The sintering of a composite of MgO-B2O3-Al2O3 glass and Al2O3 filler is terminated due to the crystallization of Al4B2O9 in the glass. The densification of a composite of MgO-B2O3-Al2O3 glass and Al2O3 filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO-B2O3-Al2O3 glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT-200 degrees C) of 4.4 x 10(-6) K-1, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.