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Magnesite is Calcined at different temperatures produces magnesium oxide of different reactivity. High temperatures (1500 – 2000 °C) diminish the available surface area and produces dead-burned (often called dead burnt) magnesia, an unreactive form used as a refractory. Calcining temperatures (1000 – 1500 °C) produces hard-burned magnesia, which has limited reactivity and calcining at lower temperature, (700 – 1000 °C) produces light-burned magnesia, a reactive form, also known as caustic calcined magnesia. Although some decomposition of the carbonate to oxide occurs at temperatures below 700 °C, the resulting material appears to reabsorb carbon dioxide from the air.