[16]The difference in the atomic weights of the various soluble Chlorides used in salting must be borne in mind. Ten grains of Chloride of Ammonium contain as much Chlorine as eleven of Chloride of Sodium, or twenty-two grains of Chloride of Barium. (See the Vocabulary, Part III.)
[16]The difference in the atomic weights of the various soluble Chlorides used in salting must be borne in mind. Ten grains of Chloride of Ammonium contain as much Chlorine as eleven of Chloride of Sodium, or twenty-two grains of Chloride of Barium. (See the Vocabulary, Part III.)
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, todoublethe 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[17]or Nitrate of Ammonia, in preparing Chloride of Silver paper. By doing so, a great increase of sensitiveness, and also of intensity of image, is obtained. This will be understood if we remember that the action of light in producing the print is of a reducing nature. Hence the substitution of Oxide for Nitrate of Silver facilitates the decomposition; just asAmmonio-Nitrateof Silver is more readily reduced by Gallic or Pyrogallic Acid than the simple Nitrate (see p. 31).
[17]The chemistry of Ammonio-Nitrate of Silver is explained in the Vocabulary, Part III.
[17]The chemistry of Ammonio-Nitrate of Silver is explained in the Vocabulary, Part III.
Ammonio-Nitrate paper has the disadvantage of soondiscolouringwhen 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 thepaper and gives additional power of bronzing, by assisting to reduce the free Nitrate of Silver. Many other organic matters, tending to absorb oxygen, would act in the same way.
Gelatineis 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 atpage 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 ofblue. 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 adarkerimage 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 bedarkerafter 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 ofOxideof 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 permanentgloss upon the paper. The sensitive albuminized paper darkens in the sun to a chocolate-brown colour, which becomes very red on immersion in the Hyposulphite. The finished prints are clear and transparent; usually of a brown tone, or with a shade of purple when the gold Bath is newly made and active; pure blacks are not easily obtained.
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 areddeninginfluence 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 anexcessof 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 todecompose, 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 howdecompositionof 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. Therationaleof these changes is explained to a certain extent by studying the composition of the Hyposulphite of Silver. The formula for this substance is as follows:—
AgO S2O2.
But AgO S2O2plainly 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 thetemperatureis high. To obviate it, observe the following directions:—It is especially in the reaction betweenNitrate of Silverand Hyposulphite of Soda that the blackening is seen; the Chloride and otherinsolubleSalts 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 brownmay 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,[18]visibleby transmitted light, may occur. Each atom of Nitrate of Silver requiresthreeatoms of Hyposulphite of Soda to form thesweet and soluble double salt, and hence, if the action be not continued sufficiently long, another compound will be formed almost tasteless and insoluble (p. 44). Even immersion in a new Bath of Hyposulphite of Soda does not fix the print when once the yellow stage of decomposition has been established. This yellow salt is insoluble in Hyposulphite of Soda, and consequently remains in the paper.
[18]The writer has noticed that when sensitive paper iskept for some timebefore being used for printing, these yellow patches of imperfect fixation are very liable to occur. The Nitrate of Silver appears gradually to enter into combination with the organic matter of the size of the paper, and cannot then be so easily extracted by the fixing bath.
[18]The writer has noticed that when sensitive paper iskept for some timebefore being used for printing, these yellow patches of imperfect fixation are very liable to occur. The Nitrate of Silver appears gradually to enter into combination with the organic matter of the size of the paper, and cannot then be so easily extracted by the fixing bath.
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 appearspottedwhen 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 thatAlbuminizedpaper, 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 isintended to remain, is washed in water to remove the excess of Nitrate of Silver. It is then immersed in a dilute solution of Chloride of Gold, acidified by Hydrochloric Acid. The effect is to reduce the intensity considerably, and at the same time to change the dark shades to a violet or bluish tint. After a second washing with water, the proof is placed in plain Hyposulphite of Soda, which fixes it and alters the tone to a pure black or a blue-black, according to the manner of preparing the paper and the time of exposure to light.
Therationaleof 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 ofChlorineis 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 analkalineinstead 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 sameinkyblueness. 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"[19](a compound which dissolves in Hyposulphite of Soda with considerable formation of the unstable red salt), the activity of the Bath will be promoted.
[19]Read the observations on the Explosive Properties of Fulminating Gold in the Vocabulary, Part III.
[19]Read the observations on the Explosive Properties of Fulminating Gold in the Vocabulary, Part III.
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,and Sulphuret of Silver, Sulphuric Acid, and metallic Gold are the results. That a minute proportion of Sulphuret of Silver is formed seems certain; but the change must be superficial, as the stability of the print is very little lessened when the process is properly performed.
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 printbefore 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 precipitateSulphur, 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.TheAGEof 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,and consequent yellowness in the white parts of the proof. It is therefore particularly recommended to wash the print in water before immersing it in the fixing and toning Bath.
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.[20]Now the Hyposulphite of Soda is a substance which can be very readily made to yield up Sulphur to any bodies which possess an affinity for that element, and as the reduced Silver compound in the print has such an affinity, there is always a tendency to absorption of Sulphur when the proofs are immersed in the Bath. Consequently in many cases a sulphur toning-process is set up, and as the picture is improved by it in appearance, losing its brick-red colour and assuming a purple shade, it was at first adopted by Photographers. Experience however has shown that colours brightened in this way are less permanent than others, and are liable to fade unless kept perfectly dry. Hence the process will be discarded by all careful operators, and the object will be to avoid sulphuration as far as possible. This can be done to a great extent, and, when the Bath is properly managed, the prints will be toned almost entirely by Gold, and will, with care, be permanent.
[20]For a more detailed account of the toning process by Sulphur, see the Third Section of this Chapter,page 145. The instability of sulphuretted prints is shown in the fourth Section.
[20]For a more detailed account of the toning process by Sulphur, see the Third Section of this Chapter,page 145. The instability of sulphuretted prints is shown in the fourth Section.
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 totonerapidly 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,and splits up into Sulphurous Acid—which remains dissolved in the liquid, communicating the characteristic odour of burning Sulphur—andSulphur, which separates in a finely divided state and forms a milky deposit.[21]
[21]From the Vocabulary, Part III., it will be seen that commercial Chloride of Gold usually containsfree Hydrochloric Acid; hence a considerable deposit of Sulphur takes place on adding it to the Hyposulphite solution, and the liquid must not be used immediately.
[21]From the Vocabulary, Part III., it will be seen that commercial Chloride of Gold usually containsfree Hydrochloric Acid; hence a considerable deposit of Sulphur takes place on adding it to the Hyposulphite solution, and the liquid must not be used immediately.
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.[22]
[22]The chemical reader will understand the decomposition of free Hyposulphurous Acid by the following equation:—S2O2= SO2and S.
[22]The chemical reader will understand the decomposition of free Hyposulphurous Acid by the following equation:—S2O2= SO2and S.
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 tonedby Sulphur more than by Gold, and will not possess the same stability; the Bath will also, after long use, be found to acquire a distinctacidreaction to test-paper, the acidity being due to a peculiar principle generated by decomposing Hyposulphite of Silver, and which is shown to have an injurious action upon the print (p. 158). To avoid this the solution should be keptneutral to test-paperby means of a drop of Ammonia, if required; and when it begins to be exhausted, and does not tone (quickly) a print from which the free Nitrate of Silver has been removed by washing, a fresh quantity of Chloride of Gold should be added.
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.[23]
[23]The preparation of a toning bath by Tetrathionate, without Gold, is described in the next Section, but it is not recommended for practical use.
[23]The preparation of a toning bath by Tetrathionate, without Gold, is described in the next Section, but it is not recommended for practical use.
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 oftenbear no resemblance to those of a metal. One Photograph may also differ essentially from another, so that we are led to infer the existence of two varieties, the first of which is less of a metallic nature than the second.
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 Silverand exposed to light, loses its characteristic property of dissolving in hot water. This experiment is conclusive.
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 ofAmmonio-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 Chlorideof Silver, although pale and feeble, may, after fixing, be immersed ill dilute solution of Cyanide of Potassium without injury. But a photograph on Chloride of Silver supported by an organic basis, is much acted upon by Cyanide of Potassium, quickly losing its finer details.
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 ofoxidizing agentsshows 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 ofdevelopedimages.—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 onCollodion. Collodion contains Pyroxyline, a substance which behaves towards the salts of Silver in a manner different from that of most organic bodies,exhibiting no tendency to assist their reduction by light. Hence Chloride of Silver on Collodion darkens far more slowly than the same salt upon Albumen, and the image, after fixing, is feeble and metallic. Iodide of Silver on Collodion, exposed and developed, gives usually a more metallic image, with less intensity, than Iodide of Silver upon Albumen, or on paper sized with Gelatine. By adding to the Collodion a body which has an affinity for low oxides of Silver, such for instance as Glycyrrhizine, the opacity of the developed image is increased.
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, andrepresents the shadows of a picture. Collodion positives developed with protosalts of Iron are nearly or quite metallic. Photographs on Albumen or Gelatine less so than those on Collodion. Developed Photographs contain more Silver than others, if the development has been prolonged. The half shadows of the image in a Positive Print are especially liable to suffer under injurious conditions, since they contain the Silver in a less perfect state of reduction.[24]