Cer-Vit aka CerVit or Cervit or Cer-Vit C-101 (from 'ceramic-vitreous'), is a glass-ceramic material made up of oxides of silicon, aluminium and lithium, with ultra low coefficient of thermal expansion, used for telescope mirrors and lenses. It was made by Owens-Illinois Owens Corning in 1967, and discontinued in 1978.  Cer-Vit and Zerodur are two-phase ceramics consisting of a high percentage of crystalline silica-based material and the remainder of non-crystalline glass. These ceramics also include silica-based glasses such as Homosil fused quartz, soda-lime glass, and the like. 
Glass-ceramics are made by heating glass to rearrange its atomic structure into a more regular pattern. As partially crystalline materials, they can tolerate high temperatures, thermal shock and chemicals. They are useful in many applications, such as heat-resistant cookware, turbine engines, electronic equipment, and missile nose cones. Other glass-ceramics are Pyroceram and ULE 7971(titanium silicate) developed by Corning Glass, and opaque white Hercuvit, closely related to Cer-Vit, developed for cooktops by Pittsburgh Plate Glass. 
Cer-Vit was flawed by the inclusion of crystals and small bubbles (0.1–0.2 mm diameter) resulting from the manufacturing process. In most applications flaws smaller than 0.1 mm are not serious, while larger defects are only considered if they lie on or very close to the working surface.  The use of Cervit has largely been superseded by that of Zerodur and Sitall.
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