PHOTOGRAPHIC MATERIALS 123 



Table II. — Thickness of Film Base 



Thickness, M^. 



Roll film " 85 



Motion-pictui'e film 140 



Cut film 200 



The emulsion will not adhere directly to cellulose nitrate or acetate. Therefore, 

 the surface of the fUm base must be specialljf prepared to ensure adhesion of the emul- 

 sion.i This is usually done by coating with a thin layer of substratum or "sub" 

 containing an adhesive, such as gelatin, a solvent for the adhesive, and a solvent for 

 the film base. Since the sub contains both solvents and nonsolvents for the gelatin 

 as well as for the cellulose esters, the relative proportions of the constituents must be 

 chosen with care. 



Paper. — Paper is an ideal support for photographic prints since it is more or less 

 flexible, opaque, economical, and durable. The requirements are rather rigid, since 

 the paper must not affect the keeping properties of the emulsion or the permanence 

 of the developed image and must be unaffected by the immersion in alkaline developers 

 and acid-fixing baths. Photographic paper is made from specially pure stock, with 

 particular attention to freedom from metallic impurities and any substances which 

 might affect the emulsion or the silver image, either directly or through decomposition 

 products. Originally rag stock was much used, but with the increasingly severe chem- 

 ical treatments given cloth fibers in dyeing, cleaning, and stripping the dyes, particu- 

 larly the use of chlorine bleaches, the rag fibers are often rather badly broken down; 

 hence high-grade sulphite pulp has been found to produce more stable papers. 



Papers for positive emulsions are made in both single weight and double weight 

 and are provided with a coating of baryta in hardened gelatin to present a smooth 

 surface for the emulsion, and to increase the reflecting power of the paper. The 

 texture of the print surface is controlled largely bj^ the composition and handling of 

 this baryta coating. 



For some applications, where very rapid processing and drying is important, the 

 paper base is waterproofed by impregnation with a lacquer of cellulose nitrate or 

 cellulose acetate. 



Requirements for Special Cases. — While certain properties, such as inertness toward 

 the emulsion or the processing solutions, are required of all support materials, certain 

 particular properties may become of great importance in materials for specialized 

 uses. Thus the motion pictures require a flexible base with considerable mechanical 

 toughness, exhibiting little shrinkage during processing or on aging. Ordinary 

 amateur photography, making use almost exclusively of roll film and film packs, 

 is based on the use of a flexible fllm support, but factors such as toughness and low 

 shrinkage are of less importance. Certain very precise fields on the other hand, such 

 as astronomy and precision cartography, can tolerate no shrinkage or distortion; 

 hence they utilize only emulsions on glass. In aerial mapping, where low shrinkage 

 is important, the factors of weight and flexibility are also of great importance, so that 

 specially prepared low-shrinkage film is used. 



In most fields of photography, however, these refinements are of no significance, 

 and the choice of support material can be made on the basis of greatest convenience 

 in use, which in most cases means film. 



Protective and Other Additional Coatings. — In addition to the base or mechanical 

 support, and the sensitive emulsion, many commercial materials have other layers 

 which improve the photographic or mechanical properties in various ways. 



Gelatin Backing (Noncurling). — Since the gelatin emulsion swells on wetting and 

 shrinks on drying, while the film base is substantially unaffected, strains are produced 



1 FucHS, E., Preliminary Preparation of Photographic Layers, Phot. Ind., 34, .552 (1930). 



