The composition is formulated as follows, in percent by weight: synthetic corundum material: 76.5 to 90.0 kaolinite: 2.1 to 5.7 feldspar: 2.8 to 7.6 adhesive and moistening ingredient: 3.0 to 4.5 calcium borosilicate frit: 2.1 to 5.7 Calcium borosilicate frit compositon, in percent by weight, is as follows: silicon oxide: 22.0 to 38.0 aluminium oxide: 14.5 to 15.5 boron oxide: 25.0 to 35.0 calcium oxide: 12.5 to 17.5 sodium oxide: 4.0 to 7.5 potassium oxide: 2.0 to 3.5 The boron oxide-to-calcium oxide ratio is within 1.5 to 2.5.
