphuretting compound, it is then a true Sulphuret. Comparative permanence of Photographs under the action of Sulphur.—Developed Positives, as a rule, stand better than those printed by direct exposure to light; but much depends upon the nature of the negative process followed; and hence no general statement can be made which will not be liable to many exceptions. The mode of conducting the development must not be overlooked. The prints, which become very red in the Hyposulphite fixing Bath from the action of the developer having been stopped at too early a period, are often sulphuretted and destroyed even more readily than a vigorous sun-print obtained by direct exposure to light. A point of even greater importance is the nature of the sensitive surface which receives the latent image. It is the print developed upon Iodide of Silver which especially resists sulphuration. In that case, not only is the preliminary toning effect of the Sulphur more slow than usual, but the impression cannot be made to fade by any continuance of the action. It loses much of its brilliancy, and is reduced in intensity, but it is not so completely destroyed as to be useless. The reason of this, as shown in the last paper, depends upon the fact that the Talbotype proofs contain the largest amount of Silver in the image. The employment of Gold in toning does not render an ordinary sun-print as permanent as a Positive developed upon Iodide of Silver. The deep shadows of the picture are protected by the Gold, but the lighter shades not so perfectly. Hence after the Sulphur has acted, in place of the universal yellow and faded aspect presented by the simple untoned print, the Positive fully toned by Gold has black shadows with yellow half-tones. Therefore, whilst recommending the use of Gold as a toning agent, it does not seem advisable to lay too much stress upon it as a preservative from the destructive action of Sulphur. Exposure of Positive Prints to a Sulphuretting Atmosphere.—In testing the action of a solution of Sulphuretted Hydrogen upon paper Positives, it did not appear that the conditions under which the prints were placed bore a sufficiently close resemblance to the case of Positives exposed to an atmosphere contaminated with minute traces of the gas; and this more particularly because it is known that dry Sulphuretted Hydrogen has comparatively little effect upon Photographic Prints. The experiments were therefore repeated in a somewhat different form. A number of Positives (about three dozen) printed in various ways, were suspended in a glass case, measuring 2½ feet by 21 inches, and containing 7½ cubic feet of air; into which was introduced, occasionally, a few bubbles of Sulphuretted Hydrogen, just sufficient to keep the air of the chamber smelling perceptibly of the gas. A polished Daguerreotype plate was hung up in the centre, to serve as a guide to the progress of the sulphuretting action. By the second day the metal plate had acquired a faint yellow hue, not easily seen except in certain positions; but the Positives were unaffected. At the expiration of three days the majority of the pictures exhibited no signs of change, but a few untoned prints of a pale red colour, some of which had been printed by development, and others by direct exposure to light, had perceptibly darkened. After the eighth day, the action, appearing to progress more slowly than at first, was stopped, and the prints removed. The general results obtained were as follows:— The Daguerreotype plate had become strongly tarnished with a film of Sulphuret of Silver, which appeared yellowish-brown in some parts and steel-blue in others. The Positives were, as a rule, toned to a slightly colder shade, but many of them had scarcely changed. No obvious difference was observed between prints developed on paper prepared with Chloride of Silver, and others printed by direct exposure to light; but in all cases the prints obtained by those methods which give a very red image after fixing, were the first to show the change of colour due to sulphuration, the proofs submitted to the test having all been previously toned with Gold. Effect of Oxidizing Agents upon Positive Prints.—It appeared of importance to ascertain to what extent Photographic Prints are susceptible of oxidation; on account of the atmospheric influences to which they are necessarily exposed. In experimenting upon this subject the following results have been obtained. Powerful oxidizers destroy Positive Prints rapidly; the action usually commencing at the corners and edges of the paper, or at any isolated point, such as a metallic speck or particle of extraneous matter, which can serve as a centre of chemical action. Air which has been Ozonized by Phosphorus, and in which blue litmus-paper becomes reddened, quickly bleaches the Positive image. Oxygen gas, obtained by voltaic decomposition of acidified water and which should contain Ozone, did not appear to have an equal amount of effect, the action being comparatively slight, or altogether wanting. Peroxide of Hydrogen obtained in solution, and in conjunction with Acetate of Baryta, by adding Peroxide of Barium to dilute Acetic Acid, Nitric Acid applied in a concentrated form acts immediately upon the darkened surface, bleaching every part of the print with the exception of the bronzed shadows, which usually retain a slight residual colour. A solution of Chromic Acid is still more active. This liquid may usefully be applied to distinguish prints toned by Sulphur from others toned by Gold; the presence of metallic Gold protecting the shadows of the picture in some measure from the action of the acid. The solution should be prepared as follows:—
A solution of Permanganate of Potash is an energetic destroyer of paper positives; and, as it is a neutral substance, may conveniently be employed in testing the relative capability of withstanding oxidation possessed by different Photographic Prints. The solution should be dilute, of a pale pink hue, and the Positives must be moved occasionally, as the first effect is to decolorize a great portion of the liquid, the Permanganate oxidizing the size Comparative permanence of Photographs treated with Permanganate of Potash.—Developed prints prepared by a Negative process withstand the action better than others. But to this rule there are exceptions; much depending upon the time of exposure to light, and the extent to which the development is carried. Those prints which, being exposed for a short time, and afterwards strongly developed, become dark in colour and vigorous in outline, are more permanent than others which having been over-exposed and under-developed, lose their dark colour and become red and comparatively faint in the Hyposulphite fixing Bath. Positives developed upon a surface of Chloride of Silver on plain paper do not resist the oxidizing action so perfectly as those on Iodide of Silver. Prints developed upon paper prepared with Serum of Milk containing Caseine stand better than those on plain paper. Of prints obtained by the ordinary process of direct exposure to light, those on plain paper are the first to fade, the oxidizing action being most seen upon the half-tones. The use of Albumen gives a great advantage. Developed prints on Albumen stand far better than the same upon plain paper; and even the Albuminized sun prints are less injured by the Permanganate than the best of the Negative prints prepared without Albumen. Caseine has the same effect, but to a less extent; and as Serum of Milk almost invariably contains uncoagulated Caseine, its efficacy is thus explained. The manner of toning the print is a point of importance; previous sulphuration in an old Hyposulphite Bath always facilitating the oxidizing action. Action of Chlorine upon Positive Prints.—Aqueous solution of Chlorine destroys the Photographic image, changing it first to a violet tint (probably Subchloride), and subsequently obliterating it by conversion into white Chloride of Silver. The impression, The Action of Chlorine water usually commences at the edges and corners of the print, in the same manner as that of oxidizing agents. The proofs upon Albumen are the least readily injured, and next, those developed on Iodide of Silver. Hydrochloric Acid.—The liquid acid of sp. gr. ·116, even when free from Chlorine, acts immediately upon the half-tones of a positive print, and destroys the full shadows in the course of a few hours; a slight residual colour however usually remains in the darkest parts. The prints developed on Iodide of Silver are the most permanent. Sulphuric, Acetic Acids, etc.—Acids of all kinds appear to exert an injurious influence upon Positive prints, and especially so upon the half-tones of the image, the effect varying with the strength of the acid and the degree of dilution with water. Even a vegetable acid like Acetic gradually darkens the colour and destroys partially or entirely the faint outlines of the picture. Bichloride of Mercury.—The most important particulars relating to the action of this test upon Photographs are well known. The image is ultimately converted into a white powder, and hence, in the case of a Positive print, it becomes invisible; immersion in Ammonia or Hyposulphite of Soda however restores it in a form often resembling in tint the original impression. A point worthy of note is the protective effect of a deposit of Gold, which is very marked, the proof, after toning, resisting the action of the Bichloride for comparatively a long time. Ammonia.—The effect of Ammonia upon a print is rather to redden the image than to destroy it; the half-tones become pale and faint, but they do not disappear. Toning with Gold enables the proof to resist the action of the strongest solution of Ammonia, and hence Ammonia may safely be employed as a fixing agent after the use of the Sel d'or Bath. Hyposulphite of Soda.—A concentrated solution of Hyposulphite of Soda exercises a gradual solvent action upon the image of Photographic Prints, at the same time tending to communicate Sulphur and to darken the colour of the impression. A faint yellow outline of Sulphuret of Silver usually remains after the solution of the image is completed. Developed prints of all kinds, but in particular the Talbotype proofs upon Iodide of Silver, are less readily dissolved by Hyposulphite of Soda than those obtained by the direct action of light. There is also a slight difference between plain and Albuminized prints, which is in favour of the former, the albuminized paper always losing somewhat more by immersion in the Hyposulphite Bath than plain Chloride paper sensitized by Nitrate of Silver. Cyanide of Potassium.—The solvent action of Cyanide of Potassium is most energetic upon Photographs formed on paper. These images, whether developed or not, do not withstand the test so well as the impressions on Collodion. Albuminized proofs are also somewhat more easily affected than prints on simple Chloride paper sensitized with Nitrate or Ammonio-Nitrate of Silver. Heat, moist and dry.—Long-continued boiling in distilled water has a reddening action upon Positive Prints. The image becomes at length pale and faint, resembling a print treated with Ammonia before toning. A deposit of Gold upon the image lessens, but does not altogether neutralize, the effect of the hot water. If the boiling be long continued, the violet-purple tone often imparted by the Gold invariably gives place to a chocolate-brown, which appears to be the most permanent colour. Prints developed by Gallic Acid upon paper prepared with Serum of Milk or with a Citrate, suffer as much as others obtained by direct action of light. Ammonio-Nitrate prints on highly salted paper, which become nearly black when toned with Gold, retain their original appearance the most perfectly; a slight diminution of brightness being the only observable difference after long boiling in water. Albumen proofs, and prints on English papers, or foreign papers prepared with Serum of Milk, Citrates, Tartrates, or any of those bodies which redden the reduced Salt, are, as a rule, rendered lighter in colour, and pass from purple to brown when boiled in water. Dry heat has an opposite effect to that of hot water, usually darkening the colour of the image. On exposing a plain paper print simply fixed, and thoroughly freed from Hyposulphite of Soda by washing, to a current of heated air, it changes gradually from red to dark brown, in which state it continues until the temperature rises to the point at which the paper begins to char, when it resumes its original red tone, becoming at the same time faint and indistinct. The Products of Combustion of Coal-gas a cause of Fading.—Coal-gas contains Sulphur compounds, which in combustion are oxidized into Sulphurous and Sulphuric Acids; other substances of a deleterious nature may also be present. A plate of polished silver suspended in a glass tube, through which was directed the current of heated air rising from a small gas jet, became tarnished with a white film in the course of twenty-four hours. Positive prints exposed to the same, absorbed moisture and faded; the action resembling that of oxidation, in being preceded by a general darkening in colour. Of four prints exposed, an Iodide-developed print was the least injured, and next, a print upon Albuminized paper. ON THE ACTION OF DAMP AIR UPON POSITIVE PRINTS. In order to ascertain this point, more than six dozen Positives, printed on every variety of paper, were mounted in new and perfectly clean stoppered glass bottles, at the bottom of each of which was placed a little distilled water, to keep the contained air always moist. They were removed at the expiration of three months, having been kept during that time, some in the dark, and others exposed to the light. As the prints were prepared by various methods, toned in different ways, and mounted with or without substances likely to exercise a deleterious action, this series of experiments will possess considerable value in determining some of the intrinsic causes of fading of Positives. The general results obtained were as follows:—Positives which had been simply fixed in Hyposulphite of Soda remained quite uninjured. Whether developed by Gallic Acid on either of the Toned Positives were found in many cases to be less permanent than Positives simply fixed. This was especially the case when the toning had been effected by Sulphur; all the sulphuretted prints, fixed in solution of Hyposulphite which had been long used, became yellow in the half-tones when exposed to moisture. Positives fixed and toned in Hyposulphite containing Gold were variously affected; some prepared when the solution was in an active state being unchanged, others losing a little half-tone, and others, again, fading badly. These latter were prepared in a Bath which had lost Gold and acquired sulphuretting properties; and it was noticed that they were more injured by the action of boiling water than those Positives which proved to be permanent under the influence of the moisture. Toning by means of Chloride of Gold appeared to be highly satisfactory, but the number of prints operated upon was small. The Sel d'or process also did not injure the integrity of the image, no commencing yellowness or bleaching of half-tones being visible after exposure to the moist air. This series of experiments confirmed the statement made in a former paper, that some tints obtained in Positive printing are more permanent than others. Violet tones produced by Sulphur invariably passed into a dull brown by the action of the moist air; and even when Gold was employed in toning, these same purple colours were usually reddened. This was especially the case when English papers were used, or foreign papers re-sized with Serum of Milk containing Caseine. The chocolate-brown tints which best stand the action of boiling water, and in particular those upon Ammonio-Nitrate paper, were least affected by the damp air; and indeed it was evident that the two agents, viz. moist air and hot water, acted alike in tending to redden the print, although the latter did so in the most marked manner. It seemed also, from the results of these experiments, to be a point of great importance that the size should be removed from the print in order to render it indestructible by damp air. This was evidently seen in two cases where Positives, toned in an old The use of improper substances for mounting proved to be another determining cause of fading by oxidation. Those bodies which combine with Oxide of Silver, are likely upon theoretical grounds to destroy the half-tones of the image; and it was found, that if the picture were left in contact with Alum, Acetic Acid, etc., or with the substances which generate an acid by fermentation, such as paste or starch, it invariably faded. The supposed accelerating influence of Light upon the fading of Positives was not confirmed by these experiments, as far as they extended. Many of the bottles containing the Photographs were placed outside the window of a house with a southern aspect during the whole of the three months with the exception of two or three weeks, but no difference whatever could be detected between Positives so treated and others kept in total darkness. It will be proper however that this part of the investigation should be repeated, allowing a longer time. An examination of the various modes employed for coating Positives, in order to exclude the atmosphere, showed that many of them were not fitted to fulfil the purpose intended. Waxed prints faded quite as much when exposed to moisture as others not waxed. White wax is a substance often adulterated, and Oil of Turpentine has been shown to contain a body resembling Ozone in properties, and possessing the power of bleaching a dilute solution of Sulphate of Indigo. Spirit varnish applied to the ON THE CHANGE IN COMPOSITION WHICH HYPOSULPHITE OF SODA EXPERIENCES BY USE IN FIXING PAPER PROOFS. It was remarked by Photographers at an early period that the properties of the Fixing Bath of Hyposulphite of Soda became altered by constant use; that it gradually acquired the power of darkening the colour of the Positive image. This change was at first referred to the accumulation of Salts of Silver in the Bath, and hence directions were given to dissolve a portion of blackened Chloride of Silver in the Hyposulphite in preparing a new solution. Careful experiments performed by the Author convinced him that an error had been entertained; since it was found that the simple solution of Chloride of Silver in Hyposulphite of Soda had no power of yielding the black tones. But it afterwards appeared that if the fixing Bath, containing dissolved Silver Salts, were set aside for a few weeks, a decomposition occurred in it, evidenced by the formation of a black deposit of Sulphuret of Silver; and then it became active in toning the proofs. The presence of this deposit of Sulphuret of Silver indicated that a portion of Hyposulphite of Silver had spontaneously decomposed, and, knowing the products which are generated by the spontaneous decomposition of this salt, a clue to the difficulty was afforded. One atom of Hyposulphite of Silver includes the elements of one of Sulphuret of Silver and one of Sulphuric Acid. Sulphuric Acid in contact with Hyposulphite of Soda produces Sulphurous Acid by a process of displacement; and Plessy has shown that Sulphurous Acid reacts upon an excess of Hyposulphite of Soda, forming two of that interesting series of Sulphur compounds designated by Berzelius the "Polythionic Acids." It appeared therefore probable, upon theoretical grounds, that the Penta-, Tetra-, and Trithionates might produce some effect in the Hyposulphite fixing Bath. Upon making the trial these expectations were verified; and it was found that Tetrathionate of Soda added to Hyposulphite of Soda yielded a fixing and toning Bath quite equal in activity to that produced by means of Chloride of Gold. It may be useful to review for an instant the composition of the Polythionic series of acids; it is thus represented:—
The amount of Oxygen in all is the same, that of the other element increases progressively; hence it is at once evident that the highest member of the series might by losing Sulphur descend gradually until it reached the condition of the lowest. This transition is not only theoretically possible, but there is an actual tendency to it, all the acids being unstable with the exception of the Hyposulphuric. The Alkaline Salts of these acids are more unstable than the acids themselves; a solution of Tetrathionate of Soda becomes milky in the course of a few days from deposition of Sulphur, and, if tested, is then found to contain Trithionate and eventually Dithionate of Soda. The cause of the change in properties of the fixing Bath being thus clearly traced to a decomposition of Hyposulphite of Silver, and a consequent generation of unstable principles capable of imparting Sulphur to the immersed proofs, it seemed desirable to continue the experiments.— There is a peculiar acid condition commonly assumed by old fixing Baths, which could not be satisfactorily explained, since it was known that acids do not exist long in a free state in solution of Hyposulphite of Soda, but tend to neutralize themselves by displacing Hyposulphurous Acid spontaneously decomposable into Sulphurous Acid and Sulphur. This point is set at rest by the discovery of a peculiar reaction which takes place between certain salts of the Polythionic acids and Hyposulphite of Soda. A It is evident that a Sulphur acid exists which has not hitherto been described, and that this acid is formed as one of the products of the decomposition of the Hyposulphite of Silver contained in the fixing Bath. The subject is an important one to Photographers, because it is found that Hyposulphite Baths which have acquired the acid reaction, although toning quickly, yield Positives which fade on keeping. The acid may perhaps combine with the reduced Silver Salt, which, if the image be allowed to contain Suboxide of Silver, is theoretically probable. The experiments were next directed towards ascertaining more carefully the effect of the acid fixing Bath upon the Positive proofs. Tetrathionate of Soda added to solution of Hyposulphite of Soda produces, at the expiration of twelve hours, a liquid which, when filtered from the deposited Sulphur, reddens blue litmus-paper slowly. Positive prints immersed in the Bath pass from red to black, dissolving in the half-tones, and becoming yellow and faded if the action be too long continued. On adding Carbonate of Soda in quantity sufficient to remove the acid reaction, the power of toning is much diminished, but dark colours can still be obtained by continuing the action. The solvent effect upon the half-tones, evidently caused in great measure by the acid, is lessened; whilst the tendency to yellowness in the white parts of the proof, almost disappears. These effects are more particularly manifested when the prints are immersed in the Bath immediately on their removal from the printing frame; and it is found almost impossible to preserve the whites of the impression clear, in the acid Bath, unless the Nitrate of Silver has been washed away. Solution of half-tones and yellowness in the lights, both a source of annoyance to the operator, are thus traced in great measure to an acid condition of the fixing and toning Bath; and the remedy is obvious. The Author's experiments upon the Tetrathionates and their It may be interesting to the scientific investigator to describe the mode of preparing a fixing and toning Bath, illustrating the above remarks:—
Dissolve the Nitrate of Silver in 2 ounces of the water, then from the total quantity of Hyposulphite of Soda, weigh out
dissolve this likewise in 2 ounces of water, and the remainder of the Hyposulphite in the other 4 ounces. Then, having the three solutions in separate vessels, pour the Nitrate of Silver at once into the 2-ounce solution of Hyposulphite, agitating the precipitated Hyposulphite of Silver rapidly. In a short time it will begin to decompose, passing from white to canary-yellow, and then to orange-yellow. When the orange-yellow begins to verge towards brown, add the 4-ounce concentrated solution of Hyposulphite, which will at once complete the decomposition, a part of the precipitate dissolving and the remainder becoming perfectly black. After filtering out the black Sulphuret of Silver, the solution is ready for use. A Bath prepared by this formula is not usually very active, but it shows clearly the process by which an ordinary fixing Bath may be converted into a toning Bath by the immersion of positives having free Nitrate of Silver upon the surface. The following formula is more economical and gives a better
Dissolve the Hyposulphite of Soda in seven ounces of the water, the Nitrate of Silver in the remaining one ounce; then pour the Perchloride of Iron into the solution of Hyposulphite, by degrees, stirring all the time. The addition of the Iron Salt strikes a fine purple colour, but this soon disappears. When the liquid has become again colourless, which it does in a few minutes, add the Nitrate of Silver, stirring briskly. Perfect solution will take place without any formation of black Sulphuret. A toning Bath prepared with Chloride of Iron will be ready for use twelve hours after mixing, but it will be more active at the expiration of a week. The solution is acid to test-paper, and milky from a deposit of Sulphur, which must be filtered out. The Perchloride of Iron should be prepared by boiling Peroxide of Iron with Hydrochloric Acid, in preference to dissolving Iron wire in Aqua-Regia. The addition of the Nitrate of Silver is made in order to produce a portion of Hyposulphite of Silver in the bath; the presence of a Silver Salt having been found to modify the tint of the Positives, and to prevent their quickly turning yellow. SECTION IV. On the Fading of Photographic Prints. For many years subsequent to the discovery of the process of Photographic Printing by Mr. Fox Talbot, it was not generally known that pictures so produced were easily susceptible of injury from various causes, and in particular from traces of the fixing-agent remaining in the paper. Hence, due care not being taken in the proper cleansing and preservation of the proofs, the majority of them faded. This matter became at last one of such importance that the Council of the Photographic Society decided upon forming a Committee for the purpose of examining the subject. The Author was honoured by being placed upon this Committee, and the researches of which an abstract has been given in the previous Section, were undertaken at the request of the Society. The present Section is intended to explain practically and in a concise manner the causes of the fading of Photographic Prints, and the precautions which should be taken to ensure their permanency. The chemistry of the subject having been fully explained in the last Section, it will suffice to refer the reader to its pages for more detailed information. Historical evidence of the permanence of Photographs.—It is a point of interest to collect information as to the existence of old Photographs which have remained many years unchanged. There are numerous instances of Positives printed more than ten years ago, which have not perceptibly altered up to the present time. These prints are mostly on plain paper, Albumen not having come into use at so early a date. The general impression of practical operators however is, that fading has occurred less frequently since the introduction of Albuminized paper. Positives printed by development on paper prepared by Talbot's method seem as a rule to have stood remarkably well, and instances of Talbotype Negatives having faded are rare. Of the prints which have proved to be permanent, some are red or brown in colour, but many, being of a dark or purple shade, have evidently been toned, although not with Gold, the use of which was unknown to the earlier Photographers. It is plain from data thus collected, that Photographs do not necessarily fade by time; and the fact that in one and the same portfolio are constantly seen prints which appear permanent, and others in an advanced state of Causes of fading.—The Author believes that the fading of Photographic Prints may almost invariably be referred to one or other of the following conditions:— a. Imperfect washing.—This is perhaps the most important of all, and the most frequent. When Hyposulphite of Soda is allowed to remain in the paper, even in minute quantity, it gradually decomposes, with liberation of Sulphur, and destroys the print in the same way and quite as effectually as a solution of Sulphuretted Hydrogen or an alkaline sulphuret. Imperfect washing may be suspected, if the Photograph, within a few months from the date of its preparation, begins to get darker in colour: the half-tints, which are the first to show the action, afterwards passing into the yellow stage, whilst the dark shadows remain black or brown for a longer time. The proper mode of washing Photographs is sometimes misunderstood. The length of time during which the print lies in the water is a point of less importance, than that the water should be continually changed. When a number of Positives are placed together in a pan, and a tap turned upon them, the circulation of fluid does not necessarily extend to the bottom. This is proved by the addition of a little colouring matter, which shows that the stream flows actively above, but at the lower part of the vessel, and between the prints, there is a stationary layer of water which is of little use in washing out the Hyposulphite. Care should therefore be taken that the pictures are kept as far as possible separate from each other, and when running water cannot be had, that they are frequently moved and turned over, fresh water being constantly added. When this is done, and especially if the size be removed from the paper in the manner presently to be advised, four b. Acid matters left in the Paper.—Upon examining collections of old Photographs, it is not uncommon to find prints which are stated to have remained unaltered for a long time after their first production, but in the course of time to have lost their brilliancy, and become pale and indistinct. This kind of fading often commences at the corners and edges of the paper, and works inwards towards the centre. The Author's experiments have shown that it is principally caused by a slow process of oxidation. The Photographic Image does not appear readily susceptible of oxidation unless it be previously darkened by the action of Sulphur, or placed in contact with acids or bodies which act as solvents of Oxide of Silver (p. 146). The materials often used in sizing papers, such as Alum and Resin, being of an acid nature, are directly injurious to the image; and the removal of the size, which may easily be effected by means of a dilute alkali or an alkaline carbonate, without injury to the tint, has the additional advantage of carrying out the last traces of Hyposulphite of Soda, and also the germs of fungi, which if allowed to remain would vegetate and produce a destructive mouldiness on exposure to damp (Chap. III. Part II.). The fact that acids facilitate oxidation of the image is likewise a hint that Photographic Prints should not be handled too frequently, or touched with the finger more than is necessary; the warm hand may leave behind a trace of acid c. Moisture as a cause of fading.—Although. Photographs properly printed are not readily injured by damp air (p. 153), yet as there are impurities of various kinds constantly floating in the atmosphere, a state of comparative dryness may be said to be essential to the preservation of all Photographs. In collecting evidence upon the subject, "wet" and "damp" are frequently alleged as having been causes of fading—the prints were hung against a damp wall during frosty weather, in a room without a fire: or the rain had been allowed to penetrate the frame! No pictures will long survive such treatment, and Photographs, like engravings and water-colour paintings, require common care to be exercised in their preservation. d. The modes of Mounting the Proof.—This subject has been alluded to in the abstract of the Author's papers at p. 155. All cements which are of an acid nature, or which are liable to become sour by acetous fermentation, should be avoided. Flour paste is especially injurious, and many cases of fading have been traced to this cause. The addition of Bichloride of Mercury, which is often made to prevent the paste from becoming mouldy, would still more unfit it for Photographic use (p. 151). Starch is not much preferable. No substance appears better than Gelatine, which does not readily decompose, and shows no tendency to absorb atmospheric moisture. The deliquescent nature of many bodies is a point which should be borne in mind in mounting Photographs, and hence the use of a salt like Carbonate of Potash, which the writer has known to be added to paste to prevent the formation of acid, would be unadvisable. e. The effect of Imperfect Fixation.—The earlier Photographers did not always succeed in properly fixing their prints, since old Photographs are often found thickly studded with spots and blotches in the tissue of the paper. These prints however are not invariably faded upon the surface, and hence it cannot be said that imperfect fixation will certainly end in the total destruction of the picture. f. Exposure to an impure Atmosphere as a cause of Fading.—The five causes of fading which precede, have mostly reference to an intrinsically faulty condition of the print. This, the sixth, explains the mode in which a Photograph carefully prepared may yet suffer injury from deleterious matters often present in the atmosphere. The air of large cities, and particularly that emanating from sewers and drains, contains Sulphuretted Hydrogen, and hence articles of silver-plate become tarnished unless placed beneath glass. The injury which a print sustains by exposure to air contaminated with Sulphuretted Hydrogen, is less than the tarnish produced upon the bright surface of a silver plate (see p. 148); but it is recommended as a precautionary measure, that Photographic Pictures be protected by glass or kept in a portfolio, and that they be not exposed too freely to the air. The products of the combustion of coal-gas are probably more likely than the cause last named, to be a source of injury to Photographs suspended without any covering. The sulphur compounds in gas burn into Sulphurous and Sulphuric Acids, the latter of which, in combination with Ammonia, produces the sparkling crystals often observed upon the shop windows. The question as to the manner in which the Photographic Image may best be protected from these extraneous causes of fading has been mooted, and many plans of coating prints with some impervious material have been devised. If the pictures are to be glazed or kept in a portfolio, this g. Decomposition of Pyroxyline a source of Injury to Collodion Photographs.—Collodion Positives and Negatives are usually esteemed permanent; but some have been exhibited which, having been put away in a damp place, gradually became pale and indistinct. The change commences at rough edges and isolated points, leaving the centre, as a rule, the last affected. On examination, numerous cracks are often visible, thus seeming to indicate that the Collodion film has undergone decomposition. The result of this would be the liberation of corrosive Oxides of Nitrogen, which destroy the image. Substitution compounds containing Peroxide of Nitrogen are known to be liable to spontaneous change. The bitter resin produced by acting upon white sugar with Nitro-Sulphuric Acid, if not kept perfectly dry, will sometimes evolve enough gas to destroy the cork of the bottle in which it is kept; the solution of the resin has then a strong acid reaction, and rapidly fades an ordinary Positive Print. These facts are interesting, and indicate that Collodion Pictures, containing in themselves the elements of their destruction, should be protected from moisture by a coating of varnish. Comparative Permanence of Photographic Prints.—There is every reason to think that the Photographic Image, however formed, is permanent, if certain injurious conditions are avoided;—in other words, that prints do not necessarily fade, in the same manner as fugitive colours, by a simple exposure to light and air. But supposing a case, which is the common one, of injurious influences which cannot altogether be removed, it may be useful to inquire what mode of printing gives the greatest amount of stability. Positives produced by a short exposure to light and The original Talbotype process, in which the latent image is formed upon Iodide of Silver, produces, next to Collodion, the most stable image; but the difficulty of obtaining bright and warm tints on Iodide of Silver, will stand in the way of its adoption. The toning of Paper Positives is the part of the process which is likely to injure their stability; inasmuch as the finest results cannot easily be obtained without incurring sulphuration, and the action of Sulphur, if carried to any extent, has been shown to be detrimental. The point to be kept in view, is to alter the original structure of the image as little as possible in toning; and it is best to use Gold in preference to Sulphur as the colouring agent. On theoretical grounds, toning by an alkaline solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best process; but the employment of Sel d'or, which gives a more agreeable colour and has not been found practically to injure the image, will be generally preferred. In using a single fixing and toning Bath the same object of working by Gold rather than by Sulphur may be best attained by The prints which are least stable are such as have been toned in acid Hyposulphite Baths, without Gold; and the difficulty of preserving such pictures from becoming yellow in the half-tones is very great. Possibly a portion of the Sulphuretted Acid may unite with the Suboxide of Silver and cannot be removed by washing (see p. 158); but even if this be not the case, it is certain that no ordinary amount of care will obviate the occasional occurrence of fading, unless the Hyposulphite Bath be kept neutral to test-paper. And all those plans of toning in which Acetic or Hydrochloric Acid is mixed with Hyposulphite of Soda, and the Positive immersed whilst the liquid is in a milky state from precipitation of Sulphur, ought studiously to be avoided. It will be well also to avoid pushing the action of the fixing and toning Bath to its utmost limits, since practice and theory both teach us that the Positives which have been long in the Hyposulphite, and consequently show a tendency to yellowness in the light parts, are most liable to lose their half-tones on keeping. Photographic Prints are found often to darken slightly in the course of years; and therefore by suspending the toning action at an earlier stage a margin is left for what some have termed "an improvement by time." The use of Albuminized in preference to plain paper gives an advantage in protecting the image from oxidation; but if constantly exposed to moisture, a putrefactive decomposition of the animal matter may occur. The proper colour of the Albumen image being a pale red, the black tones should not be sought for on that variety of paper: their production, if Hyposulphite of Soda were used in toning, would probably imply an amount of Sulphuration which would more than counterbalance any advantage otherwise derivable from the Albumen. Permanent Positives of a black colour may easily be obtained by sensitizing plain paper, free from animal matters, Mode of testing the permanence of Positives.—The tests for Hyposulphite of Soda are not sufficiently delicate to indicate with certainty when the process of washing has been properly performed. The quantity of that salt left in the paper is usually so small and so much mixed up with organic matter, that the application of Protonitrate of Mercury or of Nitrate of Silver to the liquid which drains from the corner of the print, would probably mislead the operator. A dilute solution of Permanganate of Potash, prepared by dissolving from half a grain or two grains of the salt, according to its purity, in one gallon of distilled water, affords a convenient mode of testing Positives as regards their power of resisting oxidation; and to an experienced eye it will prove the presence or absence of Hyposulphite of Soda, the smallest trace of which is sufficient to remove the pink colour of the Permanganate. The most available and simple plan of testing permanence is to enclose the pictures in a stoppered glass bottle with a small quantity of water. If they retain their half-tones after a course of three months of this treatment, and do not become mouldy, the mode of printing followed is satisfactory. Boiling water will also be found useful in distinguishing the unstable colours produced by Sulphur from those following the judicious employment of Gold; in all cases the image will at first be reddened by the hot water, but if toned without Sulphur it will, as a rule, recover much of its dark colour on drying. The characteristic appearance of prints which have been I much sulphuretted in the toning Bath, and are very liable to fade, should be known. A yellow colour in the lights is a bad sign; and if the half-tones are at all faint and indistinct, with an aspect of commencing yellowness, it is almost certain that the Positive will not last for any considerable length of time. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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