The calcined insulator of an inorganic composite obtd. by moulding and heat treating under pressurised conditions comprises a starting material contg. 10-50, pref. 20-40 wt.% mica powder as base material; 10-50, pref. 20-40 wt.% MgO powder as a charging material; and 25-60, pref. 30-50 wt.% binder comprising a mixt. of boric anhydride, ZnO and boric acid as 20-60 pts. wt.% 100 pts. binder. The mica is pref. natural phlogopite natural muscovite or synthetic mica. The MgO is pref. electrofused, sintered or baked MgO. The binder pref. consists of 20-60, esp. 30-50 pts. boric acid, 5-30, esp. 10-25 pts. boric anhydride; and 30-60 esp. 35-50 pts. ZnO. The calcined body is pref. moulded at 170-180 deg.C, 150-200Kg/cm2.
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