• Borosilicate glass has a relatively low coefficient of thermal expansion. Ohara E6 glass has a coefficient of thermal expansion of 2.8 x10-6/C. This expansion rate keeps the mirror figure accurate as the temperature varies. • The working point of the borosilicate glass is low enough that we can mold it into the complex honeycomb structures at temperatures which are easy to obtain. At 1180 C (2156 F), the viscosity of the glass is about the same as cold honey at ~ 15 C. • The borosilicate glass is relatively inexpensive compared to other types of glass and glass-ceramics used in making that mirror blanks. While the Mirror Lab orders special production E6 borosilicate glass from Ohara, mirror blanks could be, and have been, cast from common borosilicate glass, like that used in cookware, i.e., Pyrex (Corning 7740), with a coefficient of thermal expansion of 3.3 x 10-6/C. • Borosilicate glass has good resistance to attack by chemicals. This property is important because the polished gla