122 HANDBOOK OF PHOTOGRAPHY 



Glass for photographic use should be flat and free from any imperfections which 

 affect the transparency, such as bubbles or striations, and it should preferably be prac- 

 tically colorless. For most purposes, specially selected cast or drawn glass is suitable, 

 though for certain special cases, such as very large size plates used in photomechanical 

 reproduction, ground and polished plate glass is used. 



For the ordinary plate sizes, up to 8 by 10 in., glass approximately 0.05 to 0.06 in. 

 thick is generally used. The larger sizes are usually coated on heavier glass, the thick- 

 nesses used for one type of plate being shown in Table I. 



Table I. — Thickness of Glass Used in Plates for the Graphic Arts 



Plate Size Thickness, In. 

 8 X 10 0.060 



10 X 12 0.075 



11 X 14 0.085 



14 X 17 0.085 . 



16X20 to 24X31 0.130(1^) 



26X34 to SOX 40 0.190(^6) 



Occasionally plates are supplied on specially thin glass that has a certain amount of 

 flexibility, hence permitting the use of the plates in instruments, such as certain types 

 of spectrographs, where the focal plane is slightly curved. 



Since the emulsion will not stick to plain glass, plates are subbed with some mate- 

 rial svich as sodium silicate or hardened gelatin to which the film emulsion can adhere. 



Film. — Photographic film base is normally composed of a cellulose derivative, 

 usuallj^ cellulose nitrate or cellulose acetate, mixed with certain materials, called plas- 

 ticizers, which keep the film flexible and transparent. Camphor is the usual plasticizer 

 for nitrate film, while various high-boiling-point organic liquids, such as certain of the 

 hydroxy-substituted amines or certain organic phthalates, are used with cellulose 

 acetate. 



Film is light in weight, flexible, strong, and transparent. 



The flexibility allows it to be used in long strips that can be fed through the camera 

 or other apparatus, slowlj'' or rapidly as desired, bj'^ more or less simple mechanical 

 means. The light weight and mechanical toughness also give film an advantage over 

 glass by simplifying the problems of storage, handling, and transportation, particularly 

 when any considerable quantities are involved. Film is not completely inert but 

 swells slightly in the processing solutions, and shrinks on drying, but this is of impor- 

 tance only where the most extreme precision in reproduction of geometrical form 

 is required, as in some astronomical work and in aerial mapping. Materials for these 

 purposes are available on film base which has been specially treated so that the dimen- 

 sional changes are practically zero. 



Cellulose acetate film has the advantage of low imflammability, since it presents no 

 greater hazard than an equal weight of newspaper. In the past, its physical and 

 mechanical properties were not so desirable as those of nitrate film. In recent years 

 these properties have been much improved, and acetate film is finding increasing use, 

 particularly where the fire hazard offered bj^ nitrate film is of consequence. 



The thickness of film base varies according to the type of film and manner in which 

 it will be used. Thus roll film must be sufficiently flexible to pass through the camera 

 readily. Cut films, on the other hand, should be sufficiently stiff to remain flat in the 

 film holder and to handle conveniently during and after processing. Motion-picture 

 film must be sufficientlj^ flexible to pass through the camera or projector, though the 

 curves are not so sharp as in some roll-film cameras, and the film must be strong enough 

 to withstand the strain imposed by the pull-down mechanism. Some representative 

 values are shown in Table II. 



