Many metals etched with standard reagents to reveal the grain boundaries often yield only a high percentage of the boundaries rather than all of them. Color tint etchants, however, reveal the grain structure completely. In the case of metals with annealing twins, such as copper, it can be very difficult to rate the grain size when a standard etchant reveals only a portion of the grain and twin boundaries. In fact, it can be quite difficult to make a precise measurement of the grain size, even manually with such a specimen, because distinguishing between grain and twin boundaries (the latter must be ignored in the measurement) is not always simple. With a color-etched microstructure, however, it is relatively easy to separate grain boundaries from twin boundaries (at least manually). Further, the films grow as a function of crystal orientation. Therefore, one can detect any preferred crystallographic orientation by the narrowness of the color range present. If a wide range of colors is present in a random pattern, the crystal orientation is random. If a narrow range of colors is present in the grains, then a preferred orientation is present.
Specimen preparation must be better when using color methods than for black-and-white methods because the epitaxially grown films are sensitive to residual preparation-induced damage. This same level of preparation is required in image-analysis work and can be easily obtained by a knowledgeable metallographer with the proper equipment. Electrolytic polishing is not required to get damage-free surfaces.
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