This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Bromide And Negative Enlarging. When this side of the room is used for negative and bromide enlarging, the table employed for changing plates is dropped in order to make room for the enlarging easel. The enlarging camera is arranged on the previously mentioned table at the end of the room. An opening is cut in the partition to admit the condensing lenses, which are fitted in the wall and the camera is fitted flush to this partition. The electric arc light used for enlarging is hung on the outside of the wall and is operated by a switch from the inside. As the length of the room is twenty feet, an enlargement of any size can be made.
Theory Of Developing. We will next consider the process of developing and the action of the developer upon the exposed plate. When you have made your exposure and placed your plate in the developer, the developing agent builds up and renders the latent image visible. When the plate was exposed in the camera no visible change took place on the sensitive surface, still some parts were affected by the light, while other parts remained unchanged.
23. When the plate is placed in the developer it may merely change the color of the light affected parts, and render them visible to the eye, or it may build up an image on the plate. In other words, the developer is the agent which exerts an action upon the portions of a sensitive plate which has been exposed to the light, and has no effect on the parts which have not been exposed.
24. In the pyro developer the action of the pyro is assisted by an alkali, such as carbonate of soda. When you expose a dry plate to the light you have a certain amount of silver sub-bromide. Immerse this plate in plain pyrogallic acid, and there will be little or no change, but if a small quantity of alkali is added, the image will begin to develop and the plate will blacken rapidly, forming a metallic silver; therefore, it is necessary that a certain amount of alkali must be used in the developing of the plate.
25. The action of the carbonate of soda is to open the pores of the emulsion. If the action is too rapid (and this would be the case if too strong or too much carbonate' of soda was used) the pyro would penetrate the emulsion too rapidly and too deeply, and would then stain the film, and you would produce a yellow negative. To offset this and prevent staining, sulphite of soda is added. The carbonate is termed the accelerator and assists the pyro in developing, while the sulphite is the preservative of color; therefore controls the color of the negative.
Theory Of Fixing. The agent universally adopted for fixing both plates and paper is hypo-sulphite of soda. A sensitive emulsion of chloride, or iodide of silver, on which has been formed an image, either with or without the aid of the developing agent, must pass through this process to render it indestructible by diffused light. It is true that the image itself is sufficiently permanent, and it cannot be said, in correct language, to need fixing. The unchanged silver salts surrounding it are still sensitive to the light, and tend to be decomposed in their turn, and so the picture is lost. It is, therefore, necessary to remove these salts by applying some chemical agent capable of dissolving them. In order that a chemical may be employed with success as a fixing agent, it must produce no injurious effect upon the silver salts which have been affected by the light. Hypo-sulphite of soda is employed not only on account of its having these safe qualities, but because it is economical. The fact that the silver contained in an ordinary fixing bath is present in the state of hypo-sulphite must be borne in mind, because this salt is liable to undergo peculiar chemical changes. Iodide of silver is dissolved by hypo more slowly than chloride of silver, and the amount eventually taken up is less. This is explained in the following manner : -
During the dissolving of iodide of silver, iodide of sodium is formed, and this has the effect of acting as a stop to the fixing. In other words, it retards the action of the hyposulphite of soda. The time occupied in fixing will, of course, vary with the strength of the hypo-sulphite of soda solution employed.
27. The process of fixing is simply the dissolving away of the sensitive salt unacted upon by the light. We recommend the plain hypo bath for the following reasons: The addition of any acid to the hypo-sulphite of soda may cause chemical changes. It first displaces the chemical hypo-sulphurous acid from its combination with soda. This acid begins to decompose, and splits up into the sulphurous acid, remaining dissolved in the liquid, and giving the characteristic odor of burning sulphur. Sulphur which separates in a finely divided state forms a milky deposit. In other words, sulphurization has been produced, and a plate fixed in this bath will discolor, and the image will probably fade away entirely in time. It will also harden the emulsion, and it is next to impossible to doctor successfully a plate fixed in a bath of this kind.