Semi-thin sections are often stained by Richardson technique with a blue dye, Methylene blue Azure II.
Basophilic and osmiophilic structures stain blue, and metachromatic structures turn red-violet.
Semi-thin sections of tissue specimens are generally prepared for the light microscopic examination before ultra-thin sections are prepared for examination under the transmission electron microscope (TEM). Semi-thin sections are thinner than other sections commonly used for light microscopy.
The embedding agent for TEM examination is a plastic resin, which is harder than paraffin. Since plastic resin cannot be detached easily and without creating artifacts, in contrast to paraffin, there exist only few suitable staining techniques for tissue embedded in plastic resin. Richardson stain is one of them.