Silica glass or vitreous silica is of considerable technical importance as it has a very low thermal expansion. This difficult-to-make glass contains tiny holes created using acids and is used for filtration. Porous glasses of this kind are commonly known as Vycor.
Porous Glasses - Vycor
Glasses having the desirable properties of fused silica have been on the market for nearly a half century under the trademark Vycor (Corning). These glasses are almost pure vitreous silica. They are prepared by a unique process discovered by Hood and Nordberg that circumvents the need for high temperatures in melting and forming.
A relatively soft alkali-borosilicate glass is melted in a conventional manner and is then pressed, drawn, or blown into the desired but oversized shape by standard processes used in glass production. The resultant work-piece, which occasionally is given additional finishing operations, is subjected to a heat treatment above the annealing point but below the temperature that would produce deformation. During this heat treatment, two continuous closely intermingled glassy phases are produced. One phase is rich in alkali and boric oxide and is readily soluble in acids. The other phase is rich in silica and is insoluble. After heat treatment, the work-piece is immersed in a hot dilute acid solution. The soluble phase is slowly dissolved, leaving behind a porous high-silica skeleton.
The resulting porous article is commonly known as thirsty or porous glass. In the final step of the process, the work-piece is slowly heated to >1200°C (>2190°F) whereby the porous structure is consolidated into a clear impervious glass known as Vycor brand 96% SiOz glass or reconstructed glass.
Ref: "Porous and Reconstructed Glasses", Engineered Materials Handbook, Volume 4, Ceramic and Glasses, p427-32, 1992
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