FIXING, WASHING, AND DRYING 393 



standing the disruptive action of the evolved gas. Blisters are also likely to occur (a) 

 if the fixing bath contains an excess of acid and the films are not rinsed thoroughly, 

 (6) if a strongly acid rinse bath is used, or (c) especially during hot weather. 



According to Crabtree and Hartt, the tendency of the fixing bath to produce 

 blisters is governed by (a) the quantity of sulphite and carbonate contained in the 

 film when immersed in the fixing bath, (fe) the rate of agitation of the film when 

 immersed in the fixing bath, (c) the nature and thickness of the gelatin film, (d) the 

 duration of the rinse or wash used between development and fixing, (e) the degree 

 of swelling of the gelatin when first placed in the fixing bath, and (/) the acidity of the 

 fixing bath. 



3. Prevention. — Rinsing the film in plain water after development but prior to 

 fixing will remove developer alkali and reduce the amount of carbon dioxide formed. 

 The films may then be hardened, if necessary, before fixing. The fixing bath should 

 not contain an excess of acid. Strongly acid rinse baths should be avoided. Fixing 

 should be carried out, if possible, at temperatures not exceeding 70°F., and the film 

 should be agitated while immersed in the fixing bath. 



4. Remedy. — None. 



Dichroic Fog. 



1. Distinguishing Characteristic. — Yellowish-pink stain when film is viewed by 

 reflected light. 



2. Cause. — Dichroic fog raaj be caused (a) by old or exhausted fixing bath con- 

 taining an excess of dissolved silver salt or (6) by a fixing bath which does not contain 

 acid. 



3. Prevention. — Dichroic fog does not occur in fresh fixing baths. Replace fixing 

 bath with a fresh solution, and use a fixing bath containing acid. Rinse film in water 

 between development and fixing. Keep the fixing bath between 65 and 70°F. 



4. Remedy. — See Chap. XVII, Defects in Negatives and Prints, page 518. 



Failure to Harden. 



1. Distinguishing Characteristic. — Film is soft rather than solidly hard. 



2. Cause. — Insufficient hardening may be the result (a) of insufficient or impure 

 alum, or impure alum which does not contain the correct proportion of aluminum 

 sulphate; (6) of the fact that the bath is alkaline or neutral rather than acid; or (c) 

 of the presence of too much acid or sulphate. 



3. Prevention. — Depending upon the cause, prevention may be (a) use of suffi- 

 cient amount of alum containing the proper proportion of aluminum sulphate. The 

 hardening action of the alum is due to the aluminum sulphate. Since some grades of 

 alum do not contain the proper proportion of aluminum sulphate, a larger portion of 

 alum must be used with the deficient grades in order to prepare a bath of the adequate 

 sulphate concentration. The hardening increases as the quantity of alum is increased. 

 (6) The hardening bath should always be maintained acid, but excess of acid should be 

 avoided. For a given amount of alum, the hardening increases as the quantitj^ of 

 acetic acid is increased until a maximum is reached, after which increasing the acetic 

 acid content decreases the degree of hardening. In order to produce a fixing bath of 

 long life before aluminum sulphite precipitates, a certain minimum quantity of acetic 

 acid must be added to the fixing solution. However, the amount of acid required for 

 this purpose is usually greater than that required to produce maximum hardening. 

 (c) Reduce the amount of acid or sulphate, or increase the amount of alum relative to 

 other ingredients if the failure of the bath to harden the film is attributed to cause (c) 

 above. 



