398 HANDBOOK OF PHOTOGRAPHY 



chemical fog, but it may be removed by boiling the water. Sea water contains about 

 3.5 per cent of soluble salts, mostly halides, which retard action of photographic solu- 

 tion. Consequently sea water should be avoided. 



Effect of Impurities in Developtnent. — -If a developing solution is compounded with 

 water containing calcium salts, a white precipitate of calcium sulphite may form on 

 standing. If the developer contains sodium bisulphide or potassium metabisulphite, 

 fine, needlelike calcium sulphite crystals may be precipitated as a sludge. In either 

 case this sludge is harmless if it is permitted to settle, after which the clear solution 

 may be decanted from the sludge. The developer is, nevertheless robbed of the sul- 

 phite to the extent of the sludge which has formed, but the effect is negligible except 

 in the case of developers of low alkalinit^^ Borax developers are sensitive to small 

 changes in alkalinity, however, so that it maj'^ be necessar}' to make allowance for the 

 reduction of sulphite when such developers are prepared with calcium salts. If the 

 developer is agitated, the sludge may become precipitated on the photographic 

 material. 



Sodiuna or potassium sulphide in the water used for preparing developing solutions 

 will produce objectionable chemical fog, even though only verj^ small quantities of 

 sulphides are present. Removal of sulphides may be accomplished by treating the 

 water with lead acetate to produce insoluble lead sulphide. Copper sulphate is 

 sometimes added to water supplies for the purpose of killing vegetable and bacteri- 

 ological matter. The amount of copper sulphate added is not usually sufficient to be 

 harmful, although it maj^ cause aerial fog. 



The presence of chlorides or bromides in the water supply used in compounding 

 developers exerts a restraining action on the developer which may, however, be com- 

 pensated by properly adjusting the amounts of the other ingredients. Unless present 

 in excess, magnesium salts will not be precipitated. Other salts have little or no 

 effect upon the action of the developer. 



The presence of sulphur in water from which developers are made will produce 

 fog, as a result of the formation of sodium sulphide from the interaction of the sulphur 

 with the carbonate of the developer. By boiling the water, the sulphur can usually 

 be made to coagulate, after which it maj^ be removed bj' filtration. 



Animal matter in the water from which developers are compounded usually 

 precipitates when the developer is mixed. Certain tj'pes of bacteria form growths 

 which act on the sulphite to form sodium sulphite which fogs photographic materials. 



Effect of Impurities in Fixation. — Calcium and magnesium sulphite are soluble in 

 acetic acid and therefore are not precipitated in the fixing bath. Other dissolved 

 salts, such as bicarbonates, chlorides, and sulphates, are harmless. Suspended matter 

 in the form of dirt, iron rust, and certain types of vegetable and animal matter usualh^ 

 coagulates and settles out in the fixing bath if allowed to stand. While most sus- 

 pended substances have practically no effect on the photographic properties of the 

 fixing baths, the particles maj' settle on the film, retarding fixing action locally, and 

 causing spots and stains. Extracts from vegetable matter or dissolved gases do not 

 affect the photographic properties of a fixing bath but may cause blisters or stains. 



Effect of Impurities on Washing.^ — Dissolved salts of magnesium, calcium, or 

 potassium in the water used for washing often cause trouble bj'' crystallizing on the 

 film in drying and, while not always visible, maj^ detract from the transparency of the 

 image. Water which is free of dissolved salts will also cause markings on the film if 

 it is allowed to remain in droplets on either side of the film during drying. It is 

 important, therefore, to remove all excess water from the film before drying. 



Suspended mineral, vegetable, and animal matter generally leaves a scum on the 

 film unless the gelatin surface is wiped previous to drying. If the water used for the 



'This section takpn from "Motion Picture Laboratory Practice," Eastman Kodak Co. 



