SCHOTT Glas manufactured more than 200 ZERODUR segments for various Telescopes: 84 pcs. 1.900 mm diameter × 76 mm menisci for Keck I and Keck II, 96 pcs. 1.180 mm hexagonal × 56 mm blanks for HET, 42 pcs. 1.870 mm hexagonal × 83 mm blanks for GTC, and produces at present, 40 pcs. 1.100 mm hexagonal × 82 mm blanks for LAMOST. During the production period for the GTC mirror blanks SCHOTT developed improved casting techniques to generate castings with hexagonal shape to realize near net shape processing. This reduces the necessary machining time in a very cost effective manner. Additionally slicing the castings into single blanks reduces the subsequent ceramising time and cost. Results of the important quality characteristics like CTE, CTE homogeneity, internal stress and geometrical dimensions of the GTC blanks will be presented. Finally the upscaling of the present advanced production technique to the needs of the Extremely Large Telescopes will be discussed.