TECHNIQUE OF DEVELOPMENT 356 



The complete timed system is rarely applied to practical pictorial photography, 

 but it is approximated in varying degrees. Tray rocking is sometimes accomplished 

 by a shelf tipping with attached pendulum as driver, and sometimes by direct manual 

 effort, but these are rarely carried to the extent frequently used for sensitometric 

 testing. 



Brushed Development and Roller Squeegees. — A second method for thorough and 

 complete agitation of developer during tray development substitutes the brushing 

 of the emulsion surface for the washing effect obtained by the tray-rocking procedures. 

 For purposes of reproducibility this also requires a timed routine of operations. The 

 brushing of the emulsion surface also introduces new hazards to the development. 

 The brush must be soft to avoid scratching the emulsion. It may introduce foreign 

 chemicals into the developer, as it is difficult to keep it thoroughly clean. These 

 troubles are not insurmountable and brushed development is one of the best methods 

 for securing uniform development within one development group. There is little or 

 no evidence that the final comparisons between successive developments will be better 

 than can be obtained by other means as that depends so greatly on other factors, such 

 as the precision of timing, correctness of temperature, similarity of batches of devel- 

 oper used, condition of fixer, temperature of wash water, and drying conditions. 



This method of developer agitation is little used except in work where greatest 

 uniformity is required. This is much more commonly needed in scientific and sensi- 

 tometric work than in pictorial work. 



Allied to the brushing method is the use of velvet-covered roller squeegee, rolled 

 over the emulsion surface. This replaces the sliding contact of the soft brush by the 

 rolling contact of the soft squeegee. The hazards are very similar, and its use similarly 

 limited. 



Blade Squeegees. Windshield-wiper Methods. — Functionally the purposes of any 

 of these agitation methods is to secure frequent and rapid change of the developer in 

 contact with the emulsion. With this in mind, squeegees shaped like windshield-wiper 

 blades have been used with the blade nearly, but not quite, in contact with the 

 emulsion. As the blade is moved vigorously along, high turbulence is produced at the 

 emulsion surface giving the uniform high agitation desired. In some cases the velocity 

 of the moving blade has been made higher than the velocity of a wave in the tray of 

 developer. The result was very uniform development, but the technique was hard on 

 the surroundings, owing to splashing and slopping of the developer. 



Mechanically, the guiding has been sometimes accomplished by setting the plate 

 or film in a recess in the bottom of the tray and then using a solid blade, rolling or slid- 

 ing over the recess which is made of su'ch depth as to give small uniform clearance 

 between the blade and the emulsion surface. 



This system is also confined to work requiring a higher degree of development 

 uniformity than usually needed for pictorial purposes. 



Mechanical Agitation and Circulation Systems in Tank Development. — Aside from 

 mechanical devices primarily intended to mechanize the systems just described, there 

 have been many systems introduced to provide mechanically the agitation and circula- 

 tion necessary for agitated development. The details depend upon the type of work 

 being done, but in general they involve only a few basic elements. These are: (1) 

 motion of the emulsion and its support through the developer; (2) low-velocity circula- 

 tion, such as frequently associated with temperature-control systems; (3) high-velocitj^ 

 jets and circulation; (4) squeegees. These systems have been brought to their greatest 

 technical perfection for work in and associated with the motion-picture industry, and 

 consequently the details given primarily describe methods and practices found there. 

 If the expense is warranted, these methods can be adapted to other photographic 

 products, but the large amount of film processed, which is handled in long lengths, has 



