Black crazes — white crazes?

The origin of the curious “inverted contrast” often observed in transmission electron micrographs of crazes in rubber-toughened thermoplastics was determined by using scanning transmission electron microscopy with X-ray fluorescence analysis. The “black crazes” seen in the micrographs contain high concentrations of osmium metal which is introduced through OsO4 used to stain the rubber phase. Treatment of stained sections with a strong oxidizing agent (NaIO4) removes a sufficient amount of osmium to reveal the commonly observed tufty craze microstructure with the expected contrast of light crazes in a darker background.