BK-7 Glass

Bk-7 Glass, an Optical Borosilicate Crown Glass, has a high transmission and is free of bubbles and inclusions. With superior physical properties, it can be polished like leaded glass, making it suitable for lenses and prisms for lasers and optics. It weighs about 15% less than full (24%) leaded glass (crystal).
Since BK7 performs well in all chemical tests, no special handling is required, resulting in lower manufacturing costs.

SiO2=69.13% B2O3=10.75% BaO=3.07% Na2O=10.40% K2O=6.29% As2O3=0.36%

Thermal Properties
Thermal Coefficient of Expansion (0/300℃): 70 X 10 -7/℃
Annealing Point 550℃ / 1022 °F
Softening Point 719℃ / 1326 °F
Strain Point 624℃ / 1155 °F
Mechanical Properties
Density 2.51 g/cm3
Knoop Hardness 610
Young's Modulus 8.20 X 109 dynes/mm2
Poisson's Ratio 0.208
Optical Properties
Refractive Index 1.509
Chemical Properties
Hydrolytic Resistance class 2
Acid Resistance class 1
Alkali Resistance class 1
Electrical Properties
Dielectric Constant N/A
Resistivity N/A
Dielectric Strength N/A

BK-7 Glass provides excellent transmittance through-out the visible and near infrared spectra and down to 350 nm in the ultraviolet: