FIXING, WASHING, AND DRYING 401 



An excellent example of this method of washing is that in which a thin film of fresh 

 water is made to flow continuously over both sides of a film hanging vertically. The 

 water dripping from the film is discarded. A much less satisfactory example of the 

 continuous- flow method is represented by a tank in which the inlet and outlet pipes 

 are close to one another and at the same end of the tank. In this case it is possible for 

 a large part of the incoming fresh water to circulate immediately to the outlet drain 

 without becoming effective in washing the film which remains in the central portion 

 of the tank in relatively stagnant water. A better system for tank washing is to have 

 the inlet at the bottom of one end of the tank, and the outlet at the top and opposite 

 end of the tank. Any water which is discharged from such a system must have 

 passed across the tank and, in so doing, washed the film. Moreover the tank is always 

 filled with water so long as any water discharge takes place. 



In the multiple-bath method, the negatives or prints are placed in a tank or tray 

 with a fixed amount of water which is usually not changed so long as the photosensitive 

 materials remain in them. During the time the photosensitive materials remain in 

 any given tank or tray, the negatives should be thoroughly agitated; after a lapse of 

 some time, the negatives are removed to another tank of fresh water. In so doing, 

 they will carry over into the fresh bath some of the solution from the bath from which 

 they were removed. While it is unavoidable that some of the solution be carried over, 

 much more thorough and rapid washing can be effected if as much of the solution as 

 possible is removed from the film before immersing into the succeeding wash. The 

 transferring of the prints from one tank of water to another containing fresh water is 

 continued until the prints are thoroughly washed. Usually six washes of about 5 min. 

 each are sufficient to produce satisfactory washing. 



Continuous Washing.— Experiments indicate that with thorough agitation of the 

 photographic material in the wash water, the sodium thiosulphate diffuses exponen- 

 tially, so that in equal time intervals, equal percentages of hypo are removed. With 

 an exponential law representing the diffusion in the washing process it is theoretically 

 impossible to remove all the undesired product. Practically, however, we may 

 remove as much of the hypo as we desire by washing for a sufficiently long period of 

 time. 



To derive a quantitative expression for this method of washing, let Mo be the 

 amount of material, e.g., hypo, originally present on the film, and let M be the amount 

 washed out in time t. (The values of Mo and M may be expressed as initial and final 

 concentrations of the solution or as initial and final masses of material.) Then the net 

 change in the amount of material on the film, dM, during a given time interval dt 

 will be 



dM = -k{Mo - M)dt (8) 



where fc is a proportionality factor (sometimes called the constant of elimination) 

 which depends upon the degree of agitation and the type of emulsion and usually 

 varies between 0.1 and 0.15 for negatives, according to Warwick. It can be shown 

 that the solution of this equation is 



'' = ^«^^ MT^l = 2-3^259 log. J^f^) ^^^ 



from which the time required to obtain any desired degree of washing becomes 



i = (1) log. ,/"\,r = ^^^^^T^ log.o ../"\.. (10) 



Mo ^ 2.30259 Mo 



Mo - M 0.1 ^^"' (Mo - M) 



In this equation. Mo indicates the initial amount (or concentration) and {Mo — M) 

 indicates the final amount or concentration of the material which remains. 



