FIXING, WASHING, AND DRYING 399 



washing is run into a large settling tank or if it is properly filtered before use, most of 

 the suspended matter will be removed. 



Dissolved gases will sometimes produce blisters if the water is warm, or if the film 

 is not sufficiently hardened in the fixing bath. When present in sufficient quantity, 

 dissolved extracts sometimes produce stains which are very difficult to remove. 



So far as is known, small traces of impurities which may be left in the gelatin 

 coating of motion-picture negatives or positive film are not liable to impair the prop- 

 erties of the film seriously over a period of 4 or 5 years. However, as an added pre- 

 caution, valuable film should be washed finallj^ in distilled water. 



Water Purification. — Water may be brought to various states of purification bj^: 

 (1) distillation, (2) boiling, (3) filtration, and (4) chemical treatment. Distillation 

 produces the purest water, but stills of adequate capacitj^ are expensive, especially 

 where large quantities of water are required, and the distilling process is slow. When- 

 ever it can be readily obtained, it is advisable to use distilled water for the mixing of 

 photographic solutions. It is not necessary that films be washed throughout the 

 entirety of the washing process in distilled water, however. Removal of soluble 

 materials may be accomplished by washing the films in ordinary tap water, as a 

 general rule. For those cases in which complete washing with pure water is necessary, 

 these preliminary washes may be followed with one or two baths using distilled water. 



Except in those cases in which the water contains an excess quantity of dissolved 

 salts, sufficient purification can usually be obtained by boiling the water and permitting 

 this to stand while cooling. The boiling process coagulates most of the colloidal 

 vegetable and animal matter, changes certain lime salts into an insoluble condition 

 in which they settle out, and drives out dissolved gases which may be present, such 

 as hydrogen sulphide, air, etc. Dissolved substances are, of course, not removed in 

 this process. After the water has settled, it may be filtered through paper filters or 

 fine-mesh cloth. A more rapid method of separating the settled particles from the 

 supernatant liquid is to decant or siphon off the latter. 



Filtration of the water supply may be used to remove suspended or undissolved 

 particles, but the dissolved gases and colloidal matter cannot be removed by this 

 process, unless the water has been boiled or otherwise treated to drive off dissolved 

 gas and coagulate colloidal particles. Where only very small quantities of water are 

 required, the ordinary paper filter or fine-mesh cloth fitted into a funnel may be 

 employed. Where larger quantities of water are required, a barrel filled with sand, 

 charcoal, or other filtering agents may be used. Many types of filters are available 

 commercially. 



If large quantities of water are required, chemical treatment is the most practical 

 method of water purification. Coagulation of slime, which carries down suspended 

 particles, may be effected by adding potassium alum in the proportions of 0.25 g. per 1., 

 or 15 gr. per gal. This method clears the solution quickly but does not remove dis- 

 solved salts. The addition of this small quantity of alum has been found to produce no 

 harmful effects when such chemically treated water is used in the preparation of 

 developing or fixing baths. 



Solutions of sodium oxalate, sodium phosphate, or sodium sulphite may be used to 

 coagulate sHme and precipitate calcium and magnesium salts. These solutions are 

 added to the water supply until no further precipitate is formed. This method does 

 not remove dissolved salts. 



Washing. Purpose of Washing.- — The purpose of washing is to remove from the 

 negatives or prints all or a part of the soluble chemicals or solutions which adhere 

 to them in processing. Although in most cases negatives or prints are washed to 

 remove from them the chemicals of the fixing bath, this is not always the case. Some- 

 times, for instance, the photosensitive materials are washed after development but 



