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.
Acid-Fixing Bath. The plain fixing bath has the disadvantage of becoming dark and discolored when organic developers are used, and, consequently, it is liable to stain the plate. This is overcome by using the acid-fixing bath according to the following formula: -
Dissolve and then add citric acid one and one-fourth ounces. After this has been dissolved add hypo sixteen ounces. When the ingredients are all dissolved the bath is ready for use, and plates should be fixed at least twenty minutes.
Nature And Action Of Chemicals Used. Hydro-qjjinone. - The action in developing of hydroquinone is much slower than that of eikonogen, but it is more constant, lasts a great deal longer and produces more contrast. When used alone the negatives produced are apt to be too contrasty. While it gives a fine black velvety color, it does not give the blue-black as the eikonogen. It being slow in action, it allows for all color latitude in exposure, and is, consequently, easily controlled. Hydroquinone comes in the form of yellow, nearly white, needle-like prisms, and is very soluble in water.
Eikonogen. - The action of eikonogen in the developer is similar to hydroquinone. It is, however, much more energetic in developing than hydroquinone. It inclines to produce softness, and an abundance of detail. The action is so rapid when used alone, especially in the case of over-exposure, that combined with its quality in producing softness it is apt to cause fog. It is for this reason that the hydroquinone, which is much slower as a developing agent and contrast producer, is added. The color produced with eikonogen is blue-black negatives, with a very fine grain. It is non-poisonous, and does not stain the fingers. Eikonogen comes in small white-gray crystals, and dissolves slowly. From the description of these two developing agents you will readily see why they have been combined - one producing too much softness, and the other too much contrast when used alone. Eikonogen alone would also develop too rapidly, and would be apt to fog the negative; hydroquinone alone would develop too slowly, and produce too much contrast. Consequently, the two combined in the proportion given in the formula supplies a well-balanced developer.
Carbonate And Sulphite Of Soda. The sulphite and carbonate of soda act exactly as they do in the pyro developer, the carbonate opens the pores, and the sulphite controls the color of the negative.
420. Caustic Soda (Sodium Hydrate.) - Is a white, transparent, brittle substance very soluble in water, and strongly alkaline, used in the developer as an additional accelerator.