Micrograph 1 (and 2). Most etching reagents attack different phases with different rates and sharp steps will form at the phase boundaries. In the microscope they will show up as black lines. This micrograph is taken on an Fe-C alloy containing martensite, ferrite and cementite. Martensite is big white areas. A thin band of white ferrite is shown vertically. It has been attacked more than the martensite and is thus at a lower level. Thus the black contours. Cementite is found in a fine mixture with ferrite (so-called pearlite). Cementite is not at all attacked and should thus look white, also, but it is standing above the ferrite, which was attacked. The pearlite areas thus appear black when the lines cannot be resolved.

Micrograph 2. This micrograph is taken on the same area as picture 1 but with oblique illumination coming from the left. It is thus easy to see that the band of ferrite has been attacked more than the martensite.

Micrograph 4. The etching reagent may be so sensitive to the orientation of a crystal that some crystals are hardly attacked at all. This is the case for so-called nital (1 % HNO3 in alcohol) on {100} surfaces of bcc Fe (ferrite). The white area in the center is a grain of ferrite of this orientation and comparison with picture 3 reveals that it is a small proeutectoid particle of ferrite which has later developed into pearlite by collaborating with cementite on its continued growth.