Abstract Objective. To assess the effect of mould and glass casting temperatures on the biaxial flexural strength (BFS) of two different types of castable glass–ceramic, using existing laboratory equipment and techniques. Methods. Two castable glass–ceramic materials were evaluated. One glass (LG3) is based on SiO 2–Al 2O 3–P 2O 5–CaO–CaF 2, and is similar in composition to glasses used in the manufacture of glass–ionomer cements. The other glass (SG3) is based on SiO 2–K 2O–Na 2O–CaO–CaF 2, and is a canasite-based material. Both materials were used to produce discs of 12 mm diameter and 2 mm thickness using the same lost-wax casting process as used for metal castings. Mould temperatures of between 500°C and 1000°C and glass casting temperatures of between 1100°C and 1450°C were evaluated. The cast discs were cerammed and the biaxial flexural strength determined with a Lloyd 2000 R tester. Results. A significant difference was found for the BFS in the range of mould temperatures evaluated, with the optimum investment mould temperature being 590°C for LG3 and 610°C for SG3 ( p=0.0002 and p=0.019, respectively). No significant differences were seen between any of the glass casting temperatures evaluated. Significance. The mould temperature for castable glass–ceramic materials produced using the lost-wax casting process can have a significant effect on BFS. The optimum mould temperature may differ slightly depending on the type of material being used. The glass casting temperature of these materials does not appear to have a significant effect on BFS.