ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS.

The terms "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 upon Chloride of Silver, either in the Camera or by superposition, the effect must necessarily be Negative; the Chloride being darkened by luminous rays, the lights are represented by shadows.

The following simple diagrams will make this obvious.

Fig. 1 is an opaque image drawn upon a transparent ground; 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 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 opaque but 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 the rationale of 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 termed direct, 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 the two 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, to whiten the 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 a neutral Bath 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 of Nitric 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 causes haziness and 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 of Acetate of Silver is 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 small quantity for 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 picture when 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 of Nitric Acid to 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 of Protoxide of Iron appear to be in inverse ratio to the strength of the acid with which it is associated in its salts; hence the Nitrate is 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 Positive image 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 as Corrosive 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 first darkens considerably, until it becomes of an ash-grey, approaching to black; shortly it begins to get lighter, and assumes a pure white tint, 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.

The rationale of 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 is feeble though 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 too little 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 who has 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. 13 and 101). 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 into Sulphuret 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 treated with Ammonia, the effect of which is to blacken it 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.