Last Updated: Nov 22, 2016 Views: 252
From Dr. Robert Brill, Research Scientist Emeritus:
Purity -- in the sense of homogeneity -- is normally a rather subjective property. One just examines the glass by eye or under low magnification to see if it is free of "stones", "seed" (the glassmakers' nickname for small bubbles), and cords (streamers or waviness or flow lines in the glass). Cords are produced by differences in refractive index which, in turn, can be produced by even small differences in composition.
For production purposes or quality control, one might count the number of defects per unit volume of the glass and match them to arbitrary standards.