The Application of Vacuum Shadowing to the Examination and Photography of Small Objects

Abstract

The technique of vacuum shadowing small objects and the advantages of this method in light photography are detailed. The ultra-thin metal-carbon films thus prepared show greater contrast and resolution than acetate peel replicas.Two main problems arise in the photography of small objects. First, with curved surfaces the depth of field may not be sufficient for perfect focus to be attained over the entire object. Second, the relief of the object may be so slight as to result in insufficient contrast for good photographic reproduction.The first problem has been largely solved by the technique of cellulose acetate peels, a method familiar to most geologists. The second problem, that of increasing contrast, has been examined in a number of ways; Dollar (1948), for example, described a technique for use in studying low relief on etched or abraded rock surfaces. He accentuated topographic contrast by filling cavities in the rock surface with a suitable pigment, which, in the process of making a peel, adhered to the cellulose acetate. Other techniques to improve contrast have been described by Triebel (1947) andFournier (1956) ; none of these methods, however, is completely satisfactory. A further difficulty arises from the effect of light scattering by the acetate film at higher magnifications, which seriously affects the quality of the photographs.The method described below is based on a technique commonly used in electron microscopy, but which has not, as far as the authors know, been used in light photography. Initially, a peel is made of the object to be studied. With hard objects such as sand grains, it is possible to obtain very clean surfaces by repeated replication (the early replicas being discarded) which frees the surface of relatively loose material. The peel is then placed in a vacuum evaporator with a pressure of about 10' 4 mm. of mercury, and a heavy metal such as chromium is vaporized at an angle (usually 45 degrees) upon the peel. Such shadowing results in a metal film which is thickest on surfaces perpendicular to the direction of shadowing, and lightest on surfaces parallel to that direction. Carbon is then vaporized perpendicularly upon the film to give it added strength. Finally the peel is removed with acetone, leaving a very fine metal-carbon film which has a thickness of less than 200 angstroms and is, of course, transparent. The film may be picked up on a very fine GEOL. MAG. VOL. 101 NO. 4 .