Colemanite reacts to water loss (in the ceramic firing process)

50-250C (122-482F) | 80-250C (176-482F) | 120C (248F) | 150C (302F) | 180C (356F) | 185C (365F) | 200-1000C (392-1832F) | 200-450C (392-842F) | 200C (392F) | 210-280C (410-536F) | 260C (500F) | 290C (554F) | 300C (572F) | 300-330C (572-626F) | 300C (572F) | 370C (698F) | 370-700C (698-1292F) | 400-600C (752-1112F) | | 425-650C (797-1202F) | 470-1200C (878-2192F) | 500-600C (932-1112F) | 512C (953F) | 540-600C (1004-1112F) | 650-900C (1202-1652F) | 750-850C (1382-1562F) | 750-1000C (1382-1832F) | 760C (1400F) | 760C (1400F) | 787C (1448F) | 800-1100C (1472-2012F) | 815C (1499F) | 815C (1499F) | 843C (1549F) | 850-950C (1562-1742F) | 850C (1562F) | 850C (1562F) | 870-900C (1598-1652F) | 871C (1599F) | 900C (1652F) | 900-1000C (1652-1832F) | 900C (1652F) | 926C (1698F) | 932C (1709F) | 954C (1749F) | 980C (1796F) | 982C (1799F) | 990C (1814F) | 1025-1325C (1877-2417F) | 1025C (1877F) | 1050C (1922F) | 1050C (1922F) | 1065-1120C (1949-2048F) | 1082C (1979F) | 1100C (2012F) | 1100C (2012F) | 1100C (2012F) | 1100C (2012F) | 1300C (2372F) | 1325C (2417F) | 1330C (2426F) | 1360C (2480F) | 1418-1428C (2584-2602F) | 1420C (2588F) | 1510C (2750F) | 1550C (2822F) | 1565C (2849F) | 1650C (3002F) | 1785C (3245F) | 1990C (3614F) | 2300C (4172F) | 2320C (4208F)

It has a strong reaction to the state change associated with water elimination at 400C.

400C (752F)