[14]See the Vocabulary, Part III., Art. Iodoform.
[14]See the Vocabulary, Part III., Art. Iodoform.
Commercial Iodide of Cadmium is a purer salt than the Iodide of Potassium, and may be advantageously substituted for it; but it possesses the property of coagulating Albumen, and hence cannot be employed in conjunction with that substance.
e.Presence of free Iodine.—Both in the waxed paper and the Collodion processes, the solutions often contain a small quantity of free Iodine. This Iodine, in contact with the Nitrate of Silver of the Bath, produces a mixed Iodide andIodateof Silver, and liberates Nitric Acid. It thus retards the sensitiveness of the film in proportion to the quantity of Iodine present. Collodion of a full yellow colour is perceptibly less sensitive than the same rendered colourless; and when enough Iodine has been liberated to give a red or brown tint, double the original exposure will probably be required.
If brown Collodion be much used, the Nitrate Bath may by degrees become sufficiently contaminated with free Nitric Acid to interfere with the sensitiveness of the film;but if colourless or lemon-yellow tinted Collodion be employed, this evil need not be anticipated.
Certain substances may be added to coloured Collodion which possess the property of counteracting the retarding influence of the free Iodine, such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably act in virtue of their affinity for Oxygen, by preventing the formation of Iodate of Silver. In colourless Collodion they produce little or no effect, neither do they remove the insensitiveness of the film when dependent upon a too acid condition of the Nitrate Bath.
f.Addition of Bromide or Chloride to Collodion.—In the Daguerreotype a very exalted state of sensibility is obtained by exposing the silvered plate first to the vapour of Iodine, and afterwards to that of Bromine or Chlorine; but this rule does not apply to the Collodion process, which differs essentially in principle. Soluble Bromides added to Collodion lessen its sensibility to an appreciable extent, as also do Chlorides. This rule however may perhaps be liable to an exception when artificial light is used, which contains a greater proportion of the rays of small refrangibility, known to act more powerfully upon the Bromide than upon the Iodide of Silver (p. 66).
g.Density of the sensitive film.—When the proportion of soluble Iodide in the iodizing solution is too great, the film is very dense, and the Iodide of Silver is apt to burst out upon the surface, and fall away in loose flakes into the Bath. This condition, which is highly unfavourable to sensitiveness, is very common in Collodion, and constitutes what is termed "over-iodizing." The Iodide, in fact, is formed in such a case too much upon the surface, and consequently, when the fixing agent is applied, the image not being retained by the film, is washed off and lost.
On the other hand, the sensibility of the film is not lessened by reducing the amount of Iodide in Collodion to a minimum, if all the solutions are neutral; but the pale blue films formed by a dilute Collodion, and which almostrival the Daguerreotype itself in delicacy, are nearly useless in practice; for if free Iodine or other bodies of a retarding nature are present in any quantity, either in the Collodion or in the Bath, they almost destroy the action of a weak light, producing a far more injurious effect than if the film were more yellow and opaque.
h.Impurities in Ether and Alcohol.—Pure Ether should be neutral to test-paper, but the commercial samples of this article have usually either an acid or an alkaline reaction. The frequent occurrence of a peculiar oxidizing principle in Ether has also been pointed out (p. 85). Each of these three conditions is injurious to sensitiveness; the first and last by liberating Iodine when alkaline Iodides are used; and the second, by producing Iodoform under the same circumstances. In this case the Collodion remains colourless, but gives inferior results.
The Author has also observed that Ether which has been redistilled from the residues of Collodion may contain a volatile principle (probably a compound Ether?) which produces a retarding effect upon the action of light.
Commercial Spirit of Wine is not always uniform in composition, as sufficiently evidenced by the test of smell. It may contain "fusel oil" or other volatile substances, which become milky on dilution with water, and are believed to injure the quality of the spirit for Photographic use.
i.Relative proportions of Ether and Alcohol in Collodion.—It was shown atp. 84that the addition of Alcohol to Collodion lessens the contractility of the film, and renders it soft and gelatinous. This condition is favourable to the formation of the invisible image in the Camera, the play of affinities being promoted by the loose manner in which the particles of Iodide are held together. It is therefore usual to add to Collodion as much Alcohol as it will bear without becoming glutinous, or leaving the glass; the exact quantity required varying with the strength of the spirit or its freedom from dilution with water.
k.Decomposition in the Collodion.—Collodion iodized with the metallic Iodides generally, excepting the Iodide of Cadmium, becomes brown and loses sensitiveness in the course of a few days or weeks. If the free Iodine, the cause of the brown colour, be removed, the greater part, but not the whole, of the sensitiveness is regained. The experiments of the Author, and of others, have proved that a solution of Pyroxyline in contact with an unstable iodide, slowly undergoes decomposition, the result of which is that Iodine is set free, and an equivalent quantity of the base remains in union with certain organic elements of the Collodion.
Decomposition also gradually ensues when iodized Collodion is placed in contact with reducing agents, such as Proto-iodide of Iron, Gallic Acid, Grape Sugar, Glycyrrhizine, etc., so that these combinations do not retain a constant sensibility for any length of time. Even plain Collodion uniodized cannot be preserved many months without a small but perceptible amount of change.
l.Decomposition in the Nitrate Bath.—A Collodion Nitrate Bath which has been much used, often gives a less sensitive film than when newly made. It is known also that many organic substances which reduce Nitrate of Silver, if added to the Bath, produce a state which is favourable to sensitiveness whilst the decomposition is taking place, but is eventually unfavourable; hence the solution will be injured by adding either Gallic or Pyrogallic Acid, and by organic matters generally if exposed to light.
Recapitulation.—The conditions most favourable to extreme sensitiveness of the Iodide of Silver on Collodion may be condensed as follows:—perfect neutrality of the solutions employed; a soft, gelatinous state of the film; absence of Chlorides and other salts which precipitate Nitrate of Silver; an undecomposed Collodion, containing no organic matter of that kind which is precipitated by basic Acetate of Lead, and combines with oxides of Silver.
THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE.
The general theory of the development of a latent image by means of a reducing agent, having been simply explained in the third Chapter, may now be more fully examined in its application to the Iodide of Silver on Collodion.
a.The presence of free Nitrate of Silver essential to the development.—This subject has already been mentioned (p. 36). A sensitive Collodion plate, carefully washed in distilled water, is still capable of receiving the radiant impression in the Camera, but it does not admit of development until it has been redipped in the Bath, or treated with a reducing agent to which Nitrate of Silver has been added: and if the proportion of free Nitrate of Silver on a Collodion film be too small, the image will be feeble or altogether imperfect in parts, with patches of green or blue, due to deficient reduction.
b.Comparative strength of deducing Agents.—No increase of power in a developer will suffice to bring out a perfect image on an under-exposed plate, or upon a film containing too little Nitrate of Silver. But there is considerable difference in the length of time which the various developers require to act. Gallic Acid is the most feeble, and Pyrogallic Acid the strongest, producing at least four times more effect than an equal weight of the crystallized Protosulphate of Iron, and twenty times more than the Protonitrate of Iron.
c.The effect of free Acid upon the development.—Acids tend to retard the reduction of the image as well as to diminish the sensibility of the film to light. Nitric Acid especially does so, from its powerful oxidizing and solvent properties. The effect of Nitric Acid is particularly seen when the film of Iodide of Silver is very blue and transparent, and the quantity of Nitrate of Silver retained upon its surface small. Under such circumstances theproper development of the image may be suspended, and spangles of metallic Silver separate. This indicates that the quantity of the acid should be diminished, or the strength of the Nitrate Bath and of the reducing agent be increased, as a counterpoise to the retarding action of acid upon the development.
Acetic Acid also moderates the rapidity of development, but it has not that tendency altogether to suspend it possessed by Nitric Acid. It is therefore usefully employed, to enable the operator to cover the plate evenly with liquid before the development commences, and to preserve the white parts of the impression from any accidental deposit of metallic Silver due to irregular action of the reducing agent.
On comparing the retarding effects of free acid upon the light's action, and upon the development, we see that the former is the most marked,—that a small quantity of Nitric Acid produces a more decided influence upon the impression of the image in the Camera than upon the bringing out of that image by means of a developer.
d.Accelerating effect of certain organic matters.—Organic bodies, like Albumen, Gelatine, Glycyrrhizine, etc., which combine chemically with oxides of Silver, and were shown in the last Section to lessen the sensitiveness of the Iodide film,—facilitate the development of the image, producing often a dense deposit of a brown or black colour by transmitted light.
In the same way, viz. by a retention of organic matter, may partly be explained the fact, that the image developed by Pyrogallic Acid, although proved by the application of tests to contain no more than an equal quantity of Silver, possesses greater opacity by transmitted light, than that resulting from the use of protosalts of Iron: and in the case of the Collodion itself the same rule applied—if it be pure, it is liable to give a less vigorous impression than when by long keeping a partial decomposition has taken place, and products have been formed which combinewith reduced Oxide of Silver more easily than the unaltered Pyroxyline.
e.Molecular conditions affecting Intensity.—The physical structure of the Collodion film is thought to exert an influence upon the mode in which the reduced Silver is thrown down during the development. A short and almost powdery state, such as Collodion iodized with the alkaline iodides acquires by keeping, is considered favourable, and a glutinous, coherent structure unfavourable, to density. This is certainly the case when the film is allowed to dry before development, as in the process with desiccated Collodion and, to some extent, in the Oxymel preservative process.
The mode of conducting the development also affects the density; a rapid action tending to produce an image of which the particles are finely divided and offer a considerable resistance to the passage of light, whilst a slow and prolonged development often leaves a metallic and almost crystalline deposit, comparatively translucent and feeble.
The writer has observed, that with certain samples of Collodion the image is much enfeebled by keeping the plate for a considerable time,—a quarter of an hour or longer,—after sensitizing, but before development. This effect is not the result of the Nitrate of Silver having partially drained away, since a second dip in the Nitrate Bath immediately before applying the Pyrogallic Acid, does not remedy it. An alteration of molecular structure may therefore be the correct explanation, and if so, a contractile Collodion would suffer more than one possessing less coherency.
The actinic power of the light at the time of taking the picture, influences the appearance of the developed image; the most vigorous impressions being produced by a strong light acting for a short time. On a dull dark day, or in copying badly lighted interiors, the photograph will often lack bloom and richness, and be blue and inky by transmitted light.
f.Development of images upon Bromide and Chloride of Silver.—Of the three principal Salts of Silver, the Iodide is the most sensitive to light, but the Bromide and Chloride, under some conditions, are more easily developed and give a darker image. In the Collodion process the difference is principally seen when organic bodies, like Grape Sugar, Glycyrrhizine, etc., are introduced in order to increase the intensity; a far more decided effect being produced by adding both Glycyrrhizine and a portion of Bromide or Chloride, than by using the Glycyrrhizine alone.[15]
[15]See the Author's Paper on the chemical composition of the photographic image, in the eighth Chapter.
[15]See the Author's Paper on the chemical composition of the photographic image, in the eighth Chapter.
g.The intensity of the image affected by the length of exposure.—This point has been briefly alluded to in the third Chapter. If the exposure in the Camera be prolonged beyond the proper time the development takes place rapidly but without any intensity, the picture being pale and translucent. The effects produced by over-action of the light are particularly seen when the Nitrate Bath contains Nitrite of Silver, or Acetate of Silver; the image being frequently in such a case dark by reflected light, and red by transmitted light,—more nearly resembling in fact a photographic print, developed on paper prepared with Chloride of Silver. When Collodion plates are coated with honey without previously removing the free Nitrate of Silver, a slow reducing action is set up, which may give rise to the characteristic appearance above referred to, after development. Other organic substances, such as biliary matter, etc., will act in the same way.
h.Certain conditions of the Bath which affect development.—Attention may be called to a peculiar state of the Nitrate Bath, in which the Collodion image developes unusually slowly, and has a dull grey metallic appearance, with an absence of intensity in the parts most acted on by the light. This condition, which occurs only when using a newly mixed solution, is thought by the Author to dependupon the presence of an Oxide of Nitrogen retained by the Nitrate of Silver. It is removed partially by neutralizing the Bath with an alkali, more perfectly so by adding an excess of alkali followed by Acetic Acid; but most completely by carefullyfusingthe Nitrate of Silver before dissolving it.
Commercial Nitrate of Silver has sometimes a fragrant smell, similar to that produced by pouring strong Nitric Acid upon Alcohol. When such is the case, it contains organic matter, and produces a Bath which yields red and misty pictures.
Nitrate of Silver which has been sufficiently strongly fused to decompose the Salt, and produce a portion of the basic Nitrite of Silver exhibits great peculiarity of development, the image coming out instantaneously and with great force. This condition is exactly the reverse of that produced by the presence of acids, in which the development is slow and gradual.
In summing up the different conditions of the Nitrate Bath which affect the development of the image, as many asfourmight be mentioned, each of which gives a more rapid reduction than the one which precedes it. These are—the acid Nitrate Bath, the neutral Bath, the Bath of strongly fused Nitrate of Silver, and the Bath containingAmmoniacalNitrate of Silver, which is quite unmanageable, and produces an instantaneous and universal blackening of the film on the application of the developer.
Greater intensity of image is commonly obtained in a Nitrate Bath which has been a long time in use, than in a newly mixed solution: this may be due to minute quantities of organic matter dissolved out of the Collodion film, which, having an affinity for Oxygen, partially reduce the Nitrate of Silver; and also to the accumulation of Alcohol and Ether in an old Bath producing a short and friable structure of the film.
i.Effect of Temperature on Development.—Reduction of the oxides of noble metals proceeds more rapidly inproportion as the temperature rises. In cold weather it will be found that the development of the image is slower than usual, and that greater strength of the reducing agent and more free Nitrate of Silver is required to produce the effect.
On the other hand, if the heat of the atmosphere be excessive, the tendency to rapid reduction will be greatly increased, the solutions decomposing each other almost immediately on mixing. In this case the remedy will be to use Acetic Acidfreelyboth in the Bath and in the developer, at the same time lessening the quantity of Pyrogallic Acid, and omitting the Nitrate of Silver which is sometimes added towards the end of the development.
Also in the case of films which are to be kept for a long time in a sensitive condition by means of honey, etc., the modifying influence of temperature must be observed, and the quantity of free Nitrate of Silver left upon the film be reduced to a minimum if the thermometer stands higher than usual.
SECTION IV.
On certain Irregularities in the Developing Process.
The characteristics of the proper development of a latent image are—that the action of the reducing agent should cause a blackening of the Iodide in the parts touched by light, but produce no effect upon those which have remained in shadow.
In operating both on Collodion and paper however there is a liability to failure in this respect; the film beginning, after the application of the developer, to change in colour to a greater or less extent over the whole surface.
There are two main causes which produce this state of things:—the first being due to an irregularity in the action of the light; the second to a faulty condition of the chemicals employed.
If from a defect in the construction of the instrument,or from other causes which will be pointed out more particularly in the Second Part of this work, diffused white light gains entrance into the Camera, it produces indistinctness of the image by affecting the Iodide more or less universally.
The luminous image of the Camera not being perfectly pure, mereover-exposureof the sensitive plate will usually have the same effect. In such a case, when the developer is poured on, a faint image first appears, and is followed by a general cloudiness.
The clearness of the developed Collodion picture is much influenced by the condition of all the solutions employed, but particularly so by that of the Nitrate Bath. If this liquid be in the state termed alkaline (p. 88), it will be impossible to obtain a good picture; and even when neutral, care and avoidance of all disturbing causes will be required to prevent a deposition of Silver upon the shadows of the image: especially so when Nitrite of Silver or Acetate of Silver are present, both of these salts being more easily reduced than the Nitrate of Silver.
The use ofAcidis the principal resource in obviating cloudiness of the image. Acids lessen the facility of reduction of the Salts of Silver by developing agents (p. 98), and hence when they are present the metal is deposited more slowly, and only on the parts where the action of the light has so modified the particles of Iodide as to favour the decomposition: whereas if acids be absent or present in insufficient quantity, the equilibrium of the mixture of Nitrate of Silver and reducing agent which constitutes the developer is so unstable, that any rough point or sharp edge is likely to become a centre from which the chemical action, once started, radiates to all parts of the plate.
Various acids have been employed, such as Acetic acid, Citric acid, Tartaric acid, etc. Nitric acid is the most effectual of all, but is seldom used, because, although the image can often be developed with great clearness when the Bath contains a small quantity of Nitric acid, yet sucha condition is not favourable tointensity; on the other hand, films which are prone to irregular reduction, such as those prepared in a chemically neutral bath or a bath containing Acetate or Nitrite of Silver, are likely to give the greatest vigour of impression. Hence, when this quality is desired, the use of Nitric Acid will be adopted cautiously.
The state of the Collodion must be attended to as well as that of the bath; it should be either acid or neutral, not alkaline. Colourless Collodion may be used successfully as a rule, but sometimes a little free Iodine is advantageously added. Care should be taken in introducing organic substances, many of which dissolve out into the bath, and spoil it for giving clear pictures. Glycyrrhizine, however, which is recommended to produce intensity of Negatives, has no effect of that kind, and may be employed with safety.
The condition of the developing agent is a point of importance in producing clear and distinct pictures. The Acetic acid, which is advised in the formulæ, cannot be omitted or even lessened in quantity without danger. This is particularly the case in hot weather or under any other condition which favours reduction, such as neutrality of the bath, etc.; at all times, in fact, when the solutions of Pyrogallic acid and Nitrate of Silver decompose each other with unusual rapidity.
In addition to the points now mentioned, viz. the state of the Bath, of the Collodion, and of the developer, the reader should also study the remarks made in the Third Section of Chapter III. on the effect ofsurface conditionsin modifying the deposition of vapour and of metallic Silver: he will then in all probability experience but little difficulty in dealing with those numerous irregularities in the action of the developing fluid, which often prove the greatest hindrance to the successful practice of the Collodion process.
ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS.
Theterms "Positive" and "Negative" occur so frequently in all works upon the subject of Photography, that it will be impossible for the student to make progress without thoroughly understanding their meaning.
A Positive may be defined to be a Photograph which gives a natural representation of an object, as it appears to the eye.
A Negative Photograph, on the other hand, has the lights and shadows reversed, so that the appearance of the object is changed or negatived.
In Photographs taken uponChloride of Silver, either in the Camera or by superposition, the effect must necessarily be Negative; the Chloride beingdarkened by luminous rays, the lights are represented by shadows.
The following simple diagrams will make this obvious.
Fig. 1.Fig. 2.Fig. 3.
Fig. 1.Fig. 2.Fig. 3.
Fig. 1 is an opaque image drawn upon a transparentground; fig. 2 represents the effect produced by placing it in contact with a layer of sensitive Chloride and exposing to light; and fig. 3 is the result of copying this negative again on Chloride of Silver.
Fig. 3 therefore is a Positive copy of Fig. 1, obtained by means of a Negative. By the first operation the tints are reversed; by the second, being reversed again, they are made to correspond to the original. The possession of a Negative therefore enables us to obtain Positive copies of the object, indefinite in number and all precisely similar in appearance. This capability of multiplying impressions is of the utmost importance, and has rendered the production of good Negative Photographs of greater consequence than any other branch of the Art.
The same Photograph may often be made to show either as a Positive or as a Negative. For instance, supposing a piece of silver-leaf to be cut into the shape of a cross and pasted on a square of glass, the appearance presented by it would vary under different circumstances.
Fig. 1.Fig. 2.
Fig. 1.Fig. 2.
Fig. 1 represents it placed on a layer of black velvet; fig. 2 as held up to the light. If we term it Positive in the first case,i. e.by reflected light, then it is Negative in the second, that is, by transmitted light. The explanation is obvious.
Therefore to carry our original definition of Positives and Negatives a little further, we may say, that the former are usually viewed by reflected, and the latter by transmitted, light.
All Photographs however cannot be made to represent both Positives and Negatives. In order to possess this capability, it is necessary that a part of the image should be transparent, and the other opaquebut with a bright surface. These conditions are fulfilled when the Iodide of Silver upon Collodion is employed, in conjunction with a developing agent.
Every Collodion picture is to a certain extent both Negative and Positive, and hence the processes for obtaining both varieties of Photographs are substantially the same. Although however the general characters of a Positive and a Negative are similar, there are some points of difference. A surface which appears perfectly opaque when looked down upon, becomes somewhat translucent on being held up to the light; hence, to give the same effect, the deposit of metal in a Negative must be proportionally thicker than in a positive; otherwise the minor details of the image, will be invisible, from not obstructing the light sufficiently.
With these preliminary remarks, we are prepared to investigate more closely therationaleof the processes for obtaining Collodion Positives and Negatives. All that refers to paper Positives upon Chloride of Silver will be treated in a subsequent Chapter.
SECTION I.
On Collodion Positives.
Collodion Positives are sometimes termeddirect, because obtained by a single operation. The Chloride of Silver,acted upon by light alone, is not adapted to yield direct Positives, the reduced surface being dark and incapable of representing the lights of a picture. Hence a developing agent is necessarily employed, and the Iodide of Silver substituted for the Chloride, as being a more sensitive preparation. Collodion Positives are closely allied in their nature to Daguerreotypes. The difference between thetwo consists principally in the surface used to sustain the sensitive layer, and the nature of the substance by which the invisible image is developed.
In a Collodion Positive the lights are formed by a bright surface of reduced Silver, and the shadows by a black background showing through the transparent portions of the plate.
Two main points are to be attended to in the production of these Photographs.
First, to obtain an image distinct in every part,but of comparatively small intensity.—If the deposit of reduced metal be too thick, the dark background is not seen to a sufficient extent, and the picture in consequence is deficient in shadow.
Secondly, towhitenthe surface of the reduced metal as much as possible, in order to produce a sufficient contrast of light and shade. Iodide of Silver developed in the usual way presents a dull yellow appearance which is sombre and unpleasing.
The Collodion and Nitrate Bath for Positives.—Good Positives may be obtained by diluting down a sample of Collodion with Ether and Alcohol until it gives a pale bluish film in the Bath. The proportion of Iodide of Silver being in that case small, the action of the high lights is less violent, and the shadows are allowed more time to impress themselves. The dilution lessens the amount of Pyroxyline in the Collodion at the same time with the Iodide, which is an advantage, the slight and transparent films always giving more sharpness and definition in the picture.
The employment of a very thin film for Positives is not however always a successful process. The particles of the Iodide of Silver being closely in contact with the glass, unusual care is required in cleaning the plates in order to avoid stains; and the amount of free Nitrate of Silver retained upon the surface of the film being small, circular patches of imperfect development are liable to occur,unless the reducing agent be scattered evenly and perfectly over the surface. Also if free Iodine or organic substances which have a retarding effect on the action of light are present to a considerable extent, the Collodion will not work well with a small proportion of Iodide. The Author found in experimenting on this subject that with perfectly pure Collodion and aneutralBath most vigorous impressions were produced when the density of the film had been so far reduced by dilution that scarcely anything could be seen upon the glass; but with Collodion strongly tinted with Iodine, or with a Bath containing Nitric Acid, it was necessary to stop the dilution at a certain point or the film became absolutely insensitive to feeble radiations of light, and the shadows could not be brought out by any amount of exposure. In this case, by adding more Iodide a better effect was obtained.
A thicker Collodion may be used for positives if a little free Iodine be added, for the purpose of diminishing intensity and keeping the shadows clear during the development. This process is easier to practice than the last, but does not always give the same perfect definition.
No organic substance of the class to which Glycyrrhizine and Sugars belong should be added to Collodion which is to be used for Positives. By so doing the image would be rendered intense, and the high lights liable to solarization,id est, a dark appearance by reflected light.
The Nitrate Bath.—If the materials are pure, the Nitrate Bath may advantageously be diluted down at the same time with the Collodion, when Positives are to be taken; but the employment of a very weak Nitrate Bath (such as one of 20 grains to the ounce), although highly useful in obviating excess of development, has some disadvantages; it becomes necessary to exclude free Nitric Acid, and to avoid the employment of a Collodion too highly tinted with Iodine. On the other hand, with a strong Nitrate Bath, and a tolerably dense film of Iodide of Silver, a better result is often secured by the use ofNitric Acid. The sensitiveness of the plates is impaired, but at the same time the intensity is diminished, and the picture shows well upon the surface of the glass.
A new Bath is better for taking Positives than one which has been a long time in use. The latter often causeshazinessand irregular markings on the film during the action of the developer. This is due partly to the accumulation of Alcohol and Ether in the Bath, which causes the solution of Sulphate of Iron to flow in an oily manner; and partly to a reduction of the Nitrate of Silver by organic matter.
The presence ofAcetate of Silveris objectionable in a Positive Nitrate Bath as producing solarization and intensity of image; hence those precautions which obviate its formation must be adopted (p. 89).
If fused Nitrate of Silver be used for the Positive Nitrate Bath, it is very important that the fusion should not be carried too far, or the solution would contain a basic Nitrite of Silver, and yield an intense, solarized, and misty image.
The Developers for Collodion Positives.—Pyrogallic Acid when used with Acetic Acid, as is usual for negative pictures, produces a surface which is dull and yellow. This may be obviated by substituting Nitric Acid in smallquantityfor the Acetic. The surface produced by Pyrogallic Acid with Nitric Acid is lustreless, but very white, if the solution be used of the proper strength. On attempting to increase the amount of Nitric Acid the deposit becomes metallic, and the half-tones of the picture are injured; Pyrogallic Acid, although an active developer, does not allow of the addition of mineral acid to the same extent as the Salts of Iron. It requires also, when combined with Nitric Acid, a fair proportion of Nitrate of Silver on the film, or the development will be imperfect in parts of the plate.
Sulphate of Iron.—The Protosalts of Iron were first employed in Photography by Mr. Hunt. The Sulphate is a most energetic developer, and often brings out a picturewhen others would fail. To produce by means of it a dead white tint with absence of metallic lustre, it may be used in conjunction with Acetic Acid, and in a somewhat concentrated condition, so as to develope the picture quickly.
The addition ofNitric Acidto Sulphate of Iron modifies the development, making it more slow and gradual, and producing a bright sparkling surface of reduced Silver. Too much of this acid however must not be used, or the action will be irregular. The Nitrate Bath also must be tolerably concentrated, in order to compensate for the retarding effect of Nitric Acid upon the development. The blue and transparent films of Iodide of Silver, formed in a very dilute Nitrate Bath, are not well adapted for Positives to be developed in this way. They are injured by the acid, and the development of the image becomes imperfect.
Protonitrate of Iron.—This salt, first used by Dr. Diamond, is remarkable as giving a surface of brilliant metallic lustre without any addition of free acid. Theoretically, it may be considered as closely corresponding to the Sulphate of Iron with Nitric Acid added. There are however slight practical differences between them, which are perhaps in favour of the Protonitrate.
The reducing powers ofProtoxideof Iron appear to be in inverse ratio to the strength of the acid with which it is associated in its salts; hence theNitrateis by far the most feeble developer of the Protosalts of Iron.
The rules already given for the use of Sulphate of Iron acidified with Nitric Acid, apply also to the Nitrate of Iron; the proportion of free Nitrate of Silver must be large, and the film of Iodide of Silver not too transparent.
In developing direct Positives either by Pyrogallic Acid or the Salts of Iron, the colour of the image will be found liable to some variation; the character of the light, whether bright or feeble, and the length of exposure in the Camera, affecting the result.
A Process for whitening the Positive Image by means of Bichloride of Mercury.—In place of brightening the Positiveimage by modifying the developer, it was proposed some time since by Mr. Archer to effect the same object by the use of the salt known asCorrosive Sublimate, or Bichloride of Mercury.
The image is first developed in the usual way, fixed, and washed. It is then treated with the solution of Bichloride, the effect of which is to produce almost immediately an interesting series of changes in colour. The surface firstdarkens considerably, until it becomes of an ash-grey, approaching to black; shortly it begins to get lighter, and assumes apure whitetint, or a white slightly inclining to blue. It is then seen, on examination, that the whole substance of the deposit is entirely converted into this white powder.
Therationaleof the reaction of Bichloride of Mercury appears to be, that the Chlorine of the mercurial salt divides itself between the Mercury and the Silver, a portion of it passing to the latter metal and converting it into a Protochloride. The white powder is therefore probably a compound salt, as is further evidenced by the effects produced on treating it with various reagents.
SECTION II.
On Collodion Negatives.
As in the case of a direct Positive we require an image which isfeeblethough distinct, so, on the other hand, for a negative, it is necessary to obtain one of considerable intensity. In the Chapter immediately following the present, it will be shown that in using glass Negatives to produce Positive copies upon Chloride of Silver paper, a good result cannot be secured unless the Negative is sufficiently dark to obstruct light strongly.
The Collodion and Nitrate Bath for Negatives.—A Collodion containing a very small portion of Iodide and yielding a blue transparent film in the Bath is not well adapted for taking Negatives. Pale opalescent films often give toolittle intensity in the high lights, and, unless the Nitrate Bath be acid, do not admit of being exposed in the Camera for the proper length of time without cloudiness and indistinctness of image being produced under the action of the developer. The effect known as "solarization of negatives,"i. e.a red and translucent appearance of the highest lights, is also more liable to occur when operating with a very pale film. On the other hand, if the layer of Iodide be too yellow and creamy, the half-tones of the image will often be imperfectly developed, so that a middle point between these extremes is the best.
A pure and newly prepared Collodion, although highly sensitive to light, does not always give, with one application of the developer, a sufficiently vigorous image to serve as a negative matrix; and this particularly in the most brightly illuminated parts, such as the sky in a landscape photograph, or the white borders of an engraving. But on keeping the Collodion for some weeks or months it becomes yellow, if iodized with the alkaline iodides, and a decomposition takes place in it, as before shown (p. 97), which lessens the rapidity of action, but adds to the intensity of the negative.
Grape Sugar may be employed for the purpose of giving intensity to newly mixed Collodion: also Glycyrrhizine, which is a resinous body extracted from the root of Liquorice; but as both substances have an effect in lessening the sensitiveness and keeping qualities of the fluid, they should be used cautiously. In taking portraits in the open air, on bright days, and with a Bath which has been mixed for a considerable time, it will rarely be found that the intensity will be deficient; and especially so if the developer be applied a second time to the film with a few drops of solution of Nitrate of Silver added. In landscape Photography however, or in copying engravings, where extreme sensitiveness is not an object, the Glycyrrhizine may sometimes be added with advantage in order to obtain perfect opacity of the blacks.
When the use of this substance is resorted to, the mode of iodizing the Collodion appears to be of importance, the increase of intensity being greater with the Iodide of Cadmium than with the Iodides of the Alkalies; the latter probably exercising a decomposing action. An addition of a Bromide or a Chloride to the Collodion in small quantity has also a marked effect in adding to the intensity when Glycyrrhizine is used with alkaline Iodides (p. 101).
Substances which produce intensity of the Collodion image have often, if added in too large quantity, a tendency to lower the half-tone, and prevent the darker parts of the picture from being sufficiently brought out. The print from the Negative is then pale and white, or "chalky" as it is termed, in the high lights. Collodion in this condition is often preferred by the beginner, from the facility with which the Negatives are obtained, but it does not give the finest results. An excess of Glycyrrhizine in Collodion has also the effect of interfering with the precipitation of the Iodide of Silver, producing a blue and smoky film which is nearly useless for Negatives.
A judicious employment of free Iodine in Collodion which has been previously intensified with Glycyrrhizine, has a remarkable effect in improving the gradation of tone. The excessive opacity of the high lights is diminished, and hence the operator is enabled by a longer exposure of the sensitive plate to bring out the shadows and minor details of the image with great distinctness. Collodion prepared in this manner is too slow to be used for portraits, excepting in a strong light, but often gives an image with great roundness and stereoscopic effect.
The Iodine and the liquorice sugar employed conjointly, tend also to preserve the clearness of the plates under the influence of the developer, and to give sharpness to the lines and dots of engravings, etc., which, with a new and sensitive Collodion, are often imperfectly rendered. These advantages will be appreciated by the operator whohas failed from working with a too feeble Collodion; but it must be borne in mind, that all substances acting as intensifiers have a bad effect when the state of the film is not such as to call for their employment.
The Proto-iodide of Iron has been recommended as an addition to Negative Collodion. In the Nitrate Bath it forms, in addition to Iodide of Silver, Protonitrate of Iron, an unstable substance and a developer. The use of Iodide of Iron gives great sensibility, but it is difficult to preserve it pure and unchanged. It also decomposes the Collodion in the course of a few hours, becoming itself peroxidized, and producing an insensitive condition of film. In addition to this, the negatives taken by the aid of Iodide of Iron are commonly of an inferior kind, the reduction being too marked in the high lights; so that its employment is of doubtful utility.
The Nitrate Bath.—This should be prepared from Nitrate of Silver which has been melted at a moderate heat (see pp.13and101). If this point be neglected, the best Collodion will sometimes fail in producing an intense negative.
Acetic Acid must be added in minute quantity, to preserve the solution from a too ready reduction by the Alcohol and Ether of the Collodion. Also, unless the Nitrate of Silver be quite pure and free from organic matter (p. 104), clear pictures will not be obtained without the use of Acid.
Acetate of Silver has often been advised as an addition to the Negative Nitrate Bath. It is produced by dropping into the solution an alkali, such as Ammonia, followed by Acetic Acid in excess. The Negatives are rendered blacker and more vigorous by this proceeding, but especially so when the Bath is contaminated with Nitric Acid; which neutralizes itself at the expense of the Acetate of Silver, thus:—
As a rule, it will be better to avoid adding Acetate of Silver to the Bath, since with, pure melted Nitrate of Silver no Nitric Acid can be present, and perfect intensity is easily obtained. When the Bath is saturated with Acetate of Silver, it is in a more reducible state, and hence unless the glass plates are very perfectly cleaned, black lines and markings, the results of irregular action, will be produced on the application of the developer to the film (p. 104). Solarization, or reddening by over-exposure, is also promoted by the presence of Acetate of Silver.
Developing solutions for Negatives.—The Protosalts of Iron are not usually employed in developing Negative impressions. They are liable to yield a violet-coloured image, which cannot easily be rendered more intense by continuing the action.
Gallic Acid is too feeble for developing Collodion pictures. Pyrogallic Acid is much superior, and may be used of any strength, according to the effect desired. When the light is bad, the temperature low, and the Negative developes slowly and appears blue and inky by transmitted light, the proportion of the reducing agent should be increased. But with an intense Collodion, on a clear summer's day, the finest gradation is obtained with a weak solution, which does not begin to act until the plate has been evenly covered. A strong developer might in such, a case produce too much opacity in the highest lights, and would probably occasion stains of irregular reduction.
Modes of strengthening a finished impression which is too feeble to be used as a Negative.—The ordinary plan of pushing the development cannot be applied with advantage after the picture has been washed and dried. In that case, if it is found to be too feeble to print well, its intensity may be increased by one of the following methods.—
It must be premised however, that the same degree of excellence is not to be expected in a Negative Photograph which has been improperly developed in the first instance and more especially if the exposure to light was too short.Any "instantaneous Positive" may be rendered sufficiently intense for a Negative, but in that case the shadows are almost invariably imperfect.
1.Treatment of the image with Sulphuretted Hydrogen or Hydrosulphate of Ammonia.—The object is to convert the metallic Silver intoSulphuret of Silver, and if this could be done it would be of service. The mere application of an Alkaline Sulphuret has however but little effect upon the image, excepting to darken its surface and destroy the Positive appearance by reflected light; the structure of the metallic deposit being too dense to admit of the Sulphur reaching its interior.
Professor Donny ('Photographic Journal,' vol. i.) proposes to obviate this by first converting the image into the white Salt of Mercury and Silver by the application of Bichloride of Mercury, and afterwards treating it with solution of Sulphuretted Hydrogen or Hydrosulphate of Ammonia. Negatives produced in this way are of a brown-yellow colour by transmitted light, and opaque to chemical rays to an extent which would not,à priori, have been anticipated.
2.MM. Barreswil and Davanne's process.—The image is converted into Iodide of Silver by treating it with a saturated solution of Iodine in water. It is then washed—to remove the excess of Iodine,—exposed to the light, and a portion of the ordinary developing solution, mixed with Nitrate of Silver, poured over it. The changes which ensue are precisely the same as those already described; the whole object of the process being to bring the metallic surface back again into the condition of Iodide of Silver modified by light, that the developing action may be commenced afresh, and more Silver deposited from the Nitrate in the usual way.
3.The process with Bichloride of Mercury and Ammonia.—The image is first converted into the usual white double Salt of Mercury and Silver by the application of a solution of the Corrosive Sublimate. It is then treatedwith Ammonia, the effect of which is toblackenit intensely. Probably the alkali acts by converting Chloride of Mercury into the black Oxide of Mercury. In place of Ammonia, a dilute solution of Hyposulphite of Soda or Cyanide of Potassium may be used, with very similar results.
ON THE THEORY OF POSITIVE PRINTING.
Thesubject 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 colouredsubsalt; 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 havingbeen laid in contact with it, the combination has been exposed to the agency of light for a sufficient length of time. Upon removing the glass, a Positive representation of the object will be found below, of great beauty and detail. Now if this image were in its nature fixed and permanent, or if there were means of making it so, without injury to the tint, the production of Paper Positives would certainly be a simple department of the Photographic Art; for it will be found that with almost any Negative, and with sensitive paper however prepared, the picture will look tolerably well on its first removal from the printing-frame. Immersion in the bath of Hyposulphite of Soda however, which is essentially necessary in order to fix the picture, produces an unfavourable effect upon the tint; decomposing the violet-coloured Subchloride of Silver, and leaving behind a red substance which appears to be united to the fibre of the paper, and, when tested, reacts in the manner of a Suboxide of Silver.
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 andtoning, 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 forsizingthe 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, appearspotted. 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 byfloating, 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 brushingor spreading with a glass rod be adopted, 7 to 1 or 8 to 1 will not be too much.
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,bronzeor copper-colour. When thebronzedstage 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 isa want of contrastwhich 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 sensibilityof the paper is regulated up to a certain point by the amount of salt[16]used in the preparation. The quantity of alkaline Chloride determines the amount of Chloride of Silver; and with a proper excess of Nitrate of Silver, papers are to a certain point more sensitive in proportion as they contain more of the Chloride.