They have extremely low coefficients of expansion over the usual range that amateurs use their telescopes. The low coefficient of expansion is achieved through a combination of chemistry and heat-treating. The heat-treating allows a partial crystallization of the glass. The crystalline component has a negative coefficient of expansion that balances the positive coefficeint of expansion of the amorphous glass component. With coefficients of expansion more than 100 times lower than pyrex, glass ceramics make ideal telescope mirrors.
Several manufacturers make their own proprietary low expansion glass ceramics.
Zerodur properties Zerodur is a machinable glass ceramic produced by Schott Glass Technologies. Using common glass manufacturing technology, raw materials are melted, refined, homogenized, and then hot formed. After cooling and annealing, a temperature treatment transforms the glass into a glass ceramic through controlled volume crystallization. The material contains 70-78 weight percent crystalline phase with a high quartz structure and a negative linear thermal expansion. The glass phase has a positive coefficient of expansion.