The sol-gel process is a chemical approach to making optical materials at low temperature. Through hydrolysis and condensation reactions, a metal alkoxide such as tetraethyl-orthosilicate (TEOS) is converted largely to high surface area silica gel at room tempera-ture. After drying, the result is a rigid monolithic shape of bulk density about half that of conventional fused silica. The reduced weight of the shape is due to interconnected microporosity. The average pore size is generally smaller than 10 nm, and the material is transparent to visible light.
L. C. Klein,
T. A. Gallo,
G. J. Garvey,
"Transparent Microporous Silica By The Sol-Gel Process", Proc. SPIE 0505, Advances in Optical Materials, (26 December 1984); doi: 10.1117/12.964629; https://doi.org/10.1117/12.964629