Etching reveals and delineates grain boundaries and other microstructural features that are not apparent on the as-polished surface. The two most common types of etching in ceramography are selective chemical corrosion, and a thermal treatment that causes relief.  As an example, alumina can be chemically etched by immersion in boiling concentrated phosphoric acid for 30&ndash;60 s, or thermally etched in a furnace for 20&ndash;40 min at  in air.  The plastic encapsulation must be removed before thermal etching.  The alumina in Fig. 1 was thermally etched.
Alternatively, non-cubic ceramics can be prepared as thin sections, also known as petrography, for examination by polarized transmitted light microscopy.  In this technique, the specimen is sawed to ~1 mm thick, glued to a microscope slide, and ground or sawed (e.g., by microtome) to a thickness (x) approaching 30 µm.  A cover slip is glued onto the exposed surface.  The adhesives, such as epoxy or Canada balsam resin, must have approximately the same refractive index (η ≈ 1.54) as glass.  Most ceramics have a very small absorption coefficient (α ≈ 0.5 cm for alumina in Fig. 2) in the Beer–Lambert law below, and can be viewed in transmitted light.  Cubic ceramics, e.g. yttria-stabilized zirconia and spinel, have the same refractive index in all crystallographic directions and appear, therefore, black when the microscope's polarizer is 90° out of phase with its analyzer.
:  (Beer–Lambert eqn)
Ceramographic specimens are electrical insulators in most cases, and must be coated with a conductive ~10-nm layer of metal or carbon for electron microscopy, after polishing and etching.  Gold or Au-Pd alloy from a sputter coater or evaporative coater also improves the reflection of visible light from the polished surface under a microscope, by the Fresnel formula below.  Bare alumina (η ≈ 1.77, k ≈ 10) has a negligible extinction coefficient and reflects only 8% of the incident light from the microscope, as in Fig. 1.  Gold-coated (η ≈ 0.82, k ≈ 1.59 @ λ = 500 nm) alumina reflects 44% in air, 39% in immersion oil.
:  (Fresnel eqn)..