test were as shown in table 1 it was clearly understood from table 1 that the sintered article obtained in comparative experiment 1 was inferior in density and hardness to the sintered articles obtained in examples 1 to 14 sintered articles were produced by following the procedure of example 1 except that high purity not less than 99 readily sintering alumina powder produced by the bayer process in the form of minute particles having a diameter of not more than 1 u03bcm and incorporating therein mgo and tio x wherein x 1 to 2 as a sintering auxiliary were used as inorganic compounds the conditions of production and the results of test for physical properties were as shown in table 2 a sintered article was produced with the ratio of the cbn powder to the alumina powder and the sintering conditions set as shown in table 2 they were tested for physical properties the results of the test were as shown in table 2 it was clearly understood from table 2 that the sintered article obtained in comparative experiment 2 was inferior in density and hardness to the sintered articles obtained in examples 5 to 28 the powder x ray diffraction patterns obtained of the sintered articles of examples 1 5 15 and 19 to determine the crystal phases of the articles showed no discernible.