FIXING, WASHING, AND DRYING 383 



tration of the fixing baths used in practice vary so widely throughout their life that 

 an evaluation of the constants is likely to be merely of academic interest. 



So far as the physical action and effects taking place are concerned, the rate of 

 fixation is determined largely by the penetration of the sodium thiosulphate through 

 the film, because the chemical action takes place rapidly compared with the rate of 

 penetration of the hypo. Of course, the rate of fixing depends upon the agitation of 

 the fixing solution over the photosensitive material, as well as the freshness of the 

 fixing solution. If agitation is lacking, the rate of fixing will be less than that which 

 might otherwise be expected. In an unagitated fixing bath the chemical reactions 

 take place only at the surface of the material, and any additional removal of silver 

 halide then becomes dependent upon the relatively slow process of diffusion rather 

 than upon mere solubility. 



Requirements of a Fixing Bath. — To be perfectly satisfactory, a fixing bath should 

 fulfill the following six requirements: 



1. Throughout its life, the fixing bath should dissolve the silver halide from the 

 emulsion without staining the photographic material. 



2. The fixing bath should not precipitate sulphur, even when used at fairly high 

 temperatures. 



3. Throughout its useful life, the fixing bath should not precipitate aluminum 

 sulphate. 



4. The bath should not produce blisters in the gelatin coating of the film. 



5. The fixing bath should provide a satisfactory degree of hardening. 



6. The fixing bath should be economical, i.e., its cost in terms of the amount of 

 material fixed should be as low as possible. 



Uneven stains are objectionable because of the denser image they produce in the 

 final print. Uniform, colored stains may not be injurious in a negative from which 

 positive prints are to be made. They may be objectionable in positive motion-picture 

 film or in paper prints. 



If the second requirement is not met, more or less colloidal sulphur will be pre- 

 cipitated in the gelatin of the fUm. The precipitated sulphur cannot be removed by 

 washing, since sulphur is insoluble in water; it will, however, combine with the silver 

 of the fixed negative, ultimately causing silver sulphide stains and fading of the image. 



A precipitate of aluminum sulphite forms when the fixing bath becomes neutralized 

 by the alkaline of the developer which is carried over into the fixing bath. The 

 aluminum sulphite occurs as a reaction of the alum hardening agent with the alkaline 

 sulphite preservative. 



Blisters are usually produced in the fixing bath by the too rapid formation of carbon 

 dioxide gas. Bubbles or gas pockets of carbon dioxide may then form within the 

 gelatin layer which may finally break. The carbon dioxide is formed by the carbonate 

 of the developer carried over into the fixing bath reacting with the acid of the acid 

 hardening fixing solution. 



Hardening is useful primarily to retard the swelling of the gelatin coating so that 

 the film may be dried more rapidly than if it is not hardened. In unswollen and 

 hardened film a minimum of water is present, and consequently drying may proceed 

 rapidly. Hardened films may also be dried at higher temperature than unhardened 

 film without the potential danger of softening or melting the gelatin coating. Harden- 

 ing may not be required if the gelatin does not swell so much during the processing 

 that the film may be handled and dried satisfactorily. However, the hardening agent 

 may be conveniently added to the fixing bath so easily that it is worth while to employ 

 the hardener as a safeguard. 



Classification of Fixing Baths. — Several types of fixing baths are in common use 

 and may be classified as follows: 



