7.4 Striae

Striae are thread-like veins or cords that are visual indications of abruptly varying density. Striae can also be considered as a lack of homogeneity caused by incomplete stirring of the molten glass. Some glasses contain components that evaporate during melting causing layers of varying density, and therefore the appearance of parallel striae.

Striations in glass are detected by means of a striaescope consisting of a point source of light and a collimating lens. Polished samples are examined at several different angles in the striaescope. They are then compared with the standards and graded. These established standard glasses are of a high order of quality and are certified to U.S. military specification MIL-G-174B.