4. Special lenses

4.1 Glass ceramics

These are materials derived from glass melting and shaping processes. Glass is subjected to two consecutive heat treatments, which give rise to germs and then cause them to grow.

Glass-ceramics offer several advantages. Firstly, the object is shaped in a pasty state using conventional glassmaking techniques (molding, pressing). Glass-ceramics have no residual porosity. Its mechanical properties are generally far superior to those of the glass from which it is derived. The crystals developed are very small (0.05 to 0.5 µm) and the percentage of crystalline phase can reach 75 to 80% by volume.

Glass-ceramics are the result of controlled glass devitrification (