PhotoHistory

From: PHOTOGRAPHY OF TO-DAY

By H. CHAPMAN JONES, 1913

Photography as a practical art dates from the introduction of the Daguerreotype and the paper processes of Fox Talbot in 1839. There have been only two other epochs of first importance, namely the collodion process in 1851 and the gelatine process about twenty years after. The introduction of these newer processes did not mean a sudden revolution in photography as an art, for an innovation was always regarded with a measure of skepticism. It was necessary to convince those who were actively engaged in the pursuit of the advantages of the new process, then the workers had by practice to become skilled in the working of it and their apparatus had to be adapted to it. So we find that Daguerreotypes were made commercially for five or six years after collodion was used, but the facilities and economy of the collodion process finally drove the Daguerreotype process into the obsolete methods of the past. For about five-and-twenty years collodion held its own without a rival. It was collodion that popularized photography, for a Daguerreotype was a single and costly picture, while by the use of collodion those who enjoyed but slender incomes could afford to have their photographs taken now and then, and if they wished they could have copies for distribution.

There were two collodion processes, the negative and the positive, the latter giving a single picture which was always put into a frame of some sort after the style of a Daguerreotype. In the negative process, the photographer made a negative, and from it printed the desired number of copies on paper which were finally mounted on cards. It is, practically speaking, the same process, whether it is wished to make a negative or a positive, and only the details are varied according to the end in view.

Collodion is a solution of a kind of gun-cotton in a mixture of alcohol and ether, and when the solution is poured upon any surface the solvents evaporate and leave a continuous film or varnish of the gun-cotton. Cotton when pure is a definite substance called cellulose, and cotton wool and good white blotting-paper, such as is used for filtering purposes in chemical laboratories, are nearly pure cellulose. When cellulose is put into a mixture of nitric and sulphuric acids, nitrates of cellulose are produced. If the action is pushed on as far as possible a nitrate is produced which if dried and set on fire burns with a sudden burst of flame. This is the gun-cotton that is used in warfare and for blasting in mines and quarries. If the action of the acids is moderated by adding a little water or shortening the time that the cotton is subjected to their action, a less explosive compound is produced which is called “soluble gun-cotton” or “pyroxyline,” and it is this substance that is dissolved in a mixture of alcohol and ether to prepare collodion. There is no possibility of accident from the combustibility of the basis of collodion when employed in the usual manner, for however combustible a material may be, it cannot be set fire to when it is spread as a thin varnish-like film upon glass or any similar support. The old experiment of wrapping a piece of paper tightly round a poker and then holding it in a flame, shows that close contact with a considerable mass of material keeps a layer of combustible material cool even when a source of heat is applied to it.

Although the collodion film plays the same part as the paper did in the older process in being the vehicle that is impregnated with the sensitive compound, it is not convenient to prepare the film first and then treat it as the paper was treated. The sensitive compound, silver iodide in this case, is obtained by bringing together two substances that produce it by their action upon each other, and it is convenient to put one of these into the liquid collodion before the glass is coated with it. As the change produced by light during the exposure in the camera is exceedingly minute and invisible, serving only to render the silver compound amenable to the developer, great care has to be taken that the silver iodide is produced and kept in the dark, so that no light reaches it at any time whatever until it receives in the camera the image of the object to be photographed.

The collodion before use, therefore, has added to it a solution of an iodide that is soluble in the collodion solvents, generally iodide of potassium, of ammonium, or of cadmium. It is then ready for application to the glass plate. This must be scrupulously clean, for any ordinary dirty matter would be very likely to interfere with the action of the developer, either facilitating it and so causing a deposit on development where it should not be, or hindering it and so leading to the production of parts that are unduly transparent. For a similar reason the collodion cannot be applied with a brush as is often customary with varnishes, for it would be impossible to keep the brush clean. The use of a brush too would result in an uneven layer, and this clearly would be detrimental. The prepared collodion is therefore poured on to the glass plate while this is held in a horizontal position, and by a dexterous sloping of the plate the liquid is caused to flow all over it and the excess is allowed to drain off at one corner. A small plate is held in the hand, while a plate that is too large and heavy for manipulation in this way, and a piece of plate glass two or three feet square is of a considerable weight, is allowed to rest near its center on a well padded and suitably supported round cushion two or three inches in diameter. On such a contrivance the workman can manipulate large and heavy plates with ease.

When the film of collodion has set, but before it has dried, the plate is placed in a solution of nitrate of silver, and it is from this stage that the plate must be kept in the dark or in a room to which only such light is admitted as is unable to act upon it. The nitrate of silver is soluble in water and so also is the iodide that was added to the collodion, but as soon as these two substances come together in a liquid, iodide of silver is formed and deposited as a fine yellow powder, because it is not soluble in water. It will be seen that as the iodide added to the collodion mixed thoroughly with it, it must thoroughly permeate the film on the plate, and as the silver nitrate solution soaks into the film the iodide of silver is produced throughout its substance the silver compound is in the film not on it. After three or four minutes the plate is withdrawn from the nitrate of silver solution (called the “silver bath”), allowed to drain a little, and at once put into its carrier and exposed in the camera.

It will be observed that the plate is wet with the solution of nitrate of silver when it is being exposed, and indeed the plate is wet all the time until it is ready for the final operations of drying and varnishing. Hence this is often called the “wet plate process.” It is essential for the proper action of light that the plate have nitrate of silver upon it, and if it were allowed to dry this salt would crystallize, and the crystals being deposited here and there irregularly would cause an uneven action. The wetness is therefore a matter of necessity and not of convenience. After the exposure the plate is taken to the dark room, removed from its carrier, and a suitable quantity of the developer is poured upon its surface so that it mixes with the silver nitrate solution with which it is already wet. The developer may be either pyrogallic acid or ferrous sulphate, but in either case its action would be too vigorous alone and therefore it is made slower and more under control by adding a few drops of acetic acid to it. If the developer does not flow evenly on the plate a little alcohol may be added to it to facilitate its flow.

Very soon after the developer has been poured on to the plate the image begins to appear, and the operator carefully watches the growing image as he causes the liquid on the plate to move a little to and fro. When the darkening of the image begins to lag, the exhausted solution is rinsed off the plate, and a mixture of some more developer with its acid plus a little nitrate of silver solution is poured on.

The darkening of the image now goes on again, and when the operator considers that it is dark enough, the plate is rinsed, put into the fixing bath to dissolve out from the film all the silver salt, for it is no longer wanted, washed, dried, and varnished. It is then ready to be used for the making of prints.

The iodide of silver in this case is the sensitive substance. The amount of light that gains access to it while the plate is in the camera produces no visible change, but it is certain that the light does produce some change, because by the after treatment those parts that the light has impinged upon behave in a different manner from those parts that have not been subjected to its action. The nature of the changes taking place during development is not difficult to understand. If the developer proper, the pyrogallic acid or the ferrous sulphate, were added to a solution of nitrate of silver, metallic silver would at once be separated from the liquid as a fine dark powder, and in time this would settle to the bottom of the vessel. By the addition of a drop or two of acid to the mixture, the separation of the metal would be made more slow and it might be a minute or two before any change began to be visible. This is the condition of the liquid that is on the surface of the exposed plate during development. It is just ready to deposit metallic silver, but it is held back by the acid. But the acid holds it back to only a very slight extent, so that the smallest amount of disturbance from any outside source is very likely to upset the unstable equilibrium of the solution and cause metallic silver to separate from it. We have seen that light is a force and often causes changes, and it has in this case disturbed the silver iodide in the collodion film on the plate where it has fallen upon it, and this disturbed condition of the silver compound is just able to cause the deposition of silver from the solution. So silver is deposited where the light has acted, but not in the other parts of the film. After a little time the metal begins to be deposited from the developer without the action of any outside disturbing force, and the separating metal imparts a brownish tinge to the solution. If this were allowed to continue silver would be precipitated all over the plate and the negative would be “fogged.” To avoid this catastrophe, as soon as the developing liquid is seen to be beginning to be colored it is thrown off and a new solution is applied.

What is the nature of the disturbance that the light has effected in the iodide of silver? This is not known. Various theories have been suggested and the greater number or all of them have in turn been cast aside. We shall say something on this subject when we discuss exposure and development more in detail.

We have just described the production of a negative on a wet collodion plate, and seen that in the end there is a dark deposit wherever the light has acted on the plate. If this deposit can be got white instead of gray or black, we shall get at once on the plate what may be imagined to be a white pigment corresponding in place and in quantity to the whiteness or brightness of the object photographed, and if a black material is put all over behind the plate, we have much the same result as would be obtained by an artist who started with a black surface and painted on it in white pigment a picture of the object before him. Only, of course, our photograph would be far more perfect than the artist’s picture and it would lack the personality of the latter which is often so welcome and so valuable. This is a collodion positive. To get the deposit white, some ferrous nitrate and nitric acid are used in the developer instead of only ferrous sulphate and acetic acid, and it is desirable to give a shorter exposure, so that the deposit may not be so dense as to give a flat monotonous appearance even in the highest lights. The black backing may be of black velvet or a black varnish painted on. If the backing is put on the film side of the plate so that the photograph is looked at through the glass, the picture is seen correctly; but if the backing is put on the glass side so that the film is towards the spectator, the image is laterally reversed, as a Daguerreotype is when taken without the use of a mirror or reflecting prism.

A collodion positive if well made is a very beautiful production, and some have said that it may surpass the finest Daguerreotype. This of course is largely a matter of skill in the worker, but there is one circumstance that is in their favor as compared with Daguerreotypes, namely, that they have not that troublesome brightness of the polished metal that renders it necessary to turn a Daguerreotype about until the reflection is got rid of before it can be properly seen. The collodion positive is the kind of photograph supplied by the peripatetic photographer who gives you a completed and framed picture while you wait. The whole process can be done in two or three minutes, for the thin film of collodion may be rapidly washed and if necessary dried by warming the plate, without fear of damage. A “ferrotype” or “tintype” is a collodion positive made on a piece of thin enameled iron which itself constitutes the black backing necessary. In this case, of course, the photograph must be laterally reversed unless a mirror is used.

The necessity for having the plate wet and wet with a solution of nitrate of silver, when working the collodion wet plate process, has been pointed out. The disadvantages of this necessity are numerous and can hardly be appreciated by one who has not himself performed the necessary operations. Consider a photographer working at home in his own studio and about to take a portrait. After arranging the sitter and getting everything connected with the subject ready, the photographer must retire to the dark room to prepare the plate. When it has been in the nitrate of silver solution long enough it has to be transferred while wet and dripping, for it must not be drained too long, to the back portion of the camera, that is made removable for the purpose of conveying the plate in darkness from the dark room to the camera in the studio. It is hardly possible to prevent the silver solution from getting on to the fingers and on to some parts of the camera. It very speedily blackens the fingers and in time it rots the woodwork. In development there is more silver solution liable to get on to the hands, so that even the most careful and dexterous worker could hardly avoid justifying the description of photography as the “black art.” But if a photograph had to be taken away from home there were other and greater disadvantages. The plates had to be prepared and developed on the spot, and unless they were exceedingly small this necessitated the carrying of large bulks of solutions and also a “tent” to work in, a tent of special construction to exclude the light. The tent might be only large enough to put the worker’s head and hands in, instead of his whole body, but still it had to be of considerable size to permit of the various manipulations necessarily done in the dark. The most modest outfit for outdoor work would therefore generally be an arrangement on wheels and require the aid of at least one assistant.