Individual structural elements are worked out in polished metal specimen (metallographic grinding) to highlight contrasts by subtractive manufacturing.
Structural elements may be for example: carbides, the matrix (the basic structural mass), grain boundaries, various solid-solution phases, non-metal inclusions, intermetallic phases and compounds, segregations and the like.
Fundamentally, etching is divided in two different principles; etching without modification of the ground surface, which includes photo etching (dark-field, polarized light, phase contrast), and etching with modification of the ground surface, such as electro-chemical etching (anodic etching, potentiostatic etching, classic chemical etching) and physical etching (ion milling, vapor deposition on interference layers, thermal etching).
The classic principle of chemical etching is the oldest ground surface-contrasting method and thus is commonly merely referred to as etching. The different types of etchants corrode the structure elements to varying extents due to their different chemical compositions and thus create contrasts and make the matrix structures visible, which can be observed when viewed in an appropriately magnifying microscope.