380 HANDBOOK OF PHOTOGRAPHY 



to replace the exhausted solution with a new one, rather than to attempt to replenish 

 it by the addition of acid. 



Most developers contain carbonates which may be decomposed bj'' the acid in the 

 stop bath with the liberation of carbon dioxide. The evolution of carbon dioxide is 

 prone to cause the emulsion to blister, especially if the materials are not agitated upon 

 immersion in the stop bath. It may, therefore, be good practice to rinse the films or 

 prints for a few seconds in clear water before placing them in the stop bath in order to 

 remove as much of the developer as possible. This procedure will, however, dilute the 

 stop bath. 



Since the stop bath prevents development, no harm will be done if the films or 

 prints are left in the stop bath until it is convenient to transfer them to the fixing 

 solution. This procedure may be convenient where a small quantity of work is being 

 done, since it permits the operator to prevent contamination of the developer with 

 hypo which might be picked up on the fingers from the fixing solution. 



Fixing. Purpose of Fixing. — After development of the photosensitive material 

 (either film, plate, or paper) the emulsion contains the metallic silver image which has 

 been reduced from a silver halide during the process of development; it also contains 

 grains of silver haUde which have not been acted upon by light during exposure and 

 which, consequently, have not been changed to metallic silver during development. 

 This remaining silver salt is still sensitive to light, even if the photosensitive material 

 is desensitized, and the salt will therefore ultimately become dark and mask the image. 



To prevent this action from taking place and to assure that the image will remain 

 permanent, a fixing bath is employed. Permanance of the image may be accomplished 

 either by (1) rendering the unexposed silver salt as permanent as the metallic silver 

 and preventing its subsequent exposure or bj^ (2) dissolving away the salt from the 

 metallic silver image. In practice the latter method is universally employed. 



The silver salts for which the fixing bath must be solvent are silver chloride, silver 

 bromide, and silver iodide. Silver bromide is the most common, although all three 

 may be present. All three of these halides are soluble in potassium cyanide (KCN), 

 ammonium hydroxide (NH4OH), and sodium thiosulphite (Na2S203), but not all 

 these solvents are suitable for ordinary photographic work. Potassium cyanide, for 

 example, is highly poisonous, has a softening effect on the gelatin and is also a solvent 

 for metallic silver. Ammonium hydroxide produces the objectionable and pungent 

 odor of ammonia (NH3). Sodium thiosulphite, commonly known as "hypo" from its 

 other chemical name, sodium hyposulphite, is the onlj^ other alternative of the com- 

 monly available materials and is used almost exclusively in photographic fixing baths. 



Chemistry of Fixing. — The chemistry of fixation, like practically all photographic 

 chemistry, is rather involved and highly complicated, especially since many chemical 

 compounds may be formed during the fixing process. No attempt will be made to 

 indicate in any detail the possible chemical reactions which are thought to take place; 

 an equation or two indicating the rationale of the essential operations may not, how- 

 ever, be entirely out of order. 



The exposed and developed, but unfixed, photographic material contains deposits 

 of metallic silver (Ag) as well as of silver halide which we may designate as AgF, if we 

 understand the symbol Y to represent the appropriate halogen, usually bromide. 

 Thus the developed film contains the compounds Ag + AgF, and it is desired to 

 remove the halide leaving only the metallic silver. If the developed film is immersed 

 in a solution of sodium thiosulphate Na2S203, the chemical reactions may be 

 described as 



3Ag + SAgF + XNa2S203 = 3Ag + 3NaF + Na6Ag3(S203)4 + 



(X - 4)Na2S203 (1) 



